科研之友
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摘要
Howardite-Eucrite-Diogenite (HED) meteorites represent a large suit of crustal and sub-crustal rocks from the Vesta. This work presents systematic examination of the Ca isotope data on multiple varieties of HED meteorites for a better understanding of the magmatic evolution of the Vesta. Falls and finds possess similar Ca isotope compositions, and no correlation is observed between delta 44/40Ca and (Sr/Eu*)n, indicating that terrestrial weathering effect on Ca isotopes is insubstantial. According to the data in literature, the inner solar system may have a homogeneous delta 44/40Ca and the average of inner solar system bodies (0.97 +/- 0.03%o) can approximate the composition of bulk silicate Vesta (BSV). Basaltic eucrites define a cluster in delta 44/40Ca (0.95 +/- 0.07%o, 2SD, N = 15) that is higher than the terrestrial mid-ocean ridge basalts (similar to 0.85%o). Combined with partial melting and magma ocean differentiation modeling, the Ca isotope signatures suggest that eucrites represent the residual melts evolved from a magma ocean formed by primordial Vesta's moderate-to-high degree melting (20-100%). Diogenites have distinguishingly higher delta 44/40Ca (1.18 +/- 0.15%o, 2SD, N = 7) than the basaltic eucrites, which displays a negative correlation with the 1000 x Lu/Ti ratio and a positive correlation with 1/Ca. However, magma ocean crystallization can only explain diogenites with delta 44/40Ca higher than 1.17%o, suggesting that diogenites have complicated petrogenesis and are not necessarily cogenetic with eucrites. Diogenites with delta 44/40Ca < 1.17%o may result from magma-ocean-cumulate partial melts intruding the eucritic crust. Mixing models suggest that the eucritic component in these diogenites may be less than 10%. Two howardites have lower delta 44/40Ca of 0.80 +/- 0.04%o and 0.86 +/- 0.05%o than eucrites and diogenites. This signature may reflect the addition of carbonaceous chondritic materials due to impact brecciation.
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单位桂林理工大学; 中国科学院
