H2O CONCENTRATION OF NOMINALLY ANHYDROUS GARNET AND CLINOPYROXENE IN ECLOGITE

Trace amounts of water in nominally anhydrous minerals exert fundamental petrologic and rheologic controls on mantle processes, including metasomatism, melting, and deformation. The dehydration of lattice-bound OH during progressive metamorphism of mafic protoliths to eclogite is one of the main contributors of H₂O to the mantle, but relatively few studies have characterized H₂O in eclogite. We measured the H₂O contents in garnet and cpx in a global suite of eclogite from the Smithsonian Institution’s collection. Our data represent 3 xenolith eclogite samples from South Africa and Norway as well as 3 tectonic eclogite samples from the United States, Austria, and Norway. Small fragments of each specimen were crushed then handpicked for garnet and pyroxene. We mounted the crystals in epoxy, and prepared doubly-polished wafers. We then measured their H₂O content using FTIR. The FTIR measures absorbance which correlates with H₂O concentration following the Beer Lambert Law. Garnets from Norway and South Africa have no absorbance, despite an H₂O detection limit as small as 15 ppm. Garnets from the Austrian sample have small absorbances that indicate water contents range up to 40 ppm H₂O. Clinopyroxene crystals from Norway have absorbances with multiple peaks between 3500 and 3800 cm^-1. Such absorbances suggest the CPX contains up to 200 ppm. Based on the results, we infer that the dehydration reaction of basalt to eclogite has variable efficiency, but may provide a mechanism for transporting some H₂O deep into the mantle. Globally this contribution of H₂O can have disproportionately large implications on the water cycle of the mantle, and may affect the rheology, volcanic and tectonic processes of the upper mantle.