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Water in upper mantle pyroxene megacrysts and xenocrysts – a survey study
Stockholm University, Faculty of Science, Department of Geological Sciences.
Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52f, 6020 Innsbruck, Österrike.
2011 (English)In: American Mineralogist, ISSN 0003-004X, E-ISSN 1945-3027, Vol. 96, no 0809 Aug-Sept, 1215-1227 p.Article in journal (Refereed) Published
Abstract [en]

Water content, mineral chemistry and oxidation state of clino- and orthopyroxene xenocrysts and megacrysts was investigated by FTIR (Fourier Transform Infrared) spectroscopy – including FPA (Focal Plane Array) detector mapping, Mößbauer spectroscopy and electron microprobe. A variety of tectonic settings, ages and modes of emplacement are represented by 23 samples from 6 areas (Massif Central, France; Letseng, Lesotho; Colorado, USA; Kakanui, New Zealand; Oahu, Hawaii; New South Wales, Australia). The xenocrysts are from both garnet and spinel peridotites – including lherzolite and harzburgite varieties, and one sample of clinopyroxenite. Water contents vary between ~10 and 600 wt ppm. Samples from Massif Central, Colorado, Kakanui and Hawaii have rather high water contents: 180 – 600 wt ppm. The samples from Lesotho and New South Wales have considerably lower amounts: ~10 – 300 wt ppm. Water contents of xenocrysts and megacrysts from New South Wales vary within a narrow range (clinopyroxene: ~50 wt ppm, orthopyroxene: 15-20 wt ppm), whereas the water content of the Lesothian samples scatter considerably. No significant correlations are observed between water content, mineral chemistry, or oxidation state of the samples. FPA mapping reveals homogenous distribution of water in the pyroxene lattice. The results are compared to available literature and research on hydrogen diffusion in natural mantle pyroxene. Altogether, the data suggest that pyroxene found in fresh peridotite xenoliths partly reflects the water content of the mantle source region. On the other hand, variable mineral chemistry and water contents of megacryst pyroxenes indicate processes such as magmatic equilibration, magma mixing and contamination.

Place, publisher, year, edition, pages
Mineralogical Society of America, 2011. Vol. 96, no 0809 Aug-Sept, 1215-1227 p.
Keyword [en]
mantle water, pyroxene, mantle xenoliths, megacrysts, FTIR spectroscopy, FPA detector analysis, Mößbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-38665DOI: 10.2138/am.2011.3641OAI: oai:DiVA.org:su-38665DiVA: diva2:312460
Available from: 2010-04-24 Created: 2010-04-24 Last updated: 2017-12-12Bibliographically 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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