[M-TT44] Frontiers in Geochemistry

[MTT44-04] Rare-earth element imaging of fine-grained calcium and aluminum-rich inclusions in Allende meteorite

*Yuki Masuda¹, Sota Niki², Tetsuya Yokoyama¹, Takafumi Hirata² (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.Geochemical Research Center, Graduate School of Science, The University of Tokyo)

Keywords:cosmochemistry, CAI, element imaging, LA-ICP-MS

Calcium and aluminum-rich inclusions (CAIs) in chondrites are one of the oldest materials in the Solar System. The CAIs generally consist of several refractory minerals that are presumably produced through direct condensation from a nebular gas of solar abundances (e.g., Yoneda and Grossman 1995, Lodders et al. 2003). Unlike coarse-grained CAIs that have experienced remelting after their formation, fine-grained CAIs (FGs) are considered to have eluded the remelting. Therefore, the chemical and isotopic compositions in FGs would provide key information regarding the high-temperature processes associated with the condensation of refractory inclusions. One such example is the abundances of rare earth elements (REE) in FGs. The CI-normalized REE abundance patterns for FGs show a characteristic depletion in heavy-REEs (e.g., Er, or Lu) against light-REEs (e.g., La, Nd) (Group Ⅱ pattern: Mason and Tayor 1982). The fractionated REE abundance pattern for the FGs is explained either by REE partitioning into ultra-refractory minerals including hibonite (Davis et al. 2018), or by re-evaporation and re-condensation (Hu et al. 2021). Hu et al. (2021) also pointed out that the re-evaporation was associated with the earliest thermal events in the solar system such as FU Orionis. To decipher the processes responsible for generating individual REE patterns in FGs, determination of REE distributions in individual FG minerals is indispensable. However, most of the previous studies performed chemical/isotopic analyses using bulk FGs or fragments of FGs, which provided mean compositions of multiple mineral phases in FGs.
In this study, we performed an imaging analysis of REEs for two FGs (FG1 and FG2) in Allende meteorite using a LA-ICP-MS (LA system: Jupiter Solid Nebulizer, S. T. Japan Inc.; ICP-MS system: iCAP-TQ, Thermo Fisher Scientific) installed at the Univ. of Tokyo. Imaging analysis using the LA-ICP-MS has an advantage for obtaining data from a wider range within a shorter time. The laser imaging conducted from a 300 µm × 300 µm region took about 1 h. In the imaging, the abundances of major elements (MEs) and REEs were measured, and the resulting imaging data for the REEs were compared with the abundances of MEs, as well as the mineral distribution obtained from the SEM-EDS analysis previously performed in Tokyo Tech.
Fractionated REE patterns were observed for four measured regions in a spinel-rich inclusion FG1. The averaged REE patterns for the four regions were similar to each other. The highest REE abundances were found in the Ti-rich calcic pyroxene, and the abundances decreased in the order of Ti-poor calcic pyroxene, spinel, and secondary minerals (nepheline and hedenbergite). FG2 had a complicated layered structure. The rim was composed of two layers with olivine (outer rim) and aggregates of spinel and calcic pyroxene (inner rim), while the core was composed of fine-grained calcic pyroxene and nepheline. The FG2 core had a fractionated REE pattern that resembles that of FG1. However, the outer rim did not show fractionated REE pattern, and the inner-rim showed a unique REE pattern enriched in Gd to Ho. The heterogeneous REE distribution within and across individual FGs, as well as the unique REE abundance pattern with Gd–Ho enrichments, are difficult to explain only by a simple condensation model that resulted in equilibrium distribution of REEs among constituting minerals. These results suggest the REE partitioning to minerals in addition to hibonite or the thermal diffusion of REEs in the FGs. On the other hand, the differences in REE patterns among core-rims in individual FGs indicate that some of the FGs can be formed through condensation from multiple reservoirs.