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Hydrogen defect saturation in natural pyroxene
Stockholm University, Faculty of Science, Department of Geological Sciences.
Enheten för mineralogi, Naturhistoriska riksmuseet, Box 50007, 10405 Stockholm .
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
Abstract [en]

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.
Keyword [en]
Pyroxene, hydrogen accommodating defects, hydrogen diffusion, FTIR spectroscopy, Mößbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-38666DOI: 10.1007/s00269-010-0407-yOAI: oai:DiVA.org:su-38666DiVA: diva2:312461
Note
Published in Physics and Chemistry of Minerals 2011, vol. 38, pp. 335–344.Available from: 2010-04-24 Created: 2010-04-24 Last updated: 2012-06-20Bibliographically approved
In thesis
1. Water as a trace component in mantle pyroxene: Quantifying diffusion, storage capacity and variation with geological environment
Open this publication in new window or tab >>Water as a trace component in mantle pyroxene: Quantifying diffusion, storage capacity and variation with geological environment
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this study, distribution and diffusion of water in pyroxene are examined in an effort to explain the water content variation observed in natural pyroxene. Water is a common trace component in many nominally anhydrous minerals (NAMs) from the Earth's crust and mantle and greatly impacts their physical properties. Therefore, it is crucial to constrain the processes that control water incorporation in these minerals. The pyroxene group has a fairly simple mineral chemistry, the highest amount of water and the greatest water content variation measured in mantle NAMs. Therefore, they are ideal for a case study such as the present. The redox reaction: OH- + Fe2+ ↔ O2- + Fe3+ + ½H2, is believed to control water diffusion in many NAMs having sufficiently high iron contents. Nevertheless, reactions involving vacancies and charge-deficient substitutions which are mainly controlled by cation diffusion are also present and have reaction kinetics that is significantly slower than the redox exchange. Therefore, diffusion and reaction kinetics were studied in (1) synthetic diopside with ~0.7 wt % FeO which allows the study of contributions from both types of reactions (i.e. Fe-redox and cation diffusion). These results were then compared (2) with reaction kinetics in pure synthetic diopside. The diffusion rates are faster in Fe-free diopside, as reaction kinetics is dominated by cation diffusion in samples with low Fe contents. Next (3), water content variation and zonation were investigated in natural pyroxene using high resolution FTIR imaging. The results show no water zonation and a correlation between mantle source and water content. Finally (4), thermal annealing experiments in H2 on natural pyroxene show little or no re-hydration capacity for mantle samples. Altogether, the results indicate that the water contents of most mantle pyroxenes do reflect mantle conditions and that many types of reactions controlling water uptake and release seem to be present in pyroxene.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2010. 27 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 338
Keyword
pyroxene, mantle mineralogy, water in nominally anhydrous minerals, iron redox-reaction, kinetics, water zonation, FTIR imaging, re-hydration capacity
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-38701 (URN)978-91-7447-065-9 (ISBN)
Public defence
2010-06-04, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 09:30 (English)
Opponent
Supervisors
Note
At the time of doctoral defence the following papers were unpublished and had the status as follows: Papers 3 and 4: Manuscripts.Available from: 2010-05-11 Created: 2010-04-26 Last updated: 2010-04-28Bibliographically approved

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