New evidence for wet accretion of inner solar system planetesimals from meteorites Chelyabinsk andBenenitra

TL;DR

This study analyzes hydrogen isotopic compositions and water contents in meteorites to provide evidence for wet accretion processes in the inner solar system, indicating minimal water loss during parent body metamorphism.

Contribution

It offers new isotopic data from recent meteorite falls, demonstrating that inner solar system planetesimals incorporated nebular water, reducing the need for late volatile delivery to terrestrial planets.

Findings

Meteorites Chelyabinsk and Benenitra contain hydrated minerals with D-poor isotopic signatures.

Water in these meteorites is mainly derived from nebular hydrogen, with minimal loss during thermal metamorphism.

Inner Solar System bodies could hold significant water from nebular sources, impacting planetary formation theories.

Abstract

We investigated the hydrogen isotopic compositions and water contents of pyroxenes in two recent ordinary chondrite falls, namely, Chelyabinsk (2013 fall) and Benenitra (2018 fall), and compared them to three ordinary chondrite Antarctic finds, namely Graves Nunataks GRA 06179, Larkman Nunatak LAR 12241, and Dominion Range DOM 10035. The pyroxene minerals in Benenitra and Chelyabinsk are hydrated ($\sim$0.018-0.087 wt.$\%$ H$_2$O) and show D-poor isotopic signatures ($\delta$D$_{SMOW}$ from -444$\unicode{x2030}$ to -49$\unicode{x2030}$). On the contrary, the ordinary chondrite finds exhibit evidence of terrestrial contamination with elevated water contents ($\sim$0.039-0.174 wt.$\%$) and values (from -199$\unicode{x2030}$ to -14$\unicode{x2030}$). We evaluated several small parent body processes that are likely to alter the measured compositions in Benenitra and Chelyabinsk, and inferred that water-loss in S-type planetesimals is minimal during thermal metamorphism. Benenitra and Chelyabinsk hydrogen compositions reflect a mixed component of D-poor nebular hydrogen and water from the D-rich mesostases. 45-95$\%$ of water in the minerals characterized by low $\delta$D$_{SMOW}$ values was contributed by nebular hydrogen. S-type asteroids dominantly composed of nominally anhydrous minerals can hold 254-518 ppm of water. Addition of a nebular water component to nominally dry inner Solar System bodies during accretion suggests a reduced need of volatile delivery to the terrestrial planets during late accretion.