The effect of aqueous alteration and metamorphism in the survival of presolar silicate grains in chondrites

TL;DR

This study examines how aqueous alteration and metamorphism affect the preservation of presolar silicate grains in chondrites, revealing that less altered, primitive chondrites retain more presolar material, which informs early solar system conditions.

Contribution

It provides a comprehensive overview of presolar grain abundances in primitive chondrites and highlights the impact of aqueous alteration and metamorphism on their survival.

Findings

Presolar silicate grains are more abundant in unequilibrated, primitive chondrites.

Aqueous alteration correlates with decreased presolar grain abundance.

Altered chondrites show biased, lower presolar grain content due to their thermal histories.

Abstract

Relatively small amounts (typically between 2-200 parts per million) of presolar grains have been preserved in the matrices of chondritic meteorites. The measured abundances of the different types of grains are highly variable from one chondrite to another, but are higher in unequilibrated chondrites that have experienced little or no aqueous alteration and/or metamorphic heating than in processed meteorites. A general overview of the abundances measured in presolar grains (particularly the recently identified presolar silicates) contained in primitive chondrites is presented. Here we will focus on the most primitive chondrite groups, as typically the highest measured abundances of presolar grains occur in primitive chondrites that have experienced little thermal metamorphism. Looking at the most aqueously altered chondrite groups, we find a clear pattern of decreasing abundance of presolar silicate grains with increasing level of aqueous alteration. We conclude that the measured abundances of presolar grains in altered chondrites are strongly biased by their peculiar histories. Scales quantifying the intensity of aqueous alteration and shock metamorphism in chondrites could correlate with the content in presolar silicates. To do this it would be required to infer the degree of destruction or homogenization of presolar grains in the matrices of primitive meteorites. To get an unbiased picture of the relative abundance of presolar grains in the different regions of the protoplanetary disk where first meteorites consolidated, future dedicated studies of primitive meteorites, IDPs, and collected materials from sample-return missions (like e.g. the planned Marco Polo) are urgently required.