Water speciation in partially deuterated hydrous stishovite at 450 °C and 9 GPa

The high-pressure SiO2 polymorph stishovite is a major constituent of subducted oceanic crust and is stable over a wide range of pressures and temperatures ranging from the lower upper mantle to the lower mantle1. Its capacity to store water (i.e., H, OH, H2O) renders stishovite an important water conveyor to the transition zone and lower mantle2. Previous studies demonstrated that Al-free Stishovite quenched from multi-anvil experiments can incorporate up to 3.2 wt% water (<550 °C, 10 GPa3-5). This was explained by a hydrogarnet substitution mechanism, where four H+ substitute for one Si4+ in the tetragonal stishovite lattice3,4. In-situ X-ray diffraction experiments with diamond anvil cell (DAC) found stishovite containing up to 10 wt% water under pressure but showed that this water is lost upon decompression2, suggesting that water also exists in a more mobile molecular form in the stishovite structure.

We investigated the water speciation in hydrous stishovite synthesized at 450 °C and 9 GPa from partially deuterated SiO2 glass via 1H-NMR. Water comprises 1.5–2.0 wt% of the stishovite and causes up to 0.4% unit-cell volume expansion as determined by X-ray diffraction. 1H-NMR cannot rule out the presence of a hydrogarnet substitution mechanism, but spinning sideband intensities indicate that water is also stored as semi-rigid H2O defects. Spectroscopic D/H analysis of the system components suggests a preferential partitioning of deuterons into the stishovite relative to the coexisting water. We further present a method to recover and preserve the quenched fluid from the experimental capsule.

1Ono,S.(2001)EPSL,57-63; 2Lin,Y.(2020)PNAS,184-189; 3Spektor,K.(2011)PNAS,20918-20922; 4Spektor,K.(2016)Am.Min. 2514-2524; 5Nisr,C.(2017)Am.Min.,2180-2189.