Mineralogy, petrology, and oxygen-isotope compositions of magnetite ± fayalite assemblages in CO3, CV3, and LL3 chondrites

We report on the mineralogy, petrology, and O-isotope compositions of magnetite and fayalite (Fa(90-100)) from several metasomatically altered and weakly metamorphosed carbonaceous (Y-81020 [CO3.05], EET 90043 [CO3.1], MAC 88107 [CO3.1-like], and Kaba [oxidized Bali-like CV3.1]) and unequilibrated ordinary chondrites (UOCs; Semarkona [LL3.00], MET 00452 [LL3.05], MET 96503 [LL3.05], EET 910161 [LL3.05], Ngawi [LL3.0-3.6 breccia], and Vicencia [LL3.2]). In MAC 88107, EET 90043, and Kaba, nearly pure fayalite (Fa(98-100)) associates with phyllosilicates, magnetite, Fe,Ni-sulfides, and hedenbergite (Fs(similar to 50)Wo(similar to 50)), and occurs in all chondritic components-chondrules, matrices, and refractory inclusions. In UOCs, nearly pure fayalite (Fa(95-98)) associates with phyllosilicates and magnetite, and occurs mainly in matrices and fine-grained chondrule rims. Oxygen-isotope compositions of fayalite and magnetite in UOCs, COs, CVs, and MAC 88107 are in disequilibrium with those of chondrule olivine and low-Ca pyroxene phenocrysts, and plot along mass-dependent fractionation lines with slope of similar to 0.5, but different Delta O-17 (similar to+4.3 +/- 1.4 parts per thousand, -0.2 +/- 0.6 parts per thousand, -1.5 +/- 1 parts per thousand, and -1.8 +/- 0.8 parts per thousand, respectively). Based on the mineralogical observations, thermodynamic analysis, O-isotope compositions, and recently reported experimental data, we infer that (1) fayalite and magnetite in COs, CVs, MAC 88107, and UOCs resulted from aqueous fluid-rock interaction on the chondrite parent asteroids that occurred at low local water-to-rock mass ratios (0.1-0.4) and elevated temperatures (similar to 100-300 degrees C), and (2) Delta O-17 of fayalite and magnetite reflects O-isotope compositions of aqueous fluids on the host meteorite parent bodies. The observed differences in Delta O-17 of fayalite-magnetite assemblages in UOCs, CVs, COs, and MAC 88107 suggest that water ices that accreted into the ordinary chondrite and carbonaceous chondrite parent asteroids had different Delta O-17, implying spatial and/or temporal variations in O-isotope compositions of water in the protoplanetary disk.