Secondary Ca-Fe-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions

Details Secondary Ca-Fe-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions Secondary Ca-Fe-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..623k&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 4, pp. 623-645.

Mathematics

Logic

31

Scientific paper

We have characterized Ca-Fe-rich silicates [salite-hedenbergite pyroxenes (Fs10-50Wo45-50), andradite (Ca3Fe2Si3O12), kirschsteinite (CaFeSiO4), and wollastonite (CaSiO3)] in the type I chondrules and matrices in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali-like CV3 chondrites, metal is oxidized to magnetite; magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes with minor andradite and pure fayalite. We infer that Ca-Fe-rich pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (< 300C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si-Fe-Ca-O-H system and large oxygen isotopic fractionation of the coexisting magnetite and fayalite (~ 20 () (Krot et al., 1998) are consistent with this interpretation. In type I chondrules in the Allende-like CV3 chondrites and dark inclusions, magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes and ferrous olivine; low-Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca-Fe-rich pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca-Fe-pyroxene-andradite veins and surrounded by Ca-rich rims consisting of Ca-Fe-rich pyroxenes, andradite, wollastonite, and kirschsteinite. Ca-rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca-Fe-rich pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende-like chondrites and dark inclusions experienced similar aqueous alteration to the Bali-like chondrites and were subsequently metamorphosed resulting in loss of aqueous solutions and dehydration of phyllosilicates. We conclude that Ca-Fe-rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid-rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites.

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