Chromium isotope composition of organic-rich marine sediments and their mineral phases and implications for using black shales as a paleoredox archive
Chromium isotope composition of organic-rich marine sediments and their mineral phases and implications for using black shales as a paleoredox archive
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DOI：
10.1016/j.gca.2019.11.035
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发表时间：
2020-02-01
影响因子：
5
通讯作者：
Frei, Robert
中科院分区：
文献类型：
--
作者：
Frank, Anja B.;Klaebe, Robert M.;Frei, Robert
海洋沉积物中的铬（Cr）稳定同位素，特别是铁的形成，已被用来跟踪地球表面氧化还原条件的变化。近年来，已经发表了几项研究，重点是页岩托管的铬同位素，以进一步限制大气氧水平的演变。由于页岩是由碎屑和自生组分的混合物组成的，因此了解哪一相是Cr的宿主，并确定不同宿主相之间Cr浓度和分馏程度的任何变化是至关重要的。在这里，我们评估页岩是否有潜力记录铬同位素值是有意义的古环境研究，侧重于代表性的例子，富含有机质的海洋沉积物（黑色页岩，磷酸盐和燧石样品）从寒武纪的九门冲和埃迪卡拉时代的刘茶坡组（中国南方）。我们使用一种新的方法来隔离自生组件，可能记录主要的海洋氧化还原波动。散装沉积物pasts和顺序浸出（0.5 M盐酸，6 M盐酸，HF和王水），结合X射线衍射和微束矿物测绘（Nanomin）表征未反应和浸出的沉积物样品，使我们能够确定潜在的铬托管相及其同位素特征。黑色页岩块状沉积物三角洲Cr-53值在0.02%e +/- 0.15%e和0.56%e +/- 0.10%e之间变化，因此与-0.124 +/- 0.101%e的火成地球特征相比，略有正分馏。燧石样品测得的最高δ Cr-53值（0.98%e +/- 0.13%e）。浸出试验结果表明，大部分铬（75-85%）是托管在硅酸盐矿物部分的沉积物，并在6 M盐酸和HF酸浸出步骤中释放。此外，碳酸盐/磷酸盐和黄铁矿可以通过我们的顺序浸出方法（分别通过0.5 M HCl和王水浸出步骤释放）分离为潜在的Cr主体。每个连续浸出步骤的Δ Cr-53值通常在其各自体积的误差范围内。Delta Cr-53值。这是令人惊讶的，因为预计来自碎屑相的Cr与自生相中的Cr具有不同的Δ Cr-53值，自生相中的Cr被认为记录了水柱中Cr的Δ Cr-53，并且表明碎屑矿物的Δ Cr-53值可能偏离火成岩记录的值的窄范围。自生和碎屑矿物相观察到的类似的三角洲Cr-53值表明全岩成岩均质化，这使得古氧化还原条件下富含有机物的黑色页岩中Cr稳定同位素的应用和解释变得复杂。(C)2019由Elsevier Ltd.出版
Chromium (Cr) stable isotopes in marine sediments, in particular iron formations, have been used to track changes in Earth's surface redox conditions. In recent years several studies have been published that focus on shale-hosted Cr isotopes to further constrain the evolution of atmospheric oxygen levels. As shales are composed of a mixture of detrital and authigenic components, it is critical to know which phases host Cr and to determine any variations in Cr concentration and degree of fractionation between the different host phases. Here we assess whether shales have the potential to record Cr-isotope values that are meaningful to paleoenvironmental studies, focusing on representative examples of organic-rich marine sediments (black shale, phosphate and chert samples) from the Cambrian-aged Jiumenchong and Ediacaran-aged Liuchapo Formations (South China). We use a novel approach to isolate authigenic components which potentially record primary marine redox-fluctuations. Bulk sediment digests and sequential leaches (0.5 M HCL, 6 M HCl, HF and aqua regia), combined with X-ray diffraction and microbeam mineral mapping (Nanomin) characterisation of unreacted and leached sediment samples, allow us to identify potential Cr-hosting phases and their isotopic signatures. The black shale bulk sediment delta Cr-53 values vary between 0.02%e +/- 0.15%e and 0.56%e +/- 0.10%e and are thus slightly positively fractioned compared to the igneous Earth signature of -0.124 +/- 0.101%e. The highest delta Cr-53 value was measured for a chert sample (0.98%e +/- 0.13%e). The leaching test results reveal that the majority of Cr (75-85%) is hosted in the silicate mineral fraction of the sediment, and is liberated during the 6 M HCl and HF acid leaching steps. Further, carbonates/phosphates and pyrite could be isolated as potential Cr hosts by our sequential leaching approach (liberated by the 0.5 M HCl and aqua regia leaching steps respectively). The delta Cr-53 values of each sequential leach step were generally within error of their respective bulk . delta Cr-53 value. This is surprising as Cr derived from detrital phases is expected to have a different delta Cr-53 value from Cr in authigenic phases, which are thought to record delta Cr-53 of Cr in the water column, and suggests that the delta Cr-53 value of detrital minerals might deviate from the narrow range of values documented for igneous rocks. The similar delta Cr-53 values observed for the authigenic and detrital mineral phases suggest whole-rock diagenetic homogenisation, which complicates the application and interpretation of Cr stable isotopes in organic-rich black shales for paleoredox conditions. (C) 2019 Published by Elsevier Ltd.