Abstract

Pink pyropic garnet (Py 80 Gr 8 Alm 12) and black Cr-poor spinel (Mg/[Mg + Fe] = 0.84; Cr2O3 = 10 wt.%) occur in subequal amounts in a 3.5 cm alkremite xenolith from Moses Rock Dike, San Juan County, Utah, a kimberlite pipe. Primary garnet and spinel are more magnesian than these phases in similar xenoliths from kimberlite pipes in South Africa and the U.S.S.R. Subsequent alteration and hydration along cracks and grain boundaries produced secondary minerals that account for less than 5 percent of the rock. The presence of iron-bearing kornerupine in the secondary mineral assemblage suggests that alteration occurred at elevated temperatures and crustal pressures.

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