Data from: Disequilibrium oxygen isotope distribution among aqueously altered minerals in Ryugu asteroid returned samples
Data files
Apr 18, 2024 version files 39.76 MB
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A0058-C1002_RLx10_spot.pdf
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C0002-C1001_RLx10_spot.pdf
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MCFC_CycleData.csv
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MCFC_RawResult.csv
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README.md
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SEM_SIMSpit_Breunnerite.pdf
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SEM_SIMSpit_Dolomite.pdf
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SEM_SIMSpit_magnetite.pdf
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.
README: 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.