Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust
Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust
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DOI：
10.1016/j.epsl.2020.116083
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发表时间：
2020-03-15
影响因子：
5.3
通讯作者：
Anbar, Ariel D.
中科院分区：
文献类型：
--
作者：
Greaney, Allison T.;Rudnick, Roberta L.;Anbar, Ariel D.
24个组合物的冰川杂岩跨越沉积年龄为2900至300 Ma的钼同位素显示了系统的转变，以更轻的成分和减少钼浓度随着时间的推移。与大氧化事件（GOE）有关的杂岩可分为三个年龄组：GOE前（2.43 - 2.90 Ga）、同GOE（2.20 - 2.39 Ga）和GOE后（0.33 - 0.75 Ga）。前GOE复合材料具有+0.03份/千份（+/-0.18份/千份）的平均Δ Mo-98（NIST 3134），合成GOE复合材料平均-0.29份/千份（+/-0.60份/千份），并且后GOE复合材料平均-0.45份/千份（+/-0.51份/千份）。这些组在p=0.05时具有统计学差异。我们使用GOE前的数据来估计太古代上陆壳（UCC）三角洲Mo-98的平均签名为+0.03 +/- 0.18千分之一（2西格玛），这福尔斯在以前估计的现代火成岩的范围内。由于杂岩代表了火成岩和风化壳的混合物，随着时间的推移，向较轻的Mo值的转变可能反映了氧化风化过程中Mo同位素分馏，以及风化层和土壤中轻Mo同位素的保留增加。我们推测，这种分馏是由于动员的氧化钼后GOE，和随后的吸附轻钼到铁锰氧化物和/或风化层中的有机质。我们的结论是，大陆风化产物中的钼同位素记录大气氧的上升和氧化风化的开始。由于在氧化条件下形成的风化层的同位素比一般的大陆火成岩轻，质量平衡表明氧化风化过程中的Mo同位素分馏应导致同位素重的地下水和河水，这在现代系统中观察到。(C)2020爱思唯尔B. V.保留所有权利。
Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 - 2.90 Ga), syn-GOE (2.20 - 2.39 Ga), and post-GOE (0.33 - 0.75 Ga). Pre-GOE composites have an average delta Mo-98(NIST3134) of +0.03 parts per thousand (+/- 0.18 parts per thousand), syn-GOE composites average -0.29 parts per thousand (+/- 0.60 parts per thousand), and post-GOE composites average -0.45 parts per thousand (+/- 0.51 parts per thousand). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) delta Mo-98 signature as +0.03 +/- 0.18 parts per thousand (2 sigma), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems. (C) 2020 Elsevier B.V. All rights reserved.