3‐D Characterization of Detrital Zircon Grains and its Implications for Fluvial Transport, Mixing, and Preservation Bias
3‐D Characterization of Detrital Zircon Grains and its Implications for Fluvial Transport, Mixing, and Preservation Bias
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碎屑锆石颗粒的 3D 表征及其对河流输送、混合和保存偏差的影响
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发表时间：
2017
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通讯作者：
J. Shaw
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作者：
V. Markwitz;C. L. Kirkland;Andrew Mehnert;Klaus Gessner;J. Shaw
由于 U-Pb 年龄不一致、变晶化、变质叠印和河流迁移过程，碎屑锆石研究可能会遭受来源信息选择性丢失的影响。同位素组成和锆石晶粒形状之间的关系，以及晶粒形状在运输过程中如何改变，在很大程度上尚不清楚。我们将 X 射线断层扫描与 U-Pb 地质年代学相结合，量化河流输送如何影响默奇森河沿岸的 3D 锆石形状、碎屑年龄特征和颗粒密度，默奇森河的流域包括西澳大利亚的始太古代到早古生代烃源岩。我们获取了 373 个碎屑锆石的断层扫描体积和同位素数据，以记录传输方向上尺寸、形状和密度的变化，并探索颗粒形状、年龄谱和不一致材料的比例如何沿通道变化。结果表明，形状特征对传输距离、水流梯度、与源材料的接近程度以及源是否由原生锆石或回收锆石组成很敏感。随着运输距离的增加，颗粒长度的减小幅度大于宽度的减小幅度。此外，变晶晶粒的损失以接近恒定的速率发生，导致平均计算的锆石密度线性增加约。每 100 公里运输距离 0.03 g/cm3。因此，3D 颗粒形状与碎屑年龄特征密切相关，平均颗粒密度是绝对传输距离的函数。 3D 形状特征提供了有关碎屑锆石种群的宝贵信息，包括源材料与河流输送过程之间的相互作用，这显着影响保存偏差，并由此推断采样数据的代表性。
Detrital zircon studies can suffer from selective loss of provenance information due to U‐Pb age discordance, metamictization, metamorphic overprinting and fluviatile transport processes. The relationship between isotopic composition and zircon grain shape, and how grain shape is modified during transport, is largely unknown. We combine X‐ray tomography with U‐Pb geochronology to quantify how fluvial transport affects 3‐D zircon shape, detrital age signature, and grain density along the Murchison River, whose catchment comprises Eoarchean to Early Paleozoic source rocks in Western Australia. We acquired tomographic volumes and isotopic data from 373 detrital zircons to document changes in size, shape and density in transport direction, and explore how grain shape, age spectra and the proportion of discordant material vary along the channel. Results show that shape characteristics are sensitive to transport distance, stream gradient, proximity to source material, and whether the source consists of primary or recycled zircons. With increasing transport distance, grain lengths decrease more than their widths. Furthermore, the loss of metamict grains occurs at a near constant rate, resulting in a linear increase of mean calculated zircon density by ca. 0.03 g/cm3 per 100 km transport distance. 3‐D grain shape is therefore strongly linked to detrital age signature, and mean grain density is a function of the absolute transport distance. 3‐D shape characteristics provide valuable information on detrital zircon populations, including the interaction between source materials with fluvial transport processes, which significantly affects preservation bias and, by inference, the representativeness of the sampled data.