Hydrous Na-garnet from Garnet Ridge; products of mantle metasomatism underneath the Colorado Plateau

Abstract

This is the first report on amphibole exsolution in pyrope from the Colorado Plateau. Pyrope crystals delivered from mantle depths underneath the Colorado Plateau by kimberlitic volcanism at 30 Ma were collected at Garnet Ridge, northern Arizona. The garnet grains analyzed in this study occur as discrete crystals (without adjacent rock matrix) and are classified into two major groups, Cr-rich pyrope and Cr-poor pyrope. The Cr-poor pyrope group is divided into four subgroups based on exsolved phases: amphibole lamella type, ilmenite lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type. Exsolved amphibole occurs in amphibole lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type of Cr-poor pyrope. The amphibole crystals tend to have preferred orientations in their garnet hosts and occur as monomineralic hexagonal or rhombic prisms and tablets, and as multimineralic needles or blades with other exsolved phases. Exsolved amphibole has pargasitic compositions (Na2O up to 1.6 apfu based on 23 oxygen). Garnet host crystals that have undergone amphibole exsolution have low OH contents (2–42 ppmw H2O) compared to garnets that do not have amphibole lamellae (up to 115 ppmw H2O). The low OH contents of garnets hosting amphibole lamellae suggest loss of OH from garnet during amphibole exsolution. Amphibole exsolution from pyrope resulted from breakdown of a precursor “hydrous Na-garnet” composition (Mg,Na+ x)3(Al2 − x, Mgx)2Si3O12 − 2x(OH)2x. Exsolution of amphibole and other phases probably occurred during exhumation to depths shallower than 100 km prior to volcanic eruption. Based on the abundance and composition of exsolved clinopyroxene and amphibole lamellae in one garnet, hydrous Na-garnet had excess silicon (Si3.017 apfu, 12 oxygen normalization, vs. X3Y2Si3O12 for typical garnet). Comparison with experimental data suggests crystallization at pressures near 6–8 GPa. Garnet crystals that host exsolved amphibole have compositions (Pyp49-76, 3–10 wt% CaO, and up to 0.6 wt% Cr2O3) similar to garnets reported from pyroxenites, and have pyrope-almandine-grossular compositional ranges that overlap with the Cr-rich pyrope (typical lherzolitic garnet). Hydrous Na-garnet was likely formed by metasomatic reactions between Cr-rich pyrope and Na-rich aqueous fluid in the deep upper mantle. The most likely source of metasomatic Na-rich fluid is ancient oceanic crust that was subducted before subduction of the Farallon Plate beneath the Colorado Plateau.