A systematic study of OH in hydrous pyrope-grossular garnets

Garnet is a common mineral in crustal metamorphic rocks and a primary constituent of the Earth’s upper mantle. Natural silicate garnets are stable over a wide range of pressure and temperature conditions and have very complex chemical compositions. The chemical composition of a natural garnet reflects its growth environment making garnet a critical indicator mineral for many geological processes. Garnet, like olivine and many other nominally anhydrous minerals, can incorporate water as hydroxyl (OH^-) into its crystal structure. The water content in natural silicate garnets ranges from almost dry to thousands of parts per million. The compositional effect on the water content and the mechanisms of OH^- incorporation in silicate garnets are not well understood, majorly impeded by their complex compositions. Here we reported the influence of chemical compositions on the water content and OH^- substitution mechanisms in silicate garnets by studying synthetic garnet samples with compositions in the pyrope-grossular solid solution. We synthesized a series of hydrous pyrope-grossular garnets with various magnesium to calcium ratios by a multi-anvil press. Fourier transform infrared spectroscopy and Raman spectroscopy were applied to characterize the OH^- content and identify OH^- substitution mechanisms in the synthetic samples. The influence of hydration on the crystal structure of samples was investigated by the single-crystal X-ray diffraction technique. Our experimental results will help to constrain the bulk water content in the Earth’s interior and to interpret seismic data of a hydrous mantle.