Xenon isotopes in the MORB source, not distinctive of early global degassing

Abstract Although fissiogenic Xe in mid‐ocean ridge basalt (MORB) has long been used inferred to infer an early (>4 Ga) catastrophic mantle degassing, decomposition of the analyzed gas in terms of its constituents requires several assumptions. The canonical interpretation has not been tested using the full spectrum of possible initial components and radiogenic inputs. We use a Markov chain Monte Carlo approach that examines a broad range of Xe isotopic components present during Earth formation and evolution. Our best fit simulations are consistent with the preservation of much higher Pu/U in MORB source Xe than previously recognized but are equally supportive of both limited and early catastrophic loss. We show that an initially Xe depleted upper mantle that becomes progressively ingassed through both radiogenic ingrowth and subduction of evolving atmospheric Xe is equally consistent with all evidence, underscoring the need for improvements to our knowledge of fission Xe spectra, MORB Xe measurements, and Pu geochemistry.