Pyroxene Chondrules from Olivine-depleted, Dust-enriched Systems

Introduction: The principal solid precursor components of chondrules are nebular condensates and/or presolar interstellar grains. In either case, the precursors are most likely dominated by forsterite (Mg2SiO4), iron-nickel metal, Ca-aluminate phases, and a C-rich component [1-5]. It is reasonable to explore the consequences of enriching a solar gas in these components, on local scales in the early nebula, in ratios which yield CI chondritic abundances of rock-forming elements, in considering the formation of meteorite components (i.e.-chondrules) [6]. Silicate liquids and FeO-bearing silicates are the main components of ferromagnesian chondrules. Neither is stable in a vapor of solar composition under nebular conditions, except at extremely high total pressure (P > 10 bar), but both are thermodynamically stable in dust-enriched systems at 10 < P < 10 bar [6]. In chemical systems of solar bulk composition, and in such systems enriched in dust of CI bulk composition, Mg/Si = 1.074. In these systems, olivine, (Mg,Fe)2SiO4, always becomes the dominant thermodynamically stable condensed phase at a temperature at least 100° higher than that at which orthopyroxene, (Mg,Fe)SiO3, appears, regardless of P [6]. For example, in a cooling gas with a CI dust enrichment factor of 800x, P = 10 bar, olivine appears at 1980 K, opx at 1650 K, metal at 1790 K. At 500x enrichment, these T are 1940, 1700, and 1780 K, respectively (Fig. 1). Thus olivine-rich chondrules with unfractionated REE are to be expected, if these chondrules formed near thermodynamic equilibrium as liquid droplets, in systems enriched in unfractionated chondritic dust.