Calcium isotopic ratios and rare earth element abundances in refractory inclusions from the Allende CV3 chondrite

摘要

Refractory inclusions in primitive meteorites are the oldest objects formed in the Solar System. They exhibit large mass-dependent isotopic effects in many elements, including Si, Mg and Ca, indicative of their complex origin. We report data for both mass-dependent (δ) and mass-independent (ε) Ca isotopic effects, and rare earth element (REE) abundances in six refractory inclusions from the Allende CV3 meteorite. Our data reveal large mass-dependent Ca isotopic effects in these refractory inclusions, with δ ~(44/40)Ca ranging from -5.60 to +0.35 (relative to NIST SRM 915a), which are significantly lower than that of the bulk silicate Earth (δ ~(44/40)Ca=1.05±0.04). Importantly, δ ~(44/40)Ca is correlated with REE patterns: refractory inclusions with Group II REE patterns have lighter Ca isotope compositions compared to those with Group I REE patterns. Specifically, the studied refractory inclusions form negative ~(44)Ca/ ~(40)Ca-Tm/Er and ~(44)Ca/ ~(40)Ca-Nd/Er trends, implying segregation of up to 3% of an ultrarefractory evaporation residue from a chondritic reservoir prior to the formation of refractory inclusions. This residue would have to be characterized by high ~(44)Ca/ ~(40)Ca and low Tm/Er and Nd/Er. At 2σ levels of ±0.5 and ±1.5 for ε ~(40/44)Ca and ε ~(43/44)Ca, respectively, the six refractory inclusions studied show no detectable ~(40)Ca or ~(43)Ca anomalies relative to the terrestrial standards. However, five out of the six refractory inclusions do show a ~(48)Ca _(excess) of several ε-units, clearly resolvable from the terrestrial standards.