Formation conditions and chemical composition of early-condensed olivine in the solar protoplanetary disk

Abstract

The dynamic evolution of the solar protoplanetary disk led to the formation of the first solar system solids by condensation, among which amoeboid olivine aggregates (AOAs) are the most abundant. The conditions under which AOAs formed remain poorly constrained, with both equilibrium condensation under sub-solar dust/gas ratios and out-of-equilibrium condensation having been proposed. The recent identification of metal–olivine inclusions (MOIs) in carbonaceous chondrites (CCs) provides a unique opportunity to investigate the conditions of forsterite condensation. Our petrographic survey confirms the frequent occurrence of MOIs in CCs. Their occasional association with AOAs, together with the high MnO/FeO ratios and the ¹⁶O-rich composition of forsteritic rims, indicates formation by gas–solid condensation early in the disk's history. These forsteritic rims are chemically depleted in refractory elements such as Ca, Al, and Ti. Combined with existing literature data, our findings support the condensation of both AOAs and MOIs under out-of-equilibrium conditions during transient, localized heating events in a thermally heterogeneous and dynamically evolving disk. A comparison with relict olivine grains in type I chondrules suggests that AOA-MOI association represent plausible type I chondrule precursors. These results question models proposing that Ca–Al–Ti-rich refractory forsterite necessarily predates chondrule formation. Instead, we suggest that Ca–Al–Ti-poor olivine may better preserve information about the nature and conditions of early chondrule precursors.