Trace element and strontium and barium isotopic variability of CM chondrites Aguas Zarcas and NWA 11346: Insights from stepwise acid leaching

Carbonaceous chondrite meteorites represent unmelted but variably metamorphosed aggregates of a diverse range of the earliest solids of the solar system, including matrix, chondrules, calcium-aluminium inclusions and exotic presolar grains. As such, they preserve a record of the inventory of materials that were first brought together in the evolving protoplanetary disk then were subjected to processes such as thermal and aqueous alteration on their parent body. To gain further insights into the elemental and isotopic variability of primitive meteorites and their components, stepwise acid leaching experiments were carried out on pre-rain collected 2019 fall CM2 Aguas Zarcas and anomalous-CM find NWA 11346 carbonaceous chondrites. Pre-rain collected fall samples of primitive meteorites in particular are exceptionally rare; thus the Aguas Zarcas sample provides an excellent opportunity to study materials essentially unaffected by any terrestrial alteration imprint. Acid leachates and bulk samples were analysed for trace elements (including the rare earth elements) via ICP-MS, while Sr and Ba isotopes were measured using TIMS. Trace element variations reveal diverse compositions for bulk samples and their leachates, interpreted to reflect differing parent body histories and the contrasting impacts of terrestrial contamination. Sr isotopic analysis of leach fractions yields variable (primordial) µ84Sr values compared to the NBS987 terrestrial standard, ranging from +326 to -2089 ppm in Aguas Zarcas and from +46 to -559 ppm in NWA 11346. Both Aguas Zarcas and NWA 11346 show the strongest depletions of µ84Sr in the strongest acid leachates; however, the variability of µ84Sr among Aguas Zarcas leachates differs from the same leaching steps for NWA 11346. Bulk meteorite Sr isotopic analysis of Aguas Zarcas and NWA 11346 yields values indistinguishable from the standard. For Ba isotopes, bulk Aguas Zarcas shows resolvable depletions in 130Ba, 135Ba, 137Ba and 138Ba, while NWA 11346 shows similar depletions in 135Ba, 137Ba and 138Ba. Early Aguas Zarcas leachates are characterised by positive anomalies of 130Ba, 132Ba, 135Ba, 137Ba and 138Ba (up to +2295 ppm µ130Ba). In contrast, final leachates of both samples show strong depletions for the same nuclides, the largest of which are -60,000 ppm µ130Ba in Aguas Zarcas and -4370 ppm µ130Ba in NWA 11346. The Sr and Ba isotopic variability between samples and their respective leachates suggests input from multiple nucleosynthetic sources, highlighting the diversity among CM chondrites and the heterogenous distribution of nuclides in the protoplanetary disk.