Nucleation Constraints Lead To Molten Chondrule Precursors in the Early Solar System

Abstract

This paper is on an application of nucleation theory and an empirical extension of the quasichemical solution theory of Guggenheim to an ancient bit of nature and to a long standing problem on the origins of meteorites. Chondritic meteorites, which were formed 4.6 billion years ago, contain solid chondrules, which appear to have been liquid silicate droplets formed in a rainlike process in the solar nebula with a gas of solar composition. Such droplets have been experimentally subcooled up to 500 °C below their liquidus temperatures. Just as in terrestrial clouds of subcooled water, silicate droplets should readily form metastably below their liquidus temperatures. Calculations of the condensation of these metastable droplets from a solar gas with an expected nucleation constraint on the formation of metallic iron lead to calculated compositions of chondrules at two different temperatures for each particular pressure that emulate the compositions of two types of porphyritic chondrules in the well-studied meteorite Semarkona, a minimally metamorphosed (or automorphosed), i.e., “most primordial”, chondrite. The two types of chondrules, types IA and II, have very different concentrations of FeO, which are well above the equilibrium concentration because of the nucleation constraint on the formation of metallic iron. The high supersaturation of iron in the gas leads to much higher levels of iron oxide in the chondrules than would be present at equilibrium by driving the reaction H2O + Fe ↔ H2 + FeO to the right. These concepts, if correct, can explain a major part of the chemistries of primordial meteorites and the different relative sizes of the metallic cores of planets. The physics of nucleation of metallic iron and of the crystallization of silicates governs the chemistry of iron compounds such as FeO and leads to metastable liquid chondrule droplets. These significant physicochemical processes explain the origins of two chondrule classes.