New enclaves in the Vaca Muerta mesosiderite: Petrogenesis and comparison with HED meteorites

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Thirty-eight new enclaves from the Vaca Muerta mesosiderite have been studied, and detailed SEM-petrography was carried out on fourteen of them. Three are coarse-grained gabbroic ilmenite-free enclaves consisting mainly of pigeonite (En_<56-49>) and plagioclase, seven are fine- to medium-grained ilmenite-bearing enclaves (En_<49-36>) with ophitic to granular texture, and four are breccias. The most important breccias are diogenitic monomict breccias (En_<74-65>) and an olivine-orthopyroxenite monomict breccia (Fo_<73-71>). Reduction-induced orthopyroxene commonly surrounds pigeonite grains in the ilmenite-free enclaves, and occurs rarely in the marginal parts of the ilmenite-bearing enclaves. Whitlockite forms from augite lamellae as a result of reduction, and the ilmenite-free enclaves have been enriched in P_2O_5. Reduction of the ilmenite-free enclaves occurred in the parent body before mixing with Fe-Ni metal on the mesosiderite parent body. In the ilmenite-bearing enclaves reduction occurred on the mesosiderite parent body after Fe-Ni metal was mixed in. Thus, two stages of subsolidus reduction occurred in enclaves. Ilmenite and rutile occur in the ilmenite-bearing and diogenitic enclaves, and various subsolidus reaction textures of chromite, ilmenite, rutile and probably preexisting pseudobrookite are found. Al_2O_3-depleted chromite formed through the decomposition of Cr_2O_3-bearing ilmenite and pseudobrookite. Zircon and baddeleyite were also found in the ilmenite-bearing enclaves, and the latter is the first occurrence reported in mesosiderites. The ilmenite-bearing enclaves always contain baddeleyite, whereas ZrO_2-bearing phases are hardly ever found in HED meteorites. The bulk compositions of the enclaves were analyzed, and found to be more SiO_2-enriched than the HED meteorites. Highly SiO_2-enriched ilmenite-bearing enclaves are especially common, and follow the Nuevo Laredo trend of the eucrites, whereas the other ilmenite-bearing enclaves follow the Stannern trend. The highly SiO_2-enriched enclaves probably formed by fractional crystallization, whereas the other ilmenite-bearing enclaves formed through partial melting. The existence of the highly SiO_2-enriched enclaves may partly explain the SiO_2-rich nature of mesosiderites, in addition to the in situ reduction process. Our observations suggest that the parent body and the parental magmas of the enclaves in the mesosiderites were initially different from those of the HED meteorites.
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New enclaves in the Vaca Muerta mesosiderite: Petrogenesis and comparison with HED meteorites

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