Nature of hydrogen defects in clinopyroxenes from room temperature up to 1000 °C: Implication for the preservation of hydrogen in the upper mantle and impact on electrical conductivity

Abstract

Water incorporated as hydrogenated defects in mantle minerals can influence physical properties of the mantle. Knowledge of hydrogen defects at high temperatures (T) is fundamental to understand and quantify their influence on mantle physical properties. Clinopyroxene contributes significantly to the upper mantle water budget. Here, we investigate the behavior of hydrogen defects in ten natural clinopyroxene crystals at temperatures up to 1000 o C, using in situ and quenched experiments. The in situ high T Fourier transform infrared (FTIR) spectra indicate no proton transfer between point defects, but the local environments of hydrogen defects vary. Dehydration rates at 1000 o C of the six samples with different chemical compositions are calculated based on the quenched experiments. These rates are not only slightly site-specific, but also increase with Fe and tetrahedrally coordinated Al contents. Indeed, the Near-FTIR spectra suggest that the dehydration of the samples in this study involves oxidation of Fe 2+. For two diopsides with a mantle affinity, the diffusivity is about 10 −12 m 2 /s at 1000 °C. The results mainly have the following implications: (1) the different local environments of hydrogen defects between high T and low T may be responsible for the different mechanism of water impact on This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press.

Electrical Conductivity of Clinopyroxene‐NaCl‐H2O System at High Temperatures and Pressures: Implications for High‐Conductivity Anomalies in the Deep Crust and Subduction Zone

“…Two groups of OH absorption bands with peaks at 3,640 and 3,560 cm −1 were observed for the clinopyroxene in DS198 and DS205, but no absorption band existed in the DS196 spectra (Figure 4). The OH absorption bands of our recovered clinopyroxene corresponded to Groups 1 and 2 of the diopside researched by Yang et al (2019). Based on the absorption spectra in the wavenumber range of 3,000–4,000 cm −1 , the water content of the DS198 and DS205 was calculated using the modified form of the Beer‐Lambert law (Yang et al, 2011): where c , Δ, I , t , and γ represent the mass fraction of H 2 O (ppm), integrated area of the absorption spectra (cm −2 ), integral specific absorption coefficient (ppm −1 ·cm −2 ) from Bell et al (1995), sample thickness (cm), and orientation factor of 1/3 from Paterson (1982), respectively.…”

Low-temperature infrared spectrum and atomic-scale structure of hydrous defects in diopside

“…The samples are well-characterized gem-quality diopside crystals from Austria and Russia previously investigated by Ingrin et al (1989), Andrut et al (2007), and Yang et al (2019). Their composition is close to the endmember composition: Ca 0.97 Cr 0.01 Mg 0.97 Fe 0.04 Al 0.01 Si 1.99 O 6 and Ca 0.99 Mn 0.01 Mg 0.94 Fe 0.06 Al 0.01 Si 1.99 O 6 for the Russian and Austrian samples, respectively.…”

“…The infrared spectra of reference diopside samples (Fig. 1) display a series of well-resolved OH-stretching bands characteristic of OH defects in diopside (e.g., Ingrin et al, 1989;Andrut et al, 2003Andrut et al, , 2007Yang et al, 2019). An asymmetric band at 3646-3650 cm −1 with a shoulder at ∼ 3611 cm −1 is observed at near-ambient temperature in both samples.…”

“…Their absorbance follows the order E//β > E//α E//γ , and they are usually ascribed to the coupled incorporation of trivalent cations and protons at the tetrahedral Si site (e.g., Beran, 1976;Skogby et al, 1990;Johnson et al, 2002;Bromiley et al, 2004;Gavrilenko et al, 2010). In natural samples, the origin of several bands observed between 3500 and 3600 cm −1 is still debated (e.g., Yang et al, 2019;Azevedo-Vannson et al, 2020). These bands are prominent in uncommon spectra of upper-mantle xenoliths (Denis et al, 2015;Patkó et al, 2019;Azevedo-Vannson et al, 2020).…”

“…In the present study, we report new low-temperature infrared spectra of natural diopside samples previously investigated by Ingrin et al (1989), Andrut et al (2007), and Yang et al (2019). In parallel, we use a quantum-mechanical modeling approach to investigate the structure and infrared spectroscopic properties of a series of OH-bearing defects in diopside.…”