DOI: 10.5061/dryad.dv41ns246
title: Data from: Disequilibrium oxygen isotope distribution among aqueously altered minerals in Ryugu asteroid returned samples

description: 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 &gt;2‰ that was melted and interacted with
 anhydrous solids with the initial ∆17O &lt;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 &gt;2‰) and anhydrous solids (∆17O &lt;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.
description: # 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 (cps) from asc file] + [17O
 bkg adjustment] * 18O (cps) corrected for bkg: [18O (cps) from asc file] +
 [18O bkg adjustment] * d18O_RAW: [18O (cps) corrected for bkg] / [16O
 (cps) from asc file], "N/A" indicate the rejected cycles *
 d17O_RAW no tau correction:[17O (cps) corrected for bkg] / [16O (cps) from
 asc file], "N/A" indicate the rejected cycles * ∆'17O_RAW
 no tau correction:∆'17O calculated from [d18O_RAW] and [d17O_RAW no
 tau correction], "N/A" indicate the rejected cycles * 17O
 tau-cor ‰: time correction for 17O in each cycle in ‰ unit * d17O_RAW tau
 corrected:[d17O_RAW no tau correction]+[17O tau-cor ‰], "N/A"
 indicate the rejected cycles * ∆'17O_RAW tau corrected:∆'17O
 calculated from [d18O_RAW] and [d17O_RAW tau corrected], "N/A"
 indicate the rejected cycles ##### File "MCFC\_RawResult.csv"
 contains mean raw isotope ratios of each SIMS analysis from the values in
 MCFC\_CycleData.cvs" * 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 *
 d18O_RAW:　mean raw measured d18O of each analysis * d18O_RAW 2SE: twice
 the standard error of the mean for raw measured d18O * d17O_RAW: mean raw
 measured d17O of each analysis * d17O_RAW 2SE: twice the standard error of
 the mean for raw measured d17O * ∆'17O_RAW: raw measured ∆'17O
 calculated from [d18O_RAW] and [d17O_RAW] * ∆'17O_RAW 2SE: twice the
 standard error of the mean for raw measured ∆'17O * erorr correl:
 error correlation between d18O_RAW and d17O_RAW * n d18: numbers of cycles
 to calculate the mean raw measured d18O * n d17: numbers of cycles to
 calculate the mean raw measured d17O * n ∆'17: numbers of cycles to
 calculate the mean raw measured ∆'17O Files,
 SEM_SIMSpit_Breunnerite.pdf, SEM_SIMSpit_Dolomite.pdf, and
 SEM_SIMSpit_magnetite.pdf, contain SEM images of SIMS pits. Files,
 A0058-C1002_RLx10_spot.pdf and C0002-C1001_RLx10_spot.pdf are the
 reflected light microscope images that also show the locations of SIMS
 analyses on the sections A0058-C1002 and C0002-C1001, respectively.