Speaker: Lan Hu
Affiliation: EPSS, UCLA

Abstract:
Xenon isotopes trace Earth's deep volatile history, but present-day mantle Xe observations display a paradox: atmospheric-like isotopic ratios require substantial regassing, while the low concentration requires limited regassing. Previous models resolved this by invoking early limited regassing, but this is hard to reconcile with evidence for early plate tectonics, higher Xe concentrations in the ancient atmosphere, and incomplete noble gas removal from subducting slabs. We use a parameterized thermal evolution and material recycling framework to explore time-dependent degassing and regassing histories, including Xe loss through continental crust extraction, mid-ocean ridge and plume melting, and Xe recycle via subduction. We show that strong early Xe loss driven by rapid early continental crust generation reduces the bulk mantle concentration enough to make later regassing effective at reproducing the isotopic ratios. Fissiogenic Xe constraints and the required intense early degassing together favor lower initial mantle uranium concentration, pointing to starting compositions with at least an enstatite-chondrite component rather than purely CI-chondritic material. Analytical solutions with simplified approximation and machine learning analysis confirm that these requirements are linked: low uranium and large early crustal extraction are coupled, and early degassing controls the timescale over which the system transitions to moderate degassing or regassing. Our results show that a major early degassing episode with prolonged moderate regassing can explain both the isotopic ratios and concentrations, offering an alternative to limited regassing scenarios and new constraints on Earth's starting materials and early history.

Submitted by: Omar Abuassaf (oabuassaf@epss.ucla.edu)