Origin of isolated olivine grains in carbonaceous chondrites

TL;DR

This study investigates isolated olivine grains in carbonaceous chondrites, revealing they originated from chondrules and provide evidence of frequent collisions during chondrule formation.

Contribution

It provides detailed mineralogical and isotopic analysis showing that isolated olivine grains derive from chondrules and record collisional processes in the early solar system.

Findings

IOG derived from chondrules based on petrographic and isotopic similarity.

CL zoning indicates interaction with gas, suggesting partial melting and splash-out.

Refractory forsterites with specific isotopic signatures escaped equilibration.

Abstract

We report microscopic, cathodoluminescence, chemical and O isotopic measurements of FeO-poor isolated olivine grains (IOG) in the carbonaceous chondrites Allende (CV3), Northwest Africa 5958 (C2-ung), Northwest Africa 11086 (CM2-an), Allan Hills 77307 (CO3.0). The general petrographic, chemical and isotopic similarity with bona fide type I chondrules confirms that the IOG derived from them. The concentric CL zoning, reflecting a decrease in refractory elements toward the margins, and frequent rimming by enstatite are taken as evidence of interaction of the IOG with the gas as stand-alone objects. This indicates that they were splashed out of chondrules when these were still partially molten. CaO-rich refractory forsterites, which are restricted to $\Delta^{17}O < -4\permil$ likely escaped equilibration at lower temperatures because of their large size and possibly quicker quenching. The IOG thus bear witness to frequent collisions in the chondrule-forming regions.