Dehydroxylation of omphacite of eclogite from the Dabie-Sulu

摘要

Dehydroxylation of omphacites in eclogites from UHP Dabie-Sulu terrain has been investigated using a combined method of petrological observations,mineral chemistry,heat treatment,Mossbauer,and micro-FT1R 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)+1/2H2,and most likely represents loss of H bonded to 01 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 02,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.