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

• Applicant: Rachel Bezard, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 418227749

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.

• DFG Programme: Research Grants