Dehydroxylation of omphacite of eclogite from the Dabie-Sulu

Abstract

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, Fe 2+ at the M1 site is oxidized to Fe 3+, accompanied by a decrease in the intensity of the 3460 cm − 1 OH band. This reaction can be described as Fe 2+( M1) + (OH) − = Fe 3+( M1) + O 2−(O3) + 1/2H 2, 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 Fe 2+ 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.