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Data from: Disequilibrium oxygen isotope distribution among aqueously altered minerals in Ryugu asteroid returned samples

Kita, Noriko T.1

Research facility: University of Wisconsin–Madison
Published Apr 18, 2024 on Dryad. https://doi.org/10.5061/dryad.dv41ns246

Data files

Apr 18, 2024 version files 39.76 MB

Abstract

Oxygen 3-isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3-isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher δ18O (from 3‰ to 7‰) and ∆17O (~2‰) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower δ18O (~ –3‰) and variable ∆17O (0-2‰) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi-collection Faraday cup detectors with precisions ≤0.3‰. The dependence of instrumental biases on carbonate compositions was corrected using two methods, using Fe and (Fe+Mn) contents, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of ∆17O; 0.0–0.3‰ for dolomite in A0058 and 0.2–0.8‰ for dolomite and breunnerite in C0002. The majority of breunnerite, including large ≥100 µm grains, show systematically lower δ18O (~21‰) than dolomite (25–30‰ and 23–27‰ depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse-grained lithology in A0058 are calculated to be 51±11˚C and 78±14˚C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn-bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially or temporally. Initial water ice accreted to the Ryugu parent body might have ∆17O >2‰ that was melted and interacted with anhydrous solids with the initial ∆17O <0‰.  In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with ∆17O ~2‰ that was produced by isotope exchange between water (∆17O >2‰) and anhydrous solids (∆17O <0‰). Dolomite and breunnerite along with some magnetite formed at a later stage of aqueous alteration under higher water-to-rock ratios and the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ18O of carbonates decreases in the order of calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.