The stability of hibonite, melilite and other aluminous phases in silicate melts: Implications for the origin of hibonite-bearing inclusions from carbonaceous chondrites

Abstract

Phase fields in which hibonite and silicate melt coexist with spinel CaAl_(4)O_7, gehlenitic melilite, anorthite or corundum at 1 bar in the system CaO-MgO-Al_(2)O_3-SiO_2-TiO_2 were determined. The hibonites contain up to 1.7 wt% SiO_2. For TiO_2, the experimentally determined partition coefficients between hibonite and coexisting melt, D^(Hib/L)_i, vary from 0.8 to 2.1 and generally decrease with increasing TiO_2 in the liquid. Based on Ti partitioning between hibonite and melt, bulk inclusion compositions and hibonite-saturated liquidus phase diagrams, the hibonite in hibonite-poor fluffy Type A inclusions from Allende and at least some hibonite from hibonite-rich inclusions is relict, although much of the hibonite from hibonite-glass spherules probably crystallized metastably from a melt. Bulk compositions for all of these CAls are consistent with an origin as melilite + hibonite + spinel + perovskite phase assemblages that were partially altered and in some cases partially or completely melted. The duration of the melting event was sufficient to remove any Na introduced by the alteration process but frequently insufficient to dissolve all of the original hibonite. Simple thermochemical models developed for meteoritic melilite and hibonite solid solutions were used to obtain equilibration temperatures of hibonite-bearing phase assemblages with vapor. Referenced to 10^(-3) atm, hibonite + corundum + vapor equilibrated at ~1260 °C and hibonite +spinel± melilite +vapor at 1215 ± 10 °C. If these temperatures reflect condensation in a cooling gas of solar composition, then hibonite ± corundum condensed first, followed by spinel and then melilite. The position of perovskite within this sequence is uncertain, but it probably began to condense before spinel. This sequence of phase appearances and relative temperatures is generally consistent with observed textures but differs from expectations based on classical condensation calculations in that equilibration temperatures are generally lower than predicted and melilite initially condenses with or even after spinel. Simple thermochemical models for the substitution of trace elements into the Ca site of meteoritic hibonites suggest that virtually all Eu is divalent in early condensate hibonites but that Eu^(2+)/Eu^(3+) decreases by a factor of 20 or more during the course of condensation primarily because the ratio is proportional to the partial pressure of Al, which decreases dramatically as aluminous phases condense. The relative sizes of Eu and Yb anomalies in meteoritic hibonites and inclusions may be partly due to this effect.

Additional details