Dehydroxylation of omphacite of eclogite from the Dabie-Sulu

Su, W. and Zhang, M. and Redfern, S. A. T. and Bromiley, G. D. (2008) Dehydroxylation of omphacite of eclogite from the Dabie-Sulu. Lithos, 105 (1-2). pp. 181-190. DOI 10.1016/j.lithos.2008.03.006

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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, 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) + O2−(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.

Item Type:	Article
Uncontrolled Keywords:	08AREP IA56 2008 P
Subjects:	03 - Mineral Sciences
Divisions:	03 - Mineral Sciences
Journal or Publication Title:	Lithos
Volume:	105
Page Range:	pp. 181-190
Identification Number:	10.1016/j.lithos.2008.03.006
Depositing User:	Sarah Humbert
Date Deposited:	16 Feb 2009 13:03
Last Modified:	23 Jul 2013 10:08
URI:	http://eprints.esc.cam.ac.uk/id/eprint/719

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