The Origin and Significance of Reverse Zoning in Melilite from Allende Type B Inclusions

Abstract

In many Type B Allende inclusions, melilite is reversely-zoned over restricted portions of each crystal. Textural relationships and the results of dynamic crystallization experiments suggest that the reverselyzoned intervals in these Type melilites result from the co-precipitation of melilite with clinopyroxene from a melt, prior to the onset of anorthite precipitation. When clinopyroxene begins to precipitate, the Al/Mg ratio of the melt rises, causing the crystallizing melilite to become more gehlenitic, an effect which is negated by crystallization of anorthite. Because the equilibrium crystallization sequence in these liquids is anorthite before pyroxene, melilite reverse zoning can occur only when anorthite nucleation is suppressed relative to pyroxene. This has been achieved in our experiments at cooling rates as low as 0.5°C/hour. Our experiments further indicate, however, that reverse zoning does not form at cooling rates ≥50°C/hour , probably because the clinopyroxene becomes too Al-rich to drive up the Al/Mg ratio of the liquid. Type inclusions with reversely-zoned melilites must have cooled at rates greater than those at which anorthite begins to crystallize before clinopyroxene but <50°C/hour. Such rates are far too slow for the Type droplets to have cooled by radiation into a nebular gas but are much faster than the cooling rate of the solar nebula itself. One possibility is that Type B's formed in local hot regions within the nebula, where their cooling rate was equal to that of their surrounding gas. Other possibilities are that their cooling rates reflect their movement along nebular temperature gradients or the influence of a heat source. The sun or viscous drag on inclusions as they moved through the nebular gas are potential candidates for such heat sources.

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