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.

通过大洋玄武岩观测到的地幔不均匀微量元素和放射成因同位素组成的起源是近三十年来的一个活跃问题。长期以来，人们一直认为地壳返回地幔是造成所观察到的主要不均匀性的原因。然而，地幔内交代作用等过程也被提出来部分或全部解释所观察到的变化，而没有地壳的参与。早在80年代，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下，即在地球表面。因此，大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复杂，发现在俯冲带的部分熔融、岩浆分异和板片脱水/熔融等高温过程中会发生大的同位素分馏。对于大多数稳定同位素系统而言，由于缺乏对这些过程中稳定同位素行为的充分了解，无法提供令人信服的证据，也无法对大洋玄武岩源中的再循环地壳进行定性，本提案旨在发掘并随后利用Mo稳定同位素作为地壳再循环的强有力示踪剂的潜力。Mo稳定同位素体系是一种非常有前途的示踪剂，但其全部潜力目前受到以下因素的限制：（1）岛弧和陆壳分异;（2）俯冲带板状变质沉积物和变质基性岩熔融;（3）低度地幔部分熔体的产生和分异。目标是填补钼同位素高温行为方面的知识空白，并调查和描述大洋玄武岩来源中循环地壳的特征，涵盖放射性同位素确定的整个成分谱。