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1 Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
2 Institut für Geowissenschaften, Universität Tuebingen, D-72074 Tübingen, Germany
Correspondence: * E-mail: geoffrey.bromiley{at}uni-bayreuth.de
A new technique for performing long duration (up to 300 hours) high-pressure annealing experiments under water-saturated conditions has been developed. This technique has been used to investigate water-solubility and speciation in natural, gem-quality chromian diopside. Capsule design for the technique is a variant of the double-capsule technique, and relies on the use of a semi-permeable Pt membrane, which permits free hydrogen diffusion into samples, but protects samples from reacting with buffer mixtures. The investigation of a natural single crystal of chromian diopside revealed a very unusual annealing behavior: water contents increase sharply after a short annealing period and then decrease slowly to some metastable equilibrium value. The main process that takes place during the annealing experiments is hydrogen diffusion coupled with Fe3+ reduction. This essentially reverses the main mechanism for hydrogen loss from mantle samples during exhumation, and the technique therefore provides sample-specific information on original water contents. Absorption bands at 3646 and 3434 cm–1 in IR spectra from annealed samples suggest two main mechanisms for hydrogen incorporation in the diopside sample: (1) incorporation of hydrogen onto the O2 site, with vibration of the OH dipole in the direction of a nearby O3 site (along the edge of an M2 site), and (2) incorporation of hydrogen onto the O2 site with vibration of the OH dipole toward a nearby O1 site (along a shared M1–M2 edge) or O2 site (along the edge of an M1 site). The ratio of peak heights between the absorption bands at 3646 and 3434 cm–1 is independent of water fugacity but dependent on oxygen fugacity, and appears to provide a measure of the redox state "frozen" into the sample. This ratio could be used to determine whether pyroxenes from upper-mantle xenoliths had experienced concurrent hydrogen-loss and oxidation during exhumation.
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