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

近三十年来，通过大洋玄武岩观测到的地幔的不均匀微量元素和放射性同位素组成的起源一直是一个活跃的问题。长期以来，地壳返回地幔一直被认为是所观察到的主要不均质性的原因。然而，地幔内交代作用等过程也被认为可以部分或全部解释在没有地壳参与的情况下观察到的变化。早在80年代，对大洋玄武岩的稳定同位素研究就被认为有可能结束这场辩论。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，大洋玄武岩中强烈分馏特征的存在被认为是再循环地壳返回地幔的明确证据。然而，稳定同位素的解释比最初预测的要复杂得多，在部分熔融、岩浆分异和俯冲带板片脱水/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，由于缺乏对稳定同位素在这些过程中的行为的充分了解，无法提供令人信服的证据和表征大洋玄武岩来源中的再循环地壳。这一建议旨在解锁并随后利用钼稳定同位素作为地壳再循环的强大示踪剂的潜力。钼稳定同位素体系是一种很有前途的示踪剂，但由于在(1)岛弧和陆壳分异；(2)俯冲带板片变质和变质基性熔融；(3)低度地幔部分熔体的产生和分异过程中缺乏对钼稳定同位素行为的限制，其全部潜力受到阻碍。其目标是填补关于钼同位素高温行为的这些知识空白，并调查和确定大洋玄武岩来源中的再循环结壳的特征，涵盖放射性同位素所界定的整个成分谱。

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.