Aqueous alteration of opaque assemblages in the Ningqiang carbonaceous chondrite: Evidence from oxygen isotopes

Chondrules and matrix of the Ningqiang carbonaceous chondrite contain numerous opaque assemblages (OAs). These OAs are mainly composed of Ni-rich metal, magnetite, sulfides, phosphates with minor amounts of silicates and trace amounts of Pt-metal grains. Their mineral assemblage is very similar to that of OAs found in Allende refractory inclusions. In this study, we performed in situ analyses of O-isotopic compositions of magnetite and whitlockite in OAs and of olivine adjacent to OAs from Ningqiang. O-isotopic compositions of magnetite vary by 16‰ in δ¹⁸O and fall on a regression line with a slope of 0.51 and a mean Δ¹⁷O of -2.9 ± 0.3 (1σ)‰ in the three-O-isotope diagram. Magnetite grains have consistently higher Δ¹⁷O values than their adjacent olivine grains. The mineral chemistry and O-isotopic compositions of OAs in Ningqiang chondrules and matrix are most consistent with the formation of magnetites and whitlockite by aqueous alteration of preexisting metal alloys within a planetary body. Formation of magnetites and phosphates in OAs thus do not constrain processes in the early solar nebula, but rather provide information on metamorphism in a planetary environment.