High Rare Earth Element Concentrations in Rare Meteoritic Sulfides May Represent New Residual Condensate from the Early Solar System

Normalized REE abundances in oldhamites from EH and EL chondrites show the positive Eu anomaly in EH3s, indicative of evaporation then condensation, and the negative Eu anomaly in metamorphosed EHs and Els, consistent with formation during metamorphism or magmatism. Credit: Hammouda et al., 2022.

Enstatite chondrites (ECs) are a very reduced (oxygen-poor) type of primitive meteorite in which elements like Fe and S are present mainly as metal and sulfides rather than oxides and sulfates. They are of great interest because they share many isotopic characteristics with bulk Earth and may represent the majority of the material that accreted to form Earth. There are two main types of ECs distinguished by their bulk iron content: EHs (high iron) and ELs (low iron).

A new study of ECs by Tahar Hammouda, Maud Boyet, and Paul Frossard (CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Université Clermont Auvergne) and Camille Cartier (Université de Lorraine) describes an interesting message from oldhamite (CaS), an unusual mineral found in ECs. CaS is rare in less reduced materials because Ca does not have chalcophile behavior (bonding to S) in oxidized systems. Important trace elements, such as the heat-producing elements potassium and thorium, and rare-earth elements (REEs), typically get concentrated into Ca-bearing geologic materials and therefore tend to concentrate in oldhamite.

Hammouda and colleagues measured REE concentrations in oldhamite in ECs using laser ablation inductively coupled mass spectrometry. They found that in the most primitive (unmetamorphosed) EHs, REEs show patterns consistent with condensation from a nebular gas, similar to many fine-grained calcium aluminum inclusions (CAIs), the first solids to form in the solar system. This result suggests a new formation scenario for CaS in EHs – evaporation followed by condensation in the nebula. This contrasts with CaS in primitive ELs and all metamorphosed ECs, which have REE patterns consistent with CaS formation during metamorphism or magmatism. Importantly, these primitive EH CaS grains appear to represent a new early solar system solid that provides direct clues to the chemical environment and processes that occurred in the first few million years of our solar system. READ MORE