Dehydration mechanisms in synthetic ortho- and clinopyroxene
2008 (English)In: Abstract volume of the 2nd EuroMinScI Conference, Giens, France, 31 March – 02 April 2008, 2008Conference paper (Other academic)
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.
Place, publisher, year, edition, pages
IdentifiersURN: urn:nbn:se:su:diva-15951OAI: oai:DiVA.org:su-15951DiVA: diva2:182471