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3.5 Fluids and their Interaction with Silicate Melts and Minerals

Surprisingly little is known about the speciation and properties of fluids at the pressures and temperatures of the lower crust and upper mantle. This is mostly due to experimental difficulties. Conventional spectroscopic cells for in-situ measurements of fluids are usually limited to temperatures below 500°C and pressures below 5 kbars. For this reason, even very simple systems such as H2O-SiO2 or H2O-CO2 have been almost exclusively studied at near-surface pressures and temperatures, while the speciation of these fluids under more extreme conditions is largely unknown. This situation is now changing with the advent of externally-heated hydrothermal diamond anvil cells capable of reaching 100 kbars and 1200°C. One ongoing project at Bayerisches Geoinstitute focusses on the speciation of silica in water circulating in the lower crust and upper mantle. In-situ Raman spectra obtained in a diamond cell show that Si(OH)4 is the dominant silica species over a wide range of pressures and temperatures. However, at conditions close to the critical point in the system SiO2-H2O, dimer and possibly polymer species of silica become important.

A large amount of work in 1998 has centered again on the solubility of water in nominally anhydrous minerals which could provide a major reservoir of water in the interior of the Earth. These studies include measurements of water solubility, water diffusion and the partitioning of water between various high-pressure phases. Ultimately, they will provide a complete picture of water storage capabilities and exchange kinetics involving various reservoirs of the mantle. Measurements of the speciation of water in silicate melts have now advanced to a stage where they can be routinely carried out on a variety of melt compositions. Accordingly, the aim of some ongoing work is to understand the speciation and structure of hydrous silicate melts with compositions close to natural magmas.

The degassing of magmas releases large amounts of sulfur compounds and halogens to the atmosphere. Among these gases, chlorine compounds have received particular attention, as they act as catalysts in processes that destroy the stratospheric ozone layer. Since the heavier halogens, such as bromine, are much less abundant than chlorine, they were often neglected when considering the impact of volcanic eruptions on atmospheric processes. Recent investigations showed, however, that the catalytic activity of bromine in the stratosphere is orders of magnitude higher than for chlorine. This means that bromine degassing from magmas may very well have an important effect on stratospheric chemistry. Therefore, we carried out the first experimental study of the degassing behaviour of the heavy halogens bromine and iodine.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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