Measuring the age of the lathrop wells volcanic center at yucca mountain.
نویسندگان
چکیده
B. Turrin et al. (1) argue that conventional K-Ar and 4Ar/39Ar age determinations and paleomagnetic data provide a definitive age assignment of approximately 136 ka (thousand years ago) to 141 ka for the Lathrop Wells volcanic center with an error of less than 10,000 years. This conclusion is tendered despite replicate age determinations that extend over almost three orders of magnitude. Turrin (1) and other also conclude (2, 3) that the Lathrop Wells volcanic center is a simple monogenetic center, and so revert to an earlier interpretation (4, 5) that was made before studies revealed the complexity of the volcanic stratigraphy (69). The geologic map and stratigraphic nomenclature of the Lathrop Wells volcanic center presented by Turrin et al. (1) were extracted and modified without apparent reference to the original publication of these field studies (10). Stratigraphic units separated by soil-bounded unconformities were modified or not accounted for in their interpretation (6, 8). These unconformities indicate a hiatus in eruptive activity of significantly sustained time (at least 103 years) and allow the development of soil profiles that are similar to radiocarbon dated soil sequences within arid regions of the southwestern United States (11-14). Without complete stratigraphic sampling, statements regarding the complexity of Lathrop Wells eruptive history offer only an oversimplified stratigraphy. Turrin et al. state that their combined flow and scoria unit Q13/Qsu [(1), figure 11 is younger than the flows and scoria of Q15/Qs5, but they report without any implications a weighted mean of 141 + 9 ka for the younger rocks and an age of 136 + 8 ka for the older rocks. In comparison, recently reported thermoluminescence age determination (8) of a buried soil between tephra deposits of their unit Qs5 is 9.9 ± 0.7 ka. Cosmogenic 3He age determinations (8) of surface-exposed volcanic bombs of unit Qs5 yield ages of 23 + 4 ka to 44 + 13 ka. Flows that stratigraphically lie below these tephra and bomb deposits yield a thermoluminescence date of 24.5 ± 2.5 ka for baked soils that underly unit Q13 (8) and yield a cosmogenic 3He date of 64 ± 6 ka on exposed bedrock of unit Q15 (8). The weighted means of the K-Ar and 39Ar/40Ar age determinations have insufficient precision to constrain the age of these late Quatemary volcanic flows and tephra separated by soil-bounded unconformites. Our major criticism of the K-Ar and 40Ar/39Ar age determinations of the volcanic center made by Turrin et al. is of their method of averaging the age determinations, not of their analytical methods. If the data are compiled as a conventional mean, large 1-a errors are obtained that overlap and are consistent with the results of every other chronology method used to assess the age of the Lathrop Wells center (8). The use of a weighted mean gives age assignments with unrealistically small errors, in that the group age dates range from 20 ka to 947 ka. Yet Turrin et al. do not explain why the weighted mean might be more reliable than the conventional mean, nor do they test the validity of the weighted mean method. Our specific concerns are as follows. 1) The age determinations are positively skewed with a mean larger than the median, which indicates influence of the mean by older ages. 2) Turrin et al. did not examine the data set with conventional tests for outliers. Evaluation of their data shows that outliers are present where outliers are defined to be more than 1.5 times the interquantile range. The data set is nongaussian, with inclusion of the outliers, and therefore is probably not suitable for description with a weighted mean. 3) Four age determinations were discarded by Turrin et al. in the weighted mean data reduction because of "contamination." No systematic criteria were presented for doing so, and recalculation of the data set (1) with these four age determinations yields significantly older values of the weighted mean with larger uncertainty. 4) The regression plots in [(1), figure 21 show the presence of influential cases which should have been identified to check for errors and suitability to the data set. The influential cases could strongly control the y-intercept, the values of which are used by Turrin et al. to argue against the presence of excess Ar. 5) There was no discussion of data errors other than analytical in (1). Because the 40Ar/39Ar analyses were of the matrix of fine-grained basalt (15), there is a possible problem of recoil of 39Ar which could give anomalous older ages (16, 17). 6) Conventional, whole rock K-Ar data are averaged (1) with the 40Ar/39Ar to establish final values for the weighted means. However, the whole rock data are not listed in (1). Thus it is not clear whether the data set belongs to the same population as the 40Ar/39Ar data. We conclude that the reduction of the data set of Turrin et al. with a weighted mean method is unsupported at best and
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ورودعنوان ژورنال:
- Science
دوره 257 5069 شماره
صفحات -
تاریخ انتشار 1992