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

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 me..., , , # Disequilibrium oxygen isotope distribution among aqueously altered minerals in Ryugu asteroid returned samples ## Description of the data and file structure ##### File \"MCFC\_CycleData.csv\" contains raw data for SIMS oxygen isotope analyses of carbonate and tau correction parameters. * File: text filename of each analysis * Comment: Standard mount names, standard names, and grains for standard. Section names and spot names for unknown * analysis #: analysis number * cycles: cycle number (1-20), cycle 0 correcponds to Faraday Cup (FC) baseline measured during presputtering * 16O (cps) from asc file: 16O intensity recored in [File] * 17O (cps) from asc file: 17O intensity recored in [File] * 18O (cps) from asc file: 18O intensity recored in [File] * 17O bkg adjustment: Difference of FC baseline for 17O relative to the mean of 8 analyses * 18O bkg adjustment: Difference of FC baseline for 18O relative to the mean of 8 analyses * 17O (cps) corrected for bkg: [17O (...