Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Y. Ikeda et M. Prinz, Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite, METEORIT PL, 36(4), 2001, pp. 481-499
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
1086-9379 → ACNP
Year of publication
481 - 499
SICI code
Magmatic inclusions occur in type II ureilite clasts (olivine-orthopyroxene -augite assemblages with essentially no carbon) and in a large isolated pla gioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureil ite clasts (olivine-pigeonite assemblages with carbon), as well as other li thic and mineral clasts in this meteorite, are described in Ikeda et al. (2 000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyrox enes, resulting in feldspar component-enriched glass. On the other hand, th e magmatic inclusions in the large plagioclase clast consist mainly of pyro xene and plagioclase, with a mesostasis. They crystallized with a compositi on along the cotectic line between the pyroxene and plagioclase liquidus fi elds. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain pheno crysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or gla ss. Crystallization of these porphyritic clasts took place along the cotect ic line between the pyroxene and plagioclase fields. Pilotaxitic felsic cla sts crystallized plagioclase laths and minor interstitial pyroxene under me tastable conditions, and the mesostasis is extremely enriched in plagioclas e component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyrox ene-metal and pyroxene-plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene-metal crystallization trend c orresponds to the magmatic inclusions in the type II ureilite clasts and th e pilotaxitic felsic clasts, where crystallization took place under reducin g and metastable conditions, suppressing precipitation of plagioclase. The pyroxene-plagioclase crystallization trend corresponds to the magmatic incl usions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within t he ureilite parent body. The felsic clasts may have formed mainly from albi te component-rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fra ctional partial melting of alkali-bearing chondritic precursors. However, t ype II ureilites may have formed as cumulates from a basaltic melt.