Abstract
Full Text
SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks
Neal J. McNaughton, Birger Rasmussen, and Ian R. Fletcher

Supplementary Material

Selecting Samples
Samples of clastic rocks were collected, and polished thin-sections prepared. These were examined using an SEM or ESEM (Environmental Scanning Electron Microscope) to locate xenotime overgrowths on zircons, using back-scattered electron (BSE), cathodoluminescence (CL), or charge contrast (CCI) imaging. CCI is a new method (Griffin, 1997), which for some minerals gives images similar to CL, but is much faster, has higher resolution, and does not require carbon coating. Samples with the largest and more abundant xenotime overgrowths were selected for analysis.

Selecting Grains
Detrital zircon grains were separated from the rocks by conventional rock crushing and milling, and heavy liquid and magnetic methods. Zircons were then hand-picked from the final concentrate. The recovery of xenotime overgrowths is variable, and grain selection criteria are still being refined. For the analyzed Warton Sandstone sample (FB2; mount 98-16B), recovery was good, but for a second Warton Sandstone sample (FB1; 98-16A) and a corresponding sample of Pentecost Sandstone (PT-1; 98-33B), few overgrowths were recovered and their proportion bore no relation to the proportion of xenotime-bearing zircon grains identified in polished thin sections. A second sample of Pentecost Sandstone zircons was prepared (98-36A), concentrating on grains with visible overgrowths. This gave a better recovery, but still not as good as for Warton Sandstone FB2. The overgrowths are smaller, and the randomness in grain orientation resulted in few overgrowths being sufficiently exposed to permit analysis without significant primary beam overlap onto the zircon. When the Pentecost Sandstone zircons were analyzed, it was noted that those with xenotime overgrowths had the most discordant U-Pb. They also appear highly metamict in SEM images, and therefore are the grains that would normally not be selected for zircon analysis. A third picking was carried out, concentrating on dark, irregular grains. These grains were individually oriented on the mounting substrate to give the best cross section of the larger overgrowths (mount 98-78A). It is possible that some xenotime overgrowths are lost during the milling, but we have not yet found evidence of this in the fine residues.

Preparing Samples
The sample mounting and SHRIMP analytical procedures used in this study are based on those routinely used in Perth for zircon analyses, which are based on those developed at ANU (Smith et al., 1998, and references therein). Epoxy mounts were prepared with detrital zircon grains, and grains of the SHRIMP standards (CZ3 zircon and xtc xenotime). The mounts are ground and polished to expose a cross section of the zircons and xenotime overgrowths, photographed in transmitted and reflected light, and SEM imaged by BSE and CCI. CL imaging is generally avoided because it gives little additional information, it consumes more time, and the longer exposure to the electron beam usually results in damage to the resin adjacent to the sample. This damage can affect the conductivity of the sample surface after gold coating, and it causes difficulties for high-magnification optical viewing in the SHRIMP source. In extreme cases, the overgrowths or parts of zircons can fall from the mount during cleaning before gold coating.

SHRIMP Mineral Standards
CZ3 is the single zircon crystal routinely use for zircon analyses on the Perth SHRIMP (Pidgeon et al., 1994; Nelson, 1997). For xenotime, we used "xtc," a temporary in-house multigrain xenotime standard composed of high-U grains from an Archean pegmatite. It has the limitations common to any multigrain standard, including having variable U abundances; SHRIMP data show variations of a factor of ~4. Some grains have inclusions of phosphates with very high Th (and 208Pb) contents which give discordant 208Pb/206Pb versus 232Th/238U data. 207Pb/206Pb data from "clean" grain areas scatter more than expected from the counting statistics for individual analyses, but this is considered to be due to the counting statistics underestimating the precisions. The high U content and age of xtc leads to count rates for Pb which are so high that the assigned internal precisions are extremely small, and minor fluctuations in primary beam intensity and nonlinearities in the secondary beam intensity profile, which are normally insignificant, here contribute significantly to the data scatter. Some scatter might also arise from submicroscopic high-Th inclusions, though such data can usually be identified and excluded. For substantial xtc data sets, the 207Pb/206Pb age is within several million years of the 2632.3 ± 1.4 Ma age determined by TIMS on very small chips of separate grains (Table A1). The TIMS data show variable discordance, between 0 and ~3%. SHRIMP comparisons with published TIMS data for a high-U sample (Aleinikoff and Grauch, 1990) indicate U abundances of ~10,000 ppm (1%) for xtc. The chips used for TIMS analysis were too small to permit accurate weighing and subsequent element abundance determination.

Reproducibility of the U/Pb ratio for xtc during a normal analytical session is variable, ranging from close to 1% (that is, similar to the best for CZ3 zircon) to ~5%. The variability is largely due to a dependence of measured Pb+/U+ on U content, but there must also be a contribution to this scatter from real variations in Pb/U on the scale of the SHRIMP primary beam spot (see Table A1).

Comparisons between xtc and several other potential standards have never revealed discordance beyond the 2 to 3% seen in TIMS data for xtc. Apparently, variable concordance between (as well as within) samples is always explicable by matrix/impurity (as reflected in U content) effects(Fig. A1).

SHRIMP Procedures
Analytical procedures for xenotime are based on those used for zircon, as described by Smith et al. (1998) and references within. There are, however, important variations:

1. The primary ion beam spot is reduced in diameter, to =15 μm in all cases.

2. The reduction in primary beam diameter results in a reduction in intensity. The primary beam currents used in this study ranged from ~0.4 to ~1.2 nA. Because of the generally high U contents, the corresponding reduction in data quality (Pb+ count rates) was not severe, although it was significant for xenotime from the Pentecost Sandstone.

3. The retardation lens was activated for all analyses, to minimize the possibility of tailing from potentially large REE oxide peaks which are adjacent to the Pb peaks. Additional trials will be necessary to determine whether this is a general requirement for xenotime analyses.

4. Additional mass peaks were recorded. Y2O+ was used as a mass and abundance reference peak, in place of the Zr2O+ used for zircon, but Zr2O+ was still recorded to monitor overlap of the primary ion spot onto adjacent zircon. For most analyses, mass 209 was recorded to try to identify 208PbH+, should this be generated from hydrous portions of samples. No significantly elevated 209+ counts were recorded except when 204+ was also high. It is not clear whether this reflects contamination by both Bi and common Pb, or whether common-Pb contaminated zones are also hydrous and therefore generate PbH+. For present purposes, the difference is immaterial, because data with significantly elevated 204Pb+ are not used. Mass 270 was also monitored for some analyses, to see whether UO2+ provided a proxy for U+ which was less sensitive to element impurity effects. This does not seem to be the case.

Data Reduction
Data files were reorganized to resemble zircon data files and processed as such using Krill, a software package prepared by P. Kinny which is computationally equivalent to programs used previously in Perth and at the Australian National University. This necessitated using a ln(Pb/U) versus ln(UO/O) trend for Pb/U calibration. This might not be the optimum correlation for xenotime, but a better one has not been identified.

Concordance
In addition to the constraints applicable to any SHRIMP analyses, there are three factors limiting the determination of concordance, and hence 206Pb/238U ages, for xenotime overgrowths:

1. Variation of Pb+/U+ with concentration of U (and corresponding Pb, as well as Th and possibly other trace elements). Eventually, it might be possible to use a three-dimensional (Pb/U versus UO/U and U abundance) calibration, but, as yet, there are too many indeterminate factors to justify establishing "routine" procedures for this.

2. Lack of fully characterized standards. As noted above, comparisons between putative standards suggest that all are within a few percent of concordant, and it seems likely that those with lower U abundances are concordant, but until a homogeneous standard is identified, extensive TIMS analyses to test this are not justified.

3. Variable overlap of the primary beam spot onto the sample. It is for this reason that much of the Pb/U data for Warton and Pentecost xenotime samples are unreliable, and only Pb/Pb data are reported. A detailed study of the effects of variable overlap (Fletcher and McNaughton, unpublished), using the cz3 zircon standard, shows that when the sample fills <50% of the spot, Pb/U data can be severely perturbed. That is, the Pb+/U+/UO+/Zr2O+ correlations, on which Pb/U and element abundances are based, break down. The variations normally observed presumably result from variable efficiency of ion extraction for different ionic species from (?radially) different portions of the target spot, which cannot be placed in exactly the same position relative to the extraction ion optics for all spots. When the ion extraction geometry becomes extreme, as in the present case, more variation is observed in the ratios of ionic species. These effects are probably enhanced (the ratios are further degraded) because the ionized sample is not being generated from a flat surface. There can be slight "doming" of the sample during polishing, resin adjacent to the sample is sometimes damaged during SEM imaging, and the primary ion beam is likely to cut deeper into the resin than into the sample during analysis.

Zircon Corrections
When the primary ion beam overlaps onto the adjoining zircon, the data (as a measure of xenotime composition) are obviously corrupted. The extent of overlap can be monitored by recording Zr2O+/Y2O+, and the algebra for removing the zircon contribution to the recorded signals is trivial. However, the corrections are sensitive to the age difference between the zircon and the xenotime overgrowth, and the relative U (and Pb) abundance of the two, and even small degrees of overlap can sometimes require large corrections. In addition, there are a number of practical difficulties that limit the reliability of any correction applied:

1. There is always a small Zr2O+ signal in xenotime data. Although the background Zr2O+/Y2O+ appears to be very similar, at ~0.002, in xenotimes from various sources it cannot be assumed to be constant, and it is difficult to determine precisely for small overgrowths.

2. It cannot be assumed that the ionization efficiency for Zr2O+ extracted from immediately adjacent to xenotime is the same as for normal zircon analyses; that is, the apparent proportions of zircon and xenotime might not match the true contribution of the zircon to the total Pb and U signals.

3. The composition of the zircon substrate to the xenotime cannot be measured directly. In some cases, a measurement of a remote portion of the zircon grain is likely to give a good approximation, but in many grains, there are visible differences between the bulk of the zircon grain and the (in places very thin) zone on which the xenotime rests. The zircon sampled with the xenotime could be younger (or in rare cases older) than the separately measured zircon spot, and it could have different U abundance and have experienced different degrees of Pb loss.

References

  1. J. N. Aleinikoff, R. I. Grauch, Am. J. Sci. 290, 522 (1990).
  2. B. J. Griffin, Microsc. Microanal 3, 1197 (1997).
  3. D. R. Nelson, West. Aust. Geol. Surv. Rec. 1997/2 (1997).
  4. R. T. Pidgeon, D. Furfaro, A. K. Kennedy, A. A. Nemchin, W. van Bronswjk, U. S. Geol. Surv. Circ. 1107, 251 (1994).
  5. J. B. Smith et al., Precambrian Res. 88, 143 (1998).

Figure A1. SHRIMP data for trial xenotime standards H1 (selected data) and Y2, from a single analytical session (mount 98-60). U/Pb and U content are calibrated against xtc, which is taken to be 10,000 ppm of U and 2632 Ma (concordant).


Medium version | Full size version


Figure A2. Probability plots on SHRIMP data for detrital zircon grains from the Warton and Pentecost Sandstones.


Medium version | Full size version


Table A1. TIMS data for xtc xenotime standard.
chip 2 4 5 6
207Pb*/206Pb* 0.17766 0.17773 0.17782 0.17794
± (2σ,%) 0.056 0.053 0.074 0.054
206Pb*/238U 0.50090 0.49544 0.48748 0.48791
± (2σ,%) 0.47 0.23 0.20 0.018
207Pb*/235U 12.2700 12.1410 11.9520 11.9707
± (2σ, %) 0.47 0.23 0.21 0.019
207Pb*/206Pb* age (Ma) 2631.1 2631.8 2632.6 2633.8
± (2σ, Ma) 0.9 0.9 1.2 0.9
206Pb*/238U age (Ma) 2617.7 2594.2 2559.8 2561.6
± (2σ, Ma) 10.0 4.8 4.1 3.8
Analyses by A. Nemchin, Curtin University of Technology, Perth.

Table A2. SHRIMP data for detrital zircons from Warton Sandstone (sample FB2; mount UWA 98-16B).
grain-
spot
U
(ppm)
Th
(ppm)
4f206 (%) 207*
206*
208*
206*
206*
238
207*
235
%conc. 207*
206*
Age(Ma)
4-1 49 25 0.775 0.1029 ± 36 0.1335 ± 80 0.3065 ± 90 4.349 ± 212 103 1677 ± 65
4-2 45 22 0.000 0.1100 ± 18 0.1383 ± 32 0.3105 ± 92 4.710 ± 167 97 1800 ± 29
3-1 104 42 0.000 0.1326 ± 11 0.1158 ± 15 0.3935 ± 106 7.197 ± 210 100 2133 ± 14
4-3 90 51 0.224 0.1053 ± 20 0.1616 ± 44 0.3332 ± 92 4.839 ± 170 108 1720 ± 35
4-4 118 87 0.076 0.1111 ± 18 0.2161 ± 42 0.3166 ± 86 4.850 ± 162 98 1817 ± 30
26-1 164 106 0.000 0.2003 ± 14 0.1615 ± 18 0.5820 ± 155 16.070 ± 455 105 2828 ± 11
27-1 966 187 0.287 0.1380 ± 7 0.0474 ± 12 0.1621 ± 41 3.083 ± 82 44 2202 ± 9
9-1 316 185 0.101 0.1193 ± 8 0.1670 ± 16 0.3296 ± 85 5.420 ± 149 94 1945 ± 12
21-1 82 50 0.000 0.1091 ± 23 0.1796 ± 52 0.3269 ± 91 4.917 ± 182 102 1785 ± 39
21-2 77 55 0.326 0.1072 ± 21 0.2009 ± 48 0.3207 ± 90 4.738 ± 172 102 1752 ± 36
21-3 68 52 0.276 0.1057 ± 21 0.2176 ± 49 0.3184 ± 91 4.641 ± 170 103 1727 ± 36
20-1 61 167 0.215 0.1153 ± 33 0.7692 ± 106 0.2851 ± 83 4.532 ± 194 86 1884 ± 51
19a-1 278 114 0.046 0.1142 ± 9 0.1169 ± 18 0.3058 ± 79 4.815 ± 136 92 1867 ± 15
18-1 67 81 0.049 0.1105 ± 78 0.3347 ± 189 0.3139 ± 103 4.782 ± 389 97 1808 ± 128
16-1 237 322 0.151 0.1136 ± 12 0.3521 ± 31 0.2707 ± 71 4.239 ± 124 83 1857 ± 18
28-1 91 123 0.068 0.1617 ± 16 0.3714 ± 40 0.4595 ± 126 10.242 ± 311 99 2473 ± 17
29b-1 83 69 0.000 0.1194 ± 13 0.2474 ± 31 0.3436 ± 96 5.655 ± 176 98 1947 ± 19
30-1 108 33 0.284 0.1639 ± 16 0.0784 ± 29 0.4971 ± 135 11.233 ± 338 104 2496 ± 17
31-1 47 23 3.782 0.1128 ± 68 0.1547 ± 154 0.3182 ± 97 4.948 ± 351 97 1845 ± 109
32-1 152 76 0.071 0.1160 ± 13 0.1454 ± 26 0.3341 ± 89 5.344 ± 161 98 1895 ± 20
33-1 70 40 0.160 0.1096 ± 24 0.1644 ± 52 0.3217 ± 91 4.861 ± 182 100 1793 ± 39
33-2 89 62 0.496 0.1068 ± 23 0.1938 ± 52 0.3208 ± 89 4.723 ± 176 103 1745 ± 39
34-1 203 75 0.034 0.1163 ± 10 0.1026 ± 17 0.3385 ± 89 5.427 ± 155 99 1900 ± 15
35-1 46 56 0.064 0.1145 ± 34 0.3615 ± 88 0.3163 ± 95 4.994 ± 225 95 1872 ± 54
36-1 130 143 0.153 0.1091 ± 16 0.3127 ± 42 0.3245 ± 87 4.880 ± 158 102 1784 ± 27
36-2 164 106 0.000 0.1125 ± 9 0.1873 ± 19 0.3280 ± 87 5.086 ± 145 99 1839 ± 15
37-1 153 123 0.954 0.1167 ± 27 0.1646 ± 59 0.2445 ± 65 3.934 ± 146 74 1906 ± 41
38-1 96 61 0.078 0.1649 ± 15 0.1647 ± 26 0.4852 ± 132 11.029 ± 327 102 2506 ± 15
39-1 314 102 0.067 0.1214 ± 8 0.0903 ± 12 0.3631 ± 94 6.079 ± 166 101 1977 ± 11
40-1 143 103 0.121 0.1060 ± 14 0.2036 ± 32 0.3227 ± 86 4.716 ± 147 104 1731 ± 24
40-2 125 69 0.000 0.1098 ± 11 0.1575 ± 21 0.3119 ± 84 4.720 ± 141 97 1795 ± 18
40-3 125 69 0.000 0.1094 ± 10 0.1608 ± 20 0.3193 ± 85 4.817 ± 141 100 1790 ± 17
41-1 35 15 0.023 0.1124 ± 37 0.1257 ± 78 0.3195 ± 99 4.953 ± 237 97 1839 ± 59
42-1 121 75 0.086 0.1154 ± 17 0.1723 ± 36 0.3381 ± 91 5.381 ± 173 100 1887 ± 26
43-1 208 145 0.062 0.1127 ± 12 0.1960 ± 28 0.2950 ± 78 4.585 ± 137 90 1844 ± 20
44-1 488 88 0.132 0.1223 ± 7 0.0539 ± 12 0.3491 ± 89 5.886 ± 158 97 1990 ± 10
45-1 43 18 0.512 0.1061 ± 44 0.1153 ± 96 0.3193 ± 96 4.669 ± 252 103 1733 ± 75
45-2 96 69 0.391 0.1097 ± 19 0.2086 ± 44 0.3143 ± 86 4.755 ± 163 98 1795 ± 32
46-1 55 44 0.150 0.1115 ± 22 0.2291 ± 51 0.3403 ± 98 5.234 ± 193 103 1825 ± 36
47-1 85 50 0.000 0.1079 ± 13 0.1688 ± 26 0.3296 ± 91 4.902 ± 154 104 1764 ± 21
47-2 80 44 0.287 0.1050 ± 21 0.1497 ± 45 0.3183 ± 89 4.609 ± 168 104 1715 ± 37
47-3 88 48 0.111 0.1111 ± 19 0.1609 ± 41 0.3110 ± 86 4.763 ± 164 96 1817 ± 32
25-1 90 87 0.119 0.1637 ± 20 0.2641 ± 44 0.4541 ± 125 10.250 ± 323 97 2494 ± 21
23-1 113 65 0.245 0.1186 ± 16 0.1539 ± 34 0.3647 ± 99 5.964 ± 190 104 1935 ± 25
48-1 315 167 0.050 0.1101 ± 12 0.1486 ± 25 0.2996 ± 79 4.548 ± 136 94 1801 ± 20
49-1 130 72 0.100 0.1251 ± 17 0.1570 ± 35 0.3812 ± 103 6.573 ± 208 103 2030 ± 24
50-1 197 132 0.154 0.1122 ± 11 0.1875 ± 24 0.3378 ± 88 5.227 ± 152 102 1836 ± 18
51-1 172 113 0.005 0.1129 ± 11 0.1874 ± 23 0.3484 ± 92 5.422 ± 158 104 1846 ± 17
52-1 57 29 0.000 0.1217 ± 14 0.1476 ± 25 0.3658 ± 104 6.136 ± 196 101 1981 ± 21
53-1 54 38 0.000 0.1620 ± 16 0.1946 ± 26 0.4979 ± 142 11.119 ± 348 105 2476 ± 16
54-1 65 44 0.000 0.1106 ± 13 0.1928 ± 29 0.3390 ± 96 5.169 ± 166 104 1809 ± 22
54-2 35 26 0.000 0.1108 ± 18 0.2070 ± 41 0.3422 ± 106 5.228 ± 193 105 1813 ± 30
55-1 343 316 0.000 0.1188 ± 10 0.2584 ± 24 0.2935 ± 76 4.808 ± 135 86 1938 ± 14
55-2 256 178 0.239 0.1139 ± 16 0.1911 ± 35 0.4069 ± 109 6.391 ± 203 118 1863 ± 25
56-1 83 46 0.000 0.1115 ± 12 0.1611 ± 23 0.3294 ± 93 5.062 ± 159 101 1823 ± 19
57-1 123 56 0.088 0.1681 ± 12 0.1263 ± 20 0.4883 ± 130 11.316 ± 323 101 2538 ± 12
58-1 49 24 1.213 0.0954 ± 51 0.1100 ± 113 0.3518 ± 110 4.626 ± 303 127 1536 ± 101
58-2 116 91 0.287 0.1051 ± 37 0.2140 ± 85 0.2659 ± 75 3.852 ± 183 89 1715 ± 64
58-3 73 39 0.186 0.1091 ± 43 0.1525 ± 94 0.2978 ± 88 4.478 ± 232 94 1784 ± 71
59-1 40 47 0.292 0.1140 ± 39 0.3395 ± 97 0.3269 ± 98 5.140 ± 250 98 1864 ± 62
60-1 54 32 0.000 0.1112 ± 15 0.1729 ± 31 0.3006 ± 87 4.608 ± 154 93 1819 ± 25
61-1 158 125 0.000 0.1182 ± 9 0.2249 ± 20 0.3719 ± 98 6.062 ± 171 106 1929 ± 13
62-1 74 32 0.119 0.1635 ± 16 0.1168 ± 28 0.4998 ± 138 11.263 ± 344 105 2492 ± 17
63-1 55 63 0.000 0.1143 ± 14 0.3326 ± 42 0.3644 ± 104 5.743 ± 187 107 1869 ± 22
64-1 100 66 0.146 0.1136 ± 19 0.1848 ± 42 0.3640 ± 99 5.698 ± 192 108 1857 ± 30
65-1 78 52 0.320 0.1302 ± 25 0.1702 ± 54 0.4139 ± 116 7.430 ± 267 106 2101 ± 34
66-1 60 47 0.131 0.1189 ± 17 0.2194 ± 39 0.3768 ± 106 6.177 ± 206 106 1940 ± 26
67-1 242 124 0.000 0.1171 ± 7 0.1460 ± 13 0.3480 ± 90 5.618 ± 154 101 1912 ± 11
68-1 16 13 0.557 0.1093 ± 98 0.2333 ± 225 0.3359 ± 124 5.061 ± 511 104 1787 ± 164
68-2 14 11 0.000 0.1070 ± 28 0.2420 ± 70 0.3319 ± 121 4.898 ± 231 106 1749 ± 47
69-1 242 120 0.000 0.1180 ± 7 0.1414 ± 12 0.3522 ± 91 5.732 ± 156 101 1927 ± 11
70-1 120 65 0.000 0.1098 ± 9 0.1616 ± 19 0.3326 ± 89 5.036 ± 147 103 1797 ± 16
70-2 28 28 0.000 0.1110 ± 20 0.2943 ± 55 0.3473 ± 110 5.317 ± 204 106 1816 ± 33
19a-2 411 194 0.103 0.1148 ± 9 0.1101 ± 16 0.2139 ± 25 3.386 ± 50 67 1877 ± 14
19b-1 1878 483 1.461 0.1134 ± 21 0.117 ± 45 0.0459 ± 5 0.717 ± 16 16 1854 ± 33
29a-1 82 37 0.048 0.1651 ± 19 0.1292 ± 35 0.4579 ± 65 10.423 ± 204 97 2508 ± 20
56-2 83 49 0.374 0.1088 ± 28 0.1811 ± 63 0.3267 ± 50 4.902 ± 154 102 1780 ± 47
71-1 270 47 0.136 0.1568 ± 10 0.0281 ± 13 0.4325 ± 51 9.35 ± 129 96 2421 ± 10
All listed Pb isotope ratios are common-Pb corrected, on the basis of measures 204Pb, assuming a Broken Hill Pb isotopic composition.

4f206 = percentage of 206Pb calculated to be common Pb.

%conc. = concordance.

Uncertainties are 1σ, and apply to the last digits listed.

Pb/U and concentration calibrations are based on interspersed measurements of cz3 (564 Ma).


Table A3. SHRIMP data for detrital zircons from Pentecost Sandstone [sample PT-1; mount UWA 98-36A and 98-78 (last four analyses)].
grain-
spot
U
(ppm)
Th
(ppm)
4f206 (%) 207*
206*
208*
206*
206*
238
207*
235
%conc. 207*
206*
Age(Ma)
1-3 1686 1420 8.583 0.1138 ± 24 0.0981 ± 54 0.0888 ± 8 1.393 ± 34 29 1860 ± 38
2-2 369 362 1.145 0.1269 ± 11 0.1469 ± 23 0.3225 ± 31 5.645 ± 77 88 2056 ± 15
3-2 1179 1280 9.666 0.1629 ± 25 0.2473 ± 58 0.1158 ± 11 2.601 ± 50 28 2486 ± 26
4-1 1160 901 11.609 0.1135 ± 26 0.1696 ± 59 0.1462 ± 14 2.287 ± 59 47 1856 ± 41
5-2 242 519 1.824 0.1588 ± 15 0.4084 ± 36 0.3706 ± 38 8.114 ± 120 83 2443 ± 16
6-1 172 103 0.040 0.1117 ± 9 0.1691 ± 18 0.3254 ± 36 5.013 ± 71 99 1828 ± 14
7-1 226 92 0.098 0.1237 ± 10 0.0907 ± 19 0.3457 ± 79 5.894 ± 148 95 2010 ± 14
8-1 148 69 0.085 0.1589 ± 12 0.1180 ± 22 0.4387 ± 103 9.615 ± 245 96 2444 ± 13
9-1 183 89 0.019 0.1384 ± 10 0.1323 ± 18 0.4103 ± 95 7.827 ± 195 100 2207 ± 12
10-1 153 169 0.129 0.1147 ± 15 0.2474 ± 35 0.2942 ± 69 4.651 ± 131 89 1875 ± 23
11-1 210 183 2.813 0.1283 ± 23 0.1064 ± 50 0.3574 ± 82 6.325 ± 195 95 2075 ± 31
12-1 361 183 0.507 0.1460 ± 10 0.1423 ± 20 0.3561 ± 80 7.170 ± 174 85 2300 ± 12
6-2 226 84 0.015 0.1121 ± 9 0.1048 ± 16 0.3225 ± 74 4.986 ± 126 98 1834 ± 14
6-3 343 380 0.454 0.1134 ± 10 0.2148 ± 23 0.3032 ± 68 4.740 ± 120 92 1855 ± 16
14-1 94 58 0.175 0.1195 ± 17 0.1645 ± 35 0.3638 ± 91 5.994 ± 181 103 1949 ± 25
15-1 133 76 0.095 0.1558 ± 14 0.1328 ± 26 0.4291 ± 102 9.215 ± 243 95 2410 ± 15
16-1 102 51 0.000 0.1119 ± 10 0.1390 ± 18 0.3317 ± 80 5.119 ± 137 101 1831 ± 17
16-2 138 67 0.000 0.1126 ± 9 0.1400 ± 16 0.3318 ± 78 5.151 ± 132 100 1841 ± 14
17-1 64 19 0.016 0.1150 ± 19 0.0873 ± 37 0.3407 ± 87 5.404 ± 174 101 1880 ± 30
18-1 109 68 0.000 0.1141 ± 10 0.1747 ± 20 0.3400 ± 81 5.348 ± 141 101 1865 ± 15
18-2 110 40 0.162 0.1132 ± 16 0.0998 ± 32 0.3447 ± 83 5.378 ± 158 103 1851 ±25
19-1 85 103 0.209 0.1165 ± 18 0.2589 ± 42 0.3428 ± 84 5.507 ± 168 100 1903 ± 27
20-1 71 91 0.040 0.1592 ± 17 0.3242 ± 39 0.4630 ± 115 10.165 ± 287 100 2448 ±18
21-1 139 24 0.000 0.1219 ± 9 0.0494 ± 9 0.3560 ± 84 5.983 ± 153 99 1984 ± 13
22-1 97 53 0.000 0.1652 ± 11 0.1541 ± 16 0.4836 ± 116 11.017 ± 284 101 2510 ± 11
1-4 2037 1695 9.989 0.1106 ± 26 0.1019 ± 60 0.0810 ± 18 1.235 ± 43 28 1810 ± 43
23-1 265 137 0.082 0.1145 ± 10 0.1444 ± 20 0.3216 ± 74 5.077 ±1 30 96 1872 ± 16
24-1 47 30 0.000 0.1100 ± 15 0.1786 ± 31 0.3458 ± 91 5.246 ± 163 106 1800 ± 24
25-1 160 79 0.029 0.1141 ± 11 0.1407 ± 21 0.3573 ± 85 5.622 ± 150 106 1866 ± 17
26-1 268 145 0.207 0.1276 ± 9 0.1496 ± 17 0.3477 ± 79 6.115 ± 149 93 2065 ± 12
27-1 129 180 0.000 0.1162 ± 9 0.3946 ± 30 0.3588 ± 85 5.749 ± 149 104 1899 ± 14
5-1 1111 2544 8.475 0.1450 ± 37 0.4641 ± 88 0.0987 ± 11 1.974 ± 58 27 2287 ± 44
7-1 89 41 0.204 0.1154 ± 17 0.1317 ± 34 0.3516 ± 50 5.597 ± 122 103 1887 ± 27
9-1 1560 972 9.848 0.1162 ± 44 0.1525 ± 100 0.0697 ± 8 1.116 ± 46 23 1898 ± 68
10-1 872 1144 16.271 0.1190 ± 50 0.2871 ± 116 0.1471 ± 18 2.413 ± 110 46 1941 ± 76
All listed Pb isotope ratios are common-Pb corrected, on the basis of measures 204Pb, assuming a Broken Hill Pb isotopic composition.

4f206 = percentage of 206Pb calculated to be common Pb.

%conc. = concordance.

Uncertainties are 1σ, and apply to the last digits listed.

Pb/U and concentration calibrations are based on interspersed measurements of cz3 (564 Ma).


Table A4. SHRIMP data for xenotime overgrowths on zircons in Warton Sandstone (sample FB2; mount UWA 98-16B).
grain-
spot
U
(~ppm)
Th Th
U
4f206 (%) 207*
206*
208*
206*
207*
206*
Age(Ma)
Zr2O+
Y2O
adjusted
207*
206*
Age(Ma)
4-1 2120 640 0.303 0.007 0.1049 ± 7 0.0595 ± 8 1713 ± 12 0.0032 1713 ± 12
26-1 1940 780 0.398 0.016 0.1035 ± 5 0.0803 ± 7 1688 ± 10 0.0019 1688 ± 10
26-2 2460 880 0.359 0.054 0.1067 ± 10 0.0834 ± 15 1743 ± 18 0.0024 1743 ± 18
21-1 2020 1140 0.560 0.107 0.1040 ± 8 0.0782 ± 11 1697 ± 14 0.0031 1697 ± 14
24-1 2320 720 0.314 0.053 0.1047 ±7 0.0427 ± 7 1709 ± 12 0.0027 1709 ± 12
24-2 2020 660 0.323 0.005 0.1049 ± 6 0.0612 ± 7 1712 ± 11 0.0019 1712 ± 11
25-1 1640 560 0.340 0.039 0.1006 ± 9 0.0377 ± 10 1635 ± 16 0.0025 1635 ± 16
25-2 2220 1540 0.690 0.161 0.1036 ± 9 0.0483 ± 13 1689 ± 17 0.0068 1688 ± 18
16-1 1720 480 0.280 0.173 0.1016 ± 11 0.0209 ± 14 1654 ± 20 0.0024 1654 ± 20
18-1 1780 800 0.443 0.037 0.1062 ± 8 0.0715 ± 11 1736 ± 14 0.0020 1736 ± 14
20-1 2900 1060 0.362 0.405 0.0983 ± 14 0.0258 ± 22 1592 ± 28 0.0055 1592 ± 28
9-1 2240 700 0.317 0.931 0.1042 ± 14 0.0180 ± 24 1700 ± 25 0.0037 1700 ± 25
19a-1 2000 960 0.479 0.034 0.1003 ± 13 0.0511 ± 18 1629 ± 24 0.0044 1628 ± 25
19a-2 2320 740 0.316 0.250 0.1049 ± 11 0.0593 ± 17 1712 ± 20 0.0027 1712 ± 20
4-2 4040 1920 0.475 0.063 0.1052 ± 9 0.1018 ± 14 1718 ± 16 0.0034 1718 ± 16
4-3 2240 660 0.296 0.047 0.1022 ± 10 0.0610 ± 12 1664 ± 18 0.0046 1664 ± 18
28-1 2280 580 0.251 0.117 0.1029 ± 14 0.0356 ± 14 1678 ± 24 0.0022 1678 ± 24
26-3 2700 720 0.263 0.231 0.1040 ± 13 0.0367 ± 15 1696 ± 23 0.0049 1694 ± 25
58-1 2180 1260 0.574 0.197 0.0995 ± 20 0.1153 ± 34 1615 ± 37 0.0088 1615 ± 37
56-1 2520 820 0.322 0.126 0.1045 ± 12 0.0550 ± 14 1706 ± 21 0.0069 1706 ± 21
60-1 2700 1540 0.569 0.023 0.1041 ± 6 0.1211 ± 10 1698 ± 11 0.0033 1698 ± 11
32-2 2820 1620 0.574 0.153 0.1037 ± 15 0.0891 ± 21 1692 ± 26 0.0090 1692 ± 26
31-1 1980 880 0.442 0.207 0.1026 ± 13 0.0840 ± 20 1672 ± 24 0.0086 1672 ± 24
31-2 2520 1300 0.512 0.212 0.1026 ± 17 0.0770 ± 24 1672 ± 30 0.0218 1672 ± 30
71-1 2660 1040 0.391 0.142 0.1039 ± 8 0.0774 ± 11 1694 ± 14 0.0087 1690 ± 18
71-2 3980 2040 0.513 0.152 0.1050 ± 15 0.0896 ± 21 1715 ± 25 0.0066 1713 ± 27
41-1 2500 1240 0.498 0.397 0.1048 ± 20 0.0335 ± 26 1711 ± 35 0.0062 1711 ± 35
41-2 1980 640 0.319 0.039 0.1049 ± 18 0.0424 ± 20 1712 ± 31 0.0035 1712 ± 31
25-3 2140 1580 0.741 0.388 0.1027 ± 19 0.0676 ± 29 1674 ± 34 0.0104 1672 ± 36
21-3 1460 520 0.358 .098 0.1027 ± 15 0.0487 ± 17 1673 ± 27 0.0198 1673 ± 27
66-1 1640 620 0.377 0.408 0.1029 ± 11 0.0547 ± 16 1677 ± 21 0.0023 1677 ± 21
66-2 1840 1240 0.673 0.363 0.1021 ± 11 0.0522 ± 15 1663 ± 19 0.0043 1663 ± 19
19b-3 2560 940 0.364 0.055 0.1049 ± 11 0.0865 ± 15 1713 ± 19 0.0029 1713 ± 19
19b-4 2060 880 0.428 0.182 0.1032 ± 13 0.0861 ± 20 1683 ± 24 0.0037 1683 ± 24
Data not used (large zircon overlap).
21-2 2400 1500 0.626 0.193 0.1069 ± 10 0.1101 ± 17 1748 ± 17 0.6070 1748 ± 17
27-1 1480 740 0.496 0.619 0.0956 ± 15 0.0259 ± 25 1540 ± 30 0.0049 1530 ± 40
32-1 4400 2240 0.509 1.100 0.1000 ± 26 0.0588 ± 44 1625 ± 47 0.0409 1623 ± 49
29a-1 3100 2260 0.726 0.100 0.1054 ± 13 0.1326 ± 22 1721 ± 22 0.0756 1712 ± 31
All listed Pb isotope ratios are common-Pb corrected, on the basis of measures 204Pb, assuming a Broken Hill Pb isotopic composition.

4f206 = percentage of 206Pb calculated to be common Pb.

Uncertainties are 1σ, and apply to the last digits listed.

Concentration calibrations are based on interspersed measurements of xtc (10000 Ma).

Corrections for zircon 'contamination' use zircon data from Tables A2 and A3.


SHRIMP data for xenotime overgrowths on zircons in Pentecost Sandstone (sample PT-1; UWA mounts 98-36 [first 7 analyses] and 98-78)
grain-
spot
U
(~ppm)
Th Th
U
4f206 (%) 207*
206*
208*
206*
207*
206*
Age(Ma)
Zr2O+
Y2O
adjusted
207*
206*
Age(Ma)
1-1a 520 10860 21.219 0.980 0.1014 ± 15 5.4141 ± 277 1649 ± 27 0.0034 1647 ± 29
1-1b 1440 13340 9.250 3.067 0.0990 ± 18 5.2846 ± 227 1605 ± 34 0.0091 1601 ± 38
3-1c 600 3340 5.646 0.815 0.0994 ± 23 1.4708 ± 143 1612 ± 43 0.0022 1612 ± 43
3-2b 320 3240 10.230 0.953 0.1000 ± 15 2.6298 ± 142 1624 ± 28 0.0025 1621 ± 31
3-2c 320 3500 10.789 0.656 0.1012 ± 13 2.8618 ± 145 1646 ± 25 0.0023 1644 ± 27
3-2d 340 3100 9.257 1.772 0.1046 ± 22 2.5933 ± 171 1707 ± 40 0.0038 1695 ± 52
3-2f 360 1740 4.915 1.350 0.0916 ± 25 1.3656 ± 135 1459 ± 51 0.0019 1459 ± 51
9-1a 340 4680 14.007 0.343 0.1041 ± 21 3.0483 ± 252 1698 ± 36 0.0019 1698 ± 36
9-1b 360 6980 18.911 0.521 0.1054 ± 16 3.9905 ± 247 1721 ± 28 0.0022 1721 ± 28
9-1c 280 4400 15.209 1.256 0.0990 ± 39 3.5181 ± 384 1605 ± 73 0.0021 1605 ± 73
9-1d 360 5900 15.93 1.140 0.1021 ± 33 3.5845 ± 330 1663 ± 60 0.0025 1662 ± 61
7-1a 160 1040 6.396 0.233 0.1016 ± 20 1.3394 ± 170 1653 ± 37 0.0019 1653 ± 37
7-1b 200 1340 6.491 0.412 0.1035 ± 24 1.3941 ± 180 1688 ± 43 0.0025 1688 ± 43
7-1c 220 1500 6.522 0.223 0.1048 ± 28 1.4085 ± 187 1710 ± 49 0.0023 1710 ± 49
7-1d 300 2000 6.540 0.771 0.1006 ± 33 1.3614 ± 198 1636 ± 61 0.0035 1635 ± 62
7-2a 560 7060 12.443 0.040 0.1049 ± 12 2.8032 ± 169 1712 ± 20 0.0023 1712 ± 20
7-2b 600 6960 11.504 0.275 0.1044 ± 14 2.7540 ± 174 1704 ± 24 0.0019 1704 ± 24
7-2c 640 7080 11.239 0.059 0.1051 ± 12 2.6397 ± 173 1715 ± 21 0.0019 1715 ± 21
7-3a 360 11600 32.977 0.883 0.1031 ± 22 7.0476 ± 430 1681 ± 39 0.0026 1681 ± 39
7-3b 400 13140 33.071 0.559 0.1040 ± 19 7.1798 ± 421 1696 ± 34 0.0025 1696 ± 34
7-3c 480 14320 29.244 0.339 0.1031 ± 16 6.4629 ± 370 1680 ± 28 0.0023 1680 ± 28
5-1a 500 4200 8.502 0.323 0.1047 ± 12 1.7412 ± 113 1710 ± 21 0.0021 1710 ± 21
5-1b 520 3600 6.881 0.044 0.1048 ± 10 1.4895 ± 104 1711 ± 18 0.0018 1711 ± 18
5-2a 300 5300 18.240 0.453 0.1006 ± 18 4.0435 ± 293 1636 ± 33 0.0021 1636 ± 33
5-2b 300 5040 16.338 0.257 0.1028 ± 17 3.7208 ± 286 1675 ± 31 0.0018 1675 ± 31
5-2c 340 4920 14.676 0.233 0.1039 ± 20 3.3745 ± 280 1696 ± 35 0.0021 1696 ± 35
10-1a 720 5400 7.472 0.380 0.1001 ± 10 1.3928 ± 79 1627 ± 18 0.0025 1627 ± 18
10-1b 700 4940 7.008 0.187 0.1012 ± 10 1.3228 ± 82 1645 ± 18 0.0027 1644 ± 19
10-1c 740 4920 6.676 0.307 0.1017 ± 11 1.3071 ± 81 1656 ± 19 0.0024 1656 ± 19
10-1d 760 4960 6.516 0.164 0.1032 ± 9 1.2933 ± 78 1683 ± 16 0.0022 1683 ± 16
7-4a 780 9460 11.991 5.066 0.0959 ± 39 2.6705 ± 214 1546 ± 76 0.0034 1546 ± 76
7-5a 820 5960 7.264 0.154 0.1000 ± 9 1.5190 ± 88 1624 ± 17 0.0021 1624 ± 17
Data not used (large zircon overlap).
2-1a 2960 6500 2.201 9.841 0.1017 ± 49 1.4201 ± 170 1656 ± 88 0.0298 1648 ± 96
2-1b 3320 7280 2.186 9.541 0.1023 ± 47 1.5220 ± 171 1666 ± 86 0.0353 1658 ± 94
5-1a 820 4220 5.115 0.420 0.1080 ± 11 1.4154 ± 89 1765 ± 19 0.0198 1742 ± 42
5-1b 940 4640 4.956 0.548 0.1074 ± 17 1.6236 ± 131 1756 ± 29 0.0334 1722 ± 63
1-2 5400 23960 4.437 11.407 0.1031 ± 75 2.2068 ± 321 1681 ± 135 0.0411 1678 ± 138
3-1a 1360 6800 5.003 1.881 0.1313 ± 14 1.2101 ± 67 2116 ± 19 0.0314 2098 ± 37
3-1b 1000 5900 5.918 1.151 0.1121 ± 16 1.5637 ± 106 1833 ± 26 0.0081 1822 ± 37
5-1c 700 3000 4.333 0.758 0.1031 ± 40 1.4507 ± 233 1680 ± 71 0.0088 1668 ± 83
3-2a 460 3980 8.765 3.478 0.1064 ± 21 2.4243 ± 124 1739 ± 37 0.0154 1668 ± 108
3-2e 400 3100 7.814 0.619 0.1042 ± 12 2.0984 ± 110 1700 ± 22 0.0120 1635 ± 87
1-1a 8320 36960 4.440 16.357 0.0937 ± 109 1.3662 ± 339 1502 ± 222 0.1164 1490 ± 234
4-1 1020 11360 11.099 4.292 0.0887 ± 29 3.7493 ± 213 1398 ± 62 0.0255 1352 ± 108
4-2a 880 8060 9.215 3.467 0.0951 ± 39 3.2648 ± 275 1529 ± 77 0.0290 1487 ± 119
4-2b 1360 10940 7.996 5.802 0.0921 ± 40 2.3257 ± 193 1470 ± 82 0.0410 1426 ± 126
4-3 1020 11600 11.450 4.307 0.0783 ± 48 4.0109 ± 375 1154 ± 122 0.0226 1079 ± 197
5-1a 440 4040 9.280 1.075 0.1035 ± 17 2.4431 ± 149 1688 ± 31 0.0147 1652 ± 67
5-1b 460 3580 7.781 1.171 0.1041 ± 18 2.1484 ±140 1699 ± 32 0.0318 1622 ± 109
5-1c 480 3460 7.309 1.185 0.1020 ± 20 2.0339 ± 149 1660 ± 36 0.0391 1564 ± 137
5-1d 540 3360 6.182 1.218 0.1017 ± 20 1.6243 ± 129 1655 ± 37 0.0366 1575 ± 117
5-1e 660 4120 6.321 0.809 0.1037 ± 17 1.6247 ± 125 1691 ± 30 0.0363 1631 ± 90
5-2 480 3980 8.359 0.786 0.0991 ± 15 2.2064 ± 137 1607 ± 29 0.0188 1550 ± 80
All listed Pb isotope ratios are common-Pb corrected, on the basis of measures 204Pb, assuming a Broken Hill Pb isotopic composition.

4f206 = percentage of 206Pb calculated to be common Pb.

Uncertainties are 1σ, and apply to the last digits listed.

Concentration calibrations are based on interspersed measurements of xtc (10000 Ma).

Corrections for zircon 'contamination' use zircon data from Tables A2 and A3.