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Water as a trace component in mantle pyroxene: Quantifying diffusion, storage capacity and variation with geological environment
Stockholm University, Faculty of Science, Department of Geological Sciences.
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 [en]
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: urn:nbn:se:su:diva-38701ISBN: 978-91-7447-065-9 (print)OAI: oai:DiVA.org:su-38701DiVA: diva2:314576
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
List of papers
1. Dehydration-hydration mechanisms in synthetic Fe-poor diopside
Open this publication in new window or tab >>Dehydration-hydration mechanisms in synthetic Fe-poor diopside
2009 (English)In: European journal of mineralogy, ISSN 0935-1221, E-ISSN 1617-4011, Vol. 21, no 1, 17-26 p.Article in journal (Refereed) Published
Abstract [en]

Small amounts (ppm) of OH in nominally anhydrous minerals (NAMs) can have a dramatic effect on the physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to incorporate substantial numbers of protons forming hydroxyl ions. Enstatite and diopside are the most important endmembers of the pyroxenes in terms of bulk volume in the upper mantle. To further constrain the behavior of hydroxyl ions in clinopyroxene, the dehydration-hydration mechanisms of synthetic 57Fe-doped diopside were investigated. Dehydration was carried out by stepwise heating in air of crystals synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step. Mössbauer spectra were recorded for three of the crystals when there had been a significant decrease in FTIR absorbance intensity. From the Mössbauer spectra we see an increase in the Fe3+ doublet with successive dehydration, although this increase is less than the decrease in OH in terms of atoms per formula unit. This means that the dehydration only partly follows the redox reaction OH + Fe2+ = O2− + Fe3+ + ½H2, and that additional reactions occur. Hydration experiments were conducted on one crystal in the same manner as the dehydrations, with the exception that hydrogen gas was used during heating. Hydration experiments resulted in re-hydration of the sample to 73 % of the original amount of OH.

 

The calculated Arrhenius equation derived from the diffusion rates during dehydration along [010] yields an activation energy (Ea) of −292 ± 50 kJ mol−1, and D0 = 10±1.9 + 2.3 m2 s−1. The result of the rehydration experiment agrees well with the established diffusion law. Diffusion rates determined for synthetic diopside are almost two orders of magnitude slower than for synthetic enstatite with comparable Fe contents. Compared to natural diopside, diffusion rates in these synthetic samples are slower, probably because of the low iron content. Ea is similar to that of dehydration of pure and low-Fe enstatite.

Place, publisher, year, edition, pages
Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung, 2009
Keyword
diopside, hydrogen incorporation, diffusion, FTIR spectroscopy, Mössbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-29033 (URN)10.1127/0935-1221/2009/0021-1880 (DOI)
Available from: 2009-08-06 Created: 2009-08-06 Last updated: 2010-11-08Bibliographically approved
2. Hydrogen diffusion in synthetic Fe-free diopside
Open this publication in new window or tab >>Hydrogen diffusion in synthetic Fe-free diopside
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
Keyword
diopside, FTIR spectroscopy, thermal annealing, hydrogen diffusion, defects
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-32787 (URN)10.1127/0935-1221/2009/0021-1971 (DOI)
Available from: 2009-12-16 Created: 2009-12-16 Last updated: 2011-02-08Bibliographically approved
3. Water in upper mantle pyroxene megacrysts and xenocrysts – a survey study
Open this publication in new window or tab >>Water in upper mantle pyroxene megacrysts and xenocrysts – a survey study
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
Keyword
mantle water, pyroxene, mantle xenoliths, megacrysts, FTIR spectroscopy, FPA detector analysis, Mößbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-38665 (URN)10.2138/am.2011.3641 (DOI)
Available from: 2010-04-24 Created: 2010-04-24 Last updated: 2017-12-12Bibliographically approved
4. Hydrogen defect saturation in natural pyroxene
Open this publication in new window or tab >>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
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
Keyword
Pyroxene, hydrogen accommodating defects, hydrogen diffusion, FTIR spectroscopy, Mößbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-38666 (URN)10.1007/s00269-010-0407-y (DOI)
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: 2017-12-12Bibliographically approved

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