The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa

Abstract

166 garnets of dominantly mantle origin were analyzed for OH content by infrared (IR) spectroscopy. IR spectra in the 3400–3700 cm^(-1) region display consistent absorption patterns attributable to OH structurally bound within the garnet crystal, occasionally contaminated by low intensity OH absorptions from microscopic inclusions. The principal structural OH absorption occurs near 3570 cm^(-1), with the appearance of additional absorptions near 3512 cm^(-1) and 3650 cm^(-1) dependent on garnet composition or paragenesis. Samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern Africa. OH abundances, using the best currently available calibration, range from less than 1 up to 135 ppm H_2O, and increase in the general order as follows: on-craton eclogites<coarse-granular peridotites<Ti-rich deformed peridotites<some off-craton eclogites<Cr-poor megacrysts. OH abundances in garnet are closely linked to host rock paragenesis and cannot be explained purely by any crystal chemical factors which we have investigated. Cr-poor garnet megacryst nodules display striking inverse correlations between OH contents and Mg/(Mg+Fe) ratios, which we interpret to reflect the progressively increasing water content of the differentiating parental megacryst magmas. OH abundances in garnet megacrysts decrease in the host rock order Group 2 (micaceous) kimberlite>Group 1 (basaltic) kimberlite>alnöite>alkali basalt. The OH contents of common lithospheric granets from coarse peridotites, including several phlogopite-bearing samples are typically less than 20 ppm H_2O, for tectonic settings of kimberlites both on and off the Archaean Kaapvaal craton. Ti-rich garnets from deformed peridotites are richer in OH, supporting previous suggestions of association of these xenoliths with putative megacryst magmas. Subcalcic Cr-rich xenocrysts, diamond inclusion garnets and garnets from diamondiferous eclogites have very low OH contents, similar to eclogites and depleted peridotites without macroscopic diamonds. The OH content of southern African peridotite and eclogite garnets are significantly lower on average than those previously examined from the Colorado Plateau diatremes. While details of emplacement-related H mobility in garnets remain to be established, our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle. Although garnets do not appear to constitute a large reservoir of mantle hydrogen, the large stability range of OH-bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer.