A CaAl 4O 7-bearing refractory spherule from Murchison: Evidence for very high-temperature melting in the solar nebula

Abstract

We have recovered a unique refractory spherule, B6, from the Murchison C2 chondrite. Approximately 170 μm in diameter, it is concentrically zoned, with an outer rim sequence, from outermost to innermost, of aluminous diopside (10 μm thick), anorthite (3 μm), and melilite (3 μm). Inside the melilite layer is a 7 μm-thick, nearly pure layer of spinel. Inward from this layer is a 20-μm wide zone of hibonite laths enclosed in spinel. Inward from this zone, at the core of the inclusion, is CaAl 4O 7 + perovskite. This is the first reported occurrence of CaAl 4O 7 in a Murchison inclusion. The sequence of mineral assemblages from the spinel shell inward parallels that expected for crystallization of a melt with the composition of this spherule: 73.2 wt% Al 2O 3; 17.7% MgO; 6.5% CaO; and 2.5% TiO 2. From the bulk composition of the inclusion, distribution of phases, and spherical shape, we conclude that the oxide phases in it, including spinel, crystallized from a liquid. From calculated phase relations in the system Al 2O 3-MgO-CaO, we infer a melting temperature of ~2100°C, the highest ever inferred for a refractory inclusion. Evidence of a high-temperature history is also seen in the spinel, which, unlike that in most CAIs, contains 1–2 mol% excess Al 2O 3 relative to stoichiometric MgAl 2O 4. At this temperature, B6 should have been grossly out of equilibrium (by ~600°C) with a gas of solar composition at 10 −3 atm, but no evidence of evaporative loss is seen. The inclusion has a Group II REE pattern and a normal magnesium isotopic composition. The latter observation requires a very brief cooling time, on the order of several seconds. This is consistent with cooling rates calculated for a liquid spherule of the size and composition of B6 undergoing radiative cooling in the solar nebula and constrains the heating mechanism to a small-scale event. We propose the following sequence for the formation of B6: condensation of solid precursors; melting of those precursors in a hypervelocity collision which caused heating of the melt to ~2100°C; rapid formation of the spinel shell, preventing evaporation of the melt; fractional crystallization of the melt; and addition, probably by condensation, of the silicate layers. Corundum and CaAl 4O 7 are very rarely found in Murchison CAIs. Apparently, most early formed, corundum-rich condensates continued to react with the solar nebula until hibonite, perovskite, and spinel had condensed, forming aggregates which, when melted, gave rise to liquids too rich in TiO 2, MgO, and CaO to yield corundum or CaAl 4O 7 upon cooling.