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

Abstract

Abstract-Magmatic inclusions occur in type I1 ureilite clasts (olivine-orthopyroxene-augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar a1 Gani (DaG) 3 19 polymict ureilite. Type I ureilite clasts (olivine-pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al. (2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component-enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields.DaG 3 19 also contains felsic lithic clasts that represent various types of igneous lithologes. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene-metal and pyroxene-plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene-metal crystallization trend corresponds to the magmatic inclusions in the type 11 ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene-plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body, The felsic clasts may have formed mainly from albite component-rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali-bearing chondritic precursors. However, type I1 ureilites may have formed as cumulates from a basaltic melt.

Osmium isotope systematics of ureilites

“…2). The polymict ureilite DaG 319, which contains feldspathic lithologies that may be derived from the missing silicate melts from the ureilite parent body (Cohen et al, 2004;Downes and Mittlefehldt, 2006;Goodrich et al, 2004;Ikeda and Prinz, 2001;Kita et al, 2004), has 187 Os/ 188 Os of 0.1262 ± 0.0004 (2σ) indistinguishable from the monomict ureilite average [0.1257 ± 0.0024 (1σ)]. Inverse Os concentrations and Os isotopic compositions of the ureilites are (r 2 =0.66) correlated (Fig.…”

Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

“…It contains 4-5% Cl and 0.7-1.4% F, with 0.4-3.4% FeO and 0.2-0.4% Na 2 O (Table 3). Primary phosphates have not been observed in monomict ureilites, although they are abundant in FeO-rich feldspathic lithologies found in polymict ureilites (Ikeda et al 2000;Cohen et al 2004;Goodrich et al 2004).…”

Microdistributions and petrogenetic implications of rare earth elements in polymict ureilites

“…Among the various lithic and mineral clasts found in polymict ureilites, the very few chondrules and chondritic fragments (Jaques and Fitzgerald, 1982;Ikeda et al, 2000) are probably remnants of projectiles that impacted the ureilite parent body and produced the polymict ureilites. The most abundant ureilitic clasts represent the same rock type that is characteristic of most monomict ureilites.…”

“…In addition, there are six known polymict ureilites (North Haig, Nilpena, Dar a1 Gani (DaG) 164/165; DaG 319; Elephant Moraine (EET) 83309 and EET 87720) that contain a variety of mineral or lithic clasts not observed in monomict ureilites. These include mineral clasts of olivine, pyroxene and plagioclase, lithic clasts resembling carbonaceous chondrite matrix and chondrules, a Ca-Al-Ti-rich assemblage similar to the Angra dos Reis achondrite, and feldspathic melt clasts (Jaques andFitzgerald, 1982;Prim etal., 1983Prim etal., , 1986Prim etal., , 1987aPrim etal., ,b, 1988Kallemeyn, 1989, 1991;Grossman, 1997Grossman, , 1998Ikeda et al, 2000). These components may carry important petrogenetic information that is complementary to what can be extracted from the more common monomict ureilites.…”