Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon.
Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon.
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大峡谷 Tonian Chuar 群的 Re-Os 和 U-Pb 耦合地质年代学。
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发表时间：
2017
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通讯作者：
F. Macdonald
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作者：
A. Rooney;J. Austermann;E. F. Smith;Yang Li;D. Selby;C. Dehler;M. Schmitz;K. Karlstrom;F. Macdonald
在大峡谷中暴露的保存完好的晚Tonian Chuar群的地层中，有化石证据表明真核生物捕食的发展、独特的生物标志物的存在以及C、S和Mo同位素化学地层学的巨大变化。尽管这一重要的继承，放射性同位素年龄的限制很少，可以把这些记录到全球范围内。在这里，我们将锆石晶体的高精度U-Pb化学磨蚀-同位素稀释-热电离质谱（CA-ID-TIMS）与铼-锇（Re-Os）沉积和硫化物地质年代计结合起来，以细化这个关键时期的时间框架地球历史的间隔。从Kwagunt组最上部Walcott段的凝灰岩中回收的锆石获得的加权平均206Pb-238U年龄为729.0 ± 0.9 Ma（加权偏差均方[MSWD]= 0.86），与同一层位上锆石的742 ± 6 Ma的先前空气磨蚀上截距年龄存在显著差异。Galeros组Carbon峡谷段富含有机质的碳酸盐的模式1 Re-Os年龄为757.0 ± 6.8 Ma（MSWD = 0.47，n = 8），初始Os同位素（187 Os/188 Os [Osi]）组成为1.13 ± 0.02。从这个地平线上的放射性Osi值表明，盆地被限制在碳峡谷成员的沉积过程中，与沉积学和地层学数据。Kwagunt组Awatubi段白铁矿（FeS2）结核的Re-Os年代学结果为751.0 ± 7.6Ma（MSWD = 0.37，n = 5），Osi为0.44 ± 0.01。这Re-Os日期被解释为限制生长的白铁矿结核在阿瓦图比成员在沉积过程中。硫化物的形成和较少的放射性OSI值是一致的硫酸盐海水流入盆地的Kwagunt组沉积过程中。尽管Re富集度较高（> 20 ng/g），但将Re-Os地质年代计应用于丘阿尔群Walcott和坦纳成员的尝试未能获得有意义的年龄。从这些单位的Re-Os数据产生负的Osi值，这表明干扰Re-Os系统。相对于基于Re丰度升高的预期量，低Os丰度（通常<100 pg/g）表明在地质学上最近的时间尺度上，由于氧化风化，Os发生沥滤。最后，碳峡谷成员提供了一个有用的案例研究，量化输入的不确定性，在铼-锇地质年代计传播到所产生的年龄不确定性，我们讨论的协议，将产生最好的改善年龄精度为未来的研究。这里介绍的U-Pb和Re-Os年代学说明了耦合这些系统的能力以及这两种方法最近改进的重要性。我们的分析表明，对于我们的数据，最有效的方法来减少所提出的Re-Os日期的不确定性是通过提高测量的Os值的精度。
Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to place these records into a global context. Here, we couple high-precision U-Pb chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) on zircon crystals with the rhenium-osmium (Re-Os) sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of Earth history. Zircons recovered from a tuff within the uppermost Walcott Member of the Kwagunt Formation yield a weighted mean 206Pb-238U age of 729.0 ± 0.9 Ma (mean square of weighted deviates [MSWD] = 0.86), differing significantly from the previous air-abrasion upper-intercept age of 742 ± 6 Ma on zircons from this same horizon. Organic-rich carbonates from the Carbon Canyon Member of the Galeros Formation yield a model 1 Re-Os age of 757.0 ± 6.8 Ma (MSWD = 0.47, n = 8), and an initial Os isotope (187Os/188Os [Osi]) composition of 1.13 ± 0.02. The radiogenic Osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the Carbon Canyon Member, in agreement with sedimentological and stratigraphic data. The Re-Os geochronology of marcasite (FeS2) nodules from the Awatubi Member of the Kwagunt Formation yield a model 1 age of 751.0 ± 7.6 Ma (MSWD = 0.37, n = 5), with an Osi of 0.44 ± 0.01. This Re-Os date is interpreted to constrain the growth of the marcasite nodules in the Awatubi Member during deposition. The formation of sulfides and the less radiogenic Osi value are consistent with an influx of sulfate-laden seawater to the basin during deposition of the Kwagunt Formation. Attempts to apply the Re-Os geochronometer to the Walcott and Tanner Members of the Chuar Group failed to yield meaningful ages, despite elevated Re enrichments (>20 ng/g). The Re-Os data from these units yielded negative Osi values, which suggest disturbance to the Re-Os system. The low Os abundances (typically <100 pg/g) relative to the amount expected based on the elevated Re abundances suggest leaching of Os due to oxidative weathering on geologically recent time scales. Finally, the Carbon Canyon Member provides a useful case study for quantifying how input uncertainties in the Re-Os geochronometer propagate into the resulting age uncertainty, and we discuss the protocols that will yield the best improvement in age precision for future studies. The U-Pb and Re-Os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods. Our analysis suggests that for our data, the most efficient way of reducing uncertainties in the presented Re-Os dates is through improved precision of measured Os values.