A NOVEL APPROACH TO INVESTIGATE WATER INCORPORATION IN GARNET

Garnet is a common mineral in eclogite-facies rocks. It is mainly formed during prograde dehydration reactions at great depth (>30 km). Although garnet does not contain any molecular water (H₂O) or structural water (OH groups) in its structural formula, it has been shown that garnet can incorporate several hundreds of ppm of water in the form of OH groups, with H substituting for other cations, especially through the hydrogarnet point defect (Aines and Rossman 1984).

We investigated water contents in garnet with various chemistry from many localities in the Alps using Fourier Transform Infrared spectroscopy (FTIR). Determined water contents range from 100-8000 ppm wt. The incorporation of water in garnet is favoured by elements such as Mn²⁺, Ca²⁺, Ti^4+, Fe³⁺ and Cr³⁺ in the crystal structure, whereas pyrope-almandine garnets generally have low water contents. The main absorption bands shift from 3600 cm^-1 in grossular-rich garnet to 3550 cm^-1 in andradite-rich garnet, but a clear correlation between band position and garnet composition is not evident.

The use of novel FTIR mapping techniques allows investigation of water zoning in crystals and correlation with element distribution maps obtained by electron probe micro-analyses (EPMA).. Some sample show spectacular zoning in Mn, Ca, Ti and Cr but also distinct zoning in water contents, with a typical high concentration of incorporated water in the prograde core of the garnets. The fact, that water correlates with chemical zoning, indicates that there has not been significant diffusional loss of water during exhumation of the high-pressure rocks and that the measured water contents faithfully represent the water contents acquired during garnet growth at temperatures up to 600°C. The water incorporated in garnet during prograde dehydration reactions might thus be transported along subduction zones towards the deeper parts of the mantle.

Aines RD, Rossman GR (1984) The hydrous component in garnets: pyralspites. American Mineralogist 69:1116-1126