Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth
Résumé
The early evolution of the solar nebula involved substantial transport of mass, resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources. The efficiency of this mixing, as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets. Here we present Mo isotopic data for a wide range of samples, including Ca-Al-rich inclusions, chondrites and differentiated meteorites, as well as martian and terrestrial samples. Most meteorites are depleted in s-process Mo relative to the Earth, and only the IAB-IIICD irons, angrites and martian meteorites have terrestrial Mo isotopic compositions. In contrast, most Ca-Al-rich inclusions are enriched in r-process Mo, but one inclusion is characterized by a large s-process deficit. Molybdenum isotopic anomalies in the bulk meteorites correlate with those in Ru exactly as predicted from nucleosynthetic theory, but no obvious correlation is apparent between Mo and Ni anomalies. Therefore, s-process Mo and Ru seem to be hosted in the same carrier, which must be distinct from the carrier responsible for isotopic anomalies in the Fe-group elements (Ni, Cr, Ti). Furthermore, the isotopic heterogeneity in Mo (and other elements) contrasts with the isotopic homogeneity for Hf and Os, indicating that different s-process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed. The Mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies, providing evidence for a progressive homogenization of the solar nebula. However, the carbonaceous chondrites exhibit larger Mo anomalies than expected for their age, indicating that they received a greater portion of material from the outer solar system (where homogenization was slow) than other meteorite parent bodies and terrestrial planets. Compared to the meteorites, Earth is enriched in s-process Mo and must have accreted from material distinct from the meteorites. Combined Mo and O isotopic data show that the composition of the Earth cannot be reconstructed by any known combination of meteorites, implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk Earth. This is exemplified by the covariation of Mo-92 and Nd-142 anomalies in chondrites, showing that the Nd-142 deficit of chondrites compared to the accessible Earth may not unequivocally be interpreted as a signature of an early differentiation of the Earth. However, further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the Earth