The U-Th-Pb systematics of zircon from the Bishop Tuff: A case study in challenges to high-precision Pb/U geochronology at the millennial scale
The U-Th-Pb systematics of zircon from the Bishop Tuff: A case study in challenges to high-precision Pb/U geochronology at the millennial scale
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Bishop Tuff 锆石的 U-Th-Pb 系统学：千年尺度高精度 Pb/U 地质年代学挑战的案例研究
DOI：
10.1016/j.gca.2015.07.018
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发表时间：
2015
影响因子：
5
通讯作者：
S. Mulcahy
中科院分区：
文献类型：
--
作者：
R. Ickert;R. Mundil;C. Magee;S. Mulcahy
更新世毕晓普凝灰岩中的锆石U-Pb定年具有挑战性，因为放射性成因的铅浓度很低，而且需要对中间子产物中的不平衡进行大量的校正。这些日期可能很难解释，因为它们的绝对精度在大小上类似于所谓的结晶时间尺度。以前对锆石结晶持续时间的估计跨度为两个数量级，似乎取决于分析方法。为了调和不同的解释，我们给出了新的SIMS和ID-TIMS锆石Pb/U年龄，包括一些用这两种技术测定的晶体。SIMS和ID-TIMS日期的分布与之前的结果相似。归一化为Th/UMelt=22.81的SIMS斑点来自切片谷物内部区域，范围为769×31×10~(-6)ka~845×10~(28)ka(2σ)，ID-TIMS测年范围为760×10~(-6)×10~(-6)ka~793×10~6×10~(-6)ka。用这两种方法测年的单个晶体的平均差异为310ka，SIMS的测年结果始终比大块晶体测年日期更早。这种程度的可变性排除了将地质意义赋予平均(或加权平均)锆石年龄的可能性。先前对单一Bt锆石的ID-TIMS研究表明，锆石年龄在统计学上具有明显的聚集性，但由于对230Th校正的铅/铀数据的协方差结构处理不当而受到损害。我们展示了正确处理的近似和精确方法，并展示了该数据中比以前认识到的更多的散布。对现有分配系数数据的仔细审查表明，它们对<；±1010ka的修正不够精确，即使精确确定，U和Th不平衡吸收的证据也意味着平衡分配系数可能没有用处。锆石和玻璃的U-Th系统学之间的不匹配表明，我们对这两个相之间的关系的理解存在差距，这给使用测量的玻璃成分的不平衡校正的准确性带来了不确定性。考虑到不平衡修正中的这些不确定性，在修正名义精度好于±10千卡的日期时，请谨慎行事。然而，最年轻的锆石年龄很可能将主教凝灰岩的喷发年龄限制在775年龄段，这与一些人对40Ar-39Ar珊瑚岩喷发年龄的估计相矛盾。
Dating zircon by U–Pb in the Pleistocene Bishop Tuff is challenging because the concentration of radiogenic Pb is low and the correction required for disequilibrium in the intermediate daughter products is large. The dates can be difficult to interpret, because their absolute precision is similar in magnitude to purported timescales of crystallization. Previous estimates of the duration of zircon crystallization span two orders of magnitude and appear to depend on the analytical approach. To reconcile the differing interpretations, we present new SIMS and ID-TIMS zircon Pb/U dates, including some crystals that are dated by both techniques. Both the SIMS and ID-TIMS dates have similar distributions as previous results. Normalized to a Th/Umelt= 2.81, SIMS spot dates from interior regions of sectioned grains range from 769 ± 31 ka to 845 ± 28 ka (2σ) and ID-TIMS dates have a range from 760 ± 7 ka to 793 ± 6 ka. Individual crystals dated by both methods have an average difference of 31 ka, with the SIMS dates being consistently older than the bulk crystal dates. This level of variability precludes the assignment of a geological significance to a mean (or weighted-mean) zircon date. A previous ID-TIMS investigation of single BT zircon that showed apparent statistically significant clustering of zircon ages was compromised by an incorrect treatment of the covariance structure of230Th-corrected Pb/U data. We show approximate and exact methods for the correct treatment and demonstrate substantially more scatter in that data than previously recognized. Close scrutiny of available partition coefficient data show that they are not adequately precise for <± 10 ka corrections and even if determined precisely, evidence for disequilibrium uptake of U and Th implies that equilibrium partition coefficients may not be useful. Mismatches between the U–Th systematics of zircon and of glasses suggest that there are gaps in our understanding of the relationship between the two phases, lending uncertainty to the accuracy of disequilibrium corrections that use measured glass compositions. Given these uncertainties in the disequilibrium correction, caution is urged when correcting dates with nominal precisions that are better than ±10 ka. Nevertheless, the youngest zircon ages very likely constrain the eruption age of the Bishop Tuff to <775 ka, which conflicts with some estimates of the40Ar–39Ar sanidine age of eruption.
影响因子：
3.5
作者：
A. Schmitt;M. Danišík;N. Evans;W. Siebel;Elena Kiemele;F. Aydın;Janet Harvey
通讯作者：
A. Schmitt;M. Danišík;N. Evans;W. Siebel;Elena Kiemele;F. Aydın;Janet Harvey