Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.546w&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 546
Astronomy and Astrophysics
Astronomy
3
Astronomical Models, Meteoritic Composition, Petrology, Vanadium, Cooling, Crystallization, Melts (Crystal Growth), Textures
Scientific paper
Sioux County (SC) has long been classified as the most 'primitive' of the noncumulate eucrites. Stolper argued for genesis of most eucrites as little-differentiated parital melts, modified only by slight near-surface fractional crystallization. In this model, SC is the type example of a primary eucrite melt, where 'primary' denotes 'a liquid that has not changed since it was generated in its source region by melting'. Recently, Jones et al. suggested that a compositional similarity between SC and a partial melt of Murchison, confirms Stopler's model. This claim prompted us to examine the petrology of SC, which seems to have been unstudied for decades. The texture of SC is in most areas highly brecciated, with sufficient diversity in grain size to raise doubts as to whether the rock is even monomict. We find the pyroxenes are mostly pigeonites that have thoroughly exsolved. The SC bulk composition is displaced from the average noncumulate eucrite toward higher mg and V, and lower Sc and Rare Earth Elements (REE). We interpret SC as an orthocumulate. The parent melt was considerably more evolved than the observed SC composition, and had a slight (-) Eu anomaly. We note a general tendency for the more slowly cooled monomict eucrites, to feature compositions with higher mg, high V, and lower REE than their more rapidly cooled counterparts. Only a relative few eucrites are likely to be compositionally close to the melts from which they crystallized. The same parent body has also yielded many orthopyroxenites with mg ratios implying parent liquids more magnesian than any noncumulate eucrite. Absence of quenched liquids as ferroan as the cumulate parent melts suggests that the cumulates formed fairly deep within the crust.
Kallemeyn Gregory W.
Warren Harry P.
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