^1.2Ming-Chang Liu,²Nozomi Matsuda, ²Kevin D. McKeegan, ^1,2Emilie T. Dunham, ^1,2Kaitlyn A. McCain
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.08.015]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
Copyright Elsevier

Chemical and isotopic measurements of HIDALGO, a stoichiometrically pure hibonite inclusion found in the matrix of the Dar al Gani 027 meteorite, were conducted by secondary ion mass spectrometry to investigate its origin and evolution. HIDALGO is characterized by large mass-dependent isotope fractionations in O, Ca, and Ti, as well as large negative anomalies in neutron-rich ⁴⁸Ca and ⁵⁰Ti, making it the newest member of the HAL-type FUN inclusions. The highly fractionated Ca and Ti isotopes but unfractionated Mg isotopes are consistent with HIDALGO being a residue from an extensive evaporation event, during which large fractions of initial Ca and Ti, and essentially all the initial Mg, in the precursor material were lost. HIDALGO appears to have incorporated live ²⁶Al at a higher level than other HAL-type inclusions, but still at a lower amount compared to the Solar System’s initial ²⁶Al abundance typically found in non-FUN CAIs. Interestingly, the inferred ¹⁰Be abundance in HIDALGO is comparable to the values observed in the majority of CV3 CAIs but ∼ 2.5 times higher than those in HAL-type samples. HIDALGO’s unusual ²⁶Al/²⁷Al and ¹⁰Be/⁹Be ratios, together with the ⁴⁸Ca-⁵⁰Ti anomalies, can be best explained by the formation of its precursor material in the isotopically heterogeneous solar nebula. Finally, large ⁷Li excesses correlating with Be/Li were found in HIDALGO, a behavior that can be interpreted as due to in-situ decay of live ⁷Be. Charged particle spallation of initially Li-free HIDALGO can simultaneously account for the inferred ⁷Be abundance and the measured Li elemental concentration. The consistency between the measurement and spallation calculation results provides support for the prior existence of ⁷Be in HIDALGO, possibly produced by irradiation close to the Sun

¹Shingo Sugawara, ¹Wataru Fujiya, ²Noriyuki Kawasaki, ³Naoya Sakamoto, ⁴Akira Yamaguchi, ²Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.08.013]
¹Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴National Institute of Polar Research, Midoricho10-3, Tachikawa, Tokyo 190-8518, Japan
Copyright Elsevier

Aqueous alteration in planetesimals is one of the earliest geological processes in the solar system. The timing of aqueous alteration sheds light on the timescale of material evolution through water–rock interaction in small bodies. The 53Mn-53Cr decay system, where a short-lived radionuclide 53Mn decays to 53Cr with a half-life of 3.7 Myr, is a powerful tool for dating carbonates in primitive meteorites that formed during aqueous alteration. In CI chondrites and samples returned from asteroid Ryugu, a major carbonate mineral is dolomite (CaMg(CO3)2) and could be dated precisely because of their relatively high Mn abundances. However, the lack of a proper dolomite standard for secondary ion mass spectrometry (SIMS) hinders us from obtaining accurate Mn/Cr ratios of carbonates, resulting in erroneous formation ages. In this work, we synthesized Mn-, Cr-, and Fe-bearing crystalline dolomite as standard materials, and evaluated the relative sensitivity factor (RSF) of Mn/Cr for SIMS analysis, namely, the ratio of Mn/Cr obtained using SIMS to true Mn/Cr. We found that the RSF values of the dolomite standards range from 0.8 to 0.9, slightly higher than that of calcite (CaCO3) (∼0.7), and increase with their Fe contents. We used the newly evaluated RSF values to date dolomite in the Ivuna CI chondrite and obtained an initial 53Mn/55Mn ratio of (3.95 ± 0.49) × 10−6 (95 % confidence interval) and the corresponding absolute age of 4564.0 + 0.6/−0.7 Ma. Our new initial 53Mn/55Mn ratio is 26 ± 19 % higher than that obtained by a previous study for the same dolomite grain using a calcite standard. This difference is consistent with the difference between the RSF values of dolomite and calcite. Based on these results, we updated the initial 53Mn/55Mn ratio previously reported for dolomite in the Ryugu sample A0058 to be (3.21 ± 0.66) × 10−6, which corresponds to an absolute age of 4562.8 + 1.0/−1.2 Ma. This age seems to be the best estimate for the formation age of dolomite in Ryugu currently available.