DEHYDRATION-HYDRATION MECHANISMS IN SYNTHETIC DIOPSIDE
Rickard Sundvall(1,2), Henrik Skogby(1), Roland Stalder(3).
1, Dept. of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden. Rickard.email@example.com.
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.