Iron isotopic variations in basalts from oceanic crust due to low-temperature seawater alteration
Iron isotopic variations in basalts from oceanic crust due to low-temperature seawater alteration
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低温海水蚀变导致洋壳玄武岩铁同位素变化
DOI：
10.1016/j.margeo.2022.106949
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发表时间：
2022-12
期刊：
影响因子：
2.9
通讯作者：
Jin-Hua Shi
中科院分区：
文献类型：
--
作者：
Xun Yu;Zhifei Liu;Jiawang Wu;Ya-Jun An;Jin-Hua Shi
海底玄武岩的铁(Fe)同位素组成可以为研究上地幔的高温过程提供重要的信息。然而，低温海水蚀变(即海底风化)可能会对初级Fe同位素组成产生重大影响。本文报道了在南中国海IODP调查中采集的U1434号玄武岩的全岩Fe同位素、主量元素、微量元素和锶单键-Nd同位素，以及微蚀变玄武岩的原位元素图。利用这些数据研究了低温海水蚀变对海底玄武岩Fe同位素组成的影响。玄武岩的δ57Fe值为+0.12‰~+0.27‰。δ57Fe值与源区敏感的锶、钕同位素之间无相关性，δ57Fe值与熔融敏感区La/Yb比值无相关性，表明δ57Fe值不受地幔源区非均质性和部分熔融的影响。MgO与CaO、Al_2O_3含量呈负相关，表明岩浆演化过程中单斜辉石和斜长石的分馏作用有限。由于橄榄石分馏可以形成增加SiO_2的玄武岩和减少的δ_(57)Fe，从而进一步排除了橄榄石分馏对Fe同位素变化的可能影响。δ_(57)Fe值与氧化镁、氧化铝、二氧化钛的相关性很强，说明铁同位素的变化是由低温海水蚀变引起的。代表地点U1434玄武岩样品的原位元素填图也表明，低温蚀变的特征是镁、二氧化硅含量减少，氧化铝、二氧化钛含量增加。我们提出了一个两阶段蚀变模型来解释U1434玄武岩的Fe同位素变化。这种蚀变似乎是在氧化条件下发生的：可能来自洋壳内部进行蚀变的外部富铁流体在洋壳表面被氧化，导致海底玄武岩的蚀变、Fe同位素分馏和δ_(57)Fe值增加。这项研究强调了氧化条件下的低温海水变化如何改变轻微变化的洋壳的铁同位素组成，这对理解海洋中的铁循环具有重要意义。
Iron (Fe) isotopic compositions of seafloor basalts can provide important insights into high-temperature processes in the upper mantle. However, low-temperature seawater alteration (i.e., seafloor weathering) may significantly affect the primary Fe isotopic compositions. In this paper, we report whole-rock Fe isotopes, major and trace elements, and Srsingle bondNd isotopes, along with in situ elemental maps for slightly altered Site U1434 basalts collected on IODP Expedition 349 from the South China Sea. We use these data to investigate the influence of low-temperature seawater alteration on the Fe isotopic compositions of seafloor basalts. The basalts have δ57Fe values of +0.12‰ to +0.27‰. No correlation between δ57Fe values versus Sr or Nd isotopes that are source-sensitive and no correlation between δ57Fe values and La/Yb ratios that are melting-sensitive indicate the δ57Fe values have not been affected by mantle source heterogeneity and partial melting. The negative correlations of MgO versus CaO and Al2O3contents suggest limited clinopyroxene and plagioclase fractionation during magmatic evolution. Possible influence of olivine fractionation on the Fe isotopic variations can further be ruled out by a positive correlation between MgO and SiO2contents and a weak negative correlation between δ57Fe values and SiO2contents because olivine fractionation can form basalts with increasing SiO2and δ57Fe with decreasing MgO. The strong correlations between δ57Fe values and MgO, Al2O3, and TiO2contents indicate the Fe isotopic variations were caused by low-temperature seawater alteration. This is also evident from in situ elemental mapping of representative Site U1434 basalt sample, which shows that low-temperature alteration was characterized by decreases in MgO and SiO2contents, and increases in Al2O3and TiO2contents. We propose a two-stage alteration model to explain the Fe isotopic variations of the Site U1434 basalts. The alteration appeared to occur under oxidizing conditions: Fe2+-rich external fluid likely derived from the progressive alteration inside the oceanic crust was oxidized at the surface of the oceanic crust, which caused the alteration, Fe isotopic fractionation, and increase in δ57Fe values of the seafloor basalts. This study highlights how low-temperature seawater alteration under oxidizing conditions can modify the Fe isotopic compositions of slightly altered oceanic crust, with critical implications for understanding the Fe cycle in the oceans.
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影响因子：
5
作者：
Caroline R. Soderman;S. Matthews;O. Shorttle;M. Jackson;Saskia Ruttor;O. Nebel;S. Turner;C. Beier;M. Millet;E. Widom;M. Humayun;H. Williams
通讯作者：
Caroline R. Soderman;S. Matthews;O. Shorttle;M. Jackson;Saskia Ruttor;O. Nebel;S. Turner;C. Beier;M. Millet;E. Widom;M. Humayun;H. Williams
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影响因子：
5.3
作者：
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通讯作者：
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影响因子：
5.3
作者：
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通讯作者：
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影响因子：
5.8
作者：
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通讯作者：
A. Draut;P. Clift