Thematic Articles

Geological fluids affect deformation of rocks both physically and chemically. The presence of fluids can lead to faulting (earthquakes) or enhance flow, depending on the level of stress. At higher stresses, fluids with a density less than their host generate Mode I microcracks, whereas fluids with a density greater than their host generate Mode I microanticracks; both can self-organize and cause faulting. At lower stresses, fluids segregate to grain boundaries at small strains and, at large strains, fluid-enriched zones develop that experience a higher strain rate than the bulk. Dissolved H2O enhances flow (e.g., by water-weakening). Consequences include earthquakes, differentiation, melt separation/volcanism, and seismic anisotropy.

Subducting slabs transport water stored in hydrous minerals into the transition zone and lower mantle. The water storage capacity of the upper and lower mantles is less than 0.2 wt%. The transition zone has a storage capacity of approximately 0.5–1 wt% due to a water solubility of about 1–3 wt% in wadsleyite and ringwoodite, which are the major con- stituents of the transition zone. Thus, the transition zone may be a major water reservoir in the Earth’s interior. Recent geophysical observations suggest the existence of the hydrated transition zone beneath subduction zones. Water or hydrogen may be transported as far as the bottom of the lower mantle by reacting with metallic iron in the lower mantle to form hydrous phases or iron hydride.

The abundance and flux of volatiles are important to the hazards associated with explosive volcanic eruptions. Volatiles in magmas can be determined from investigations of melt and fluid inclusions in combination with volatile solubility data. A comparison of the newly deter- mined solubilities of H 2O, SO2, and Cl in a molten Vesuvius phonolite with the compositions of Mt. Somma-Vesuvius melt and fluid inclusions further elucidates degassing and eruptive processes for this volcano.

Ore-forming (hydrothermal) fluids, consisting largely of H 2O, CO2, and NaCl, formed most of Earth’s ore deposits. The fluids exist as largely unconfined systems in meteoric, seawater, and basinal settings, or locally and intermittently confined systems in magmatic, metamorphic, and basinal settings, and they are driven largely by differences in temperature, elevation or density. Temperatures are highest (~600°C) in magmatic and lowest in basinal and meteoric (~100°C) systems. Salinities well above that of seawater are reached by boiling, evaporation, and evaporite dissolution, largely in magmatic and basinal systems. Today, research is focused on establishing the concentrations of metals in these fluids, the volume and duration of hydrothermal flow, and the links between ore systems and larger, regional fluid systems.

At our first inaugural meeting last April, we sifted through several pages of potential topics. Some had been suggested by the councils of the par- ticipating societies; others had been gathered at meetings, while talking to colleagues. We strived to choose topics that would be of interest to a wide part of our membership – in each of these issues, mineralogists, geochemists, and petrologists should find at least one article of great interest to them – but also topics where exciting developments are underway. We also needed to find guest editors and authors willing to work under very tight deadlines. It you are interested in proposing a topic, you can download a propos- al form from our web site. We are already developing themes for 2006 and 2007. Here is what you can look forward to in 2005.