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Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5-3 kbar)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Swedish Museum Nat Hist, Dept Geosci, Box 50007, SE-10405 Stockholm, Sweden.
Univ Innsbruck, Inst Mineral & Petrog, A-6020 Innsbruck, Austria.
Swedish Museum Nat Hist, Dept Geosci, Box 50007, SE-10405 Stockholm, Sweden.
2016 (English)In: American Mineralogist, ISSN 0003-004X, E-ISSN 1945-3027, Vol. 101, 2233-2247 p.Article in journal (Refereed) Published
Abstract [en]

Water is a key parameter in mantle rheology, magma genesis, magma evolution, and resulting eruption styles, because it controls the density and the viscosity, as well as the melting and crystallization behavior of a melt. The water content in nominally anhydrous minerals (NAMs) such as clinopyroxene recently has been used as a proxy for magmatic water contents. NAMs, however, may dehydrate during magma degassing and eruption. We performed rehydration experiments on potentially degassed clinopyroxene phenocrysts from various volcanic settings. The experiments were conducted in hydrogen gas at 1 atm or hydrothermal pressures ranging from 0.5 to 3 kbar to test the incorporation of water into natural clinopyroxene under water fugacities similar to those in a volcanic system. Our results show a dependence of the water content in the clinopyroxene crystals with pressure as the phenocrysts begin to dehydrate upon lower water fugacities in the experiments. Water loss or gain in a crystal occurs according to the relatively fast redox-reaction OH- + Fe2+ ↔ O2– + Fe3+ + ½ H2, which was confirmed by Mössbauer spectroscopy. The kinetics of this redox-process are independent of pressure and thus water fugacity. Water contents in rehydrated clinopyroxene crystals can be related to magmatic water contents at various levels in a volcanic system. Our results thus show that the water content in erupted clinopyroxene phenocrysts cannot be taken for granted to be representative of magmatic water contents prior to magma degassing. The conducted experiments indicate the simultaneous dehydration of clinopyroxene along with magma ascent and degassing. Rehydration experiments under hydrothermal pressures, however, may be able to reconstruct clinopyroxene water contents at crystallization prior to dehydration.

Place, publisher, year, edition, pages
2016. Vol. 101, 2233-2247 p.
National Category
Geosciences, Multidisciplinary
Identifiers
URN: urn:nbn:se:uu:diva-304398DOI: 10.2138/am-2016-5711CCBYNCNDISI: 000385605100027OAI: oai:DiVA.org:uu-304398DiVA: diva2:1032919
Funder
Swedish Research Council
Available from: 2016-10-05 Created: 2016-10-05 Last updated: 2017-11-30Bibliographically approved
In thesis
1. Hydrogen in nominally anhydrous silicate minerals: Quantification methods, incorporation mechanisms and geological applications
Open this publication in new window or tab >>Hydrogen in nominally anhydrous silicate minerals: Quantification methods, incorporation mechanisms and geological applications
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to increase our knowledge and understanding of trace water concentrations in nominally anhydrous minerals (NAMs). Special focus is put on the de- and rehydration mechanisms of clinopyroxene crystals in volcanic systems, how these minerals can be used to investigate the volatile content of mantle rocks and melts on both Earth and other planetary bodies (e.g., Mars). Various analytical techniques for water concentration analysis were evaluated.

The first part of the thesis focusses on rehydration experiments in hydrogen gas at 1 atm and under hydrothermal pressures from 0.5 to 3 kbar on volcanic clinopyroxene crystals in order to test hydrogen incorporation and loss from crystals and how their initial water content at crystallization prior to dehydration may be restored. The results show that extensive dehydration may occur during magma ascent and degassing but may be hindered by fast ascent rates with limited volatile loss. De- and rehydration processes are governed by the redox-reaction OH- + Fe2+ ↔ O2- + Fe3+ + ½ H2. Performing rehydration experiments at different pressures can restore the water contents of clinopyroxene at various levels in the volcanic systems. Subsequently water contents of magmas and mantle sources can be deduced based on crystal/melt partition coefficients. This thesis provides examples from the Canary Islands, Merapi volcano in Indonesia and the famous Nakhla meteorite. Using NAMs as a proxy for magmatic and mantle water contents may provide a very good method especially for planetary science where sample material is limited.

The thesis’ second part focusses on analytical methods to measure the concentration of water in NAMs. Specifically the application of Raman spectroscopy and proton-proton scattering are tested. The hydrated mineral zoisite is thoroughly analyzed in order to be used as an external standard material. Polarized single crystal spectra helped to determine the orientation of the OH-dipole in zoisite. Further, Transmission Raman spectroscopy and a new method for the preparation of very thin samples for proton-proton scattering were developed and tested. The results provide new possibilities for the concentration analysis of water in NAMs such as three dimensional distribution and high spatial resolution.                       

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 64 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1448
Keyword
NAMs, clinopyroxene, hydrogen, hydrothermal pressure, magmatic water content, zoisite, OH-dipole, Raman spectroscopy, FTIR, luminescence, proton-proton scattering
National Category
Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-306212 (URN)978-91-554-9740-8 (ISBN)
Public defence
2016-12-14, Lilla Hörsalen, Naturhistoriska Riksmuseet, Frescativägen 40, 11418 Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2016-11-21 Created: 2016-10-26 Last updated: 2016-11-28

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