Noncumulate Versus Cumulate Eucrites: Heterogeneity of 4 Vesta

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Achondrites, Crystallization, Feldspars, Heterogeneity, Meteorites, Meteoritic Composition, Vesta Asteroid, Pyroxenes, Ion Probes, Olivine, Lanthanum, Samarium, Yttrium, Ytterbium, Chondrites

Scientific paper

Eucrites, diogenites, and howardites are generally believed to have been derived from the same source. Although the parent body of these meteorites has not been identified, a strong argument was made in favor of the asteroid 4 Vesta. Noncumulate eucrites are thought to be near-surface basalts, and cumulate eucrites plutonic dikes. Despite the debate of whether eucrites are the products of either fractional crystallization of more magnesian liquids or low-pressure partial melting of an olivine and pyroxene and plagioclase assemblage, most investigators believe that noncumulate and cumulate eucrites are closely related. This concept was first challenged by the Schmitt group, who argued that the magmas with which cumulate eucrites may have equilibrated were LREE-rich and that such melts could not be related to noncumulate eucrites by fractional crystallization. Unfortunately, their plagiociase separates were contaminated by pyroxene; furthermore, pyroxene was not included in their study. As cumulate eucrites experienced extensive thermal metamorphism, it is legitimate to ask whether their results truly reflect cumulate eucrite characteristics or whether they are the consequence of subsolidus equilibration. Here we studied individual grains of pyroxene and plagioclase, the first two minerals to crystallize from a eucritic melt, in an attempt to derive independent information about the nature of the parent melts they grew in contact with. We made more than 400 in situ ion microprobe measurements of plagioclase and pyroxene in 12 eucrites representing all the chemical groups previously defined (Stannem and Ibitira) from the Stannem trend, Nuevo Laredo and Lakangaon from the Nuevo Laredo trend, Chervony Kut and Sioux County from the main group, the Moore County and Moama cumulate eucrites, the mainly unequilibrated eucrites Pasamonte and ALHA76005, and the anomalous eucrites Peramiho and Pomozdino). We evaluate the effects of subsolidus equilibration and examine the relationships between noncumulate and cumulate eucrites. Our study supports the suggestion that noncumulate and cumulate eucrites are not genetically related.

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