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GMC Data Report No. 140
Preliminary results of 24 apatite fission track analyses of samples from four wells in the National Petroleum Reserve of Alaska, which are as fol 1 ows' Husky Oil NPR Operations Inc. Tunalik Test Well No. 1; Husky Oil NPR Operations Inc. Walakpa Test Well No. 1; Husky Oil NPR Operations Inc. Walakpa Test Well No. 2; and Husky Oil NPR Operations Inc. Inigok Test Well No. 1. APPENDIX A Fission track analysis' a summary of the technique and interpretation of results From Paul O'Sullivan APPENDIX B Apatite fission track analysis sample preparation From Paul O'Sullivan Received 14 August 1989 Total of 32 pages in report ...... - ...... ~' ~- ~-~ "~ '- Additional--32 :pgs .in'Appendix A Geologi'c Mat~riais"]Cente'f'-'Dal~a RePOrt No.-140 !'- -. Preliminary Results of 24 -Apatit.e Fission Track Analyses of Samples From Four Wells in the National Petroleum Reserve of .Alaska. Husky Tunalik Test Well #1 Husky Walapka Test Wells #1 and 02 Husky Inigok Test Well 01 GMC by Paul B.O'Sullivan Alaska Division of Geological and Geophysical Surveys August, 1989 Data Report No.. 114-0 Page 1/32. Contents and Well Ix)cation Data Introduction List of Samples Tunalik #1 (8 samples) Tunalik Length Distributions Walapka #1 and #2 (7 samples) Walapka Length Distributions Inigok #1 (9 samples) Inigok Length Distributions CONTENTS Page 2 3 4 5 13 15 21 22 31 WELL LOCATION DATA TunaHk Test Well #1 - located SW t/4 of Section 20, T10N, R36W, Umiat Meridian Walapka Test Well #1 - located SE 1/4 of Section 9, T20N, R19W, Umiat Meridian Walapka Test Well #2 - located SW 1/4 of Section 30, T20N, R19W, Umiat Meridian Inigok Test Well #1 - located NE 1/4 of Section 34, T8N, RSW, Umiat Meridian GMC Data Report No. 140 Page 2/32 3 INTRODUCIION This is a preliminary report of apafite fission track analysis data of samples from four wells drilled in the National Petroleum Reserve of Alaska. During 1988, sandstone, siltstone, and conglomerate samples were collected from drill-~ore located at the State of Alaska's Geologic Materials Center in Eagle River. Apatite grains were separated from the samples and analyzed in Melbourne Ausualia at the LaTrobe University Fission Track Research Laboratory. All separatiohs and analyses were completed by the author as part of an ongoing PhD project funded by the U.S. Minerals Management Service Continental Margins Program. Each analysis includes two parts: 1) age report; and 2) track length distributions. The age report shows a listing of the individual grain ages, the resulting age and pertinent information used in determining the age. A guide to read the information is as follows: pos 07A-KEMIK Irradiation: Crystal NS NI NA Ratio U(ppm) RHOs F.T. Age(Ma) CHI Squared p(chi squared) Variance of SQR NS/NI Mean Ratio Pooled Age Mean Age -Sample number and unit collected -h-house number for grouping samples from the same kradiafion package -Number of each grain counted -Number of spontaneous tracks counted -Number of induced tracks counted -Number of area units counted in grain -Ratio of (NS/NI) for each grain -Uranium concentration of each grain -Density of spontaneous tracks (per em2) -Density of induced tracks (per em2) -Individual grain ages -Statistical test for determining multiple grain populations -probability of less than 5% indicates multiple grain populations -Statistical comparison of values of NS or NI for all grains -Pooled ratio of (NS/ND. Uses total number of spontaneous and induced tracks counted for whole sample. Value used in age calculation if sample is of a single population -Average ratio of (NS~ for grains -Age calculated using NS/NI(single population) -Age ealedated Using "Mean Ratio" (multiple populations) The track length distributions for each sample are histograms showing the relative numbers of tracks measured at a particular length, the mean length of the tracks measured, the standard deviation of the tracks measured, and the total number of tracks measured for the sample (N). :GMC' Data-Report"No. 140' · . '.Page -3/32 LI~T OF SAMPLE~ (by depth) Tunalik Sampf 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS # 100A 102A 99A 108A 10lA 103A 104A 107A 106A 105A Unit Depth fit) Results (data) Eehooka Fm. 16,946 Age only Sag River SS. ' 15,418 Age and Length Ivishak Fm. 14,852 Age and Length Kingak Shale 11,692 Age and Length Kingak Shale 10,932 Age and Length Torok Fm. 9,501 Not Dateable Torok Fro. 7,880 Not Dateable Nanushuk Gp. 6,506 'Age and Length Nanushuk Gp. 5,558 Age and Length Nanushuk Gp. 3,294 Age and Length Walapka #1 and g2 Sample 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS 88 POS # li2A lllA l15A l14A ll0A l13A 109A Unit Depth (ft) Pebble Shale 3,749 Pebble Shale 3,707 Argillite Basement 3,659 Barrow SS. 3,100 Pebble Shale 2,632 Pebble Shale 2,087 Torok Fm. 262 Results (data) Age and Length Combined w/112A Age and Length Age and Length Age and Length Age and Length Age and Length Inigok #1 Sample # Unit 88 POS 116A Kekikutk Cong. 88 POS 117A Kekiktuk Cong. 88 POS 118A Echooka Fm. 88 POS 119A Fire Creek SS. 88 POS 120A Fire Creek SS. 88 POS 12lA Kingak Shale 88 POS 122A Kingak Shale 88 POS 123A Torok Fro. 88 POS 124A Torok Fro. 88 POS 125A Torok Fro. 88 POS 126A Nanushuk Gp. 88 POS 127A Nanushuk Gp. Depth fit) 20,092 19,369 13,832 12,735 12,501 10,296 9,435 8,849 8,237 5,006 3,078 2,632 Results (data) Not Dateable Age Only Not Dateable Age and Length Age Only Not Dateable Age and Length Age and Length Age and Length Age and Length Age and Length Age and Length GMc': Data Report' No. 140 Page'4/32 - TUNALIK WELL (in numerical order) 88 POS 99A - IVISHAK FM. - 14,852' IRRADIATION LU021 COUNTED BY: POS SLIDE NUMBER Ol No. Ns Ni Na RATIO U(ppm) RHOs RHOi I 0 17 12 0.000 9.0 0.000E+00 1.656E+06 2 0 14 12 0.000 7.4 0.000E+00 1.364E+06 3 0 16 14 0.000 7.3 0.000E+00 1.336E+06 4 0 18 15 0.000 7.6 0.000E+00 1.403E+06 5 2 16 6 0.125 17.0 3.896E+05 3.117E+06 6 0 15 10 0.000 9.5 0.000E+00 1.753E+06 7 1 14 15 0.071 5.9 7.792E+04 1.091E+06 8 0 20 12 0.000 10.6 0.000E+00 1.948E+06 9 0 18 12 0.000 9.5 0.000E+00 1.753E+06 10 1 10 12 0.100 5.3 9.740E+04 9.740E+05 11 0 20 15 0.000 8.5 0.000E+00 1.558E+06 12 0 19 12 0.000 10.1 0.000E+00 1.851E+06 13 2 24 15 0.083 10.2 1.558E+05 1.870E+06 14 0 14 21 0.000 4.2 0.000E+00 7.792E+05 15 0 16 12 0.000 8.5 0.000E+00 1.558E+06 16 0 19 12 0.000 10.1 0.000E+00 1.851E-fi)6 17 1 17 15 0.059 7.2 7.79'2E+04 1.325E+06 18 0 15 20 0.000 4.8 0.000E+00 8.766E+05 , 7 302 7.9 3.381E+04 1.459E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 19.44956 WITH 17 DEGR~FS OF FREEDOM P(chi squared) = 303 % CORRELATION COEFFI~ = 0.097 VARIANCE OF SQR(Ns) = .3007498 VARIANCE OF SQR(Ni) = .1459386 Ns/Ni = 0.023 4- 0.009 MEAN RATIO = 0.024 ____. 0.010 POOLED AGE = 9.8 4- 3.8 Ma MEANAGE = 10.44- 4.2Ma Ages calculated using a zeta of 352.7 4- 3.9 for SRM612 glass RHO D = 2.408E+06cm-2; ND = 11421 V.T. GE(Ma) 0.04- 0.0 0.0~ 0.0 · 0.0~ 0.0 0.0~ 0.0 53.04- 39.7 0.0~_ 0.0 30.34- 3lA 0.04- 0.0 0.04- 0.0 42.44- 44.5 0.04- 0.0 0.04- 0.0 35.44- 26.O O.O4- 0.0 0.0~_ 0.0 0.04- 0.0 25.04- 25.7 0.04- 0.0 -. GMC' Data Report No. '140 page: 5/3'2 - 6 88 POS 100A - ECHOOKA bM. - 16,946' IRRADIATION LU021 SLIDE NUMBER 02 COUNTED BY: POS No. Ns Ni Na RATIO U(l~m) RHOs RHOi 1 0 16 12 0.000 8.5' 0.000E+00 1.558E+06 2 1 15 36 0.067 2.6 3.24TE+04 4.870E+05 3 0 10 6 0.000 10.6 0.000E+00 1.948E+06 4 0 18 14 0.000 8.2 0.000E+00 1.503E+06 5 2 33 6 0.061 35.0 3.896E+05 6.428E+06 6 0 50 12 0.000 26.5 0.000E+00 4.870E+06 7 1 9 14 0.111 4.1 8.348E+04 7~514E+05 8 0 23 16 0.000 9.1 0.000E+00 1.680E+06 9 0 19 18 0.000 6.7 0.000E+00 1.234E+06 10 0 31 20 0.000 9.9 0.000E+00 1.812E+06 11 0 17 12 0.000 9.0 0.000E+00 1.656E+06 12 0 8 6 0.000 8.5 0.000E+00 1.558E+06 13 0 20 8 0.000 15.9 0.000E+00 2.922E+06 14 0 31 16 0.000 12.3 0.000E+00 2.264E+06 15 0 16 8 0.000 12.7 0.000E+00 2.338E+06 F.T.AGE(Ma) 0.02:0.0 28.3+ 29.2 0.0-~: 0.0 0.0~_ 0.0 25.7+ 18.8 0.05:0.0 47.1+ 49.7 0.0-~_ 0.0 0.0~-_ 0.0 0.05:0.0 0.0~-_ 0.0 0.0-~-_ 0.0 0.0-~_ 0.0 0.0-~_ 0.0 0.0~-_ 0.0 4 316 9.8 2.292E+04 1.810E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED-- 18.37251 WITH 14 DEGREES OF FREEDOM P(chi squared) = 19.0 % CORRELATION COEFFICIENT = 0.062 -'VARIANCE OF SQR(Ns) = :2302054 - · VARIANCE OF SQRCNi)= 1317921 Ns/NJ = 0.013 + 0.006 MEAN RATIO = 0.016 +__ 0.009 POOLED AGE = 5.5 + 2.8 Ma MEAN AGE = 6.9+ 3.9 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.480E+06cm-2; ND = 11421 GMC Dal~a Report.No. i40 Page 6/32 88 POS 10lA - KINGAK SHALE - 10,932' IRRADIATION LU021 SLIDE NUMBER 03 COUNTED BY: POS No. Ns Ni Na RATIO U(,ppm) RHOs RHOi 1 0 7 6 0.000 7.2[ 0.000E+00 1.364E+06 2 I 34 15 0.029 14A 7.792E+04 2.649E+06 3 I 6 6 0.167 6.4 1.948E+05 1.169E+06 4 0 12 12 0.000 6.4 0.000E+00 1.169E+06 5 0 4 6 0.000 4.2 0.000E+00 7.792E+05 6 I 9 9 0.111 6.4 1.299E+05 1.169E+06 7 0 4 4 0.000 6.4 0.000E-I-00 t.169E+06 8 2 13 12 0.154 6.9 1.948E+05 1.266E406 9 2 17 -9 0.118 12.0 2.597E+05 2.208E+06 10 0 9 6 0.000 9.5 0.000E+00 1.753E+06 11 I 14 18 0.071 4.9 6.493E404 9.090E+05 12 0 7 8 0.000 5.6 0.000E400 1.023E+06 13 0 10 12 0.000 5.3 0.000E+00 9.740E+05 14 I 7 8 0.143 5.6 1.461E+05 1.023E+06 15 1 25 18 0.040 8.8 6.493E+04 1.623E+06 16 0 5 6 0.000 5.3 0.000E+00 9.740E+05 17 1 23 12 0.043 12.2 9.740E+04 2.240E~ 18 1 6 8 0.167 4.8 1.461E+05 8.766E+05 19 0 10 8 0.000 7.9 0.000E+00 1.461E+06 20 1 15 10 0.067 9.5 1.169E+05 1.753E+06 F.T.AGEO~m) 0.0-~_ 0.0 12.5i 12.7 70.5+ 76.2 0.0~: 0.0 0.0~-_ 0.0 47.1+ 49.7 0.0-~-_ 0.0 65.1+ 49.5 49.9~-_ 37.3 0.0-~_ 0.0 30.3+ 31.4 0.0-Z-_ 0.0 0.0~-_ 0.0 60.5+ 64.7 17.0-Z-_ 17.3 0.0-~_ 0.0 18.5+ 18.9 70.5+ 76.2 0.0~-_ 0.0 28.3+ 29.2 13 237 7.8 7.873E+04 1.435E+06 Area of bask: unit = 8.789E-07 ¢m-2 CI-ff SQUARED = 11.1525 WITH 19 DEGREES OFFREEDOM P(chi squared) = 91.9 % CORRELATION COEFFICIENT= 0.451 VARIANCE OF SQR(Ns) = .3160219 VARIANCE OF SQR(Ni)= 1.069143 Ns/bYl = 0.055 + 0.016 MEAN RATIO = 0.055 + 0.014 POOLED AGE = 24.2 + 6.9 Ma MEAN AGE = 24.5 + 6.3 Ma Ages calc.lated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.507E+06cm-2; ND = 11421 .. . .GMC Data Report NO. 140 'page 7/32 8 88 POS 102A - SAG RIVER SS. - 15,418' IRRADIATION LU021 SLIDE NUMBER 4 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) RHOs RHOi 1 0 16 12 0.000 8.5 0.000E+00 1.558E+06 2 1 15 6 0.067 15.9 1.948E+05 2.92ZE+06 3 0 9 6 0.000 9.5 0.000E+00 1.753E+06 4 0 19 8 0.0(~) 15.1 0.000E+00 2.776E+06 5 0 24 10 0.000 15.3 0.000E+00 2.805E+06 6 1 10 6 0.100 10.6 1.948E+05 1.948E+06 7 0 18 8 0.000 14.3 0.000E+00 2.630E+06 8 0 19 12 0.000 10.1 0.000E+00 1.851E+06 9 0 17 6 0.000 18.0 0.000E+00 3.312E+06 10 0 14 8 0.000 11.1 0.000E+00 2.045E+06 11 0 16 12 0.000 8.5 0.000E+00 1.558E+06 12 1 15 12 0.067 7.9 9.740E+04 1.461E+06 13 1 23 6 0.043 24.4 1.948E+05 4.480E+06 0.0~-_ 0.0 28.3+ 29.2 0.0-+ 0.0 0.0-+ 0.0 0.0~-_ 0.0 42.4_+ 44.5 0.0-+ 0.0 0.0~_ 0.0 0.0~_ 0.0 0.0-~_ 0.0 0.0~_ 0.0 28.3+ 29.2 18.5_+ 18.9 4 215 12.2 4.174E+04 2.244E+06 Area of basic unit = 8.789E-07 cm-2 CI-H SQUARED = 10.29022 WITH 12 DEGR~RS OF FREEDOM P(chi squared) = 59.1% CORRELATION COEFFICIENT = -0.127 VARIANCE OF SQR(Ns) = .2307692 VARIANCE OF SQR(Ni) = .2966563 Ns/NJ = 0.019 + 0.009 MEAN RATIO = 0.021 +_ 0.010 POOLED AGE = 8.3 + 4.2 Ma MEAN AGE = 9.5_+ 4.4 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.534E+06cm-2; ND = 11421 GMC Data Report No..140. · Page 8/32- - 9 88 POS 105A - NANUSHUK GROUP - 3,294' IRRADIATION LU021 SLIDE NUMBER 06 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) 1 11 28 28 0.393 6.~ 2 7 46 15 0.152 19.5 3 1 5 12 0.200 2.6 4 5 31 40 0.161 4.9 5 25 147 36 0.170 26.0 6 1 12 9 0.083 8.5 7 5 11 18 0.455 3.9 8 26 129 18 0.202 45.6 9 9 66 15 0.136 28.0 10 1 20 10 0.050 12.7 11 5 31 20 0.161 9.9 12 7 57 12 0.123 30.2 13 3 13 12 0.231 6.9 14 6 35 30 0.171 7.4 15 11 42 24 0.262 11.1 16 5 32 24 0.156 8.5 17 7 45 30 0.156 9.5 18 9 61 12 0.148 32.3 19 1 17 12 0.059 9.0 20 26 138 15 0.188 58.5 RHOs RHOi F.T.AOEOVIa) 4.592~+05 1.169E+0a 165.~ 58.8 5.45~-+05 3.584E+0a 64.4+ 26.2 9.740E+04 4.870E+05 84.6d: 92.6 1.461E+05 9.058E+05 68.3+ 32.9 8.116E+05 4.77ZE+06 72.0-+ 15.6 1.299E+05 1.558E+0~ 35.4+ 36.8 3.247E+05 7.142E+05 190.6-+102.8 1.688E+06 8.376E+06 85.9-+ 18.4 7.013E+05 5.143E+06 57.8-+ 20.5 1.169E+05 2.338E+06 21.~ 21.8 2.922E405 1.812E+06 68.3+ 32.9 6.818E+05 5.552E+06 52.1+ 20.9 2.922E+05 1.266E+06 97.5+ 62.5 2.338E+05 r.364E+06 72.6-+ 32.1 5.357E+05 2.045E+06 110.5_+ 37.5 2.435~+05 1.558E+0a ~.~ 31.8 2.727E+05 1.753E+06 65.9-+ 26.8 8.766~+05 5.941E+0~ 62.5_+ 22.3 9.740E+04 1.656E+06 25.0-+ 25.7 2.026~+0a 1.075~+07 79.7_+ 17.1 171 966 15.7 5.098E+05 2.880E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED= 16.1911 WITH 19 DEGRh-~-S OFFREEDOM P(chi squared) = 64.4 %' CORRELATION COEF~CIENT = 0.955 VARIANCE OF SQR(Ns) = 1.637335 VARIANCE OF SQR(Ni) = 7.792015 Ns/Ni = 0.177 _+ 0.015 MEAN RATIO = 0.183 + 0.022 POOLED AGE = 80.3 -+ 6.7 Ma MEAN AGE = 82.9 _+ 9.9 Ma Ages ealculated using a zeta of 352.7 _+ 3.9 for SRM612 glass RHO D = 2.588E+06cm-2; ND = 11421 GMC'Data 'Report No. 140 Page-'9~32 .c 10 88 POS 106A - NANUSHUK GROUP - 5,558' IRRADIATION LU021 SLIDE NUMBER 07 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) RHOs 1 3 19 28 0.158 4.5 1.252E+05 2 3 14 21 0.214 4.2 1.670E+05 3 4 20 20 0.200 6.4 2.338E+05 4 3 17 10 0.176 10.8 3.506E+05 5 6 37 12 0.162 19.6 5.844E+05 6 I 7 12 0.143 3.7 9.740E+04 7 5 58 21 0.086 17.6 2.783E+05 8 3 9 10 0.333 5.7 3.506E+05 9 1 8 12 0.125 4.2 9.740E+04 10 1 9 12 0.111 4.8 9.740E+04 11 5 58 18 0.086 20.5 3.247E+05 12 0 3 9 0.000 2.1 0.000E+00 13 1 13 15 0.077 5.5 7.792E+04 14 2 21 12 0.095 11.1 1.948E+05 15 2 12 16 0.167 4.8 1.461E+05 16 3 19 12 0.158 10.1 2.922E+05 17 4 21 20 0.190 6.7 2.338E+05 18 1 8 12 0.125 4.2 9.740E+04 19 5 41 15 0.122 17.4 3.896E+05 20 2 17 12 0.118 9.0 1.948E+05 RHOi F.T.AOE(Ma) 7.931E+05 66.9~: 41.5 7.792E+05 90.62 57.6 1.169E+06 84.6-+ 46.3 1.987E+06 74.7_+ 46.8 3.604E+06 68.7+ 30.2 6.818E+05 60.5_+ 64.7 3.228E+06 36.6-+ 17.1 1.052E+06 140.3+ 93.6 7.792E+05 53.0-+ 562. 8.766E+05 47.1_+ 49.7 3.766E+06 36.6:!: 17.1 3.896E+05 0.0-+ 0.0 1.013E+06 32.7+ 33.9 2.045E+06 40.4_+ 29.9 8.766E+05 70.5_+ 53.9 1.851E+06 66.9-+ 41.5 1.227E+06 80.6_+ 44.0 7.792E+05 53.0-~_ 56.2 3.195E+06 51.7+ 24.5 1.656E+06 49.9-+ 37.3 55 411 8.7 2.150E+05 1.607E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED= 6.974223 WITH 19 DEGREES OF FREEDOM P(chi squared) = 99A % CORRELATION COEFFI~ = 0.829 VARIANCE OF SQR(Ns) = .3558942 VARIANCE OF SQR(Ni)= 2.544674 Ns/Ni = 0.134 + 0.019 MEAN RATIO = 0.142 _+ 0.015 POOLED AGE = 61.4 __+ 8.8 Ma MEAN AGE = 65.3 _+ 6.9 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.616E+06cm-2; ND = 11421 · GMC' Data-RepOrt 'No. :ltl0 Page 10/32. 11 88 POS 107A - TOROK FM. - 6,506' IRRADIATION LU021 SLIDE NUMBER 08 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) . RHOs RHOi 1 0 11 30 0.000 2.3 0.000E+00 4.285E+05 2 I 6 24 0.167 1.6 4.870E+04 2.922E+05 3 3 18 12 0.167 9.5 2.922E+05 1.753E+06 4 28 280 28 0.100 63.6 1.169E+06 1.169E+07 5 4 24 8 0.167 19.1 5.844E+05 3.506E+06 6 3 23 15 0.130 9.7 2.338E+05 1.792E-t06 7 I 7 12 0.143 3.7 9.740E+04 6.818E+05 8 1 10 10 0.100 6.4 1.169E+05 1.169E-+06 9 8 49 24 0.163 13.0 3.896E+05 2.386E+06 10 11 77 18 0.143 27.2 7.142E+05 5.000E+06 11 5 17 10 0.294 10.8 5.844E+05 1.987E+06 12 3 22 8 0.136 17.5 4.383E+05 3.214E+06 13 1 10 28 0.100 2.3 4.174E+04 4.174E+05 14 3 19 12 0.158 10.1 2.922E+05 1.851E+06 15 4 23 10 0.174 14.6 4.675E+05 2.688E+06 16 1 8 10 0.125 5.1 1.169E+05 9.350E+05 17 8 50 24 0.160 13.2 3.896E+05 2.435E+06 18 5 27 12 0.185 14.3 4.870E+05 2.630E+06 19 12 85 15 0.141 36.0 ' 9.350E+05 6.623E+06 20 1 10 10 0.100 6.4 1.169E+05 1.169E+06 103 776 15.4 3.762E+05 2.834E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 8.110324 WITH 19 DEGRh-~.S OF FREEDOM P(chi squared) = 98.6 % CORRELATION COEFFICIENT = 0.976 VARIANCE OF SQR(Ns) = 1.391811 VARIANCE OF SQR(Ni) = 10.92243 Ns/Ni = 0.133 + 0.014 MEAN RATIO = 0.143 + 0.012 POOLED AGE = 61.8 + 6.5 Ma MEAN AGE = 66A -+ 5.7 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.653E+06em-2; ND = 11421 F.T. AGE(I~) 0.0-+ 0.0 70.5+ 76.2 70:5+ 44.0 42.4+ 8.4 70.5+ 38.1 55.3+ 33.9 60.5+ 64.7 42.4+ 44.5 69.1+ 26.4 60.5_+ 19.5 124.0-+_ 63.1 57.8_+ 35.6 42.4+ 44.5 66.9-+ 41.5 73.6-+ 39.9 53.0-+ 56.2 67.7-+ 25.8 78.3-+ 38.2 59.8-+ 18.5 42.4_+ 44.5 GMC Data Report No. 140 -- 88 POS 108A - KINGAK SHALE - 11.692 IRRADLATION LU021 SLIDE NUMBER 09 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) RHOs RHOi 1 6 44 40 0.136 7.0 1.753E+05 1.286E+06 2 1 25 12 0.040 13.2 9.740E+04 2A35E+06 3 0 25 15 0.000 10.6 0.000E+00 1.948E+06 4 1 15 12 0.067 7.9 9.740E+04 1.461E+06 5 0 19 15 0.000 8.1 0.000E+00 1.480E+06 6 0 7 6 0.000 7.4 0.000E+00 1.364E+06 7 1 30 15 0.033 12.7 7.792E+04 2.338E+06 8 0 12 8 0.000 9.5 0.000E+00 1.753E+06 9 2 19 12 0.105 10.1 1.948E+05 1.851E+06 10 1 25 15 0.040 10.6 7.792E+04 1.948E+06 11 1 12 10 0.083 7.6 1.169E+05 1.403E+06 12 1 8 12 0.125 4.2 9.740E+04 7.792E+05 13 0 15 10 0.000 9.5 0.000E+00 1.753E+06 F.T.AGE0Vla) 57.8+ 25.2 17.0-A: 17.3 0.0~-_ 0.0 28.3+ 29.2 0.0~ 0.0 0.O! 0.0 14~_t: 14.4 0.0L-_ 0.0 44.6± 33.2 17.0L-_ 17.3 35.4+ 36.8 53.0~ 56.2 0.0! 0.0 14 256 8.9 8.991E+04 1.644E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 11.07058 WITH 12 DEGREES OF FREEDOM P(chi squared) = 52.3 % CORRELATION COEFFI~ = 0.721 VARIANCE OF SQR(Ns) = .5429959 VARIANCE OF SQR(Ni) = 1.247556 Ns/NJ = 0.055 ± 0.015 MEAN RATIO = 0.048 + 0.014 POOLED AGE = 25.8 + 7.1 Ma MEW AGE = 22.9 ± 6.6 Ma Ages calcL,!a~ed using a zeta of 352.7 + 3.9 for SRM612 gla~ RHO D = 2.680E+06cm-2; ND = 11421 GMC Data: Report' No. 140 Page 12/32 40 30 ~20 - -- 88 POS 9~A - 13~ISHAK $$. MEAN -- 7.39 4- O.55 S.D. = 2.46 I 5 10 15 TRA(~K LEN(~TH (MI~-~ONS) 88 POS 102A- SAG ~le.R SS. MEAN = 10.04 + 1.29 S.D. = 2.88 N=5 ! I I 1 u vos ~o~- ~_$~: sma.~ ~.A~= ~0.00+~ s.~.=o.s9 [~] N=9 '""~ ! ! 88 POS 10SA - NANUSHUK GP. MEAN = 13.87 + 0.12 S.D. = 1.22 N= 106 .. ~-:-:-:-::::::: :-'5: ~:/~! ::::: ! 88 POS 106A - NANUSHUK GP. MEAN = 13.02 4- 0.16 S.D. = lA5 N= 80 '.'GMC 'Data Report No.' 140' 88 POS 107A- TOROK FM. MEAN = 11.80 + 0.19 S.D. = 1.72 N=78 .. ~..:~ ~.-..' ~&:..::::::~ ~. . . -.. ! ::::::::::::::::::::: · _ - - - ':' .... ".Pa§e.'13/32- . 30 I0 MEAN= 8.57 :~5 S.D. = 1.43 ~ N= 10 lam -- ~. 5 10 15 TRACK LENGTH (MICRONS) I ! --I' GMC Data' RePort No.' '140 Page 14/32 .- Walapka #1, 02 (in numerical order) 88 POS 109A - TOROK FM. - 262' - WALAPKA #1 IRRADIATION LU021 SLIDE NUMBER 10 COUNTED BY: POS No. Ns Ni Na RATIO U0pm) RHOs RHOi F.Ta~aE(Ma) 1 6 14 12 0.429 7.4 5.844E+05 1.364E+06 2 5 10 8 0.500 7.9 7.305E+05 1.461E+06 3 6 13 12 0.462 6.9 5.844E+05 1.266E+06 4 5 36 15 0.139 153 3.896E+05 2.805E+06 5 4 18 24 0.222 4.8 1.948E+05 8.766E+05 6 12 22 12 0.545 11.7 1.169E+06 2,143E+06 7 6 23 42 0.261 3.5 1.670E+05 6.400E+05 8 16 52 25 0.308 13.2 7.480E+05 2.431E+06 9 10 43 25 0.233 10.9 4.675E+05 2.010E+06 10 8 18 8 0.44~. 14.3 1.169E+06 2.630E+06 11 5 67 27 0.075 15.8 2.164E+05 2.900E+06 12 4 10 18 0.400 3.5 2.597E+05 6.493E+05 13 6 39 24 0.154 10.3 2.922E+05 1.899E+06 14 10 29 32 0.345 5.8 3.652E+05 1.059E+06 15 35 101 20 0.347 32.1 2.045E+06 5.902E-t06 16 4 8 15 0.500 3.4 3.117E+05 6.233E+05 17 2 8 16 0.250 3.2 1.461E+05 5.844E+05 18 6 15 14 0.400 6.8 5.009E+05 1.252E+06 19 6 14 12 0A29 7.4 5.844E+05 1.364E+06 20 4 18 20 0.222 5.7 2.338E+05 1.052E+06 179.92_ 87.8 209.4+114.7 193.5+ 95.5 58.8+ 28.1 93.92 51.9 228.1+ 81.9 110.1_+ 50.5 129.6d: 37.1 98.22 34.5 186.4+ 79.3 31.7+ 14,7 168.0+- 99.4 65.1+ 28.6 145.1+ 53.3 145.8+_ 28.7 209.4_+128.2 105.5+- 83.4 168.0i-_ 81.2 179.92 87.8 93.92 51.9 160 558 9.3 4.908E+05 1.712E+06 Area of basic unit = 8.789E-07 em-2 CHI SQUARED = 25.81069 wrITI 19 DEGREES OF FREEDOM P(chisquared) = 13.6 % CORRELATION COEFFICIENT = 0.798 VARIANCE OF SQR(Ns) = .9302874 VARIANCE OF SQR(Ni) = 3.684916 Ns/Ni = 0.987 +- 0.026 MEAN RATIO = 0.333 +- 0.030 POOLED AGE = 135.5 + 12.2 Ma MF, AN AGE = 157.1 + 14.2 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.707E+06cm-2; ND = 11421 GMC'Data Report No. 140 ".Page~ 15/3'2 '::--. '. 88 POS 110A - - 2,632'- WALAPKA #2 IRRADIATION LU021 SLIDE NUMBER 11 COUNTED BY: POS RHOs RHOi No. Ns Ni Na RATIO U(ppm) 1 3 30 25 0.100 7.6 1.403E+05 1.403E+06 2 7 58 21 0.121 17.6 3.896'E4~5 3.228E+06 3 8 41 25 0.195 10.4 3.740E+05 1.917E+06 4 8 24 25 0.333 6.1 3.740E+05 1.122E+06 5 1 11 40 0.091 1.7 2.922E+04 3.214E+05 6 13 60 42 0.217 9.1 3.618E+05 1.670E+06 7 8 48 21 0.167 14.5 4.452E+05 2.671E+06 8 35 113 30 0.310 23.9 1.364E+06 4.402E+06 9 18 21 24 0.857 5.6 8.766E+05 1.023E+06 10 3 13 30 0.231 :2.8 1.169E+05 5.065E+05 11 10 16 16 0.625 6.4 7.305E+05 1.169E+06 12 35 176 40 0.199 28.0 1.023E+06 5.143E+06 13 4 24 24 0.167 6.4 1.948E+05 1.169E+06 14 3 30 25 0.100 7.6 1.403E+05 1.403E+06 15 7 45 24 0.156 11.9 3.409E+05 2.191E+06 16 2 9 40 0.222 1.4 5.844E+04 2.630E+05 17 9 50 21 0.180 15.1 5.009E+05 2.783E+06 18 20 25 24 0.800 6.6 9.740E+05 1.217E+06 19 9 15 16 0.600 6.0 6.574E+05 1.096E+06 20 4 21 24 0.190 5.6 1.948E+05 1.023E+06 42.45:25.7 51.23:20.5 82.5+ 31.9 140.3+ 57.3 38.6+ 40.3 91.6+ 28.0 70.5+ 27.0 130.5+ 25.3 354.95:114.1 97.5+ 62.5 260.75:105.1 84.1+ 15.6 70.5+ 38.1 42.4+ 25.7 65.95:26.8 93.95:73.4 76.25:27.6 331.8+ 99.7 250.4+105.7 80.6+ 44.0 207 830 9.8 4.505E+05 1.806E+06 Area of basic unit = 8.789E-07 cra-2 CI-II SQUARED = 61.21191 WITH 19 DEGREES OF FREEDOM P(chi squared) = 0.0 % CORRELATION COEFFICIENT -- 0.809 VARIANCE OF SQR(Ns) = 1.826539 VARIANCE OF SQR(Ni) = 6.367232 Ns/NH - 0.249 5:0.019 MEAN RATIO = 0.293 + 0.052 POOLED AGE = 119.2 + 9.3 Ma MEAN AGE = 139.8 + 24.9 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.735E+06em-2; ND = 11421 GMC'-Data Report No ·'140 -Page 16/32 ' 88 POS 11 lA + 112A - - 3,70T+3,749' - WALAP~ #2 IRRADIATION LU021 SLIDE NUMBER 13 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) RHOs 1 16 49 21 0.327 14:8 8.905E+05 2 23 114 25 0.202 29.0 1.075E+06 3 3 22 15 0.136 9.3 2.338E+05 4 4 11 20 0.364 3.5 2.338E+05 5 8 28 10 0.286 17.8 9.350E+05 6 8 49 48 0.163 6.5 1.948E+05 7 9 23 40 0.391 3.7 2.630E+05 8 14 83 30 0.169 17.6 5.454E+05 9 17 60 24 0.283 15.9 8.279E+05 10 10 70 14 0.143 31.8 8.348E+05 11 2 9 12 0.222 4.8 1.948E+05 12 15 48 20 0.312 15.3 8.766E+05 13 10 41 30 0.244 8.7 3.896E+05 14 19 61 25 0.311 15.5 8.883E+05 15 8 26 18 0.308 9.2 5.195E+05 16 3 12 6 0.250 12.7 5.844E+05 17 27 81 40 0.333 12.9 7.889E+05 18 9 21 30 0.429 4.4 3.506E+05 19 17 51 20 0.333 16.2 9.935E+05 20 8 38 12 0.211 20.1 7.792E+05 RHOi 2.727E+06 5.330E+06 1.714E+06 6.428E+05 3.273E+06 1.193E+06 6.720E+05 3.234E+06 2.922E+06 5.844E+06 8.766E+05 2.805E+06 1.597E+06 2.852E+06 1.688E+06 2.338E+06 2.367E+06 8.181E+05 2.980E+06 3.701E+06 F.T.AGE~a) 137.5+ 39.6 85.3+ 19.5 57.84- 35.6 152.9-!-_ 89.3 120.5+ 48.3 69.1+ 26.4 164.44. 64.7 71.44. 20.7 119.54. 32.9 60.5+ 20.5 93.94. 73.4 131.64. 39.0 103.04. 36.4 131.9+ 34.5 129.64. 52A 105.5+ 68.1 140.34. 31.2 179.94. 71.7 140.3+ 39.4 89.04. 34.6 Area of 230 897 12.4 5.844E+05 2.279E+06 basic unit= 8.789E-07 cm-2 CHI SQUARED = 16.77851 wrrH 19 DEGREES OF FREEDOM P(chi squared) = 605 % CORRELATION COEFFICIENT = 0.838 VARIANCE OF SQR(Ns) = 1.079595 VARIANCE OF SQR(Ni) = 4.372144 Ns/Ni = 0.256 + 0.019 MEAN RATIO = 0.271 4- 0.019 POOLED AGE = 124.6 4- 9.3 Ma MEAN AGE = 131.5 + 9.1 Ma Ages calculated using a zeta of 352.7 4. 3.9 for SRM612 glass RHO D = 2.782E+06cm-2; ND = 11421 GMC~ Data RePort No."140 page- 18 88 POS l13A - - 2,087' - WALAPKA #1 IRRADIATION LU021 SLIDE NUMBER 14 COUNTED BY: POS No. N.s. Ni Na . RATIO U(ppm) RHOs RHOi . 1 5 15 12 0.333 7.6 4.870E+05 1.461E+06 2 2 7 12 0.286 3.7 1.948E+05 6.818E+05 3 4 I0 12 0.400 5.3 3.896E~5 9.740E+05 4 2 8 10 0.250 5.1 2.338E+05 9.350E+05 5 45 180 18 0.250 63.6 2.922E+06 1.169E+07 6 6 19 8 0.316 15.1 8.766E+05 2.776E+06 7 3 6 12 0.500 3.2 2.922E+05 5.844E+05 8 1 4 4 0.250 6.4 2.922E+05 1.169E+06 9 2 7 12 0.286 3.7 1.948E+05 6.818E+05 10 1 8 15 0.125 3.4 7.792E+04 6.233E+05 11 1 5 9 0.200 3.5 1.299E+05 6.493E+05 12 8 23 12 0.348 12.2 7.792E+05 2.240E+06 13 23 109 16 0.211 43.3 1.680E+06 7.962E+06 14 17 33 12 0.515 17.5 1.656E+06 3.214E+06 15 7 22 24 0.318 5.8 3.409E+05 1.071E+06 16 6 17 24 0.353 4.5 2.922E+05 8.279E+05 17 6 18 24 0.333 4.8 2.922E+05 8.766E+05 18 47 188 28 0.250 42.7 1.962E+06 7.847E+06 19 9 28 20 0.321 8.9 5.259E+05 1.636E+06 20 3 11 16 0.273 4.4 2.191E+05 8.035E+05 198 718 15.2 7.714E+05 2.797E+06 Area of basic unit = 8.789E-07 cm-2 CH~ SQUARED = 9.675831 WITH t9 DEGREES OF FREEDOM P(chi squared) = 96.0 % CORRELATION COEFFICIENT = 0.986 VARIANCE OF SQR(Ns) = 2.959536 VARIANCE OF SQR(Ni) = 12.10107 Ns/Ni = 0.276 + 0.022 MEAN RATIO = 0.306 + 0.021 POOLED AGE = 134.3 + 10.9 Ma MEAN AGE = 148.8 _+ 10.2 Ma Ages calculated using a zeta of 352.7 _+ 3.9 for SRM612 glass RRO D = 2.790E+06cm-2; ND = 11421 F.T.AGE(Ma} 140.3± 72.5 120.5+ 96.6 168.0~ 99.4 105.5± 83A 105.5± 17.7 133.0-3:62.3 209.4±148.1 105.5+ 118.0 120.5± 96.6 53.0± 56.2 84.6± 92.6 146.4± 60.1 89.~+ 20.5 215.6± 64.4 134.0± 58.2 148.5+ 70.5 140.3± 66.2 105.5± 17.3 135.4+ 51.9 115.0-,2-_ 75.0 GMC Data--'Re'port..No '1-40 -Page 18/32' ' 19 88 POS l14A - - 3,100' - WALAPKA #1 IRRADIATION LU023 SLIDE NUMBER 01 COUNTED BY: POS N9. Ns Ni Na RATIO U(ppm) RHOs RHOi 1 8 51 12 0.157 24.4 7.792E+05 4.967E+06 2 16 51 21 0.314 13.9 8.905E+05 2.838E+06 3 5 18 12 0.278 8.6 4.870E+05 1.753E+06 4 8 28 15 0.286 10.7 6.233E+05 2.182E+06 5 9 25 40 0.360 3.6 2.630E+05 7.305E+05 6 17 59 24 0.288 14.1 8.279E+05 2.873E+06 7 5 9 8 0.556 6.5 7.305E+05 1.315E+06 8 10 40 12 0.250 19.1 9.740E+05 3.896E+06 9 8 22 20 0.364 6.3 4.675E+05 1.286E+06 10 27 86 40 0.314 12.3 7.889E+05 2.513E+06 11 17 62 30 0.274 11.9 6.623E+05 2.415E+06 12 26 10! 24 0.257 24.1 1.266E+06 4.919E+06 13 5 11 20 0.455 3.2 2.922E+05 6.428E+05 14 8 50 40 0.160 7.2 2.338E+05 1.46 IE+06 15 19 75 30 0.253 14.3 7.402E+05 2.922E+06 16 17 50 40 0.340 7.2 4.967E+05 1.461E+06 17 8 24 12 0.333 11.5 7.792E+05 2.338E+06 F.T.AGEOVIa) 73.6.~. 28.0 146.3+ 42.0 129.7+ 65.6 133.4+ 53.5 167.6+ 65.2 134.5+ 37.1 256.9~-_143.3 116.9~_ 4lA 169.3+ 69.9 146.4+ 32A 128.1+ 35.1 120.3+ 26.5 210.9-~_ 113.8 75.0~_ 28.6 118.4+ 30.5 158.4+ 44.5 155.4+ 63.5 213 762 10.9 6.224E+05 2.226E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 9.33778 WITH 16 DEGR~I::-S OF FREEDOM P(chi squared) = 89.9 % CORRELATION COEFFTCIENT = 0.926 VARIANCE OF SQR(Ns) = .9294977 VARIANCE OF SQR(Ni) -- 4.162098 Ns/Ni = 0.280 + 0.022 MEAN RATIO = 0.308 + 0.023 POOLED AGE = 121.8 +_ 9A Ma MEAN AGE = 133.2_+ 10.1 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.477E+06cm-2; ND = 11864 GMC Data. Report No 140 -Page- 19/32 20 88 POS 115A - ARGILL1TE BASEMENT - 3,659' - WALAPKA #I IRRADIATION LU0'23 SLIDE NUMBER 02 COUNTED BY: POS No. Ns Ni Na RATIO U(j~m) RHOs RHOi 1 8 30 9 0.267 19.1 1.039E+06 3.896E+06 2 5 14 18 0.357 4.5 3.247E+05 9.090E+05 3 5 8 8 0.625 5.7 7.305E+05 1.169E+06 4 4 10 10 0.400 5.7 4.675E+05 1.169E+06 5 8 14 9 0.571 8.9 1.039E+06 1.818E+06 6 7 27 9 0.259 17.2 9.090E+05 3.506E+06 7 9 24 8 0.375 17.2 1.315E+06 3.506E+06 8 8 21 12 0.381 10.0 7.792E+05 2.045E+06 9 10 52 16 0.192 18.6 7.305E+05 3.798E+06 10 5 12 14 0.417 4.9 4.174E+05 1.002E+06 11 8 20 10 0.400 11.5 9.350E+05 2.338E+06 12 17 45 20 0.378 12.9 9.935E+05 2.630E+06 F.T. AGEOV~a) 124.65:49.6 166.3+ 86.7 288.35:164.4 186.0~110.1 264.1+117.1 121.25:51.4 174.55:68.3 177.3+ 73.7 90.15:31.1 193.65:103.1 186.05:77.8 175.85:50.1 94 277 11.1 7.683E+05 2.264E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 6.826693 WITH 11 DEGREES OF FREEDOM P(ch~ s~uare~ = 81.3 % CORRELATION COEFFICIENT = 0.790 VARIANCE OF SQR(Ns) = .3189163 VARIANCE OF SQR(Ni) = 1.859627 Ns/Ni = 0339 5:0.040 MEAN RATIO = 0.385 + 0.035 POOLED AGE = 148A 5:17.8 Ma MEAN AGE = 168.2 5:15.4 Ma Ages calc,l~t_ed using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.509E+06cm-2; ND = 11864 · G~qC -Data' Report NO. 140: Page"20/32 40 30 10 88 POS 109A - TOROK FM. MEAN = 13.82 + 0.13 $.D. = 1.32 N=99 5 10 15 TRACK LENGTH (MICRONS) 88 POS 111A + l12A - BARROW SS. MEAN = 12.18 _+ 0.36 S.D. = 1.90 N= 28 ! 88 POS I10A - PEBBLE SHALE MEAN = 12.55 + 0.19 S.D. = 1.42 N=53 ! ! 88 POS I13A - PEBBLE SHALE MEAN = 11.90 + 0.30 S.D. = 1.51 N= 26 88 POS l14A - BARR MEAN = 12.24 _.+ 038 S.D. = 1.49 N= 15 SS. 88 POS 115A - ARGILL1TE BASEMENT MEAN = 12.61 + 0.39 S.D. = 1.61 N= 17' · 22 INIGOK #1 WELL (in numerical order) 88 POS 117A - KEKIKTUK CONG. - 19,369' IRRADIATION LU023 SLIDE NUMBER 04 COUNTED BY: POS No. Ns Ni Na RATIO U(,ppm,) RHOs RHOi F.T.AOE(Ma) 1 0 16 12 0.000 7.6 0.000E+00 1.558E+06 2 0 15 30 0.000 2.9 0.000E+00 5.844E+05 3 0 10 6 0.000 9.6 0.000E+00 1.948E+06 4 0 18 14 0.000 7.4 0.000E+00 1.503E+06 5 0 33 12 0.000 15.8 0.000E+00 3.214E+06 6 1 50 12 0.020 23.9 9.740E-fi}4 4.870E+06 0.0~_ 0.0 0.04- 0.0 0.04- 0.0 0.0&-_ 0.0 0.05:0.0 9.44- 9.5 1 142 9.5 1.359E+04 1.930E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED= 1.816625 WITH 5 DEGREES OF FREEDOM P(chi squared) = 87.4 % CORRELATION COEFFICIENT = 0.857 VARIANCE OF SQR(Ns) = .1666667 VARIANCE OF SQR(Ni) = 2.091782 Ns/Ni = 0.007 _+ 0.007 MEAN RATIO = 0.003 _ 0.003 Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.573E+06cm-2; ND = 11864 GMC Data Report-'No. 140 Page 22/32 93 88 POS 119A - FIRE CREEK SS. - 12,735' IRRADIATION LU023 SLIDE NUMBER 05 COUNTED BY: POS No. Ns Ni Na RATIO U(pt)m) RHOs RHOi 1 0 16 20 0.000 4.6 0.000E-+O0 9.350E+05 2 1 10 9 0.100 6.4 1.299E+05 1.299E-t06 3 0 5 9 0.000 3.2 0.000E+00 6.493E+05 4 0 10 12 0.000 4.8 0.000E+00 9.740E+05 5 0 35 20 0.000 10.0 0.000E+00 2,045E+06 6 2 31 12 0.065 14.8 1.948E+05 3.019E+06 7 0 8 8 0.000 5.7 0.000E+00 1.169E+06 8 I 20 12 0.050 9.6 9.740E+04 1.948E+06 9 2 20 16 0.100 7.2 1.461E+05 1.461E+06 10 0 16 12 0.000 7.6 0.000E+00 1.558E+06 11 0 9 9 0.000 5.7 0.000E-fi)0 1.169E+06 12 0 15 16 0.000 5.4 0.000E+00 1.096E+06 13 I 19 16 0.053 6.8 7.305E-fi)4 1.388E+06 14 1 36 20 0.028 10.3 5.844E+04 2.104E+06 15 0 14 18 0.000 4.5 0.000E+00 9.090E+05 16 0 9 12 0.000 4.3 0.000E+00 8.766E+05 17 0 10 12 0.000 4.8 0.000E+00 9.740E+05 18 0 5 9 0.000 3.2 0.000E+00 6.493E+05 19 1 17 12 0.059 8.1 9.740E+04 1.656E+06 20 0 8 9 0.000 5.1 0.000E+00 1.039E+06 F.T.AGE(Ma) 0.0-+ 0.0 47.0~_ 49.3 0.0-+ 0.0 0.0-+ 0.0 0.0-+ 0.0 30.4+ 22.2 0.0-+ 0.0 23.5+ 24.1 47.0-+ 34.9 0.0-+ 0.0 0.0-+ 0.0 0.0~-_ 0.0 24.8_+ 25.4 13.1+_ 13.3 0.0~_ 0.0 0.0-+ 0.0 0.0-+ 0.0 0.0-+ 0.0 27.7+ 28.5 0.0-+ 0.0 9 313 6.8 4.000E+04 1.391E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 11.87335 WITH 19 DEGREES OF FREEDOM P(chi squared) = 89.1% CORRELATION COEFFICIENT = 0.527 VARIANCE OF SQR(Ns) = .3124098 VARIANCE OF SQR(Ni) = 1.217401 Ns/Ni -- 0.029 + 0.010 MEAN RATIO = 0.023 + 0.008 POOLED AGE -- 13.2 + 4.5 Ma MF_a6NAGE = 10A+ 3.6Ma Ages calcu!_at_ed using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.605E+00cm-2; ND = 11864 GMC Data Repo'rt No. 140 -page. 23/32 '-... 24 88 POS 120A - FIRE CREEK SS. - 12,501' IRRADIATION LU023 SLIDE NUMBER 06 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) · 1 1 35 20 0.029 10.0 2 1 20 16 0.050 7.2 3 0 14 16 0.000 5.0 4 0 10 9 0.000 6.4 5 0 15 12 0.000 7.2 6 ! 21 16 0.048 7.5 7 2 19 12 0.105 9.1 8 0 9 8 0.000 6.5 9 0 30 12 0.000 14.3 10 0 36 20 0.000 10.3 11 0 9 12 0.000 4.3 12 0 6 9 0.000 3.8 13 1 9 9 0.111 5.7 14 0 17 20 0.000 4.9 15 1 17 12 0.059 8.1 16 0 12 9 0.000 7.6 RHOs RHOi 5.844E+04 2.045E+06 7.305E+04 1.461E+06 0.000E+00 1.023E+06 0.000E+00 1.299E+06 0.000E+00 1.461E+06 7.305E+04 1.534E+06 1.948E+05 1.851E+06 0.000E+00 1.315E+06 0.000E+00 2.922E+06 0.000E+00 2.104E+06 0.000E+00 8.766E+05 0.000E+00 7.792E+05 1.299E+05 1.169E+06 0.000E+00 9.935E+05 9.740E+04 1.656E+06 0.000E+00 1.558E+06 7 279 7.5 3.859E+04 1.538E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 12.37848 WITH 15 DEGREES OF FREEDOM P(chi squared) = 65.0 % CORRELATION COEFFICIE~ = 0.206 VARIANCE OF SQR(Ns) = .2952411 VARiAN~ OF SQR(Ni) = 1.134424 Ns/Ni = 0.025 + 0.010 MEAN RATIO = 0.025 + 0.010 POOLED AGE = 11.7 + 4.5 Ma MEAN AGE = 11.7 + 4.5 Ma Ages calculated using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.637E+06cm-2; ND = 11864 F.T.AGE~/m) 13.5_+ 13.7 23.5_+ 24.1 0.0-+ 0.0 0.0-~_ 0.0 0.0-2:0.0 22.4-+ 23.0 49.5-+ 36.8 0.0-~_ 0.0 0.0-+ 0.0 0.0-+ 0.0 0.0-+ 0.0 0.0-+ 0.0 52.2-+ 55.0 0.0-+ 0.0 27.7-+ 28.5 0.0-+ 0.0 GMC Data Report No. 140 Page. 24/32 - . :..: .. . -. :. . . .-.. 25 88 POS 122A - KINGAK SHALE - 9,435' IRRADIATION LU023 SLIDE NUMBER 08 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) RHOs RHOi 1 0 13 8 0.000 9.3 0.000E+00 1.899E+06 2 1 12 12 0.083 5.7 9.740E+04 1.169E+06 3 3 7 6 0.429 6.7 5.844E+05 1.364E+06 4 2 31 6 0.065 29.6 3.896E+05 6.039E+06 5 14 91 10 0.154 52.2 1.636E+06 1.064E+07 6 3 24 12 0.125 11.5 2.922E405 2.338E+06 7 15 95 12 0.158 45.4 1.461E+06 9.253E+06 8 0 14 8 0.000 10.0 0.000E-lO0 2.045E+06 9 1 25 6 0.040 23.9 1.948E+05 4.870E+06 10 13 83 15 0.157 31.7 1,013E+06 6.467E+06 11 0 14 16 0.000 5.0 0.000E+00 1.023E406 12 0 4 6 0.000 3.8 0.000E+00 7.792E+05 13 0 6 10 0.000 3.4 0.000E+00 7.013E+05 14 1 16 9 0.062 10.2 1.299E+05 2.078E+06 15 1 11 6 0.091 10.5 1.948E+05 2.143E+06 16 10 86 6 0.116 82.2- 1.948E+06 1.675E+07 17 7 95 30 0.074 18.2 2.727E+05 3.701E+06 18 0 15 9 0.000 9.6 0.000E+00 1.948E+06 19 5 74 12 0.068 35.4 4.870E+05 7.207E+06 20 5 59 12 0.085 28.2 4.870E+05 5.746E+06 0.0~ 0.0 39.9-+ 40.8 199.1+137A 30.4+ 22.2 72.2-+ 20.7 58.7+ 36.0 74.1+ 20.6 0.0-+ 0.0 18.8+ 19.2 73.5+ 21.9 0.0-+ 0.0 0.0~_ 0.0 0.0-+ 0.0 29.4+ 30.3 42.7+ 44.6 54.6-+ 18.3 34.7+ 13.6 0.0-+ 0.0 31.8_+ 14.7 39.9-+ 18.6 81 775 21.1 4.487E+05 4.293E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED= 21.58282 WI'ITI 19 DEGRF:F-S OF FREEDOM P(chi squared) = 30.6 % CORRELATION COEb'FICIENT = 0.909 VARIANCE OF SQR(Ns) = 1.834569 VARIANCE OF SQR(Ni) = 7.775956 Ns/IXli -- 0.105 + 0.012 MEAN RATIO = 0.085 -+ 0.022 POOLED AGE = 49.6 -+ 5.8 Ma MEANAGE = 40.5-+ 10.4Ma Ages calculated using a zeta of 352.7 _+ 3.9 for SRM612 glass RHO D = 2.701E+06cm-2; ND = 11864 GMC Data Report No. 140' Page' 25/32- 26 88 POS 123A - TOROK FM. - 8,849' IRRADIATION IXI023 SLIDE NUMBER 09 COUNTED BY: POS No. Ns Ni Na RATIO U(~m) 1 0 23 24 0.000 5.5 2 1 32 16 0.031 11.5 3 3 ~2 20 0.042 20.7 4 1 10 12 0.100 4.8 5 0 4 12 0.000 1.9 6 1 8 16 0.125 2.9 7 3 31 15 0.097 11.9 8 2 ~0 20 0.100 5.7 9 6 61 40 0.098 8.7 10 3 21 30 0.143 4.0 11 2 22 16 0.091 7.9 12 0 11 20 0.000 3.2 13 5 34 40 0.147 4.9 14 6 62 30 0.097 11.9 15 2 25 16 0.080 9.0 16 1 8 20 0.125 2.3 17 1 8 12 0.125 3.8 18 3 32 40 0.094 4.6 19 2 21 18 0.095 6.7 20 0 15 20 0.000 4.3 RHOs RHOi 0.000E+00 1.120E+06 7.305E+04 2.338E+06 1.753E+05 4.208E+06 9.740E+04 9.740E+05 0.000E+O0 3.896E+05 7.305E+04 5.844E+05 2.338E+05 2.415E+06 1.169E+05 1.169E+06 1.753E+05 1.782E+06 1.169E+05 8.181E+05 1.461E+05 1.607E+06 0.000E+00 6.428E+05 1.461E+05 9.935E+05 2.338E+05 2.415E+06 1.461E+05 1.826E+06 5.844E+04 4.675E+05 9.740E+04 7.792E+05 8.766E~ 9.350E+05 1.299E+05 1.364E+06 0.000E+00 8.766E~5 42 520 6.8 1.123E+05 1.391E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 10.28085 WITH 19 DEGRg-I=-S OF FREEDOM P(chi squared) = 94.6 % CORREI_~TION COEFFICIENT = 0.777 VARIANCE OF SQR(Ns) = .602412 VARIANCE OF SQR(Ni) = 3.195221 Ns/Ni = 0.081 + 0.013 MEAN RATIO = 0.079 + 0.011 POOLED AGE = 38.8 + 6.2 Ma MEAN AGE = 38.2 + 5.3 Ma Ages calcnht~ using a zeta of 352.7 + 3.9 for SRM612 glass RI-IO D = 2.733E+06cm-2; ND = 11864 0.0!- 0.0 14.7+ 15.0 19.6-+ 11.6 47.0!-_ 49.3 0.0-+ 0.0 58.7+ 62.3 45.5+ 27.5 47.03:34.9 46.2-+ 19.8 67.0-~_ 41.4 42.7+ 31.6 0.0-+ 0.0 69.0-+ 33.1 45.5+ 19.5 37.6+ 27.7 58.7+ 62.3 58.7+ 62.3 44.1_+ 26.6 44.8-+ 33.1 0.0-+ 0.0 GMC Data-Report No. 140 .. Page 26/32 88 POS 124A - TOROK FM. - 8,237' IRRADIATION LU023 SLIDE NUMBER 10 COUNTED BY: POS 27 No. Ns Ni Na RATIO U(ppm) 1 3 27 12 0.111 12.9 2 0 20 15 0.000 7.6 3 7 102 16 0.069 36.6 4 2 9 21 0.222 2.5 5 5 53 12 0.094 25.3 6 0 15 27 0.000 3.2 7 9 58 20 0.155 16.6 8 2 14 50 0.143 1.6 9 6 128 15 0.047 49.0 10 3 15 9 0.200 9.6 11 0 16 24 0.000 3.8 12 6 130 30 0.046 24.9 13 2 13 40 0.154 1.9 14 9 68 20 0.132 19.5 15 0 12 25 0.000 2.8 16 5 61 14 0.082 25.0 17 2 15 20 0.133 4.3 18 7 90 15 0.078 34.4 19 0 12 18 0.000 3.8 20 3 26 12 0.115 12.4 Area of RHOs RHOi F.T.AGE(Ma) 2.92ZE+05 2.630E+06 52.2& 31.8 0.000E+00 1.558E+06 0.0-Z_ 0.0 5.113E+05 7.451E+06 32.3+ 12.6 1.113E+05 5.009E+05 104.04__-_ 81.3 4.870E+05 5.162E+06 44.4+ 20.8 0.000E+00 6.493E+05 0.0-!-_ 0.0 5.259E+05 3.389E+06 72.8+ 26.1 4.675E+04 3.273E+05 67.0~-_ 50.7 4.675E+05 9.974E+06 22.1+ 9.2 3.896E+05 1.948E+06 93.7+ 59.3 0.000E+00 7.792E+05 0.0-!-_ 0.0 2.338E+05 5.065E+06 21.7+ 9.1 5.844E+04 3.798E+05 72.2& 54.8 5.259E+05 3.974E+06 62.1+ 22.1 0.000E+00 5.610E+05 0.0~_ 0.0 4.174E+05 5.092E+06 38.6~_ 17.9 1.169E+05 8.766E+05 62.6+ 47.1 5A54E+05 7.013E+06 36.6d: lnA 0.000E+00 7.792E+05 0.0-~_ 0.0 2.922E+05 2.532E+06 54.9.+ 33.1 71 884 12.2 2.000E+05 2.490E+06 basic unit = 8.789E-07 cm-2 CHI SQUARED = 21.8865 wrrI-I 19 DEGR~-S OF FREEDOM P(chi squared) = 29.0 % CORRELATION COEFFICIENT = 0.765 VARIANCE OF SQR0Xls) = 1.123067 VARIANCE OF SQR(Ni) = 8.050961 Ns/NJ = 0.080 + 0.010 MEAN RATIO = 0.089 + 0.015 POOLED AGE = 39.4 + 4.9 Ma MEAN AGE = 43.7 + 7.6 Ma Ages calcul_at__,~d using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.789E+06cm-2; ND = 11864 GMc 'Data' Report No. 140 -.. 'Page 27/32 28 88 POS 125A - TOROK FM. - 5,006' IRRADIATION LU023 SLIDE NUMBER 11 COUNTED BY: POS No. Ns Ni Na RATIO U(ppm) 1 6 21 40 0.286 3.0 2 5 22 50 0.227 2.5 3 15 78 40 0.192 11.2 4 4 19 24 0.211 4.5 5 4 15 24 0.267 3.6 6 5 21 40 0.238 3.0 7 29 152 25 0.191 34.9 8 2 15 12 0.133 7.2 9 5 22 21 0.227 6.0 10 6 60 40 0.100 8.6 11 5 18 42 0.278 2.5 12 4 20 40 0.200 2.9 I3 5 22 40 0.227 3.2 14 6 23 30 0.261 4.4 15 6 20 30 0.300 3.8 16 2 13 40 0.154 1.9 17 4 15 50 0.267 1.7 18 4 16 40 0.250 2.3 19 8 29 30 0.276 5.5 20 5 21 30 0.238 4.0 RHOs RHOi F.T~GE~vla) 1.753E+05 6.136E+05 133.452 61.8 1.169E+05 5.143E+05 106.3+ 52.7 4.383E+05 2.279E+06 90.152 25.4 1.948E+05 9.253E+05 98.6+ 54.2 1.948E+05 7.305~+05 ~24.6~ 70.1 1.461E+05 6.136E+05 111.452 55.4 1.356E+O6 7.1O6E+06 89.45:18.2 1.948E+05 1.461E+06 62.65:47.1 2.783E+05 1.224E+06 106.352 52.7 1.753E+05 1.753E+O6 47.0X- 20.1 1.391E+05 5.009E+05 129.7+ 65.6 1.169E+05 5.844E+05 93.7+ 51.3 1.461E+05 6.428E+05 106.352 52.7 2.338E+05 8.961E+05 121.952 55.9 2.338E+05 7.792E+05 140.052 65.2 5.844E-fi)4 3.798E+05 72.9+ 54.8 9.350E+04 3.506E~5 124.652 70.1 1.169E+05 4.675E+05 116.952 65.4 3.117~+05 1.~30E+O6 128.852 51.5 1.948E+05 8.181E+05 111.452 55.4 130 622 5.2 2.208E+05 1.057E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 6.736065 WITH 19 DEGREES OF FRg-~DOM P(chi squared) = 99.5 % CORRELATION COEFFICIENT = 0.964 VARIANCE OF SQR(Ns) = .746221 VARIANCE OF SQR(Ni)= 4.450305 Ns/NJ = 0.209 5:0.020 MEAN RATIO = 0.226 5: 0.012 POOLED AGE = 102.3 + 9.9 Ma MEAN AGE = 110.6 + 5.8 Ma Ages calculated ming a zeta of 352.7 5:3.9 for SRM612 glass RHO D = 2.797E+06cm-2; ND = 11864 -. GMC Data Report No. 140 Page 28/32' 29 88 POS 126A - NANUSHUK GROUP - 3,078' IRRADIATION LU023 SLIDE NUMBER 12 COUNTED BY: POS No. Ns Ni Na RATIO U(~m) RHOs RHOi 1 12 40 40 0.300 5.~/ 3.506E+05 1.169E+06 2 5 22 50 0.227 2.5 1.169E+05 5.143E+05 3 7 21 12 0.333 10.0 6.818E+05 2.045E+06 4 4 20 24 0.200 4.8 1.948E+05 9.740E+05 5 2 15 18 0.133 4.8 1.299E+05 9.740E+05 6 5 22 15 0.227 8.4 3.896E+05 1.714E+06 7 6 27 15 0.222 10.3 4.675E+05 2.104E+06 8 2 12 12 0.167 5.7 1.948E+05 1.169E+06 9 9 45 18 0.200 14.3 5.844E+05 2.922E+06 10 6 45 40 0.133 6.5 1.753E+05 1.315E+06 11 8 54 30 0.148 10.3 3.117E+05 2.104E+06 12 4 20 36 0.200 3.2 1.299E+05 6.493E+05 13 4 35 36 0.114 5.6 1.299E+05 1.i36E+06 14 6 24 30 0.250 4.6 2.338E+05 9.350E+05 15 2 10 28 0.200 2.0 8.348E+04 4.174E+05 16 2 10 40 0.200 1.4 5.844E+04 2.922E+05 17 6 41 16 0.146 14.7 4.383E+05 2.995E+06 18 4 16 40 0.250 2.3 1.169E+05 4.675E+05 19 3 20 12 '0.150 9.6 2.922E+05 1.948E+06 20 5 19 30 0.263 3.6 1.948E+05 7.402E+05 102 518 5.5 2.200E+05 1.117E+06 Area of basic unit = 8.789E-07 cm-2 CHI SQUARED = 7.63678 WITH 19 DEGR~-S OF FREEDOM P(chi squared) = 99.0 % CORRELATION COEFFICIENT = 0.757 VARIANCE OF SQR(Ns) = .3122036 VARIANCE OF SQR(Ni) = 1.520991 Ns/Ni = 0.197 + 0.021 MEAN RATIO = 0.203 + 0.013 POOLED AGE = 97.5 + 10.6 Ma MEANAGE = 100.6:t: 6.5Ma Ages calc.!ot__ed using a zeta of 352.7 + 3.9 for SRM612 glass RHO D = 2.829E+06cm-2; ND = 11864 140.0~: 46.1 106.3+ 52.7 155.4+ 67.8 93.7+ 51.3 62.6+ 47.1 106.3+ 52.7 104.0-~_ 47.0 78.1+ 59.7 93.7+ 34.2 62.6d: 27.2 69.5+ 26.4 93.7+ 51.3 53.7+ 28.3 116.9-~_ 53.4 93.7+ 72.6 93.7+ 72.6 68.7+ 30.0 116.9~_ 65.4 70.4+ 43.6 123.0~-_ 61.8 GMC Data Report. No'. 140 - . . - 'Page 29/32- "-' 30 88 POS 127A - NANUSHUK GROUP - 2,632' IRRADIATION LU023 SLIDE NUMBER 13 COUNTED BY: POS No. Ns Ni Na ... RATIO U(ppm) · 1 4 14 24 0.286 3.3 2 12 41 40 0.293 5.9 3 5 29 16 0.172 10.4 4 7 21 12 0.333 10.0 5 9 24 36 0.375 3.8 6 2 16 18 0.125 5.1 7 16 77 21 0.208 21.0 8 6 28 15 0.214 10.7 9 4 43 21 0.093 11.7 10 9 41 18 0.220 13.1 11 10 41 36 0.244 6.5 12 8 71 30 0.113 13.6 13 2 14 8 0.143 10.0 14 4 38 36 0.105 6.1 15 9 40 20 0.225 11.5 16 2 15 28 0.133 3.1 17 6 25 10 0.240 14.3 18 6 42 15 0.143 16.1 19 4 18 30 0.222 3.4 20 3 19 12 0.158 9.1 RHOs RHOi F.T.AGE(Ma) 1.948E+05 6.818E+05 133.4+ 75.7 3.506E+05 1.198E+06 136.6+ 44.9 3.652E+05 2.118E+06 80.8+ 39.2 6.818E+05 2.045E+06 155.4+ 67.8 2.922E+05 7.792E+05 174.5_+ 68.3 1.299E+05 1.039E+06 58.7+ 44.0 8.905E+05 4.285E+06 97.3_+ 26.8 4.675E+05 2.182E+06 100.3+ 45.1 2.226E+05 2.393E+06 43.7+ 22.9 5.844E+05 2.662E+06 102.7+ 37.8 3.24TE+05 1.331E+06 114.0-+ 40.3 3.117E+05 2.766E+06 52.9-+ 19.8 2.922E+05 2.045E+06 67.0-+ 50.7 1.299E+05 1.234E+06 49.5+ 26.0 5.259E+05 2.338E+06 105.3_+ 38.9 8.348E+04 6.261E+05 62.6_+ 47.1 7.013E+05 2.922E+06 112.2-+ 51.0 4.675E+05 3.273E+06 67.0-+ 29.3 1.558E+05 7.013E+05 104.0-+ 57.5 2.922E+05 1.851E+06 74.1_+ 46.0 128 657 8.4 3.354E+05 1.722E+06 Area of basic unit= 8.789E-07 cm-2 CHI SQUARED = 14.7022 WITH 19 DEGREES OF FREEDOM P(chi squared) = 74.1% CORRELATION COEFFICIENT = 0.736 VARLA~CE OF SQROWs) = .5058557 VARIANCE OF SQR(Ni) = 2.123895 Ns/N-i = 0.195 _+ 0.019 MEAN RATIO = 0.202 + 0.018 POOLED AGE = 97.6 + 9.5 Ma MEAN AGE = 101.2 + 8.8 Ma Ages calculated using a zeta of 352.7 :!: 3.9 for SRM612 glass RHO D = 2.861E+06cm-2; ND = 11864 GMC Data Report No. 140 Page 30/32 4O 3O 10 88 POS I19A - FIRE CREEK SS. MEAN = 7.56 :t: 1.43 S.D. = 3,50 N=6 5 10 15 TRACK LENGTH (MI(~TRONS~ 88 POS 123A - TOROK FM. MEAN = 11.30 + 0.21 $.D. = 1.63 N=60 I 20 88 POS 122A - KINGAK SHALE IMEAN = 10.68 __ 0.45 S.D. = 2.33 N=27 88 POS 124A - TOROK FM. MEAN = 12.19 + 0.17 S.D. = 1.52 N= 77 88 POS 125A - TOROK FM. MEAN = 12.87 + 0.22 S.D. =2.24 N = 105 nlm __88 POS 126A -NANUSHUK GROUP MEAN = 13.13 + 0.16 S.D. = 1.63 N= 102 40 30 10 88 POS 127A- NANUSHUK GROUP ~ = 13.38 + 0.17 S.D. = 1.73 N= 102 5 I0 15 TRACK LF_/q'GTH (MICRONS'} ! 20 GMC Data Report No. 140 Page 32/3'~ APPENDIX A FISSION TRACK ANALYSIS: A SUMMARY OF THE TECHNIQUE AND INTERPRETATION OF RESULTS INTRODUCTION Fission tracks are damage zones formed as charged particles, produced by fission of a heavy atom (232Th, 235U, and 238U), pass through a crystal lattice. It is assumed for all practical purposes that all fission tracks have come from 238L1 because 232Th and 235U possess very low fission decay rates compared to 238U (Naeser, 1979a). 238U decays by both spontaneous fission and alpha particle emission but alpha particles themselves do not create tracks in natural minerals (Fleischer et a/., 1975). The density of spontaneous tracks (fossil tracks) is proportional to the length of time during which tracks have accumulated and to the 238U concentration of the sample. The concentration of 238U can be determined by irradiating the sample alongside a standard in a nuclear reactor, using a monitored thermal neutron flux. Thermal neutrons in the reactor induce fission in a fraction of 235U present in the sample. A count of the induced tracks produced from the decay of 235U can be related to the 238U concentration using the constant 235U/238U ratio in natural materials (7.252 x 10-3). By determining the ratio of fossil tracks to induced tracks, a geological age can be determined. The ratio of 238U to fission track density is analogous to the ratio of parent to daughter isotopes in other radiometric dating systems. FISSION TRACKS AND FISSION TRACK TECHNIQUES The Formation of a Fission Track A fission'track is-fOrmed:-When:.a nucleus of a:heav, y: element:- sUch..- aS '-uranium _ . undergoes fission. Fission decay results in two fast-moving highly-charged fission .'..':. . - - · .-' ..' ~- . ,: ' "'-. /."~' i' ". i:' 76 ! ' · . . ~ i_ . . - -: ' . - -i - ~ ' "~- '- - ' . ': .... . '-'. .-- '° '' ' 2~.' - -' '. -.. '. : '- "- '- .. ".- 77 fragments recoiling from each other in opposite directions due to electrostatic repulsion. The best explanation for the formation of fission tracks is the "ion explosion spike" (Fleischer et al., 1975) (Fig. Al). A "burst" of ionization along the path of the charged particles creates an electrostatically unstable array of adjacent ions which eject one another from their normal sites into interstitial positions. As the fission fragment passes, it strips electrons and leaves a zone of net positive charge in its wake. This causes the remaining positively-charged ions along the particles path to repel each other, forming the track or damage zone. Some have speculated that a phase change occurs in the vicinity of the fission track. 0 0 0 0 0 0 0 ' 0 O 0 0 0 0 0 0 0 0 0 0 ® ® 0 0 ~k oeo 0 0 ® 0 0 0 o o o o o 0 -(N---, 0 0 0 0 0 /e 0 0 0 0 0" NifO 0 0 a b Figure Al. The ion explosion spike mechanism for track formation in a simple crystalline solid; (a) atoms are ionized by passage of a massive charged particle (fission fragmen0, (b) causing instability and ejecting ions into the lattice by mutual repulsion (after Fleischer et at., 1975). The resultant track is only a few angstroms wide and -10-20 grn long (Naeser, 1979a). It remains stable in all insulating solids, but conducting and semi-conducting solids do not retain.' tracks as movement, of electrons-rapidly neutralizes 'the ions :prodUced. ..These .track can be observed by transmission electron microscopy but the electron beam anneals them 78 quickly. It is also possible to observe them under a petrographic microscope once the tracks have been revealed by chemical etching. Track Etchine Fission tracks are made visible by chemical etching because the etchant preferentially attacks the highly disordered (glassy?) material along the track. The geometry of track etching is determined by the simultaneous action of two etching processes (Gleadow, 1984). These are the rate of etching along the particle track surface at a linear rote (VT) and the rate of etching along an undamaged surface, or bulk etching rate (VG). Selective etching of a track depends on VT being greater than VG, with the shape of the resulting track being dependent on the difference between these two etching rotes. For example, if VT >>VG. a narrow conical track is produced (Fig. A2). If VT is only slightly greater · t thing · s olu tion ortgin(:::! surface etched surface VGt ...-......'.-::r'-.-.-.-.-.-.- - ,', ' - ' - ' ' ' ' .<.:-.i..X- ~ .-.-'...-.-. -'-'.'..'.'.' i ':':[:':':-:' ::::::::::::::::::::::::::::::: .-.-...-.-.-.-,-.-T ..- ........ . ...... '.'.' -:-:.:-:-'-:-2 I-:-V t:'": :!:iS?i: -'"-'-'-'-'-'- ~"-'-'-'-'-'-'-' :-'.-:-:-:-:-:-'. C-:-'-:-:..:.'-.'- .:.:.:.:.:.:.:.~:.:.:.'.:.:.:.: ...........-.......-........:... i · .".*.';'.' I ','".'.-.- :::-:.-:-:-:: ', :.:.-.:.: .:.:.:.:.:. Time 0 Time ti2 Time t Time 2t Figure.A2, Track. g. e0.metry showing'VGt (bulk-surface etching with. time) and VTt (track 6tching"with:fime). TracEs int6rsecting the surface .at angles.less ihan. the-critical-. angle 0 will not be revealed. See text for explanation (from Gleadow, 1984). 79 than VG, a shallow, wide, poorly defined track results. Another factor that controls the observation of tracks is the angle at which a track intersects the surface. Tracks intersecting the surface at less than the minimum intersection angle 0 are not revealed by etching because the vertical component of VT is not as great as VG (Fleischer and Price, 1963) (Fig. A2). Therefore, where 0 is greater than zero, an etching efficiency (r~) exists, defined as the fraction of tracks intersecting a given surface that are actually etched on the surface. Only those tracks intersecting at angles greater than 0 (def'med by sima = VT/VG) will be revealed by etching. The concepts of VT and VG explain track etching well for isotropic minerals, however VG is anisotropic in most minerals (Fleischer et al., 1975). This anisotropy is reduced by accumulating radiation damage from the alpha decay of uranium and thorium in the host mineral (Gleadow, 1978). Working with sphene, Gleadow, (1978) found that the mineral became more isotropic with accumulating radiation damage causing progressive change in the etching characteristics of fission tracks. The consequences of anisotropic etching (shape of etched fission tracks with different crystallograpkic orientations and different VG values on different crystallographic planes) and accumulated radiation damage are discussed in more detail by Gleadow (1981) and Gleadow (1984). Etching Conditions In order to reveal tracks clearly it is important to establish proper etching conditions. Under-etching results in tracks being faint and easy to miss so that track density is underestimated. Over-etching makes it difficult to distinguish tracks from other etch features or intersecting tracks. Apatites of widely different composition and age seem to be very consistent in their etching behavior, unlike sphene and zircon, therefore an etching . . . . t/me of approximately 20 seConds in 5 mol HNo3 at 20°C'is sufficient.to re'veal fission . tracks (Gleadow, 1984). Sphene and zircon have highly variable etching times dependent · · 80 on general radiation damage plus a number of other compositional and crystallographic features (Gleadow, 1978; Hurford and Green, 1982; Gleadow, 1984). Fleischer et al. (1975) has summarized the characteristics of fkssion tracks which make them easily distinguishable from dislocations and other spurious etch pits. Fission tracks are randomly orientated linear defects of finite length with a limited thermal stability, and should have a statistical distribution related to spatial variation of uranium concentration. Fissiol~ Track Datin~ Methods Different fission track dating methods are described by Hurford and Green (1982) and Gleadow (1981). These include the population method and the external detector method (EDM). In the population method, the spontaneous and induced track densities are measured on two aliquots of the separated mineral grains. In the EDM, spontaneous and · induced tracks are measured in exactly matching areas from the same internal surface of an individual crystal. The EDM is now the most commonly used technique for fission track mineral dating. There are many advantages of the EDM, including the ability to date and analyze individual grains and its adaptability to automation. It also requires less counting times, gives more reproducible results, and requixes less complicated handling after irradiation. The External Detector Method In this method spontaneous tracks are measured on an internal surface of a mineral grain. During irradiation induced tracks from 235U are registered on an external surface of a detector mineral held in contact with the same surface on which the spontaneous tracks are counted. This detector, usually a sheet of low-uranium muscovite (<5 ppb), is subsequently etched to reveal the registered tracks. SpOntaneous and induced tracks are . Countedin exaCflymatching areas -from the Same-surface plane oran individual .cryStal 81 that inhomogeneity in uranium concentration between grains and within grains is not a problem as it is with the population method. Since dating involves determining ages for individual grains it is important to avoid selecting grains that axe badly etched or contain dislocations. When selecting grains one should count only grains which have a low VG, identified by the presence of sharp polishing scratches. Scratches indicate a very slow bulk etching rate for that exposed surface and hence a high etching efficiency for tracks (Gleadow, 1978). Other features one looks for when selecting grains include alignment of etch pits elongated along the c-axis, and optical characteristics which indicate that the surface is parallel to the c-axis. Gleadow (1978) and Hufford and Green (1982) discuss the EDM in more detail. Hurford and Green (1982) also conclude that sphene and zircon, which are known to accumulate alpha-recoil damage, can only be dated.reliably by the EDM, This is because laboratory annealing used in the population method removes the alpha-recoil damage as well the spontaneous fission tracks thereby restoring the initial highly anisotropic pattern of bulk etch rates. An overestimation of age with sphene can result because etching of the induced tracks in the annealed sphene will be anisotropic and weakly etched tracks may be overlooked during counting. THE FISSION TRACK EQUATION, ZETA CALIBRATION, AND ERROR ANALYSIS The Fission Track Et~uation _ The fission track age equation is a specialized form of the general age equadon used in all other forms of radiometric dating (Gleadow, 1984). In fission track dating however, the ratio of daughter atoms, to parent .atoms remaining is. a function .of the ratio: of spontaneous to induced 'track rienZi'ties of the form (Price'and Walker, 1963): 82 T= l"~ln il +~'D~(~IO~ I~'D ~.fOi I = isotopic ratio 235U/238U = 7.252 x 10.3 (Conran and Adler, 1976). ~.D =total decay constant for 238U = 1.55125 x 1010yr-1 (Jaffey et aL, 1971). )~f = spontaneous fission decay constant of 238U; two values, 6.85 or 8.42 x 10-17yr-1 (Fleischer and Price, 1964; Galliker et al., 1970); see following page for explanation. c~ = thermal neutron cross section for 235U = 580 x 10'24cm2 (Hannah et al., 1969). ~b = thermal neutron fluence. Ps = spontaneous track density. Pi = induced track density. . Two or more standard glass/mica pairs are included in each irradiation package to monitor neutron fluence and the possible: presence of a gradient along the package. The standard glass (NBS glass SRM612) contains uniform U concentration (-50 ppm) that produces manageable track densities in the mica detectors. The flux is directly related to the track density in the mica Pd by: (l) = BPd (2) where B is a calibration constant for the stan~rd glass (- 5.736 x 109; Hufford and Green, 1983). To determine an age using equation (1) requires the measurement of Ps and Pi, the d. etermination of neutron fluence ({), and the use of the constants B and ),.f. The use of this equation, its systematics, and calibrations have been reviewed by Hurford and Green (1982). ' The value of the. spontaneoUsfission decayc0nstant for 238U-0~f) has been in-doubt for. -. . some time. Fleischer and Price (i964) rePorted'a value 6.85 (+ 0.20) x 10-17yr-I based on .. a comparison of fission track dates of t.ekfi.tes' with KrAr dates.. However? ~hen: the..effect- . · . 83 of track fading, causing decreased ages, is taken into consideration, a value of 8.4 x 10- t7yr-t is obtained (Storzer and Wagner, 1971). A more precise determination is by Galliker et al., (1970), who reported a value of 8.46 (+ 0.06) x 10-17yr-1. This value for Xf was con£zrmed by Storzer (1970) and Wagner et al., (1975). Both values (6.85 x 10- 17yr-1 and 8.46 x 10-17yr-I) have been used for dating by the fission track method (e.g. Gleadow and Lovering, 1974; Bar et ai.,1974). The value of B is often determined against independent measurements of the fluence or by reference to fission mack dating of an age standard using an assumed value of Xf. Green and Hurford (1984), report that neutron dosimetry using activation foils can be extremely unreliable. They also report that reproducibility of fluence calibrations between ' different laboratories is poor and that this is expected because in many cases determinations of ~.f have depended of fluence measurements (Hurford and .Green,' 1982). They concluded that any value of ~.fcalculated using a system of thermal neutron dosimetry is only valid for that system. Zeta Calibration Fission track dates are subject to systematic errors arising from the uncertainty of Xf and from difficulties with measurements of the neutron dose ((~). Hufford and Green (1982) proposed that until independent values of kf and ~ are lcnown, fission u'ack dating should be empirically calibrated against independently known ages. Substituting equation (2) into equation (1) gives: (3) 84 The constants in equation (3), except for kD (which effectively cancels out for young ages under 100 Ma) can be grouped into a single factor, "zeta" (~) which is calibrated directly from age standards. e~DTSTD - 1 ~O (Ps/Pi)b~Pd (4) Equation (3) becomes: 1 !nll+~'D~P---~ Pd) (5) T = 3,D Ratios of counts obtained over different standard glasses in common use in laboratories have been given by Hurford and Green (1983) and Green (1985). Personal Zeta Calibration I used three apafite standards for zeta calibration before any unknowns were counted. The three standards were the Fish Canyon Tuff (27.9 +_ 0.7 Ma), Durango apatite (31.4 _ 0.5 Ma), and Mt. Dromedary apatite (98.7 + 1.1 Ma). These three standards are discussed by Hurford and Green (1983) and Green (1985). My results are listed Table Al; the weighed mean zeta of 352.7 was used when determining unknown apatite ages in this study. Determination of Error The fundamental assumption of fission track statistics is that track counts, like radioactive decay, will follow a poisson distribution. The "conventional analysis" of errors(e.g, Lindsay et al., 1975) assumes that no further sources of variation are.presentin the measurement 0'f track densities. Green (-1'981); in his discussi°n on the use:'0f statistics in fission track dating, discusses this assumption in detail. For a poisson disu-ibution the . :. : ... .. - .. _--.. -..-~ ._ ~.-. - ..' - . -. : - -. . . . · · : . 85 Table A 1. Fission Track Countin~ of Standards for Personal Zeta Determination i Sample Number number of grains Durango apatite 8122-3B 20 8122-3A 1 5 8122-3B 1 5 Fish Canyon tuff 72N8-24 20 72N8-01 20 Standard track Fossil track density density (xl06cm-2) (xl05cm-2) 1.422 1.987 (321) (2353) 1.422 1.700 (218) (1927) 1.422 2.200 (234) (1734) Induced track Correlation Zeta density coefficient (xl06cm-2) 1.456 0.824 324.5 1.503 0.354 391.3 1.630 0.981 328.1 Sample Mean Zeta = 348.0 1.422 2.192 1.863 0.754 333.1 (336) (2856) 1.422 1.860 1.775 0.903 374.1 (298) (2845) Sample Mean Zeta = 353.6 Mt. Dromedary apatite 8322-39 20 1.422 10.57 2.651 0.781 350.7 (884) (2216) 8322-42 20 1.422 8.706 2.275 0.771 365.5 (767) (2004) 8322-39 20 1.422 12.64 3.201 0.857 354.1 (775) (1962) Sample Mean Zeta = 357.8 Overall Mean Zeta =352.7 Number of tracks counted are given in parenthesis. Standard and induced track densities are measured on mica detectors1 and fossil track densities on internal mineral surfaces. standard deviation S of a track count is given by the s.quare root of the total number of tracks counted N: S=~N (6) A standard deviation can be assigned to each track density measurement used in calculating a fission track age. These errors are combined to give the standard deviation of the age ST'. ST =T ~(1 / Ns) +(1 / Ni) +(1 / Nd) (7) where Ns, Ni, and Nd are the total number of tracks counted for spontaneous, induced, and standard' glass track.densitieS: Other n0n-POissonian- sources-of, variation. are PosSible 86 (Green, 1981; see below) so the conventional analysis (equation 7) is actually a limiting best case. The EDM is designed so that sampling problems should be eliminated because both Ps and Pi ideally originate from the same amount of uranium. Therefore, Ps and Pi should give approximately the same ratio within the variation allowed by the poisson distribution. However, when using the EDM, some experimental factors can make this "ideal case" unattainable: [ 1 ] Careless counting of track-like features as tracks leads to an overestimate of Ps. This results in determination of an incorrect older age. Experience in the careful identification of tracks is necessary to overcome this factor. [2] Poor contact between the grain mount and mica detector results in a lower Pi as fewer induced tracks are recorded in the detector. This leads to a higher Ps/Pi ratio and hence an older age. Bad contact over a large region of the mount can be recognized by the absence of shallow-dipping tracks and blurred replicas of grain boundaries in the mica. Apafite grains adjacent to zircon grains should not be counted as the higher relief of the zircon may result in poor contact locally and a ~ in Pi. [3] High track densities make determination of true Ps/Pi difficult. A high spontaneous track density makes determination of the correct Ps difficult whereas a high induced track density does the same for Pi. [4] A low Pi makes location of the grain replica and the correct counting area difficult and in some cases, next to impossible. This can be overcome by subjecting the sample to sufficiently large neutron fluences in the reactor. [5] Incomplete etching of tracks leads to. an underestimate of either Pi or Ps- Overetching . · may also-result in-an":underestirriaie:.of:ei~he~' 0i oi~ P'~'as it-is difficult.in this-, case tO 87 distinguish between tracks when they overlap. Overetching also results in the loss of short [6] Spontaneous tracks may not be completely revealed. Zircon or sphene grains containing different spontaneous track densities or compositions will etch at different rates due to differing degrees of alpha damage. Therefore, at any given etch time, tracks will be completely revealed only for a limited range of Ps. Below this range, tracks are incompletely revealed, whereas at high Ps tracks are lost. Therefore at either low or high track densities, the measured values may be depressed, leading to low Ps/Pi ratios. [7] Incorrect identification of the crystal or mica "mirror image" may yield totally incorrect Ps/Pi ratios. This problem can be eliminated by using a meticulous and careful technique. [8] Spatial variation of the thermal neutron fluence may occur in the nuclear reactor and cause problems (Burchart, 1981). Standard glasses, included within a package of irradiated samples, are used to determine Pd and check on uniformity of fluence on a scale of centimeters. If the neutron fluence is not consistent on this scale, this might introduce an additional variation in Ps/Pi. [9] Uranium may be vertically heterogeneous in the apatite grain (Burchart, 1981): Pi is measured in a mica detector exposed to fissions occurring below the sample surface, while Ps originates from fissions occurring both above and below the exposed sample surface. Therefore, in relating Ps to Pi, it is assumed that the amount of uranium above and below the sample surface is identical over the range of a fission event (-15-20 gm), All of the above exper/mental factors are capable of introducing non-poissonian variations to or errors in the measured values of Pi and Ps, and, therefore, to the final age determination. However, with experience and careful sample preparation, factors [1] -through [7]'.'Should'be neutralized.:.. FaCtors-[8] and-[9] plUs cOntamination'may be impossible to identify. The conventional method (equation 7) allows no check to be made 88 on the way in which the data are affected by the above factors (Green, 1981). Thus, the final estimate of Ps/Pi may be strongly affected by data with a non-poissonian variation. A Z2 test can be used to test whether variation is present in excess of that predicted from poisson statistics and determines whether or not the data represents a single population (Galbraith, 1981). In geological situations, failure of the Z2 test usually indicates that some external factor is acting on the variation of Ps/Pi. This is not always the case. Green (1985) has shown that an age determination can fail the Z2 test by chance alone when non-poissonian errors were not present. For those instances when the value of Z2 was not acceptable, Green (1981) determined that the mean of indiVidua1 grain ratios of Ps/Pi (- 1 c0 takes into account non-poissonian variation where present and gives a more realistic estimate of the precision of the determination of Ps/Pi. The value of Ps/Pi is then: and its stan~ deviation is: a(ps/pi) = ~ Y. (ps/Pi)2 - ( I; (ps/Pi))2 n(n-1) (9) The same analysis is often applied to results obtained by both the EDM and the population method. While this is valid for the EDM in most cases, it will be valid for the population method only in the case where the uranium concentration is the same for all the . .- . . grainS.---' .Where'.the~e.iS .a'va/riafi0n in uranium between grain' s; . WhiCh is likelY in most - cases, the uncertainty calculated from equation (7) will be an underestimate. · . : : . 89 APATITE FISSION TRACK LENGTHS The length of fission tracks is an important parameter because tracks decrease in length (anneal) in response to time and temperature (Wagner and Storzer, 1972) and hence can be used as geothermometers. During the annealing of fission tracks, the effects of both time and temperature are important. A higher temperature for a shorter period of time can anneal tracks the same amount as a lower temperattae over a longer time span. Fission Track Annealine _ Annealing has been discussed in detail by several authors' laboratory annealing by Naeser and Faul (1969), Storzer and Wagner (1971), and Wagner (1986); natural annealing by Naeser (1979a), and Gleadow and Duddy (1981); and fission track length annealing by Green et al. (1985a, 1986). Laborato~_ Annealing Studies:: In laboratory annealing studies, a mineral is heated for .. varying periods of time at different temperatures. The degree of observed track density reduction with time and temperature is presented on an Arrhenius plot which relates the logarithm of time to the inverse of tem?era~ure. Early studies investigating the annealing properties of apatite found that heating apafite for a period of one hour produced total track annealing between 250° and 350°C (Naeser and Faul, 1969; Wagner, 1968). An Arrhenius plot representing the data (Figure A3: from Naeser and Faul, 1969), could then be used to extrapolate to a time period of 1 m.y. where 100% annealing would occur at ~175°C. The slope of the lines on the plot increase from that for 100% track retention to that for total track loss, with the difference in the slope of these two extremes ranging by a factor of 2 or 3 (Green et al., 1985a). This fanning array has been interpreted in terms of activation energies increasing with degree of annealing (Storzer and Wagner, 1971). However, in .. another study, wagner'(1986),, found a paraliel-type'-ArrheniuS-pi°t-describing a-single activation energy when he ca_..rvied out an annealing experiment on a single apatite crystal. · .. . 90 Natural Annealing Studies: A more direct way to study fission track annealing in apatite under geologic conditions is to look at the change in apatite age with depth in a drill hole (Naeser, 1981; GleadowandDuddy, 1981). In three studies (Naeser and Forbes, Figure A3. Results of a laboratory study of fading of fission tracks in apatite and sphene. The lines marked 0% indicate temperature and time periods in which no tracks are lost. The lines marked 100% indicate temperature and time periods at which all tracks are lost (from Naeser and Faul, 1969). 1976; Naeser, 1979a; Gleadow and Duddy, 1981), apatite fission track ages. relative to · ,, '' - . . .' - -_ .. '." . . -'- '~.- . .7 ' , ---- ..-' -' .~-... - . _ ,- : 'dePth in drill' hOles'Were ~eported:- N'~tesei:'and. $or'beS'(i976i' foUnd'th&~- apatite riss-iOn: track ages decreased from 100 Ma at the surface to 12 Ma at 3000 m (-95°C present ..... '.-... . .: .... . . .--: -: · . . . .-. ... -. - . 91 downhole temperature) in the Eielson Air Force Base, Alaska, deep drill hole. Naeser (1979a) reported a zero apparent age at 2000 m depth (~135°C present downhole temperature) from the Los Alamos, New Mexico, geothermal test wells 1 and 2. Gleadow and Duddy (1981), studying apatites from drill-holes located in the Otway Basin of southeastern Australia, identified both the top and the base of the track annealing zone. In this basin, stratigraphic evidence indicates that the sediments reached their maximum depth of burial at-30 Ma and have essentially remained at this depth in a uniform temperature regime since then. Apatite ages start to decrease downhole at -60°C (Fig. A4), are reduced by about half at ~95°C, and reach zero at ~125°C. The entire partial stability zone was therefore revealed. Combining their results with laboratory data.from Wagner (1968) and Naeser and Faul (1969), Gleadow and Duddy (1981) constructed an Arrhenius plot (Fig. '*" 6O e~e/® · ) 0 ¢2 0~4 C6 C6 Figure A4. Variation in apparent apatite fission track age with down-hole temperature in wells of the Otway Basin, South Australia. Ages here are expressed as a fraction of their ori .ginal age.(120 Ma)giving a measure of P/Po, the ratio of fission tr-'ack density after and- ' 'before. na .tural annealing (fromGleadow and Duddy,:t981)i' · '. ' - ........ · - 1(')'1 300 200 100 50'C , s~s,',.,s / I I ! // /,' /I ./ r~~///l/! · / · / ~.~r~n~u~~ra( n~sutr% . . I 1 2 1000/1 ~K 92 Figure A5. Arrhenius plot for fission track fading in apatitc.' Shows results from the Otway Group drill-hole data, laboratory annealing results of Naeser (1969) and results from the Eielson, Alaska, and Los Alamos, New Mexico deep drill-holes (Naeser, 1979). Dashed lines represent the 0 and 100% track loss lines extrapolated from laboratory annealing data alone (from Gleadow and Duddy, 1981). A5). This plot shows that the temperature interval over which annealing occurs at geological time intervals is less than that predicted from laboratory studies. Thc variation of mean track length of confined tracks with change in depth and temperature down a drill hole has also been studied (Gleadow and Duddy, 1981; Gleadow et al., 1983). Proportional lengths were expressed as a ratio of L (present measured length) over Lo (the average length of fresh induced tracks in apatite). The results showed the mean lengths to be reduced relative to the original length of 16.4 + 0.8 gm even in surface samples and that -. some 10ng tracks still existed in-Sam, pies that-Wei'e 90% ' reduced i/a-' age. '.Plotting track--- length reduction versus temperature, they were able to locate the apafite partial stability zone . . · .. 93 in the Otway Basin as being between the temperatures of-60°-70° and 125°C for times in the order of 10 m.y. They concluded that the unique contribution of apatite fission track analysis is the ability to define maximum paleotemperatures and variations in temperature through time. Gleadow and Duddy (1981) also observed that single grain ages varied considerably for samples in the partial stability zone. They suggested that this indicated that different apatites can have different annealing properties, presumably controlled by apatite composition. The Effect of Comoosition 9n Apatit~ hlattealin? Gleadow and Duddy (1981) proposed that chemical composition of individual apatite grains must play some part in the considerable spread of single grain ages from apafites subjected to temperatures within the partial stability zone. Green et al. (1985a) analyzed' apatite grains from a single sample residing within the partial stability zone in an Otway Basin borehole. This sample displayed wide variation in single gra/n ages. The age for the bulk sample was 53 _+ 2 Ma, but single grain ages ranged from 0 to 120 Ma. The single grain ages were plotted against the number of C1 atoms per Cai0(PO4)6(F,OH,C1)2 molecule (Fig. A6). Cl-fich grains can be seen to be more resistant to annealing than Fl- rich grains. Fission Track Length Annealing Studig,S In a laboratory study of confined induced fission track lengths in a single, previously annealed, Durango apatite crystal, Green et al., (1985a), determined a single activation energy (-1.64 eV). This implied a near parallelism of lines on the Arrhenius plot for various degrees of length reduction. Because the annealing characteristics of individual apafite gini'ns are strongly controlled by C1 content, they suggest .eql.. that the .widely fanning . · ArrheniUs pl°ts cOUld-be ~he reStilt of the superp°sition Of'a-_series of near'parallel' · 94 DEPOSITIONAL AGE t " t , I ~ I ~ I 0.2 0.4 o.e o.8 Humber of CI 8toms per molecule Figure A6. Variation of apatite single grain ages with composition. This sample is from the Otway Basin. Composition is expressed as a number of Cl-atoms in the (F, OH, C1) group of the apatite molecule. This shows Cl-rich grains are more resistant to annealing than Fl-rich grains (from Green et al., 1985a). Arrhenius curves. Each curve would correspond to the range of compositions present and represent slightly different activation energies. Green et al., (1986) observed that in all annealed samples, the mean confined track length is always less than that in unannealed control samples. As annealing progresses, the mean length is reduced and the length distribution broadens, slowly at first, and then more rapidly below a length reduction (I.dLo) of-0.65. In addition, the anisotropy of annealing becomes more pronounced in apatite as annealing progresses. Tracks aligned parallel to the crystallographic c-axis are more resistant than tracks perpendicular to it. As the mean length decreases, the only tracks preserved are those more closely aligned parallel to the c- axis. In heavily annealed samples (IJLo < 0.65) sequential etching indicated the presence. - . . -... _ : ' - . . .. - .. - ~_::.:~ -- . '-.' .' . - . Of n°n'etChable (~ te'rrns Of'nOrmal etch fi'riles) gaps ai°flg the-'length Of a Small proportion "- 95 of tracks. These gaps, which delay the progress of the etchant during that process, are not common and may be breached with continued etching Green et al., (1986). A two-stage model for the annealing of fission tracks in apatite emerges. For mean lengths above ~10.5 gm (L/Lo _>0.65) the form of the track length distribution changes only slightly with the degree of annealing because the anisotropy is not very pronounced. Below -10.5 gm, the form of the confined track length distribution changes rapidly as annealing progresses. The dominant process causes a shortening of the etchable portion of the track from each end, with the rate of shortening increasing with increasing angle to the c-axis. For a given combination of temperature and t/me them is a characteristic maximum etchable length, which depends on the orientation of the track. As annealing becomes severe, gaps may appear in the etchable portions which may delay the progress of the etchant. With continued etching the gaps may be breached, allowing the characteristic etchable length to be revealed. The observed length distributions thus result from a combination of the anisotropy of annealing, and to a much lesser degree, the presence of gaps (Green et al., 1986). Laslett et al., (1987) used the results of Green et al., (1986) to determine whether the results were best explained by a parallel or sLightly fanning Arrhenius plot. The best fitting parallel model accounted for only 96.7% of the variation of transformed length reductions. A slightly fanning model gave the best match, accounting for 98.0% of the variation. For a slightly fanning Arrhenius model: In(t) = A(r) + B(r) T-1 where: - T = absolute.tem~ramre_ (equation 20, Laslett et al, 1987) annealing time A(r) = an unknown function of'r subject to constraints that when t = 0 or T = r = 1 so that: A(1)= 96 B(r) = a function where B is normally interpreted in terms of E/K where K is Boltzrnanns constant and E is an activation energy This model is difficult to fit since A(r) and B(r) are unknowns. Therefore, by statistical means they derived the following preferred model (see also Fig. 7): [ { (1-r2-7) / 2.7 }0.35_ 1]/0.35 =-4.87 + 0.000168T [In(t) + 28.12] (equation 27, Laslett et al, 1987) Measurin~ Fission Track Leneths There are three ways of measuring fission track lengths: [ 1] By measuring the projected lengths of tracks intersecting an exposed internal surface (Wagner and Storzer, 1972); [2] measuring the true length of tracks in an internal surface by measuring the vertical as well as the 'horizontal component of the track length and correcting for the dip of the track; or [3] measuring the true length of internal confined track$ (Fig. A7) which do not intersect the surface. Confined tracks, as defined by Lal et al. (1969), are tracks etched either via contact with a track whiCh reaches the exposed surface or via a fracture or crack. ETCHED SURFACE -. Figure A7. Confined fission track lengths as etched through fractures or other tracks (modified '1983) ..... · . from Gteadow.et al., . ~. ~'' . .... -.. ~ . ~ 97 To measure the true length of a confined track, one measures tracks present in the grain in a horizontal plane perpendicular to the line of sight..However, from a practical standpoint, it is possible to measure tracks that are not quite horizontal. Laslett et al. (1982) considered confined tracks with true dips <15° as horizontal because their measurement resulted in an underestimate of the actual length by only a few percent. With reflected light illumination, horizontal confined macks exhibit a very bright image and can be easily located. This only occurs for tracks very close to horizontal. In transmitted light, a track is considered horizontal only if it remains in sharp focus along its entire length. Confined tracks should be measured in prismatic grains (parallel to the c- crystallographic axis). This is because annealing in apafite is anisotropic causing tracks perpendicular to the c-axis to shorten faster than macks para, el to the c-axis (Laslett et al., 1984). Thus in a grain orientated parallel to the c-axis the whole range of track lengths will be present. In a basal section the mean track length will be shorter because the longest tracks will not be present. In this study, confined fission track lengths in apafite were measured using the criteria outlined in Laslett et al. (1982, 1984) and Gleadow et al. (1986a, 1986b). To properly calibrate track length measurements I measured many track length standards with known distributions. Only after I displayed an acceptable level of competence determined by P.F. Green and A.J.W. Gleadow were the Alaskan samples measured for this study. APATITE FISSION TRACK THERMAL HISTORIES As previously ckiscussed on page 29, annealing of fission tracks in apatite can be used to determine.the thermal history of-a'samPle:. Gleadow and Duddy.(1981), in a study of.the annealing properties of apatite from subsurface samples in the Otway basin of southeastern Australia, .defined. a'temperature'range, over'which fission tracks anneal. -In the .Otw.ay · . .... : . ~ . . .: . . . 98 Basin, apatite ages and mean track lengths began to decrease at -60°C and reached 0 at --125°C (Fig. A4). The entire apatite partial stability zone was therefore revealed and defined as --60-125°C (based on the data in Fig. A4). Green (1986), presented data in which some annealing occurs even at ambient surface temperatures. Track Leneth Distributions -- Heating through the apatite partial stability zone, as track densities and mean track lengths decrease with increasing temperature, track length distributions have characteristic distributions relative to their length of residence time within the partial stability zone (Fig. A8). The transition from unaffected ages through partial overprints to total resetting is reflected in the shape of the track length distribution. For samples that were subjected to lower temperatures in the partial stability zone, track length distributions show a high mean ~0 :Or I05~ 14 % ..~--.~-I 0 $ :0 BANYIJLA- 1 PORT ~0 100% FLAXMANS - 1 :.-?'or ,,,.o ' 0 , "x . ,o~- 83% ~ 93% ,,! L i' 98% ', .-. o · , ?4% .-, 80% 92' 57% . T3 108· ~ 13% 3 3 S Figure A8. Fission track length distributions observed in apatites from the Otway Basin, South Australia, at various depths (tempemtures).in three drill,holes. EStimated.formatiOn' temperature and the percentage to which the ap _paren('-fi~si0n track agehas been reduced is. also shown (from Gleadow et al:, 1986a). 99 length with a low standard deviation. As temperature increases the length distribution broadens. This results in a decrease in the mean length and an increase in standard deviation. Samples from the base of the partial stability zone show very broad and relatively flat length distributions with mean lengths -50% of the unannealed mean length. The maximum track length seen in the samples (-16 gm) doesn't change because new tracks are continually being formed. Several .possible patterns of track lengths in apatites arising from distinct thermal histories as described by Gleadow et al., (1983) are shown in Figure A9. This Figure shows a number of hypothetical temperature vs. time plots and the resulting track-length distributions. The examples A-C show simple burial histories giving unimodal apatite length patterns essentially in equilibrium with different levels in the partial stability zone. TIME Stabili~ TIME -Figure .A9.- Idealized time-temperature paths with the resultant apatite u-ack.length-. .- -disu'ibutions, .See text for explanation (mOdified Gleadow eral.,-1-983).~ The'60°C:value for'the upper boundary' of the partial annealing zone is' based on the 'work of Wagner et'al., 1977. 100 Examples D-F show a bimodal length distribution resulting from a past heating event, a skewed distribution typical of slow cooling through the partial stability zone, and an entirely shortened unimodal distribution produced by a recent temperature increase. Bimodal distributions consist of two major length components' those that were annealed during a heating event and those that have formed since cooling to lower temperatures. By statistically separating the two components and estimating the contribution of the later group to the age of the mineral, the timing of the heating event can be estimated. Skewed distributions are essentially the summation of the three length distributions shown in simple burial. The shortened distribution resulting from a recent temperature increase is produced when all previous tracks are shortened together. Gleadow et al. (1986b) determined apatite fission track length distribution patterns for a number of geologic environments. They divided confined track length distributions into five characteristic shapes (Fig. Al0): (1) Induced Track Length Distributions. Induced track length distributions from many apafite samples have mean track lengths which fall within a narrow range between 15.8 gm and 16.6 gm and have standard deviations of 4).9 gm (Fig. A10a). It is reasonable to · conclude that induced tracks in all apatite samples will have a distribution typified by a mean of ~16.3 gm and a standard deviation <1 gm (Gleadow et al. 1986b). Track length distributions of spontaneous tracks from a Wide variety of different apafites can therefore be compared without the need to refer back to lengths of induced tracks in the same apatite sample (Gleadow et al. 1986b), as had been previously suggested by Green (1980). (2) "Undisturbext Volcanic" Distributions. These distributions are characterized by mean lengths between 14.0 and 15.7 gm and standard deviations from 0.8 to 1.3 gm (Fig. A t0b) :alth0tigh m0st'irange from'0:8-tO', i-~0. gml Thi~: tyPe':ofdis'tributi6n reflects rapid cooling after formation, and subsequent exposure to temperatures <50°C (Gleadow et al. -- · - ~ . -- . -... . :- : ' ~_ __' . ~ · .. .. 101 1986b). An undisturbed volcanic-type" length distribution can also be found in rocks of non-volcanic origin, where this distribution is diagnostic of a rapid cooling, followed by residence at low temperatures (<50°C). (3) "Undisturbed Basement" Distributions. This form of distribution (Fig. A10c)is typical of plutonic rocks and high-grade metamorphic rocks that formed at high temperatures (>500°C) deep within the earths crust, were uplifted, and have now cooled to ambient surface temperatures. They are characterized by a distinct negative skewness, mean track lengths of ~12.5 to 13.5 gm, and standard deviations of-l.3 to 1.7 gm. (4) Bimodal and, (5) Mixed Distributions. These are characteristic of thermally affected, but not totally overprinted, samples. A "mixed" distribution .reflects a partial thermal overprint which may become more prominent to form a "bimodal" distribution (Fig. A10d and A10e). ~0 40 I-- 30 0 ~0 ~I: ~0 INDUCED 8IMODAL MIXED 1,,1 .... 1,, I UNDIS TUREED VOLCANIC UNDISTURBED BASEMENT -- -C d 0 5 10 15 20 0 :,5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 TRACK LENGTH (microns) Figure Al0. Characteristic apatite confined track length distributions for different thermal histories. See text for explanation (from Gleadow et al., 1986b) 'Upon' .comPleti°n of..my results; the-Alaskan 'samPle track' length' distributions' were. compared to the above length distribution models and to interpretations of various thermal .. . : · . .. - . ~ ..-. -~ .. :- : . . . · .. . . . . . . .. 102 histories (Green et al., 1985b; Gleadow et al.,1986b; Gleadow et al., 1986a) in order to work out the thermal histories for the Alaskan sedimentary sequences. Thermal Modeline -- Thermal modeling is an important tool in predicting a thermal history for a sample based on its apparent fission crack age and the shape of its track length distribution. Figure A 11 illustrates three examples with different rates of cooling and their resultant apatite ages and track length distributions as predicted by a thermal modeling program written by P.F. Green and A.J.W. Gleadow based on Laslett et al.'s. (1987) preferred model. In (A), rapid uplift resulting in cooling at a uniform rate from 130°C to 40°C over 10 m.y. is followed by much slower cooling to 0°C. The computer model produces a volcanic-type distribution for this cooling history. In (13) and (C), decreasing the rates of cooling (130°C to 40°C over times of 20 m.y. and 40 m.y. respectively) results in decreasing the mean track length and increasing the standard deviation. These effects result from the sample spending longer periods of time within the annealing zone (-60°-125°C) where more short tracks accumulate. Figure Al2 shows how sensitive this thermal modeling can be. In (A), the sample was at 130°C prior to a rapid cooling to 40°C between 50 and 60 Ma. The resultant track length distribution is a volcanic-type and contains no short tracks. In (B) and (C), the sample was at 120°C and 110°C, respectively, prior to rapid cooling and results in the preservation ofhigher percentages of shortened tracks. A small difference in the temperature prior to cooling (110-130°C) is easy to distinguish by the shape of the track length distribution. The sample at 130°C prior to cooling shows no short cracks. The sample at 120°C has a very small tail of cracks <12grn (-4%) and the sample at 110°C shows a larger tail of tracks _- . <i2gm (-8%).' .'.~- '-~- - · -" - ... - . · . . -. ._-. .-..- : : 103 .8 1/10 .8 0 o 20 40 ~o 'c 80 ~oo ~2o .8 .6 0 0 20 40 6O 80 100 ~20 100 . 100 ! I I ! I I I I I 1 80 ~ 20 0 Ha i I I I i I ~ (A) t ! I I I I I I t 80 80 40 20 0 Ha (B) Time = 40 Number 30 20 10 100 Ma, FIflge = Final Temperature = 0 5 10 15 Track Length (lzm) 76.81 O°C MEAN= 15.29 um S.D. = 1.04um SKEW = -1.45 KURT= 13.31 .I 2O Time= I00 Ma, FTAge = 74.58 Final Temperature = 0 °C 40 Number 30 10 0 5 10 15 Track Length (l. Lm) ,1 2O MEAN = 14.99 urn S.O. = 1.44 um SKEW = -1.53 KURT = 6.28 Figure All, FigUre'capfion on next.page. 104 1 .8 I/Io .G .4 .2 0 0 2O 4O GO 'C 80 100 120 Time = I00 Ma, FTAge= 67.37 Final Temperature = 0 °C 40 Number 3O (c) MEAN = 14.49 L~m S.D. = !.90 SKEW = -i.42 KURT = 5.8O Figure All. Apatitc track length distributions resulting from different assumed thermal histories. Examples show expected distributions resulting from decreasing cooling rates (interpreted in terms of increasing rates of uplift). See text for explanation. These diag/ams axe produced from a progrmn written by P.F. Green and A.J.W. Gleadow based on Laslett et al.'s (1987) preferred model of apatite fission track annealing. The thermal history being modelled is shown on the bottom left diagamm in terms of a time-temperature path. "Time" represents the total time elapsed since fission tracks start forming. The history of track shortening is shown for 20 hypothetical tracks at different times (in each top left diagram), expressed as 1/lo (measured length/length of original track). The length distributions from each are summed to give the histogram on the right. 105 1 .8 !/lo .6 .4 .2 0 0 20 40 6O 'C 80 100 120 I I I I I I I I (A) 1 .8 I~o .6 .4 .2 0 80 84 48 32 16 0 Ha 0 I I 20 - 40 - 60 80 - 100 - 120 (B) Figure .t_12. Figure caption next page. Time= 80 Ha, FTflge= Final Temperature = 56.45 O°C 40 Number 30 20 10 0 0 5 MEAN =15.I1 um S,D, = i.11 urn SKEW = -0.85 KURT= 4.46 10 15 20 Track ~ngt h (I.~m) Time = 80 Ha, FTRge= 58.24 Final Temperature = 0 °C 40 Number 30 t0 5 10 15 Tr ack Length (~) MEAN = 14.95 u~n S.D. = 1.52 urn SKEW = -2.43 KURT = 12.42 .I 20 106 Time= 80 Ma, FTAge = 60.63 Final Temperature = 0 °C 40 Number 30 0 2O 80 84 48 32 16 0 Ma 0 I _ .- 2O 4O 60 8O 120 10 0 ..I.. .I 0 5 10 15 20 Track Length (l~m) MEAN = 14.84 urn S.D. = 1.67um SKEW = -2.33 KURT - I ! .29 (c) Figure Al2. Thermal modeling of different thermal histories prior to identical cooling histories. Examples show expected distributions resulting from rapid cooling from different initial temperatures. See text for explanation. These diagrams are produced from a program written by P.F. Green and A.J.W. Gleadow based on Laslett et al.'s (1987) preferred model of apafite fission track annealing. The thermal history being modelled is shown on the bottom left diagram in terms of a time-temperature path. The history of track shortening is shown for 20 hypothetical tracks at different times, (in each top left diagram), expressed as 1/Io (measured length/length of original track). The length distributions from each are summed to give the final distribution histogram on the right of each diagram. 107 Aot~iications An age and a track length distribution from a single sample can yield a relatively minor amount of information compared to a sequence of samples selected to reveal variations within a sedimentary section. The sequence may be taken from drill holes (e.g. Gleadow and Duddy, 1981), or taken from long vertical profiles in mountain ranges (e.g. Gleadow and Fitzgerald, ! 987). Apatite fission track analysis has been used to constrain the thermal histories of many different geologic settings. These include dating the emplacement of igneous bodies (e.g. Gleadow and Oilier, 1987), evolution of sedimentary basins (e.g. Glcadow and Duddy, 1984), evolution of continental margins (e.g. Moore et al., 1986), uplift of mountain ranges (e.g. Fitzgerald et al., 1986), and "regional thcrmo-tectonic evolution" (Green, 1986). By using the observed fission track parameters and apatite thermal history models discussed above, it is possible to constrain the thermal history of a terrane. This approach has been applied to the Alaskan sexlimen~ rock units in this study. · & APPENDIX B APATITE FISSION TRACK ANALYSIS SAMPLE PREPARATION INTRODUCTION Sample preparation techniques described below are those in routine use by the Fission Track Research Group of the University of Melbourne Geology Department, recently described by Fitzgerald (1987). They have evolved over a 17 year period and have been improved by many people during that period. The techniques described below are those developed by A.J.W. Gleadow. SAMPLE TREATMENT Rock samples used in this study varied from about 3 to 6 kg. Sample numbers contain the year collected (87), my initials (POS) or those of John Decker (JD), and then number of the field location. Multiple samples from the same location were designated by A, B, or C. Sample numbers ranged from 87POS1A through 87POSl17A and 87JD3B through 87JD87A. Weathering rinds were trimmed and the initial rock crushing was performed in the Geochronology Laboratory of the Geophysica1 Institute, University of Alaska, Fairbanks. The samples were then shipped to Melbourne for the remainder of the sample preparation. At every stage of the preparation, from crushing through mineral separation, pre-irradiation and then post-irradiation handling, measures were taken to insure cleanliness of equipment to prevent any possible contamination. A reference hand specimen was retained and a thin section was prepared for each sample. ROCK CRUSHING AND MINERAL SEPARATION Rock crushing and mineral separation procedures.are stmmmrized in Table B ! followed ... : by adiscussiOn of'the'subseq{aent steps. -' ~ ': : '- '~" 109 Table B 1. Rock Crushin~ and Mineral Separation Summar[ (1) Crush to size range 200-75 gm. (a) Large jaw crusher (b) Bico Braun disk mill (2) Wash thoroughly in water to remove fines (dust), or if the sample is too large, use a Wilfley Table to wash sample and concentrate heavy minerals. (3) Oven dry at low temperatures (< 60© C). (4) First magnetic separation. (a) Frantz-full scale (1.9A), vertical paper funnel, removes ferromagnesian minerals (b) Frantz-0.4A, vertical feed, removes biotite (5) First TBE. Nonmagnetic fraction from (4b) into TBE, remove quartz, feldspar etc. as float. (6) Second magnetic separation-sink from (5). Frantz at forward slope of 25° · and side slope of 10°. (a) Frantz-0.5A, removes biotite and epidote as magnetic fraction (b) Frantz-0.8A, removes sphene and monazite as magnetic fraction (c) Frantz-l.lA, removes sphene composites as magnetic fraction (d) Frantz-full scale, magnetic cleanup (7) Second TBE. Nonmagnetic fraction from (6d) into TBE, removes remaining quartz, feldspar, etc. as float. (8) DIM. Sink from (7) into DRvI, sink is zircon fraction, float is apatite fraction. (9) Clean up apatite and zircon fraction. (a) Apafite, Frantz-full scale, 2© to 5° side slope (b) Zircon, Frantz-full scale, 0° side slope TBE = Tetrabromoethane (sym) (specific gravity = 2.95-2.97) DIM = Di-iodo methane (specific m-avitv = 3.32) ¥ · . _ . . ... 110 MINERAL MOUNTING, GRINDING, POLISHING, AND ETCHING TECHNIQUES The following methods outline the procedure for mounting and preparing apatite for fission track analysis. The purpose of the mounting medium is to support the grains while they are ground, polished, etched, and eventually counted. Grinding exposes an internal surface, polishing removes the grinding scratches plus any surface imperfections, and etching reveals the tracks so they can be seen with a petrographic microscope. At all stages of' preparation the sample numbers were labeled on the mounts. Summary of Materials and Methods Mounting medium: epoxy on a glass slide. Araldite MY753 resin and HY956 hardener, 5:1 by volume or weight. Grinding: 400 and 600 grit SiC waterproof abrasive paper on a wet rotating lap (400 rpm). Polishing: 0.3 Ixm corundum polishing powder in a H20 slurry on a nylon cloth lap rotating at 400 rpm (1:5 ratio, AI203:H20). Etchant: 5 M HNO3 for -20 (15-30) seconds at 20-22°C. Apatite grains were mounted in Araldite on a hot-plate at 120°C. Approximately 10 mg of 100-200 [xm size grains are enough to adequately cover a 1 cm x 1.5 cm area so that the grains are not touching, yet not excessively isolated. Mounting with excessive grain density makes locating the grain image on the mica replica difficult. Once in the warm epoxy, the slurry was stirred with a needle to ensure that apatite grains sank to the bottom of the araldite. The slide was then left on the hot-plate for 5 minutes to cure the Araldite. Heating to 120°C for this very short period of time has been shown not to affect track lengths (Gleadow, 1984). · . .;. 111 Grinding and Polishing During this stage the slide was held in a specially made recessed brass holder. Excess epoxy was removed using the 400 grit paper. Internal surfaces of the grains were exposed using the 600 grit paper. In the grinding stage the slide is held stationary in one orientation. Mounts were polished for at least two periods of 45 seconds each. Mounts were etched by placing two slides back to back, holding them together using tweezers and submerging them for 20 seconds in a beaker of 5M nitric acid. To stop the etching, they were submerged in a beaker of tap water and then rinsed with tap water. At this stage the mounts were trimmed down to a 1 x 1.5 cm size by snapping off the excess slide glass after scoring with a diamond pencil and ruler. Final trimming was accomplished using a rotating diamond-impregnated metal grinding wheel. PRE-IRRADITATION SAMPLE HANDLING FOR THE EXTERNAL DETECTOR METHOD Mount- Mica Pairs and Wrilooing The mounts were all trimmed to I x 1.5 cm at this stage. Pre-packaged rectangles (-50 gm thick x 1.3 cm x 0.85 cm) of low-uranium muscovite (<5 ppb) were used for the external detector. The sample number was scribed on the back of the mica prior to wrapping. The mount-mica pair was then placed in an envelope of heat shrink polythene/polyester laminate plastic using tweezers and then sealed using a heat sealer. The envelope comers were trimmed off to allow a/r to escape when the heat-shrinking takes place. Heat shrinking was done between two pm-heated glass slides on the hot plate at -IO0°C. Considerable care was necessary at this stage to ensure the mica was aligned 'correctly over the mOunt, not overlapping .the edges of. the 'glasS,: and that gOOd'contact was achieved between the mount and the mica. Pinpricks were used to mark the comers of the . - . . . . .. .. . 112 mount-mica pair for use during the coarse alignment procedure on the AutoscanTM stage. Standard glass-mica pairs (for determining Pd, the standard track density) were wrapped in the same way. The Irradiation Packaee -- Up to 15 mount-mica pairs, all labeled and in known order, were placed in a stack with a standard glass-mica pair at either end for monitoring the neutron fluence. The stacks were wrapped tightly in Al-foil, the top labeled and Placed in a high-purity aluminum irradiation tube. It is important to know the order of the samples within the tube so that if a fluence gradient is measured by the standard glasses at either end of the package then individual standard track densities can be assigned to individual mount-mica pairs. Neutron Irradiation and Fluence Monitorine _ Neutron irradiations were carded out in the X-7 position of the Australian Atomic Energy Commission HIFAR Research Reactor, Canberra, which has a well thermalized flux of .--3 x 1012 ncm-lsec-1. No flux gradients were detected from any of the 4 packages when using between 12-15 mount-mica pairs and 2 standard glasses (package thickness -5 cm). Thermal neutron fluences were monitored by recording the standard track density in the mica external detectors placed over standard glass discs of the National Bureau of Standards (NBS) reference glass SRM612 (-50 ppm U). The standard neutron dose requested for the apatite packages was 1 x 1016 neutrons cm-2. The apatites from the ANWR region were of such low uranium concentration (5-30 ppm U) that future irradiations should be ~ven higher neutron fluences. POST-IRRADIATION SAMPLE HANDLING 'After.irradiation,' the mount'mica'pairs, were. removed from .the irradiation tube. and- '- .. · _. unwrapped'when a safe 'level of radiation Was achieved. Micas were etched in 40% HF for 20 minutes, thoroughly washed.overnight in tap water, and then allowed to stand for at i .-' 113 least 12 hours. This allowed evaporation of any residual HF and therefore prevented any possible damage to microscope objectives. The mount-mica pairs were mounted on 1 x 3 inch glass slides using epoxy (Figure B 1). Care was necessary at this stage to ensure the pairs were aligned in a mirror-image configuration and that the mica was not glued face down. The micas that were put over the standard glasses were mounted together on a separate slide. ! ir~radiation number lapatite mount in'ica external detector - . . pin -prick reference cross sample number Figure BI. Mount/mica pair as mounted on a glass slide ready for counting and measuring track lengths. Pin pricks are used during coarse alignment and the scribed cross is used as the reference point for automated alignment (from Fitzgerald, 1987). MICROSCOPE EQUIPMENT AND COUNTING PROCEDURES Fission Track A~es -- A Zeiss Universal microscope system, equipped with dry, epiplan objectives mounted on a revolving nose turret, was used for all fission track counting. Objectives used were corrected for use without cover slips. The 80x objective used for counting had a numerical aperture of 0.95 and the condenser had a numerical aperture of 1.4. The tube factor was . . .- ..:. -_ : . ~..-. -. -. . - .- .. .. . .. . ... '- .: . -.. _. 'variable using'an OPt;var magnification chamber; either 1.:25x;1.6X', 0r.2.0X being available. A Zeiss KPL wideangle binocular eyepiece with a magnif-~cation of 12.5x was · ... . . _ . . . 114 used. A 10 x 10 grid located in one of the eyepieces was used for counting tracks over a known area. Fission tracks were counted at a total magnification of 1250x (80 x 1.25 x 12.5 = objective magnification x tube factor x eyepiece magnification). The size and area of the grid in the eyepiece was calibrated using a diffraction grating (stage micrometer), one line on the diffraction grating being equal to 1.5678 gm. Standard track densities (Pd) were obtained by scanning across the mica external detector and counting the number of tracks in a given area at a given number of locations. Tracks lying on the lines defining the top and right side of each square in the grid were counted as being within that square. The exact position of a track was defined by the position of the track head (the intersection of each track and the exposed surface of the grain). The requested dose of 1 x 1016 neutrons cm-2 for NBS glass sRM612 usually gave a standard'track density (Od) of-l.35-1.45-x 106 tracks per cm-2, A microcomputer-contrOlled AutoscanTM 3 axis motorized stage System (Gleadow et al., 1982; Smith and Leigh-Jones, 1985) mounted on the Zeiss Universal microscope was used for counting fission tracks. This stage system permits the automatic location of matching points on the mineral mount and its mirror image on the mica detector. The co- ordinates of all counting sites are recorded relative to the position of a small cross scribed .. on the slide (Fig. B 1). The mount and the mica were first roughly aligned using the pin pricks and then fine-tuned using distinctive grains and their images. Alignment usually took about 10 minutes and had a precision of 5-10 gm. The mount was usually scanned using the 16x objective under reflected light to look for grains with good polishing scratches (indicating proper etching conditions on the crystal . " _ -. face). -.OnCe' a good grain was. found it was ex-amined under -transmitted light-'to determine l .- whether it was suitable for counting (dislocation free, and not zoned). The area counted on '-' '- ~"'3 2 '. .'.. ~. ._- ~: _ . ~... -.: -:-.. '.- ' . . ..- . --~- ._ - .- ~ _ . ....-. - ~ : :. · -2- - ..- -' ' ' - - ( . _- . - -: . . . . -. · .. . . : ..-' .. . - - . . . ' . . [ '"-: ..' . ........ . - .. ~' '-?.. ..' .-_. .. ;' -_ .. 115 each ~ varied according to the suitable area available (area on individual grains ranged from 4 to 70 grids with each grid-8.548 x 10-7 cm2). This area had to lie within a one- track-length distance of the grain margin. Usually 20 suitable grains were located (or as many grains as possible up to 20) before counting tracks in the apatite (Ns) and in their muscovite replica (Ni). A mechanical hand counter was used to record the number of tracks. Confined Fission Track Lengths -- A drawing tube was attached to the Zeiss Universal microscope and the images of confined fission tracks were measured on a HipadTM digitizing tablet. This tablet had been previously calibrated using a stage micrometer marked at 10 and 2 gm intervals. A light emitting diode was attached to the cross hairs of the the digitizing tablet's cursor so it could be more clearly seen. Only horizontal, confined fission tracks with clearly defined ends were measured. Confined tracks were located by scanning the mount using a 40x objective under reflected light. Horizontal confined tracks usually appear as highly birefringent, cigar-shaped tubes. Track lengths were measured using an 80x dry objective. 100 track lengths (or as many as possible up to I00) were measured in each sample. Locating this many on most of the samples took 2-3 hours but some particularly young or low-U samples took up to 4 hours.
