Water incorporation in garnet under mantle transition zone conditions: A unique infrared absorption band

Abstract

Garnet is a critical mineral constituent of the mantle transition zone, where it is in general pyrope-rich (though with a Si-rich majorite component). The water (hydroxyl) storage of garnet in the mantle transition zone has been studied by several experiments, however, the incorporation mechanism remains obscure. We experimentally examined the incorporation of water in garnet under mantle transition zone conditions, by carrying out H-annealing experiments at 15–21 GPa, 1300–1500 °C and Fe-FeO buffered redox conditions. Three natural, gem-quality, pyrope-rich (single crystal) garnet samples with different chemical compositions were chosen as the starting materials, and coexisting assemblages and fluids in the experiments were also controlled. The species and amount of water in the samples were analyzed by infrared spectroscopy. The infrared spectra of the recovered garnet samples consistently display a similar single asymmetric band at ∼ 3630 cm⁻¹, regardless of significant differences in their compositions, original spectral shapes (e.g., band positions or patterns) and experimental conditions. It is expected that this band is a unique feature for water in garnet in the mantle transition zone. The band is essentially the same as the one observed for end-member pure pyrope, and is probably dominated by the well-accepted hydrogarnet substitution. By comparing with available annealing experiments at lower pressures and temperatures where the band positions of water in various garnet samples are essentially the same as those in their starting crystals, we suggest that pressure and temperature are two important factors in affecting the band patterns. The high pressure and temperature corresponding to the mantle transition zone may reshape the local bonding environments of H in garnet in a way different from that at relatively low pressures and temperatures, leading to the unique band at ∼ 3630 cm⁻¹. Under otherwise comparable conditions, the water contents of different garnet samples exhibit complex correlations with other components, due to their composition-regulated incorporation, and the most profound dependences are observed for FeO and MgO, with less or no dependences for others. Very likely, Fe and Mg are also important in the water incorporation; however, the general single band at ∼ 3630 cm⁻¹ indicates that the infrared signals by Fe and Mg substitutions are superimposed with the hydrogarnet substitution.