Xiaozhi Yang

Fe3+-rich augite and high electrical conductivity in the deep lithosphere

Magnetotelluric soundings have detected high-conductivity zones in some regions of the deep lithosphere, and these conductive areas were usually interpreted to reflect the presence of hydrous phases, silicate/carbonatite melts, or water incorporated in olivine. Augite megacrysts are the most abundant single-crystal mantle samples entrained by volcanic basalts/kimberlites in a wide variety of tectonic environments, and are usually characterized by higher Fe (∼5%–12% in total FeO), Fe 3+ (∼25%–45% in Fe 3+ /Fe total ), and H 2 O (∼200–1800 ppm) contents. Based on experimental work, we show that Fe 3+ - and H 2 O-rich augites have electrical conductivities that are several orders of magnitude higher than those of dry olivine. High conductivities in at least some regions of the lithospheric mantle can be explained by the presence of small amounts of augite (and other pyroxenite analogues, e.g., ∼10% to even less depending on composition, temperature, and models used for the estimation), which may ultimately indicate the infiltration of silicate melts and their subsequent crystallization and interaction with wall rocks in the deep lithosphere.

In-situ infrared spectra of OH in olivine to 1100 °C

Abstract The infrared spectra of hydrated San Carlos olivine were measured from room temperature to 1100°C at 1 bar using a heating stage. The spectra show that even at the highest temperatures studied, there are still two well-separated groups of OH bands centered between 3200 and 3300 cm-1 and between 3500 and 3600 cm-1, respectively. The distinction between “group I” (ν > 3450 cm-1) and “group II” bands (ν < 3450 cm-1) is therefore still meaningful at upper mantle temperatures. However, a prominent band at 3612 cm-1 already loses intensity by 100°C and nearly disappears by 300°C, suggesting that it corresponds to a particular H environment that only formed during cooling of the sample and that it is not stable at high temperature. A similar band is prominent in many olivines from mantle xenoliths and previously it has sometimes been assigned to OH groups surrounding Si vacancies. We show that structural models of water dissolution in olivine derived from infrared spectra at ambient conditions may not be fully applicable for the upper mantle

In-situ infrared spectra of hydroxyl in wadsleyite and ringwoodite at high pressure and high temperature

Abstract The infrared spectra of hydroxyl in synthetic hydrous wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4) were measured at room temperature up to ∼18.8 GPa for wadsleyite and up to ∼21.5 GPa for ringwoodite. High-temperature spectra were measured in an externally heated diamond-anvil cell up to 650 °C at ∼14.2 GPa for wadsleyite and up to 900 °C at ∼18.4 GPa for ringwoodite. The synthetic samples reproduce nearly all the important OH bands previously observed at ambient conditions. Only subtle changes were observed in the infrared spectra of both minerals, both upon compression at room temperature and upon heating at high pressure. For wadsleyite, upon compression to ∼18.8 GPa, the frequencies of the bands at ∼3600 cm-1 remain almost unchanged, while the main bands at 3200-3400 cm-1 shift to lower frequencies. During heating at 14.2 GPa to 650 °C the bands at 3200-3400 cm-1 broaden and shift to slightly lower frequencies. For ringwoodite, upon compression to ∼21.5 GPa, the main bands at 3115 cm-1 progressively shift to lower frequencies. During heating at 18.4 GPa to 900 °C, no frequency shift was observed for the band at ∼3700 cm-1, but the band initially at ∼3115 cm-1 shifts very slightly to higher frequencies, which should yield almost the same band positions at 1300-1400 °C as those measured at ambient conditions. Our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions, except perhaps for subtle changes in hydrogen bonding. The low OH-stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large H/D fractionation during degassing of the deep mantle. This may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean.

Quantitative analysis of H-species in anisotropic minerals by polarized infrared spectroscopy along three orthogonal directions

Infrared spectroscopy is a powerful technique for probing H-species in nominally anhydrous minerals, and a particular goal of considerable efforts has been providing a simple yet accurate method for the quantification. The available methods, with either polarized or unpolarized analyses, are usually time-consuming or, in some cases, subjected to larger uncertainty than theoretically expected. It is shown here that an empirical approach for measuring the concentration, by determining three polarized infrared spectra along any three mutually perpendicular directions, is theoretically and in particular experimentally correct. The theoretical background is established by considering the integrated absorbance, and the experimental measurements are based on a careful evaluation of the species and content of H in a series of gem-quality orthogonal, monoclinic and triclinic crystals, including olivine, orthopyroxene, clinopyroxene, orthoclase and albite (natural and H-annealed). The results demonstrate that the sum of the integrated absorbance from two polarized spectra along two perpendicular directions in any given plane is a constant, and that the sum of the integrated absorbance from three polarized spectra along any three orthogonal directions is of essentially the same accuracy as that along the principal axes. It is also shown that this method works well, with a relative accuracy within 10%, even at some extreme cases where the sample absorption bands are both intense and strongly anisotropic.