FeO‐rich rims and veins in Allende forsterite: Evidence for high temperature condensation at oxidizing conditions

Abstract

Forsteritic olivine grains in the Allende meteorite are commonly rimmed by FeO-rich olivine. New evidence is presented in this paper that the fayalitic rims formed by condensation from a gas and not by thermal equilibration of forsterite with FeO-rich metal or FeO-rich olivine in the interior of a parent body. A similar origin is inferred for fayalitic veins within forsterite crystals.A good correlation ofFeO with MnO was observed along profiles from forsteritic cores to fayalite-rich rims, excluding oxidation of metal as a source of rim FeO. The fayalite content of the rim is generally lower than that of the adjacent matrix olivines excluding formation of FeO-rich rims by equilibration with present matrix. This is also supported by systematic differences in Cr between rim and matrix olivines.Chromites, associated with FeO-rich olivine, were found at the boundary between forsterite and fayalite-rich rim, within the rim itself and in fayalitic veins. Condensation of chromite from a gas phase is likely for the latter two occurrences since no other suitable source of Cr is available. Chromite at the forsterite-fayalite interface was formed by diffusion of Cr from forsterite. Increasingly oxidizing conditions, apparently enabled formation of chromite.The steep compositional gradient between forsterite and fayalite provides severe constraints on the thermal history of forsterite grains. The width of oxidized zones (halos) around metal inclusions in the interior of forsterite grains are not compatible with this steep gradient. Their development requires longer diffusion times than would be allowed by the rims, indicating that these forsterite grains must have had an independent history of oxidation prior to formation of the rim.Condensation calculations indicate that FeO-rich olivine can be formed by condensation from a gas with enhanced oxygen fugacity. Increasing the oxygen fugacity would provide conditions allowing the thermodynamic stability of chromite.High temperatures and oxidizing conditions must have prevailed for some time in that part of the solar nebula where the Allende meteorite formed.

Cometary Refractory Grains: Interstellar and Nebular Sources

“…3; Palme and Fegley 1989;Weinbruch et al 1990). Vaporization of a region enriched in dust compared to solar composition yields raises the f O 2 so that slightly Fe-enriched grains can condense (Fedkin and Grossman 2006;Sect.…”

Metasomatism in the Early Solar System: The Record from Chondritic Meteorites

“…15.20c,d; see also Figs. 1-2 in Kimura and Ikeda 1995;Hua et al 1988;Weinbruch et al 1990), (3) a replacement product of Mg-rich, low-Ca pyroxene phenocrysts and magnetite nodules (Figs. 15.20bÀd,4; see also Figs.…”

“…Nebular and asteroidal models have been proposed to explain the origin of secondary minerals in chondrules, matrices, and dark inclusions in CV OxA chondrites (e.g., Housley and Cirlin 1983;Johnson et al 1990;Weinbruch et al 1990;Kojima and Tomeoka 1996;Brearley and Prinz 1996;Buchanan et al 1997;Dohmen et al 1998;Brearley 1997a;Krot et al 1997aKrot et al , 2004b. The nebular scenario involves condensation from a hot (>1,450 K), highly oxidizing nebular gas produced by evaporation of local nebular regions with high (silicate dust + water ice)/gas ratios (e.g., Weinbruch et al 1990;Johnson et al 1990;Dohmen et al 1998).…”

“…The nebular scenario involves condensation from a hot (>1,450 K), highly oxidizing nebular gas produced by evaporation of local nebular regions with high (silicate dust + water ice)/gas ratios (e.g., Weinbruch et al 1990;Johnson et al 1990;Dohmen et al 1998). This mechanism, however, is inconsistent with (1) the petrographic observations described above (e.g., postaccretionary formation of ferroan olivine and ferroan diopside -hedenbergite pyroxene veins, chondrule pseudomorphs, and Ca-rich rims around Allende dark inclusions); (2) the presence of abundant, randomly oriented (i.e., not crystallographically controlled) inclusions (< 100 nm) of pentlandite, poorly-graphitized carbon (PGC), chromite (FeCr 2 O 4 ), and ilmenite in ferroan olivine and ferroan diopside -hedenbergite pyroxenes in the matrix and dark inclusions in Allende 7 (Brearley and Prinz 1996;Brearley 1997a) (Fig.…”

Characterization of individual atmospheric particles by element mapping in electron probe microanalysis

“…The diameter of each particle is taken as the equivalent diameter derived from SE images (see above). Table 1 of [18] for details), the mean values determined with conventional particle analysis procedures, and those returned by the method outlined above.…”