Is orthopyroxene really a reliable recorder of mantle water signatures? Insights from an experimental study

<p>Experiments have been conducted to assess the effects of temperature, oxygen fugacity, crystallographic orientation, silica activity and chemical composition on the diffusivity and substitution mechanisms of hydrogen in orthopyroxene (opx). Axially oriented ~cuboids of natural Tanzanian opx were dehydrated at 1 bar in a gas mixing furnace (H<sub>2</sub>-CO<sub>2</sub> mix) at three different oxygen fugacities (~QFM-1,~QFM+1, ~QFM-7), and two different silica activity buffers (olivine+pyroxene or pyroxene+quartz) between 700°C and 1000°C. Profiles of hydrogen content versus distance were extracted from experimental samples using Fourier-Transform Infrared (FTIR) spectroscopy, with diffusion coefficients extracted using relevant analytical solutions and numerical approximations of Fick’s second law. Diffusion is the fastest along [001] ( D<sub>[001]</sub>>D<sub>[010]</sub>>D<sub>[100]</sub>). Fitting the diffusion coefficients to the isobaric Arrhenius relationship (logD=logD<sub>0</sub>+(-Q/(2.303RT)) gives activation energies (Q) and pre-exponential factors (logD<sub>0</sub>) between 127 to 162 kJmol<sup>-1</sup> and –4.29 to -5.42  m<sup>2</sup>s<sup>-1</sup> , respectively, for ~QFM-1.</p><p>The extracted hydrogen diffusivities are faster than previously measured by 0.5 to 5 orders of magnitude at ~1000 °C and ~700°C, respectively (Carpenter (2003), Stalder and Skogby (2003), Stalder and Behrens (2006), Stalder and al. (2007)) and are slightly slower, but strikingly close, to those of the fastest experimentally-determined diffusivity of H in olivine (Kohlstedt and Mackwell, 1998), suggesting a mechanism akin to proton-polaron exchange. This presents a paradoxical decoupling between natural and experimental observations. In most cases for mantle xenoliths, natural olivine has low water contents (<35 ppm), or are dry, and show H diffusive loss of water, where natural opx contains between 10 and 460 ppm and rarely show H diffusive loss (Demouchy and Bolfan-Casanova (2016), suggesting opx is more capable of recording the mantle water signature. With hydrogen diffusivities of olivine and opx being quite similar, however, both minerals should suffer from the same rate of dehydration during ascent, thus show low or zero water content in natural settings, which is not the case. Therefore, the inference that pyroxenes are better recorder of water in the mantle (e.g. Warren et Hauri (2014), Peslier (2010)) cannot be a simple function of diffusivities. A case study on an opx crystal showing a dehydration profile from a spinel-peridotite xenolith, hosted in an alkaline magma, from Patagonia supports this. Using the H diffusion coefficients from this study, the calculated rates of ascent of the mantle xenolith in alkaline magma are comparable to those associated with kimberlite magmas. The two suggestions we present are the following: i) Changing the boundary conditions may modify the hydrogen diffusive flux through the xenolith history and ii) The measured diffusivities would be apparent diffusivities as there might be different pathways or mechanisms of diffusion.</p>

Behavior of hydrogen defects in olivine at high temperature and high pressure: disordering, re-configuration and interation

<p>Water in the form of hydrogen defects in olivine strongly influences the physical properties of olivine, thereby being responsible for physical/chemical processes in the deep Earth. Knowledge of hydrogen defects in olivine is fundamental to understand water distribution and its impact on the upper mantle. However, the current explanations of water effects on processes in the deep Earth are mainly based on hydrogen defects observed at ambient conditions. Since hydrogen is highly mobile, the migration of hydrogen between lattice sites at high temperature and high pressure may not be quenchable. Therefore, there arises a question: whether the hydrogen defects in olivine obtained from infrared spectra at ambient conditions are the same as those at the temperature and pressure conditions of the upper mantle? Here, we carry out <em>in situ</em> high-temperature and high-pressure infrared spectroscopic investigations on hydrogen defects in the natural olivine and synthetic Fe-free forsterite. We find that hydrogen defects exhibit disordering at temperature-pressure conditions of the upper mantle, and hydrogen defects corresponding to pure Si vacancies display re-configuration under compression. Interestingly, dehydrogenation experiments of the natural olivine indicate interactions of hydrogen defects. The lost hydrogen of the titanium-clinohumite defects does not completely release out of the crystal. It can migrate to pure Si vacancies and, also, can move to Mg vacancies coupling with trivalent cations. Thus, dehydrogenation and interactions of hydrogen storage sites may be very complex. There may be other reactions among storage sites during dehydrogenation, depending on the chemical compositions, hydrogen storage sites, and the annealing conditions. In conclusion, we report disordering and reconfiguration of hydrogen storage sites at high temperature and high pressure, and also interactions of hydrogen storage sites during dehydrogenation. These are vital for understanding water distribution and processes in the deep Earth.</p>

Present-day volcanism on Venus as evidenced from weathering rates of olivine

At least some of Venus’ lava flows are thought to be <2.5 million years old based on visible to near-infrared (VNIR) emissivity measured by the Venus Express spacecraft. However, the exact ages of these flows are poorly constrained because the rate at which olivine alters at Venus surface conditions, and how that alteration affects VNIR spectra, remains unknown. We obtained VNIR reflectance spectra of natural olivine that was altered and oxidized in the laboratory. We show that olivine becomes coated, within days, with alteration products, primarily hematite (Fe2O3). With increasing alteration, the VNIR 1000-nm absorption, characteristic of olivine, also weakens within days. Our results indicate that lava flows lacking VNIR features due to hematite are no more than several years old. Therefore, Venus is volcanically active now.