Unlocking and using the potential of Mo stable isotopes as a tracer of crustal recycling

释放和利用钼稳定同位素作为地壳循环示踪剂的潜力

• 批准号: 418227749
• 负责人: Dr. Rachel Bezard, Ph.D.
• 金额: --
• 依托单位: Abteilung Isotopengeologie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/418227749?language=en
• 关键词: Unlocking; using; potential; Mo; stable

The origin of the heterogeneous trace element and radiogenic isotopic compositions of the Earth’s mantle observed through oceanic basalts has been an active question for the last three decades. The return of crust back to the mantle has long been suggested to be responsible for the dominant heterogeneities observed. However, processes such as intra-mantle metasomatism were also proposed to partly or entirely explain the observed variations without crust involvement. Already in the 80s, stable isotope investigations of oceanic basalts were suggested to have the potential to close this debate. This is because the largest stable isotope fractionations occur at low temperature, i.e. at the surface of the Earth. Hence the presence of strongly fractionated signatures in oceanic basalts was expected to be the unambiguous proof of recycled crust back to the mantle. However, the interpretation of stable isotopes revealed more complex than initially predicted, with large isotope fractionation found to occur during high temperature processes such as partial melting, magma differentiation and slab dehydration/melting at subduction zones. For most stable isotopic systems, the lack of a full understanding of the behaviour of stable isotopes during these processes precludes convincing evidence and characterization of recycled crust in the source of oceanic basalts.This proposal aims at unlocking and subsequently using the potential of Mo stable isotopes as a powerful tracer of crustal recycling. The Mo stable isotopic system is a very promising tracer but its full potential is currently hampered by the lack of constraints on Mo stable isotope behaviour during (1) island arc and continental crust differentiation; (2) slab metasediment and metabasite melting at subduction zones and (3) the production and differentiation of low-degree mantle partial melts. The goal is to fill these knowledge gaps on the high temperature behaviour of Mo isotopes and to investigate and characterize recycled crust in the source of oceanic basalts covering the whole compositional spectrum defined by radiogenic isotopes.

近三十年来，通过海洋玄武岩观测到的地幔非均质微量元素和放射性同位素组成的来源一直是一个活跃的问题。长期以来，地壳向地幔的回归一直被认为是观测到的主要非均质性的原因。然而，也有人提出地幔内交代作用等过程可以部分或完全解释没有地壳参与的观测变化。早在80年代，对海洋玄武岩的稳定同位素研究就被认为有可能结束这场争论。这是因为最大的稳定同位素分馏发生在低温，即在地球表面。因此，海洋玄武岩中强烈分馏特征的存在被认为是地壳再循环回到地幔的明确证据。然而，稳定同位素的解释比最初预测的更为复杂，在俯冲带的部分熔融、岩浆分异和板块脱水/熔融等高温过程中发现了大的同位素分馏。对于大多数稳定同位素系统，缺乏对稳定同位素在这些过程中的行为的充分了解，妨碍了对海洋玄武岩来源中再循环地壳的令人信服的证据和表征。该建议旨在解锁并随后利用Mo稳定同位素作为地壳循环的强大示踪剂的潜力。Mo稳定同位素系统是一种非常有前途的示踪剂，但目前由于缺乏对岛弧和大陆地壳分化过程中Mo稳定同位素行为的约束，阻碍了Mo稳定同位素系统的充分发挥；(2)俯冲带板块变质岩和变质岩的熔融作用；(3)低程度地幔部分熔体的产生和分异作用。目标是填补Mo同位素高温行为方面的知识空白，并研究和表征海洋玄武岩来源中的回收地壳，涵盖由放射性成因同位素定义的整个成分谱。