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

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‐Fg MM s). 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‐Fg MM s 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‐Fg MM s span a range of ∆ 17 O values from +1.3‰ to +4.2‰, which is intermediate between magnetites measured in carbonaceous and ordinary chondrites ( CC s and OC s). The δ 18 O values of magnetites in one H‐Fg MM 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 ∆ 17 O values of magnetite indicate that H‐Fg MM s sampled a different aqueous fluid than ordinary and carbonaceous chondrites, implying that the source of H‐Fg MM s is probably distinct from the asteroidal source of CC s and OC s. Additionally, we analyzed the oxygen isotopic composition of a magnetite‐dolomite assemblage in one of the H‐Fg MM s (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 CR 2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibrated OC s, CI s, and CM s. 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 OC s, CI s, and CM s, which implies an asteroidal origin for this particular H‐Fg MM .