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

1Elena Dobrică,2Ryan C. Ogliore,3Cécile Engrand,4Kazuhide Nagashima,1Adrian J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13366]
1Department of Earth and Planetary Sciences MSC03‐2040, 1 University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
2Department of Physics, Washington University in St. Louis, St. Louis, Missouri, 63117 USA
3Centre de Sciences Nucléaire et de Sciences de la Matière, Université Paris Sud, Université Paris‐Saclay, 91405 Orsay Campus, Saint‐Aubin, France
4Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, HI, 96822 USA
Published by arrangement with John Wiley & Sons

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.

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