EPSS Colloquium winter-2019

Xenon is a unique tracer of volatile transport. The modern deep Earth Xe budget reflects primordial volatiles delivered during accretion, radiogenic ingrowth, outgassing in association with mantle processing, and regassing as Xe is carried within hydrous phases in downwelling lithologies. The Xe isotopic composition of the mantle thus reflects the integrated long-term history of volatile transport between the deep Earth and surface reservoirs. We present a numerical model of concurrent mantle degassing, regassing and fissiogenic production. We test a wide variety of outgassing and regassing rates and take the sequestration of Pu and U into the continental crust and the evolution of the atmospheric Xe isotopic composition into account. Model realizations that satisfy Xe isotopic constraints from mantle-derived rocks indicate that significant recycling of atmospheric Xe into the deep Earth could not have occurred prior to 2.5 Ga. Because Xe is carried into Earth’s interior in hydrous mineral phases, our results indicate that downwellings were drier in the Archean era relative to the present. Our results indicate that the mantle experienced net degassing throughout the Archean and transitioned to net regassing at some point after 2.5 Ga. Progressive drying of the Archean mantle would allow for slower convection and decreased heat transport out of the mantle, suggesting non-monotonic thermal evolution of Earth’s interior. If plate tectonics and plate subduction were initiated before 2.5 Gyr ago, then early downwelling subducted material was either hydrated to a lesser extent at the surface than in the modern-day, or volatiles were more efficiently expelled from Archean downwellings slabs at shallow depths and returned to the surface.