Preservation of chemical residue-melt equilibria in natural anatexite: the effects of deformation and rapid cooling

A combined petrologic, textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the Bergell Pluton (Central Alps). In contrast to most regional metamorphic anatexites, chemical equilibrium between residue and segregated melt was attained and preserved by several elements, as shown by their present distribution. Equilibrium involved both the major and the trace elements hosted in the rock-forming minerals and the light rare earth elements (LREE), hosted primarily in allanite. Equilibrium with respect to LREEs was achieved because allanite had a reactive behaviour during water-present melting, in contrast to the refractive behaviour of xenotime, which mostly controls the distribution of the HREE's. The attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions. We propose that the achievement of equilibrium between melt and residue was promoted by deformation, operating via a mechanism of melt present granular flow (i.e. dissolution-accommodated grain-boundary sliding active during partial melting). Microstructural observations indicate that melt-present granular flow was the dominating deformation mechanism in these anatexites. The preservation of residue-melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus.     

Near-surface earthquakes in the Baltic shield, notably Sweden

A recent series of Swedish earthquakes at a focal depth not exceeding 2–3 km, the largest with I_O = V + (MSK scale) andM_L = 3.2 shows that relatively strong seismic activity can occur in the uppermost part of the Baltic Shield. During the last 15 years several near-surface earthquakes have occurred in this region, as indicated by recorded Rg-waves and/or macroseismic data. Many events are located along the coast of central Sweden, suggesting a seismic belt of minor, near-surface activity, which should be considered in connection with the radioactive waste storage in the Swedish bedrock. The appearance of Rg, common in seismic records of explosions and rockbursts, is not a sufficient discriminator between artificial events and earthquakes.     

Lower hybrid instability driven by charged dust beam

This brief note investigates a lower hybrid instability driven by a beam of negatively charged dust in a collisionless magnetized dusty plasma. Application to the generation of fluctuations by Space Shuttle exhaust in the ionosphere is discussed.     

Climate Change Impacts for the Conterminous USA: An Integrated Assessment

Human activities have altered the distribution and quality of terrestrial ecosystems. Future demands for goods and services from terrestrial ecosystems will occur in a world experiencing human-induced climate change. In this study, we characterize the range in response of unmanaged ecosystems in the conterminous U.S. to 12 climate change scenarios. We obtained this response by simulating the climatically induced shifts in net primary productivity and geographical distribution of major biomes in the conterminous U.S. with the BIOME 3 model. BIOME 3 captured well the potential distribution of major biomes across the U.S. under baseline (current) climate. BIOME 3 also reproduced the general trends of observed net primary production (NPP) acceptably. The NPP projections were reasonable for forests, but not for grasslands where the simulated values were always greater than those observed. Changes in NPP would be most severe under the BMRC climate change scenario in which severe changes in regional temperatures are projected. Under the UIUC and UIUC + Sulfate scenarios, NPP generally increases, especially in the West where increases in precipitation are projected to be greatest. A CO₂-fertilization effect either amplified increases or alleviated losses in modeled NPP. Changes in NPP were also associated with changes in the geographic distribution of major biomes. Temperate/boreal mixed forests would cover less land in the U.S. under most of the climate change scenarios examined. Conversely, the temperate conifer and temperate deciduous forests would increase in areal extent under the UIUC and UIUC + Sulfate scenarios. The Arid Shrubland/Steppe would spread significantly across the southwest U.S. under the BMRC scenario. A map overlay of the simulated regions that would lose or gain capacity to produce corn and wheat on top of the projected distribution of natural ecosystems under the BMRC and UIUC scenarios (Global mean temperature increase of +2.5 °C, no CO₂ effect) helped identify areas where natural and managed ecosystems could contract or expand. The methods and models employed here are useful in identifying; (a) the range in response of unmanaged ecosystem in the U.S. to climate change and (b) the areas of the country where, for a particular scenario of climate change, land cover changes would be most likely.     

Intermittency of riometer auroral absorption observed at South Pole

Auroral radio-wave absorption values measured at South Pole for 3 years using a riometer are analyzed in order to test whether they show evidence of intermittency. The properties of the parameters of the probability density functions determined for several magnetic local time sectors are found to be significantly different. The probability density functions for the pre-midnight sector show the typical shape associated to intermittency. No results are given for the afternoon sector because few auroral absorption events meet the selection criteria to give statistically significant results. It is suggested that if the precipitating particle population responsible for the riometer auroral absorption shared the intermittency features of the absorption then the present results would allow the study of the properties of the induction component of magnetospheric turbulence.     

Chemical reaction rates and entrainment within the Endeavour Ridge hydrothermal plume

The aging of the hydrothermal plume over the Endeavour segment of the Juan de Fuca Ridge was estimated by measuring the²²²Rn³He ratio in the plume as it dispersed. Despite uncertainties in the source function of hydrothermal input, it wa determined that the relative sequence of removal from the plume isH₂ > Δc >²²²Rn>CH₄ Mn, whereΔc is a measure of particle concentration and the mean life of²²²Rn is 5.5 days. H₂ is removed from the plume within hours of input while Mn is not removed within the two-week timescale of the radon-helium clock.Entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone. This is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient-like profile in the deep ocean. Thus we found that significant fractions of the Si and²²⁶Ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height.