FORMATION OF AN UNUSUAL COMPACT TYPE-A REFRACTORY INCLUSION FROM ALLENDE

We report the results of a study of TS2, an unusual compact Type A inc lusion from Allende. A distinctive, major feature of this inclusion is thar. many of its melilite crystals have no dominant core-rim zoning but instead consist of 50-200 mu m patches of Mg-rich melilite (Ak(32- 62), median Ak(51)) set in or partially enclosed by, and optically con tinuous with, relatively Al-rich melilite (Ak(25-53), median Ak(38)). The Al-rich regions have jagged, dendritic shapes but occur within cry stals having straight grain boundaries. Another unusual feature of thi s inclusion is the size and spatial distribution of spinel. In many pl aces, especially in the interior of the inclusion, the aluminous melil ite encloses numerous, fine (0.5-5 mu m) inclusions of spinel and mino r perovskite and fassaite. The latter phases also occur as isolated gr ains throughout the inclusion. Coarse-grained spinel, similar to 50-15 0 mu m across, occurs in clumps and chains enclosed in relatively Mg-r ich melilite, whereas none of the fine spinel grains are clumped toget her. The sample also contains a spinel-free palisade body, 1.7 x 0.85 mm, that consists almost Ak-rich of (45-65 mol%) melilite. Within the palisade body are two grains of perovskite with extremely Nb-rich (sim ilar to 4-8 wt% Nb2O5) cores and rims of typical composition. All phas es in this inclusion have chondrite-normalized FLEE patterns that are consistent with crystal/melt partitioning superimposed upon a bulk mod ified Group II pattern. We suggest that TS2 had an anomalous cooling h istory and favor the following model for the formation of TS2. Precurs ors having a bulk modified Group II pattern melted. Rapid growth of la rge, dendritic, nonstoichiometric melilite crystals occurred. The meli lite trapped pockets of melt and incorporated excess spinel components and TiO2. Bubbles formed in the residual melt. lis crystallization sl owed, coarse spinel grew. Some spinel grains collected against bubbles , forming spherical shells, and others formed clumps and chains. Relat ively Ak-rich melilite crystallized from the residual melt between den dritic melilite crystals and from melt trapped in pockets and between arms of dendrites, and incorporated the clumps and chains of coarse sp inel. Bubbles broke and filled with late-stage melt, their shapes pres erved by their spinel shells. Slow cooling, or perhaps an episode of r eheating, allowed the early melilite to become stoichiometric by exsol ving fine grains of spinel, perovskite and fassaite, and allowed the m elilite to form smooth grain boundaries. Dendritic crystals are indica tive of rapid growth and the melilite crystals in TS2 appear to be den dritic. Coarse, dendritic melilite crystals have been grown from Type B inclusion melts cooled at similar to 50-100 degrees C/h. If those re sults are applicable to Type A inclusions, we can make the first estim ate of the cooling rate of a Type A inclusion, and it is outside the r ange (2-50 degrees C/h) generally inferred for Type B inclusions. The rapid cooling inferred here may be part of an anomalous thermal histor y for TS2, or it may be representative of part of a normal thermal his tory common to Types A and B that involved rapid cooling early (at hig h temperatures) as inferred for TS2, and slower cooling later (at lowe r temperatures), as inferred for Type B inclusions. We prefer the form er explanation; otherwise, the unusual features of TS2 that are report ed here would be common in Type A inclusions (which they are not).