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A synthetic laser ruby crystal (HD-LR1) is introduced as a new matrix-matched reference material for secondary ionization mass spectrometry (SIMS) analysis of oxygen isotopes in corundum. Laser fluorination isotope ratio mass spectrometry (LF-IRMS) bulk analyses of multiple mg-sized fragments are homogenous, averaging δ18O = +18.40 ± 0.14‰ (95% confidence interval, n = 23) and Δ′17O = −0.368 ± 0.005‰ (as deviation from slope 0.528 for δ′17O vs. δ′18O at 95% conf., n = 11) relative to Vienna Standard Mean Ocean Water (V-SMOW). SIMS spot analyses show homogeneous O-isotopic values at the ng-scale independent of the location in the HD-LR1 single crystal and in four different crystallographic orientations. However, sample surface topography as an artefact of polishing corundum embedded in epoxy creates excess variability in δ18O within ∼100 μm from the edges of the grains. HD-LR1 is a chemical pure crystal with only Cr as a trace component detected at 276 μg g−1 by EPMA, whereas Be, often introduced in artificial gem enhancement, is <0.002 μg g−1 based on SIMS analyses. Therefore, HD-LR1 can also be used as a reference material for Cr, or as a blank for other trace element analyses of corundum by SIMS or LA-ICP-MS. 

Asteroid Ryugu and Ivuna-type carbonaceous meteorites may have originated from the outskirts of the Solar System. 

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 instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, 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 the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. 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 the later stage of aqueous alteration under higher water‐to‐rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ¹⁸O of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.