Tracing crustal recycling and mantle mixing using novel stable isotopes

Abstract

The degree and cause of chemical heterogeneity within the Earth’s mantle remains an enigmatic topic in geochemistry. Chemical heterogeneity may be caused by the recycling of chemically altered and subduction zone processed crustal material from the Earth’s surface into the mantle. Novel stable isotope systems offer insights into this process, as low temperature isotopic fractionations at the Earth’s surface can survive subduction back to the mantle, potentially polluting sources of mantle derived basalts.
This thesis documents the use of uranium, molybdenum, and potassium isotopes to trace crustal recycling. The uranium isotopic composition of modern-day oceanic crust altered in oxygen-rich deep oceans is shown to be heterogeneous. Comparing uranium compositions in modern sections of altered oceanic crust to ancient pieces preserved as ophiolites, shows that the deep oceans have potentially been oxygen-rich since 750 million years ago. This constrains how long isotopically distinct (from the bulk silicate Earth) uranium has been recycled into the mantle.
Utilising constraints from uranium and molybdenum isotopes, we show that sources of enriched mid-ocean ridge basalts may not contain recycled oceanic crust. Instead, isotopic data favours a model of low-degree partial melting of ancient mantle sources, pre 750 million years ago, at the asthenosphere-oceanic lithosphere boundary. Such melts, enriched in incompatible elements, metasomatise overlying oceanic lithosphere that is eventually recycled and sampled as enriched-mid-ocean ridge basalts.
We show that potassium isotopes trace subduction zone processes and can distinguish between slab derived components, such as fluids released from subducting slabs during dehydration, types of subducting sediments with or without altered oceanic crust, that are added to the sources of arc lavas.
We produce models of crustal recycling to show that uranium and molybdenum isotopic heterogeneity within the mantle can be created in geologically feasible timescales and use them to assess the utility of potassium to tracing crustal recycling.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Tim Elliott (Supervisor), Morten B. Andersen (Supervisor) & Tomas L Martin (Supervisor)