Hydrogen defect saturation in natural pyroxene
2011 (English)In: Physics and chemistry of minerals, ISSN 0342-1791, E-ISSN 1432-2021, Vol. 38, no 5, 335-344 p.Article in journal (Refereed) Published
Dehydration via the redox reaction: OH- + Fe2+ ↔ O2- + Fe3+ + ½H2, is believed to be a commonly occurring process in pyroxenes and other nominally anhydrous minerals (NAMs) from the upper mantle and appears to be fast enough to allow significant dehydration during magma ascent. Nevertheless, the mobility of hydrogen incorporating defects is controlled by cation diffusion with approximately two orders of magnitude slower reaction kinetics than the iron redox-reaction, and host defects have a much higher likelihood to be preserved than the hydrogen itself. Therefore, restoring hydrogen into the structure would be possible by driving the redox-reaction backwards, as long as temperature and time are limited so not to change the defect state of the crystal structure. Here we investigate the re-hydration capacity of megacryst and xenocryst ortho- and clinopyroxene by stepwise thermal annealing of crystallographically oriented samples in 1 atm. H2. H concentration was measured by FTIR spectroscopy after each annealing step. Most samples show only a small increase in water content up to a presumed saturation level, after which further heat treatments in H2 resulted in a slight decrease in water contents. However, two of the studied samples, both fairly Fe rich megacrysts, are significantly rehydrated. Some samples or crystal sections exhibit a practically inert behavior, with minor fluctuations around initial water concentrations. Present results indicate that most mantle pyroxene has not been substantially dehydrated during late stage magma processes, and that restoring water is possible in samples which have lost considerable amounts of water.
Place, publisher, year, edition, pages
Springer, 2011. Vol. 38, no 5, 335-344 p.
Pyroxene, hydrogen accommodating defects, hydrogen diffusion, FTIR spectroscopy, Mößbauer spectroscopy
Research subject Mineralogy, Petrology and Geochemistry
IdentifiersURN: urn:nbn:se:su:diva-38666DOI: 10.1007/s00269-010-0407-yOAI: oai:DiVA.org:su-38666DiVA: diva2:312461
Published in Physics and Chemistry of Minerals 2011, vol. 38, pp. 335–344.2010-04-242010-04-242012-06-20Bibliographically approved