Enclaves in the Mt.Padbury and Vaca Muerta mesosiderites: Magmatic and residue (or cumulate) rock types

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Nine eucritic enclaves from mesosiderites and one possible enclave were petrologically and chemically studied. Three are from Mt. Padbury, six from Vaca Muerta, and one eucritic meteorite (AMNH 4627) was found in northern Chile and may be an enclave from Vaca Muerta. The enclaves studied were classified into two groups based on the presence or absence of primary ilmenite : (1) an Il-bearing group ranging in texture from medium-grained ophitic to coarse-grained gabbroic, and (2) an Il-free group from gabbroic to coarse-grained equigranular texture. Pigeonites of the Il-bearing group are more ferroan than those of the Il-free group. The peripheral portions of pigeonite in the Il-free group have remarkable textures not found in HED meteorites. They are replaced by more-magnesian hypersthene and troilite with a minor amount of silica-mineral, indicating that reduction of pigeonite took place by H_2S under subsolidus conditions prior to formation of the mesosiderite, metal-silicate mixing. The major element compositions of the ten whole rock samples indicate that the Il-bearing group is similar to the non-cumulate eucrites, but the Il-free group differs from the cumulate eucrites. The Il-free group is significantly enriched in silica-mineral component in comparison with the cumulate eucrites. INAA and RNAA analyses for five whole rock samples were performed. Two Il-free enclaves have low REE contents with remarkable positive Eu-anomalies (chondrite-normalized Eu/Sm=13.6), and two Il-bearing enclaves have higher REE contents with slight positive Eu-anomalies (chondrite-normalized Eu/Sm=1.88-2.22). The high Eu/Sm ratios are partly due to terrestrial alteration of whitlockites, and the original Eu/Sm ratios of the Il-bearing and Il-free enclaves might be similar to those of non-cumulate eucrites and cumulate eucrites, respectively. These suggest that the Il-bearing enclaves were formed in the similar way to the non-cumulate eucrites, and the Il-free enclaves formed as cumulates from silica-rich magmas or as residues by partial melting of silica-bearing cumulate eucritic sources. A eucritic sample (AMNH 4627) has the highest REE content with no Eu-anomaly and is similar to the non-cumulate eucrites, and may be an independent eucrite.
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Enclaves in the Mt.Padbury and Vaca Muerta mesosiderites: Magmatic and residue (or cumulate) rock types

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