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 Earth1,2. For example, the relative proportions of 129Xe derived from radioactive-decay of extinct 129I and 136Xe from fission of extinct 244Pu as well as extant 238U provide crucial timestamps for mantle volatile loss and for the formation of a habitable surface environment3,4,5. 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 evolution6,7. Here, we employ dynamic mass spectrometry8,9 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 136Xe from 244Pu-Xe (denoted 136XePu/136XeTF) 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 129Xe excess from 129I decay implies that the lack of XePu cannot be ascribed to an extensive loss of short-lived radionuclide products via mantle degassing. Instead, missing XePu may require (i) a reevaluation of the initial (chondritic) Pu/U within Earth10 or (ii) significant fractionation of Pu/U during early Earth processes, both of which would have far-reaching implications for modeling of terrestrial Xe evolution11. Notably, quantitative incompatible element extraction to the Hadean crust and subsequent reintroduction of 238U via subduction (after 244Pu had become extinct, i.e., after ~ 500 Myr) could have contributed to lowering the ultimate 136XePu/136XeTF 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.