Implications of Chondrule Formation in a Gas of Solar Composition

Introduction: It is widely believed that chondrules formed in the solar nebula. If this is true then they must have been exposed to a gas of near-solar composition. Here, we explore some implications of chondrule formation in such an environment. The equilibrium distribution of major gas species in a solar composition gas is shown for temperatures relevant to chondrule formation in Fig.1 at a total pressure of 10 bars. We use the abundances of [1] with a portion of the total O removed to form major silicate minerals. It is estimated that few chondrules with liquidus temperatures over 2023 K were completely melted, and few with liquidus temperatures under 1673 K were incompletely melted [2]. However, [3] showed that a short heat pulse, with a peak temperature well above the liquidus, can also produce porphyritic chondrule textures. As a result, the maximum temperature of chondrule formation is estimated to be ~2373 K. Chondrule precursors are thought to have equilibrated with the nebular gas down to 650 K, the 50% condensation temperature for S in a canonical solar nebula [4]. We focus on the most abundant species in the gas phase: H2, CO, H2O, and the S-bearing species H2S, HS, and S. H2: Many type-I chondrules contain large Fe-based metal grains. Some of these grains are trapped at the chondrule boundary, with a significant surface area exposed to any external fluid. If chondrules formed in the solar nebula than these metal grains would have been exposed to H2(g). At the highest temperatures at which chondrules are thought to have formed (2273 K) [3], significant concentrations of H(g) may have also been present. The ability of metals to absorb hydrogen has been known since the mid 1800s [5]. The properties of metal-hydrogen systems have been the subject of intense research because small concentrations of H in metal results in significant changes in mechanical and metallurgical properties, particularly the brittleness of stainless steels and other industrial metals [6]. Within a metal, hydrogen molecules are dissociated and hydrogen atoms occupy interstitial sites in the host-metal lattice. Hydrogen atoms jump from one interstitial site to a neighboring vacant one and diffuse this way over large distances through the metal, resulting in rapid H saturation in a metal grain. The concentration of H in metal (XH) is