Multiple formation mechanisms of ferrous olivine in CV carbonaceous chondrites during fluid-assisted metamorphism

Abstract

The CV carbonaceous chondrites experienced alteration that resulted in formation of secondary ferrous olivine (Fa40-100), salite-hedenbergite pyroxenes (Fs10-50Wo45-50), wollastonite, andradite, nepheline, sodalite, phyllosilicates, magnetite, Fe,Ni-sulfides and Ni-rich metal in their Ca,Al-rich inclusions, amoeboid olivine ag-gregates, chondrules, and matrices. It has previously been suggested that fibrous ferrous olivine in dark inclusions in CV chondrites formed by dehydration of phyllosilicates during thermal metamorphism (T. Kojima and K. Tomeoka, Geochim. Cosmochim. Acta, 60, 2651, 1996; A.N. Krot et al., Meteoritics, 30, 748, 1995). This mechanism has been subsequently applied to explain the origin of ferrous olivine in the CV chondrules and matrices (A.N. Krot et al., Meteoritics, 32, 31, 1997). It is, however, inconsistent with the lack of significant fractionation of bulk oxygen isotope compositions of the CV chondrites and the Allende dark inclusions and the common occurrences of ferrous olivine in the aqueously-altered and virtually unmetamorphosed oxidized CV chondrites of the Bali-like subgroup. Based on the petrographic observations and the isotopic compositions of ferrous olivine and coexisting Ca,Fe-rich silicates in CV chondrites and their dark inclusions, we infer that ferrous olivine formed during a fluid-assisted metamorphism by several mechanisms: (i) replacement of Fe,Ni-metal±sulfide nodules, (ii) replacement of magnesian olivine and low-Ca pyroxene, and (iii) direct precipitation from an aqueous solution. Dehydration of phyllosilicates appear to have played only a minor (if any) role. Although our model does not address specifically the origin of ferrous olivine rims around forsterite grains in Allende, the observed homogenization of matrix olivines (which have comparable sizes to thicknesses of the ferrous olivine rims in Allende) from Kaba to Allende suggests that compositions of ferrous olivine rims in Allende cannot be primary and must have been modified by asteroidal alteration