Water content of mantle garnets

Abstract

Garnet megacrysts from Colorado Plateau diatremes (Green Knobs, Garnet Ridge) and the Wesselton kimberlite, South Africa, commonly contain a structural hydrous component. The Colorado Plateau samples range from 0.0 to 0.26 wt% H_(2)O, and the Wesselton samples contain from 0.01 to 0.07%. Concentrations were measured using P_(2)O_5 cell coulometry, H_2 gas manometry, and thermogravimetry. These were used to calibrate infrared integrated absorbance in the 3-µm region, which is a more sensitive measure of total O-H content than the other analytical methods. Infrared absorbance patterns were also used to differentiate structural hydrous component from water contained in alteration and included phases. The structure of the hydrous component in these garnets appears to be the classic H_(4)O_(4)^(4−) = SiO_(4)^(4−). Profiles at 100-µm intervals across these samples show flat concentration profiles or slightly increasing concentration toward the center. A large range of water content among samples appears to represent real differences in water fugacity at the point where the garnets equilibrated. Garnets in eclogite nodules from South Africa and the Solomon Islands were also studied but were either anhydrous or too badly altered to determine the content of structurally bound water. The high concentration of hydrous component in the Colorado Plateau samples is consistent with other indicators of high volatile content in that region of the mantle. The water content of mantle garnets may prove to be an accurate indicator of mantle-water fugacities.

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