Mineralogy and oxygen isotope systematics of magnetite grains and a magnetite-dolomite assemblage in hydrated fine-grained Antarctic micrometeorites

Abstract

We report the mineralogy and texture of magnetite grains, a magnetite-dolomite assemblage, and the adjacent mineral phases in five hydrated fine-grained Antarctic micrometeorites (H-FgMMs). Additionally, we measured the oxygen isotopic composition of magnetite grains and a magnetite-dolomite assemblage in these samples. Our mineralogical study shows that the secondary phases identified in H-FgMMs have similar textures and chemical compositions to those described previously in other primitive solar system materials, such as carbonaceous chondrites. However, the oxygen isotopic compositions of magnetite in H-FgMMs span a range of ∆17O values from +1.3‰ to +4.2‰, which is intermediate between magnetites measured in carbonaceous and ordinary chondrites (CCs and OCs). The δ18O values of magnetites in one H-FgMM have a ~27‰ mass-dependent spread in a single 100 × 200 μm particle, indicating that there was a localized control of the fluid composition, probably due to a low water-to-rock mass ratio. The ∆17O values of magnetite indicate that H-FgMMs sampled a different aqueous fluid than ordinary and carbonaceous chondrites, implying that the source of H-FgMMs is probably distinct from the asteroidal source of CCs and OCs. Additionally, we analyzed the oxygen isotopic composition of a magnetite-dolomite assemblage in one of the H-FgMMs (sample 03-36-46) to investigate the temperature at which these minerals coprecipitated. We have used the oxygen isotope fractionation between the coexisting magnetite and dolomite to infer a precipitation temperature between 160 and 280 °C for this sample. This alteration temperature is ~100–200 °C warmer than that determined from a calcite-magnetite assemblage from the CR2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibrated OCs, CIs, and CMs. This suggests that the sample 03-36-46 could come from a parent body that was large enough to attain temperatures as high as the OCs, CIs, and CMs, which implies an asteroidal origin for this particular H-FgMM.