![]() ![]() Previous work has shown that these signatures cannot simply result from shallow atmospheric contamination, mixing between subducted atmospheric Xe and MORB Xe, and/or different closure ages of Xe loss between the shallow and deep mantle reservoirs ( 7, 9, 13). According to recent high-precision analyses of Xe isotopes, ocean island basalt (OIB) samples display a uniformly low 129Xe*/ 136Xe* Pu (by a factor of ~2.8) compared to mid-ocean ridge basalt (MORB) samples (upper mantle sources) ( 7– 9, 13). Hence, the study of the 129Xe*/ 136Xe* Pu in silicate reservoirs of Earth has the potential to place strong constraints on Earth’s accretion and evolution ( 7– 13). Because 129Xe* comes from radioactive beta decay of now extinct volatile 129I ( t 1/2 = 15.7 Ma) and 136Xe* Pu comes from spontaneous fission of extinct refractory 244Pu ( t 1/2 = 80 Ma), the 129Xe*/ 136Xe* Pu ratio evolves as a function of both time and reservoirs compositions (i.e., I/Pu ratio) early in Earth’s history. To investigate these questions, the isotopes of xenon (Xe), the heaviest stable noble gas, are particularly useful. This implies that Earth inherited part of its volatiles, including its water, from late accretion of chondrites, with a notable carbonaceous chondrite contribution.Įarth must have accreted from diverse materials, but the nature and temporal sequence of delivery of these potential building blocks remain heavily debated ( 1– 6). Instead, our results reveal a heterogeneous accretion history, whereby predominant accretion of volatile-poor differentiated planetesimals was followed by a secondary phase of accretion of volatile-rich undifferentiated meteorites. Using multistage core formation modeling, we show that core formation alone is unlikely to explain the iodine/plutonium difference between mantle reservoirs. Here, we use first-principles molecular dynamics to quantify the metal-silicate partition coefficients of iodine and plutonium during core formation and find that both iodine and plutonium partly partition into metal liquid. Understanding whether this difference stems from core formation alone or heterogeneous accretion is, however, hindered by the unknown geochemical behavior of plutonium during core formation. The observation that mid-ocean ridge basalts had ~3× higher iodine/plutonium ratios (inferred from xenon isotopes) compared to ocean island basalts holds critical insights into Earth’s accretion. ![]()
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