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During the last year, several studies at Bayerisches Geoinstitut were concerned with the distribution of water between mantle phases. While water is a trace constituent of the upper mantle, there are good reasons to believe that some phases in the transition zone and the lower mantle could concentrate water and therefore provide major hydrous reservoirs deep inside the Earth. In particular, it had been speculated that OH groups could readily enter the structure of MgSiO₃ perovskite, the major constituent of the lower mantle. Since this phase accounts for about 50% of the mass of the Earth, even traces of OH defects in its structure could represent an amount of water comparable to the combined mass of all oceans. First experiments in the system MgO-SiO₂ show that the ilmenite polymorph of MgSiO₃ indeed easily incorporates several thousand ppm weight of water, while the perovskite phase in equilibrium with MgSiO₃ ilmenite is virtually anhydrous. This could change, however, in the presence of chemical defects that may facilitate the incorporation of OH groups. Indeed, a systematic study on MgSiO₃ enstatite shows that water solubility greatly increases in the presence of aluminum, presumably due to the substitution of H⁺ + Al³⁺ for Si⁴⁺.

Even in the upper mantle, where the water content usually is low, enormous quantities of hydrous fluid can be released by dehydration reactions in subduction zones. Previous work in Bayreuth has shown that at least in the albite-H₂O system, there is complete miscibility between silicate melt and hydrous fluid under the PT conditions prevailing in the deeper part of subduction zones. By extending this work to more complex systems with compositions relevant to the mantle, we were now able to show that this holds for a wide range of natural silicate melt compositions as well. Progress has also been made in studying the speciation of water in complex aluminosilicate melts as a function of pressure and temperature. A common aspect of these projects is the use of a special type of externally heated diamond cell for infrared studies previously developed in Bayreuth.

Aside from water, carbon dioxide and sulfur compounds are the most important fluid constituents in the crust and mantle. An exploratory study aimed at measuring the speciation of CO₂ in silicate melts directly at high P and T by Raman and infrared techniques has just been started. A study of the distribution of sulfur between silicate melts and hydrous fluids, on the other hand, is almost completed. These measurements explain why increasing sulfur emissions from a volcano often precede an eruption. Moreover, the data can be used to estimate the mass of a free fluid phase inside a magma chamber prior to eruption.