Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration

Abstract

Abstract-The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali-like (cV3,,~) and d), in which chondrules, calcium-aluminum-rich inclusions (CAIs) and matrices show characteristic alteration features Krot et al., 1997d;Kimura and Ikeda, 1997). The cv30,B lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low-Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni-rich sulfides, fayalite (Fa>go), Ca-Fe-rich pyroxenes (Fs 10-5oW045-50) and andradite. Phyllosilicates replace primary Ca-rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca-Fe-rich pyroxenes and minor andradite. Fayalite veins crosscut fine-grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 "C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large A1*Ofayal,te-magnetlte fractionation (-20%0). We infer that phyllosilicates, magnetite, fayalite, Ca-Fe-rich pyroxenes and andradite formed at relatively low temperatures (<300 "C) by fluid-rock interaction in an asteroidal environment.Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3,d lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low-Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath-shaped grains in matrix. Secondary Ca-Fe-rich pyroxenes are abundant. We infer that the CV3,d lithology experienced alteration at higher temperatures than the cv3,.& lithology. The presence of the reduced and CV3,d lithologies in the Vigarano breccia and W 3 , d and cV3,,~ lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid-rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.

Near infra-red spectroscopy of the asteroid 21 Lutetia

“…It has undergone some aqueous alteration, which has resulted in the formation of intimately intergrown phyllosilicates in the matrix and in the chondrules (Keller & McKay 1993). The lithological studies of CV3 meteorites further divide this oxidized group into two distinct subgroups (Krot et al 1998). Based on the matrix abundance, metal to magnetite ratio, phylosillicates and pure fayalite presence, Grosnaja belongs to the CV3 oxB (Krot et al 1998).…”

Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite‐free spinel inclusions in CM chondrites

“…Under these conditions, melilite altered directly to phyllosilicate. The CM-chondrite style of alteration is different than that undergone by CO chondrites (which experienced fluid-assisted thermal metamorphism; e.g., McSween 1977; Scott and Jones 1990;Rubin 1998;Brearley 2006) or CV chondrites (which experienced oxidation and aqueous metasomatism; e.g., Krot et al 1995Krot et al , 1998aKrot et al , 1998bKrot et al , 2000Brearley 1999). …”

UV-Vis spectroscopy of stardust

“…Spectra were acquired from thin sections of two carbonaceous chondrites, Allende (CV3) and Cold Bokkeveld (CM2). Despite a certain amount of aqueous alteration, the former is known for its large, well-delineated chondrules (Krot et al 1998), whilst the latter is known to have been altered heavily by fluids (Greenwood et al 1994) and is rich in fine-grained matrix which is intimately mixed with a crosslinked macromolecular organic material. The sections analysed were regular thin sections (30 mm thick) mounted on soda-glass slides in Lakeland TM resin.…”