Chondrule heritage and thermal histories from trace element and oxygen isotope analyses of chondrules and amoeboid olivine aggregates

TL;DR

This study combines oxygen isotope and trace element analyses of chondrules and amoeboid olivine aggregates in a carbonaceous chondrite to explore their origins and thermal histories, revealing insights into their formation processes.

Contribution

It provides the first LA-ICP-MS trace element data for olivine in amoeboid olivine aggregates and links isotopic and geochemical signatures to formation conditions.

Findings

Olivine in AOA shows a condensation origin based on trace element geochemistry.

Correlation between 16O enrichment and incompatible elements in chondrules.

Evidence suggests varying olivine crystallization during chondrule formation, influenced by local solid/gas ratios.

Abstract

We report combined oxygen isotope and mineral-scale trace element analyses of amoeboid olivine aggregates (AOA) and chondrules in ungrouped carbonaceous chondrite Northwest Africa 5958. The trace element geochemistry of olivine in AOA, for the first time measured by LA-ICP-MS, is consistent with a condensation origin although the shallow slope of its rare earth element (REE) pattern has yet to be physically explained. Ferromagnesian silicates in type I chondrules resemble those in other carbonaceous chondrites both geochemically and isotopically, and we find a correlation between 16O enrichment and many incompatible elements in olivine. The variation in incompatible element concentrations may relate to varying amounts of olivine crystallization during a sub-isothermal stage of chondrule-forming events, the duration of which may be anticorrelated with the local solid/gas ratio if this was the determinant of oxygen isotopic ratios as recently proposed. While aqueous alteration has depleted many chondrule mesostases in REE, some chondrules show recognizable subdued group II-like patterns supporting the idea that the immediate precursors of chondrules were nebular condensates.