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 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.

含水蚀变碳质球粒陨石中磁铁矿与碳酸盐的氧三同位素比值，为探究小行星母体流体的演化过程提供了关键线索。本研究利用二次离子质谱仪（secondary ion mass spectrometer, SIMS），对小行星龙宫（Ryugu）返回样品的两个切片A0058与C0002中的磁铁矿、白云石及菱锰铁矿（breunnerite）开展了氧三同位素分析。针对磁铁矿的分析采用较低的一次离子能量，以降低晶体取向效应带来的仪器偏差。通过SIMS溅射坑的形貌特征，我们将磁铁矿数据划分为两组：(1) δ¹⁸O值较高（3‰~7‰）、Δ¹⁷O值约为2‰，其SIMS溅射坑多呈多孔状，主要来自球粒状磁铁矿（spherulitic magnetite）；(2) δ¹⁸O值较低（约-3‰）、Δ¹⁷O值存在波动（0~2‰），主要来自自形磁铁矿（euhedral magnetite）。白云石与菱锰铁矿的分析采用多接收法拉第杯检测器（multi-collection Faraday cup detectors），分析精度≤0.3‰。由于龙宫样品中的白云石锰含量高于地球参考标准物质，因此我们通过两种方法（基于Fe含量以及(Fe+Mn)含量）对仪器偏差随碳酸盐成分的变化进行校正。白云石与菱锰铁矿的分析结果显示其Δ¹⁷O值分布范围较窄：A0058切片中的白云石Δ¹⁷O值为0.0~0.3‰，C0002切片中的白云石与菱锰铁矿Δ¹⁷O值为0.2~0.8‰。多数菱锰铁矿（包括粒径≥100μm的大颗粒）的δ¹⁸O值系统性低于白云石：根据仪器偏差校正方案的不同，白云石的δ¹⁸O值分别为25~30‰与23~27‰，而菱锰铁矿的δ¹⁸O值约为21‰。基于A0058切片中粗粒岩性的磁铁矿与白云石，我们通过计算得到其平衡温度分别为51±11℃与78±14℃，具体数值取决于白云石的仪器偏差校正方案；目前尚无含锰白云石标准物质用于评估该校正方案，因此暂无法得到可靠的温度估算结果。上述结果表明，龙宫小行星母体中的水流体氧同位素组成在空间或时间上均存在非均质性。吸积至龙宫母体的初始水冰Δ¹⁷O值可能大于2‰，该冰发生熔融后与初始Δ¹⁷O值<0‰的无水固体发生了同位素交换反应。在水蚀变的早期阶段，球粒状磁铁矿与方解石（calcite）由Δ¹⁷O约为2‰的水流体形成，该流体由Δ¹⁷O>2‰的水与Δ¹⁷O<0‰的无水固体经同位素交换反应生成。白云石、菱锰铁矿与部分磁铁矿形成于水蚀变的后期阶段，当时水岩比更高，流体与固相之间的氧同位素组成几乎达到平衡。结合已发表的文献数据，碳酸盐的δ¹⁸O值按方解石、白云石、菱锰铁矿的顺序依次降低，这表明蚀变温度可能随水蚀变程度的加深而升高。