title: Assessing Element Recycling within Subduction Zones and the Deep Mantle based on Molybdenum Isotopes
Creators: name: Ahmad, Qasid
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 550 Earth sciences &amp; geology

description: Subduction zones play a crucial role in the chemical exchange between Earth's surface and interior, which influences the geochemical reservoirs during Earth's evolution. The bidirectional mass transport between Earth's crust and mantle creates mineralogical and chemical heterogeneities within Earth's silicate reservoirs. The quantification of mass and element fluxes into subduction zones helps to constrain processes and conditions that control element transfer from the slab to the mantle wedge, which is important to understand the evolution of the continental crust and mantle through Earth's history. This thesis investigates this element transfer by applying high-precision molybdenum (Mo) isotopes as a source and process tracer. It includes individual studies based on the determination of stable Mo isotope variations on different Earth (silicate) reservoirs.
The first study presents a combined approach that characterizes the isotopic composition of subduction input and output of the Tongan subduction zone together with subducted analogs from the Western Alps and Alpine Corsica to gain insights into the behavior of elements and their isotopes during prograde subduction metamorphism. The results suggest that Mo loss and isotope fractionation occur mainly before and during early subduction in the forearc, resulting in an isotopically light and Mo-depleted subducted slab before reaching subarc depths. This highlights the importance of the forearc mantle as a carrier of heavy Mo and other fluid mobile elements. The study proposes alternative and multistage processes that involve the recycling of the metasomatized forearc mantle to subarc regions that can explain the Mo isotope variations observed at the Tongan subduction zone.
The second study investigates the Mo isotope composition of mid-ocean ridge basalts from the South Atlantic that have interacted with the Shona and Discovery mantle plumes that carry an enriched mantle 1 (EM-1) affinity. Mo isotopes correlate with radiogenic isotopes with isotopically heavy Mo in the enriched samples indicating the contribution of Proterozoic anoxic sediments in their mantle source. This is supported by previous studies based on redox-sensitive stable S and Se isotopes suggesting the recycling of reduced sediments. The preservation of pre-subduction sediment signatures indicates that Mo was retained in the sedimentary unit supporting the immobile behavior of Mo during subduction, which in turn implies reducing conditions during subduction in the Precambrian.
The third study focuses on the Mo isotope composition of the type locality of the EM-1 endmember in Pitcairn Island, whose origin is the least constrained among other heterogeneous mantle endmembers. Heavy Mo isotope compositions are observed in Pitcairn Island lavas and extend the relationship with radiogenic isotopes of EM-1-influenced MORBs from the South-Atlantic (second study) suggesting the widespread contamination of Earth’s deep-seated large low shear velocity provinces with Precambrian anoxic sediments. This suggests limited recycling of redox-sensitive elements such as Mo, S, Se, and U into arc magmas in the Precambrian and can further explain the Pb isotope systematics (high and low time-integrated Th/U and U/Pb, respectively) of the EM-1 endmember.