Water in omphacite fingerprints the thermal history of eclogites

Abstract

Omphacite is a diagnostic mineral of high- and ultrahigh-pressure rocks, and its association with garnet is characteristic of eclogites from subduction-related massifs and volcano-entrained xenoliths. Omphacite can accommodate significant amounts of water as structurally bound hydroxyl (OH) groups, and is able to convey water into Earth’s interior. We show, for the first time, experimental evidence that the infrared absorption patterns of water in omphacite are temperature sensitive. This provides a new framework for discriminating water in natural omphacite equilibrated at different temperatures. We also demonstrate that in low-temperature omphacite, the integral absorbance ratio between the infrared OH absorption bands at 3620 cm–1 and 3450 cm–1 is linearly related to temperature. Water in omphacite of massif eclogites records the temperature of OH equilibrium, allowing reconstruction of the fluid-involved thermal history and tectonics of rock evolution.

Electrical Conductivity of Eclogitic Omphacite and Garnet at Water-Rich Conditions

“…FT-IR spectra of typical pre-annealed samples are presented in Figure 1, along with the spectra of low water content samples from the study by Liu et al (2019) for comparison. All spectra show OH bands at 3,700-3,100 cm −1 , with peaks located at ~3,620, 3,520, and 3,450 cm −1 in omphacite and ~3,605 cm −1 (asymmetric) in garnet, and shapes and frequency positions of the bands agree with those in previous reports for the corresponding natural and H-annealed minerals (Langer et al, 1993;Katayama and Nakashima, 2003;Xia et al, 2005;Katayama et al, 2006;Sheng et al, 2007;Schmädicke and Gose, 2017;Jiang et al, 2022). H 2 O content is ~775, 1,395, and 2,000 ppm in omphacite and 705 and 1,460 ppm in garnet (Table 2).…”

“…The different behaviors of conduction between water-poor and water-rich omphacite, concerning the activation enthalpy (~82 vs. 70 kJ/mol) and water content exponent (~1.10 vs. 1.45) as also mentioned before, suggest that the mobility of the charged protons is probably different. As documented by Jiang et al (2022), the infrared absorption bands of structural hydroxyl in omphacite evolve with increasing temperature under otherwise identical conditions, in that the band at ~3,620 cm −1 is gradually enhanced and the broadening of the whole bands becomes significant. This is probably related to the temperature-driven cation rearrangements in the lattice and the transition of the space group from the ordered P2/n at low temperature to the disordered C2/c at high temperature, which influences the incorporation of water into the mineral structure (Jiang et al, 2022).…”

“…As documented by Jiang et al (2022), the infrared absorption bands of structural hydroxyl in omphacite evolve with increasing temperature under otherwise identical conditions, in that the band at ~3,620 cm −1 is gradually enhanced and the broadening of the whole bands becomes significant. This is probably related to the temperature-driven cation rearrangements in the lattice and the transition of the space group from the ordered P2/n at low temperature to the disordered C2/c at high temperature, which influences the incorporation of water into the mineral structure (Jiang et al, 2022). The transition temperature of the space group is pressuredependent, for example, 750 °C-800 °C at 1 GPa and 850 °C-900 °C at 2 GPa (Jiang et al, 2022).…”

“…This is probably related to the temperature-driven cation rearrangements in the lattice and the transition of the space group from the ordered P2/n at low temperature to the disordered C2/c at high temperature, which influences the incorporation of water into the mineral structure (Jiang et al, 2022). The transition temperature of the space group is pressuredependent, for example, 750 °C-800 °C at 1 GPa and 850 °C-900 °C at 2 GPa (Jiang et al, 2022). This produces different spectral shapes, and thus types, of hydroxyl between the water-rich and water-poor omphacite (Figure 1A) due to H-annealing at high and low temperatures (Table 2).…”

Redox‐Independent Low Water Solubility in Quartz and Implications for Water Storage in the Crust

“…Moreover, the annealed crystals are usually large enough for reliable water analysis. This method has been often applied to, e.g., quartz (Stalder et al., 2017), rutile (Colasanti et al., 2011), feldspars (Mosenfelder et al., 2020; Yang, 2012), pyroxenes (Bromiley et al., 2004; Jiang et al., 2022; Liu & Yang, 2020), olivine (Bai & Kohlstedt, 1993; Kohlstedt et al., 1996; Yang, 2015, 2016; Yang, Liu, & Xia, 2014), and garnet (Lu & Keppler, 1997; Zhang et al., 2022). For this approach, however, gem‐quality starting crystals of high purity and homogeneous composition are important.…”