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Hydrogen diffusion in synthetic Fe-free diopside
Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
Sektionen för Mineralogi, Naturhistoriska Riksmuseet.
Institut für Mineralogie und Petrographie, Universität Innsbruck, Österrike.
2009 (English)In: European journal of mineralogy, ISSN 0935-1221, E-ISSN 1617-4011, Vol. 21, no 5, 963-970 p.Article in journal (Refereed) Published
Abstract [en]

Hydrogen is a widespread trace element in many nominally anhydrous minerals (NAMs) in the Earth's crust and mantle and has profound influence on the physical properties of the host mineral. Of all NAMs from the upper mantle, clinopyroxenes have been shown to contain the highest amount of hydrogen. This study focuses on the dehydration kinetics of pure diopside along [010] and [100]* by thermal annealing under normal atmospheric pressure. The diopside crystals used were synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step, including untreated samples. The Arrhenius equation gives an activation energy (Ea) of -331 ± 50 kJ mol-1 and D0 = 100.9 ± 2.3 m2 s-1 for diffusion along [010]. Diffusion along [100]* gives an Ea-value of -312 ± 55 kJ mol-1 with D0 = 100.5 ± 2.4 m2 s-1. Therefore, our experimental results show no difference between diffusion along [010] and [100]* (within error limits). The diffusion rate in pure synthetic diopside is about one order of magnitude faster than for synthetic diopside with very low Fe contents. A suitable explanation for this behavior is that in the case of low Fe diopside, the rate-limiting process for the protons associated with Fe is probably Fe-diffusion. In contrast, in pure diopside all protons are associated to Mg-defects, which are more mobile than Fe. Nevertheless, compared to natural diopside with appreciable Fe contents, diffusion rates in these synthetic samples are several orders of magnitude slower.

Place, publisher, year, edition, pages
Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung , 2009. Vol. 21, no 5, 963-970 p.
Keyword [en]
diopside, FTIR spectroscopy, thermal annealing, hydrogen diffusion, defects
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-32787DOI: 10.1127/0935-1221/2009/0021-1971OAI: oai:DiVA.org:su-32787DiVA: diva2:281552
Available from: 2009-12-16 Created: 2009-12-16 Last updated: 2011-02-08Bibliographically 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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