| 1. |
- Skogby, Henrik, et al.
(författare)
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Dehydration mechanisms in synthetic ortho- and clinopyroxene
- 2008
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Ingår i: Abstract volume of the 2nd EuroMinScI Conference, Giens, France, 31 March – 02 April 2008.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Pyroxenes are among the many nominally anhydrous minerals that have been shown to contain low but significant amounts of water, structurally bound as OH defects. The pyroxenes appear to be important hosts for water in the upper mantle, and it is therefore relevant to study the thermal stability of hydrogen and related exchange mechanisms.In two parallel studies we have investigated the dehydration and rehydration behaviour in Fe-doped ortho- and clinopyroxene, synthesised by piston-cylinder techniques at 20 - 25 kbar, by step-wise heat treatments in air and H2 atmosphere at 800 - 1000ºC. The change in OH content and Fe valence state was traced by FTIR and Mössbauer spectroscopy.Results show that hydrogen exchange to a substantial extent occurs via the redox reaction OH- + Fe2+ = O2- + Fe3+ + 1/2H2, but that additional reactions are also active. Some of the OH absorption bands in ortho- and clinopyroxene spectra have previously been shown to be related to Fe3+. These bands show a different behaviour during heat treatment in air and H2 atmosphere compared to bands that are not related to Fe3+. The different behaviour of the bands can be explained by a model with two reactions with different kinetics by which Fe3+-related defects are being produced and consumed. Application of the results to natural samples is not straightforward, as natural samples generally do not exhibit Fe3+-related absorption bands. Therefore only a maximum value can be estimated for the amount of water which has left the crystal by redox dehydration.
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| 2. |
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| 3. |
- Sundvall, Rickard, et al.
(författare)
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Dehydration-hydration mechanisms in synthetic diopside
- 2007
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Ingår i: Abstract volume of the 6th European Conference on Mineralogy and Spectroscopy, Stockholm 2007, p. 77.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- DEHYDRATION-HYDRATION MECHANISMS IN SYNTHETIC DIOPSIDERickard Sundvall(1,2), Henrik Skogby(1), Roland Stalder(3).1, Dept. of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden. Rickard.sundvall@nrm.se.2, Dept. of Geology and Geochemistry, Svante Arrhenius väg 8C, 106 91 Stockholm, Sweden.3, Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Angewandte und Experimentelle Mineralogie, Goldschmidtstraβe 1, 37077 Göttingen, Germany.Small amounts (ppm) of OH in nominally anhydrous minerals (NAM:s) can have a dramatic effect on the chemical and physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to carry substantial amounts of water in the form of hydroxyl ions. Enstatite and diopside are the most important 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 three crystals from the same synthesis run. The crystals were synthesized in a piston-cylinder apparatus at 20 kbar by slow cooling from 1330oC to 1100oC under water-saturated conditions with excess silica.A series of FTIR-spectra and corresponding Mössbauer-spectra were obtained after each step.From the Mössbauer-spectra we see an increase in the Fe3+ doublet with successive dehydration, although the increase in Fe3+ 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 are also active. Hydration experiments were conducted in the same manner as the dehydrations, with the exception that hydrogen gas was used during heating. Hydration experiments were only conducted on one crystal.The calculated diffusion for the dehydration reactions yields a -logD (D in m2/s) of 13.0 (1000oC), 15.1 (900oC), and 15.3 (800oC). Re-hydration of #218 (1000oC) gives a –logD of 13.3. All crystals were oriented parallel to (010). Diffusion rates are in the same order of magnitude as 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. The Arrhenius equation yields an activation energy (Ea) of -300 kJ mol-1, from the mean of the three reactions at three different temperatures. This Ea is very similar to that of dehydration of pure and low-Fe enstatite.
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| 4. |
- Sundvall, Rickard, 1976-, et al.
(författare)
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Dehydration-hydration mechanisms in synthetic Fe-poor diopside
- 2009
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Ingår i: European journal of mineralogy. - Stuttgart : E. Schweizerbart'sche Verlagsbuchhandlung. - 0935-1221 .- 1617-4011. ; 21:1, s. 17-26
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Sundvall, Rickard, 1976-, et al.
(författare)
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Hydrogen diffusion in synthetic Fe-free diopside
- 2009
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Ingår i: European journal of mineralogy. - Stuttgart : E. Schweizerbart'sche Verlagsbuchhandlung. - 0935-1221 .- 1617-4011. ; 21:5, s. 963-970
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Sundvall, Rickard, 1976-, et al.
(författare)
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Water in upper mantle pyroxene megacrysts and xenocrysts – a survey study
- 2011
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 96:0809 Aug-Sept, s. 1215-1227
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Weis, Franz A., et al.
(författare)
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Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5-3 kbar)
- 2016
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Ingår i: American Mineralogist. - 0003-004X .- 1945-3027. ; 101, s. 2233-2247
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Weis, Franz A., et al.
(författare)
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Polarized IR and Raman spectra of zoisite : insights into OH-dipole orientation and the luminescence
- 2016
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Ingår i: European journal of mineralogy. - : Schweizerbart. - 0935-1221 .- 1617-4011. ; 38:3, s. 537-543
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Tidskriftsartikel (refereegranskat)abstract
- The OH-dipole in the mineral zoisite has been a topic of discussion regarding its general orientation and vibrational modes. We present new polarized single-crystal Raman and infrared spectra and verify the orientation of the OH-dipole along the crystallographic c axis with a slight deviation towards the crystallographic a axis. Polarized Raman and FTIR spectra confirm that the OH band at 3150 cm(-1) corresponds to the O(10)-H center dot center dot center dot O(4) hydrogen bridge and exclude a previously suggested second hydrogen bridge O(10)-H center dot center dot center dot O(2). Further, Raman spectra provide insights on the luminescence of zoisite and the interference of luminescence peaks in the OH-region.
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| 9. |
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| 10. |
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