Dehydroxylation of omphacite of eclogite from the Dabie-Sulu

Dehydroxylation of omphacites in eclogites from UHP Dabie-Sulu terrain has been investigated using a combined method of petrological observations, mineral chemistry, heat treatment, Mössbauer, and micro-FTIR spectroscopy. Progressive heating of omphacite in air up to 1100 °C results in complex, multi-stage processes involving oxidation and dehydrogenation at different temperatures. During heating, Fe²⁺ at the M1 site is oxidized to Fe³⁺, accompanied by a decrease in the intensity of the 3460 cm^{- 1} OH band. This reaction can be described as Fe²⁺(M1) + (OH)^- = Fe³⁺(M1) + O^2-(O3) + 1/2H₂, and most likely represents loss of H bonded to O1 adjacent to the M1 site. At higher temperatures, continued oxidation results in further dehydroxylation due to H loss from OH bands 3530 cm^{- 1} and 3630 cm^{- 1}. These bands represent H defects with higher thermal stability/slower diffusivity. In this case, oxidation of Fe²⁺ provides a means for charge-balancing H loss over the whole crystal structure, rather than locally. OH bands at 3530 and 3630 cm^{- 1} are consistent with H incorporation onto O2, with vibration of OH dipoles towards adjacent tetrahedral vacancies. Rapid loss of H during annealing at even low temperatures implies that significant H loss from eclogites occurs during exhumation. Furthermore, changes in relative OH band intensity are expected during exhumation due to different thermal behaviour of H defects in omphacite. As such, the ratio of OH bands in omphacite spectra could be used to deduce exhumation conditions.