Work

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

Year: 2019

Type: article

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 andOCs). The δ18O values of magnetites in one H‐FgMMhave 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 ofCCs andOCs. 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 theCR2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibratedOCs,CIs, andCMs. This suggests that the sample 03‐36‐46 could come from a parent body that was large enough to attain temperatures as high as theOCs,CIs, andCMs, which implies an asteroidal origin for this particular H‐FgMM.

Source: Meteoritics and Planetary Science

DOI: 10.1111/maps.13366

Authors: Elena Dobrică, Ryan C. Ogliore, Cécile Engrand, Kazuhide Nagashima, Adrian J. Brearley

Institutions: University of New Mexico, Washington University in St. Louis, Université Paris-Sud, Université Paris-Saclay, University of Hawaiʻi at Mānoa

Language: English

Topic: Astro and Planetary Science

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: bronze