[PPS10-09] Al−Mg systematics for partial melting of an Allende type B CAI, Golfball
The Golfball CAI has a type B CAI bulk composition  and a unique structure : a fassaite-rich mantle enclosing a melilite-rich core. Most of melilite crystals (Åk30−70) occur as laths at the inclusion rim and in the core or as blocky shapes poikilitically enclosed in the fassaite grains in the core. In the core, gehlenitic melilite grains (Åk5−12) are enclosed in strongly zoned (Åk15−70) overgrowths with small compositional discontinuity at the boundary. These gehlenitic melilite grains (Åk5−12) clearly could not have formed from a melt with bulk composition of Golfball or that of its core [6, 9], indicating they are relict grains that survived later melting event(s). High-precision in situ Al−Mg systematics with SIMS  was applied for the gehlenitic relict grains and the other melilite crystals as well as spinel, a liquidus phase. An Al−Mg isochron regression line for the data of gehlenitic relict grains and spinel gives (26Al/27Al)0 = (4.41 ± 0.22) × 10−5, while that for the other melilite crystals gives (26Al/27Al)0 = (4.41 ± 0.20) × 10−5. These results indicate that partial melting event(s) for the Golfball CAI occurred in very short order, probably shorter than a few tens of Kyr, after a precursor formation.
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