New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics

Abstract

When corrected for the effects of cosmic ray exposure, Mo and Ru nucleosynthetic isotope anomalies in iron meteorites from at least nine different parent bodies are strongly correlated in a manner consistent with variable depletion in s-process nucleosynthetic components. In contrast to prior studies, the new results show no significant deviations from a single correlation trend. In the refined Mo-Ru cosmic correlation, a distinction between the non-carbonaceous (NC) group and carbonaceous chondrite (CC) group is evident. Members of the NC group are characterized by isotope compositions reflective of variable s-process depletion. Members of the CC group analyzed here plot in a tight cluster and have the most s-process depleted Mo and Ru isotopic compositions, with Mo isotopes also slightly enriched in rand possibly p-process contributions. This indicates that the nebular feeding zone of the NC group parent bodies was characterized by Mo and Ru with variable s-process contributions, but with the two elements always mixed in the same proportions. The CC parent bodies sampled here, by contrast, were derived from a nebular feeding zone that had been mixed to a uniform s-process depleted Mo-Ru isotopic composition. Six molybdenite samples, four glacial diamictites, and two ocean island basalts were analyzed to provide a preliminary constraint on the average Mo isotope composition of the bulk silicate Earth (BSE). Combined results yield an average μ 97 Mo value of +3 ± 6. This value, coupled with a previously reported μ 100 Ru value of +1 ± 7 for the BSE, indicates that the isotopic composition of the BSE falls precisely on the refined Mo-Ru cosmic correlation. The overlap of the BSE with the correlation implies that there was homogeneous accretion of siderophile elements for the final accretion of 10 to 20 wt% of Earth's mass. The only known cosmochemical materials with an isotopic match to the BSE, with regard to Mo and Ru, are some members of the IAB iron meteorite complex and enstatite chondrites.

Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth

“…Mass-independent  54 Fe and  57 Fe and mass-dependent  56 Fe values for terrestrial and extraterrestrial materials relative to IRMM-14. References (39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)…”

Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites

“…The pre-exposure ε i Mo values of this study are a factor of ∼5 more precise than previous results (Bermingham et al 2018), and only for IIAB irons have values with comparable precision been previously reported (Yokoyama et al 2019; Table 1). Finally, pre-exposure ε i Mo values for IVA irons were calculated by averaging data for samples having no CRE effects (Poole et al 2017) and CRE-corrected data (Bermingham et al 2018).…”

Transforming Dust to Planets

“…The important observation from the isotopic data is that although Nd, Mo and Ru reflect different stages of Earth's accretion, they all are characterized by an s-process excess relative to meteorites. In fact, the Earth's mantle plots on Nd-Mo (Render et al 2017) and Mo-Ru (Dauphas et al 2004;Bermingham et al 2018) isotope correlation lines together with meteorites, demonstrating that the isotopic nature and, hence, genetic heritage of Earth's accreting material has not greatly changed over time. This implies that the inner solar system was homogeneous (and thus well-mixed) during the early stages of Earth's accretion and/or that Earth's main building blocks averaged out existing small-scale heterogeneities.…”