Primordial and 244Pu-derived xenon missing from Earth's convecting mantle

Abstract Mantle-derived noble gases are exceptional recorders of the history of planetary volatile elements (C, N, water), which are key to the development of life on Earth 1,2 . For example, the relative proportions of 129 Xe derived from radioactive-decay of extinct 129 I and 136 Xe from fission of extinct 244 Pu as well as extant 238 U provide crucial timestamps for mantle volatile loss and for the formation of a habitable surface environment 3,4,5 . However, the low abundance of heavy noble gases (Ar, Kr, Xe) in mantle-derived samples constitutes a major analytical challenge that has thus far precluded a comprehensive understanding of mantle volatile evolution 6,7 . Here, we employ dynamic mass spectrometry 8,9 to measure convecting mantle-derived Ar-Kr-Xe isotopes in Mt. Etna (Sicily) and Eifel (Germany) volcanic gases at ultra-high precision. Our data reveal that the fractions of (i) total fissiogenic 136 Xe from 244 Pu-Xe (denoted 136 Xe Pu / 136 Xe TF ) and (ii) stable Kr and Xe from primordial (i.e., accretionary) sources are both significantly lower than previous estimates suggest. The latter point requires pervasive and almost complete overprinting by subduction-derived surface components. The large 129 Xe excess from 129 I decay implies that the lack of Xe Pu cannot be ascribed to an extensive loss of short-lived radionuclide products via mantle degassing. Instead, missing Xe Pu may require (i) a reevaluation of the initial (chondritic) Pu/U within Earth 10 or (ii) significant fractionation of Pu/U during early Earth processes, both of which would have far-reaching implications for modeling of terrestrial Xe evolution 11 . Notably, quantitative incompatible element extraction to the Hadean crust and subsequent reintroduction of 238 U via subduction (after 244 Pu had become extinct, i.e., after ~ 500 Myr) could have contributed to lowering the ultimate 136 Xe Pu / 136 Xe TF of the depleted upper mantle. This indicates that subduction quantitatively overprinted the compositions of both volatile (e.g., noble gases) and refractory (e.g., uranium) elements within Earth's mantle.