Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater
Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater
复制标题
低温洋壳蚀变以及海水中钾和镁的同位素收支情况
DOI：
10.1016/j.epsl.2020.116290
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发表时间：
2020-07
影响因子：
5.3
通讯作者：
Danielle P. Santiago Ramos;L. A. Coogan;Jack Geary Murphy;J. Higgins
中科院分区：
文献类型：
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作者：
Danielle P. Santiago Ramos;L. A. Coogan;Jack Geary Murphy;J. Higgins
摘要洋壳低温(<；100℃)蚀变在决定海洋化学成分中起着重要作用。低温玄武岩蚀变对海水钾同位素组成(δ41K∼0‰)的影响尚不清楚，相对于块状硅酸盐地球(∼41 K‰=-0.5 4‰)，∼0.5 0‰相对块状硅酸盐地球(δ41K=-0.5 4‰)富集。本文介绍了白垩纪(特罗多斯蛇绿岩)和侏罗纪(大洋钻探计划孔801C)洋壳上部火山岩段的块状岩石、脉体和矿物中的一套同位素体系(δ41K，δ26 mg，δ7Li，87Sr/86Sr)和主次元素。我们利用这些数据估计了与洋壳低温蚀变有关的K同位素分馏，并对这一过程在全球海水中镁和钾的地球化学循环中的作用提供了新的约束。我们发现，来自特罗多斯蛇绿岩和ODP801C孔的热液蚀变玄武岩的δ-41K值分别为+0.0 1‰~-1.0 7‰(n=83)和+0.0 4‰~-0.88‰(n=17)。来自特罗多斯和801C孔的块状岩石样品的平均δ41K值在∼-0.50‰时彼此没有区别，表明低温玄武岩蚀变是海水的39K汇，并部分解释了海水的41K/39K高于BSE值的原因。与K相反，特罗多斯(δ0.00‰)和801C孔(∼0.20‰)的平均δ26 mg值表明，蚀变洋壳从海水中吸收了26 mg，可能是海水中轻∼26 mg组分(‰-0.8∼)相对于BSE(‰-0.2)的贡献。我们在玄武岩样品中观察到重同位素δ26 mg，其特征是总体镁含量变化很小或没有变化，这与洋壳低温蚀变期间海水和洋壳之间广泛的镁同位素交换相一致。最后，我们发现δ7Li和δ41K在特罗多斯蛇绿岩三个地点的变化可以用不同的蚀变类型来解释，这些蚀变似乎与沉积的时间及其对海水和洋壳之间的化学和同位素交换的影响有关。
Abstract Low-temperature (< 100° C) alteration of oceanic crust plays an important role in determining the chemical composition of the oceans. Although a major sink of seawater potassium, little is known about the effects of low-temperature basalt alteration on the potassium isotopic composition of seawater (δ 41 K∼ 0‰), which is∼ 0.50‰ enriched relative to bulk silicate Earth (BSE, δ 41 K=-0.54‰). Here, we present a suite of isotopic systems (δ 41 K, δ 26 Mg, δ 7 Li, 87 Sr/86 Sr) and major/minor elements in bulk rock, veins and mineral separates from the upper volcanic section of Cretaceous (Troodos ophiolite) and Jurassic (Ocean Drilling Program Hole 801C) oceanic crust. We use these data to estimate the K isotopic fractionation associated with low-temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of Mg and K in seawater. We find that hydrothermally altered basalts from the Troodos ophiolite and ODP Hole 801C, most of which are enriched in K relative to the unaltered glass compositions, have δ 41 K values both higher and lower than BSE, ranging from+ 0.01‰ to-1.07‰(n= 83) and+ 0.04‰ to-0.88‰(n= 17), respectively. Average δ 41 K values of bulk-rock samples from Troodos and Hole 801C are indistinguishable from each other at∼-0.50‰, indicating that low-temperature basalt alteration is a sink of 39 K from seawater, and explaining, in part, why seawater has a higher 41 K/39 K than BSE. In contrast to K, average δ 26 Mg values for both Troodos (∼ 0.00‰) and Hole 801C (∼ 0.20‰) indicate that altered oceanic crust (AOC) is a sink of 26 Mg from seawater, likely contributing to the light δ 26 Mg composition of seawater (∼-0.8‰) relative to BSE (∼-0.2‰). We observe isotopically heavy δ 26 Mg values in basalt samples characterized by small to no changes in bulk Mg content, consistent with extensive isotopic exchange of Mg between seawater and oceanic crust during low-temperature oceanic crust alteration. Finally, we find that variability in δ 7 Li and δ 41 K across three sites in the Troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust.