The petrogenesis of type B1 Ca-Al-rich inclusions: The spinel perspective

Abstract

Minor element variations in MgAl_2O_4 spinel from the type B1 calcium-aluminum-rich inclusion (CAI) Allende TS-34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases. These correlations result from a combination of crystallization of a liquid produced by re-melting event(s) and local re-equilibration during subsolidus reheating. The correlation of the Ti and V in spinel inclusions with the Ti and V in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite, following a partial melting event. There are, however, difficulties in quantitative modeling of the observed trends, and it is easier to explain the Ti correlation in terms of complete re-equilibration. The correlation of V in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re-melting, but it can be explained by partial re-equilibration. The distinct V and Ti concentrations in spinel inclusions in melilite from the edge regions of the CAI are best explained as being affected by only a minor degree of re-equilibration. The center melilites and included spinels formed during crystallization of the liquid produced by re-melting, while the edge melilites and included spinels are primary. The oxygen isotope compositions of TS-34 spinels are uniformly ^(16)O-rich, regardless of the host silicate phase or its location within the inclusion. Similar to other type B1 CAIs, clinopyroxene is ^(16)O-rich, but melilite is relatively ^(16)O-poor. These data require that the oxygen isotope exchange in TS-34 melilite occurred subsequent to the last re-melting event.

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