Heavy δ57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle
Heavy δ57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle
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DOI：
10.1016/j.gca.2020.09.033
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发表时间：
2021-01
影响因子：
5
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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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
岩性非均质性是地球地幔的一种普遍接受的特征，洋岛玄武岩(OIB)地球化学中的非均质性很大程度上来自于再循环的地壳物质。由于其矿物特定的平衡分馏效应或其组成，如从核幔边界携带的动力学分馏的核心液体的并入，铁同位素被用来将OIB来源的地球化学不均一性与不同的地幔岩性联系起来。在这里，我们提供了萨摩亚地幔和亚速尔群岛火山的铁同位素数据，以及相平衡和平衡矿物熔体同位素分馏模型。这些洋盆熔岩使我们能够研究核衍生和再循环地幔组分在产生重δ57Fe熔体中的作用。重δ57Fe与地壳成分富集的放射性同位素特征相关，而与Fe/Mn或任何核参与指标无关。然而，单阶段熔融类似于MORB的榴辉辉石岩不能产生皮特凯恩、亚速尔群岛和恢复活力的萨摩亚熔岩中所见的重δ57 Fe。来自与橄榄岩混合的榴辉岩熔体的反应带辉石岩的熔体(通常被认为是OIB来源的一部分)也不能产生OIB中所见的最重的Fe同位素组成。相反，OIB中重δ57 Fe熔体的产生需要：(1)使俯冲榴辉岩的同位素比其原始前身δ更重的过程(如热液蚀变、变质作用、沉积物输入)；(2)岩石圈处理，例如先前冻结的小度熔体的再动员，或被硅酸盐熔体交代的岩石圈物质的贡献；和/或(3)限制环境中较低的δ57 Fe物质对重δ57 Fe熔体的稀释的熔融条件。我们认为，没有一个单一的过程可以产生亚速尔群岛、皮特凯恩和恢复活力的萨摩亚熔岩中所见的重δ57Fe。因此，不能假设来自再循环地壳物质的辉石岩岩性是大洋中海区重δ57 Fe熔体的唯一生产者，也不能认为这些特征与地核的贡献有关。相反，观察到的重δ57 Fe OIB熔体不能归因于一个独特的来源或过程。这种模糊性反映了从再循环岩性的产生到其地幔熔融的多种过程：从MORB的产生、其低温变化、地幔的不均质性发展和岩石圈处理，到海洋岛屿的喷发。
Lithological heterogeneity is a widely accepted feature of the Earth’s mantle, with recycled crustal material accounting for a significant part of heterogeneity in ocean island basalt (OIB) geochemistry. Fe isotopes have been used to link geochemical heterogeneity in OIB sources to distinct mantle lithologies due to their mineral-specific equilibrium fractionation effects, or their composition, such as incorporation of kinetically-fractionated core liquids entrained from the core-mantle boundary. Here we present Fe isotope data for Samoan shield, and Azores volcanoes, together with a combined phase-equilibria and equilibrium mineral-melt isotope fractionation model. These OIB lavas allow us to study the roles of core-derived and recycled mantle components in generating heavy δ 57 Fe melts. Heavy δ 57 Fe correlates with radiogenic isotope signatures of enrichment by a crustal component and not with Fe/Mn or any indicator of core involvement. However, single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the heavy δ 57 Fe seen in Pitcairn, Azores, and rejuvenated Samoa lavas. Melts of a reaction-zone pyroxenite (commonly suggested to form part of the OIB source), derived from eclogite melts hybridised with peridotite, also fail to generate the heaviest Fe isotopic compositions seen in OIB. Instead, the generation of heavy δ 57 Fe melts in OIB requires:(1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (eg, hydrothermal alteration, metamorphism, sediment input);(2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with heavy δ 57 Fe by ambient lower δ 57 Fe materials. No single process we consider can generate the heavy δ 57 Fe seen in the Azores, Pitcairn, and rejuvenated Samoan lavas. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of heavy δ 57 Fe melts in OIB, nor can these signatures be related to contributions from the Earth’s core. Instead, the observation of heavy δ 57 Fe OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands.