Thermochemical Stability of Low-Iron, Manganese-Enriched Olivine in Astrophysical Environments

TL;DR

This study investigates the stability and formation conditions of low-iron, manganese-enriched olivine in astrophysical environments, revealing its thermodynamic stability in high-temperature, low-pressure solar vapor and its implications for primitive solar system materials.

Contribution

It provides new insights into the thermochemical stability and compositional evolution of LIME olivine in primitive solar system materials and cometary samples.

Findings

LIME olivine is thermodynamically stable in solar vapor at high temperatures.

Enrichment of vapor causes increased FeO/MnO ratio in olivine.

LIME olivine's composition indicates formation in very reducing conditions.

Abstract

Low-iron, manganese-enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission to comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to that of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without reequilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.