Fluid-induced mineral composition adjustments during exhumation: the case of Alpine stilbite

Stilbite is locally present as a very late mineral on fractures and fissures of granitic basement in the Central Swiss Alps. Stilbite samples from the Gotthard rail base tunnel provide evidence that they originally formed as a K-absent variety at depth. However, all stilbite samples from surface outcrops above the tunnel display significant potassium concentrations. Interestingly, water from fractures in the tunnel (at 50 °C) is oversaturated with respect to stilbite and essentially potassium-free whereas waters from high-Alpine brooks above the tunnel (and at other high-Alpine areas) have unusually high K/Na ratios. The data suggest that stilbite that may actively form on fissures at tunnel level as a K-absent variety by precipitation from water. Older stilbite that originally formed as coatings on fracture walls was gradually exhumed and uplifted and finally reached the today’s erosion surface about 2,000 m above the tunnel. However, the stilbite reaches the erosion surface as a K-rich variety as a result of interaction of the original low-K stilbite with surface water and near-surface groundwater. This leads to the conclusion that minerals once formed at depth may significantly change their composition once they reach the ground water zone on their way to the erosion surface. In the case of the stilbite, if surface outcrops would have been the only source of samples and data, the K-rich composition could have been mistaken for the composition of the mineral when it formed, which is not the case. Late-stage compositional readjustments may be difficult to discern in samples from surface outcrops. The provided data show that original mineral compositions may be adjusted by late-stage water–rock interaction in a highly selective way.     

Increasing levels and biomagnification of persistent organic pollutants (POPs) in Antarctic biota

Representatives of the Antarctic food web (krill, cephalopod, fish, penguin, seal) of the area around Elephant Island and from the Weddell Sea were analysed for the most recalcitrant organochlorine compounds. Due to sorption of the compounds to sinking particles and accumulation in sediments, two benthic fish species (Gobionotothen gibberifrons, Chaenocephalus aceratus) feeding on benthos invertebrates and fish reflected significantly increasing concentrations within a decade (1987-1996), while a benthopelagic species (Champsocephalus gunnari) feeding on krill did not. In the pelagic food chain, lipid normalised concentrations of all compounds increased from Antarctic krill to fish proving that biomagnification of highly lipophilic pollutants (log octanol-water partition coefficient>5) occurs in water-breathing animals. As top predators Weddell and southern elephant seals (Leptonychotes weddellii, Mirounga leonina) biomagnified the persistent organic pollutants relative to krill 30-160 fold with the exception of hexachlorobenzene, the levels of which were lower than in fish indicating its intense specific elimination.     

Bristen granite: a highly differentiated,fluorite-bearing A-type granite from the Aar massif,Central Alps,Switzerland

Bristen granite is a body of fine-grained leucogranite occurring in the Gotthard rail base tunnel in the Central Alps. During construction of the tunnel, Bristen granite (Brgr) has been drilled along a 600 m long section. The aplite-granite belongs to the suite of Variscan granitoid intrusions of the Aar massif and contains a variety of accessory minerals typical of highly differentiated granites. Rock forming fluorite, partly enriched in yttrium (Y) and rare earth elements (REE), is intergrown with the late Y- and REE-bearing carbonate mineral synchysite. The granite contains a variety of Ti- and Y-REE-niobates, thorite, and zircon. Compared with the calc-alkaline central Aar granite (cAgr), Bristen granite is strongly depleted in Ti, P, Mg, Sr, and Ba and shows a remarkable enrichment in incompatible elements such as Rb, Th, U, Nb, Y, HREE and F. Bristen granite is the most evolved granitoid rock of the Aar massif. The composition of Brgr is typical of post-collisional reduced (ferroan) A-type granites. The Brgr melt formed in the lower crust and crystallized from a highly differentiated melt at the cotectic point in the quartz-feldspar system close to 100 MPa and 700 °C. The Brgr intruded as a small isolated stock pre-Variscan gneisses with sharply discordant contacts. The primary igneous host of Nb, Ta, Y, U, Th and REE is biotite in addition to minor amounts of allanite, and zircon. The presence of Y-REE-fluorite, synchysite, parisite and Y- and Ti-niobates and other REE-minerals can be related to reaction of igneous biotite and primary fluorite with hydrothermal fluids. The reaction is associated with alpine metamorphism, because Y-bearing fluorite and synchysite have been reported from Alpine fissures. The transformation of primary biotite to chlorite and muscovite released the heavy metal oxides under lower greenschist facies conditions that formed the Alpine diagnostic mineral stilpnomelane at about 300 °C.     

Diatom assemblages from Costa Rican lakes: an initial ecological assessment

We compared the distributions of 59 diatom species in surface sediments of 25 Costa Rican lakes with 21 environmental variables using canonical correspondence analysis (CCA). The distribution of taxa was related to the chemical and physical characteristics of the lakes. The most influential chemical variables were cation concentrations (especially magnesium) and related variables such as water hardness, pH, and temperature. Lake area and lake depth were among the most important physical variables.A number of taxa were identified as potential environmental indicators. The diatoms Brachysira serians var. brachysira and Frustulia rhomboides seem to be associated with low values of alkalinity, hardness, Ca, Mg, and SiO2. Cymbella minuta var. silesiaca is associated with low to moderate values of alkalinity, hardness, Ca, and Mg. Nitzschia cf. amphibia may be an indicator of moderate-to-high concentrations of Mg. Pinnularia braunii var. amphicephala seems to prefer low values of hardness, Ca, Mg, and SiO2.In many closed lakes, these environmental variables (Mg/Ca/hardness/alkalinity) increase with effective evaporation. Consequently, these diatoms may be indirectly tracking P:E ratios. Results from this initial, small data set indicate the potential of diatoms for inferring lake paleochemistry, and perhaps P:E ratios, in Costa Rica.     

Optical Observations of the Nearby Isolated Pulsar PSR 0656+14 at the SAO 6-meter Telescope.

BVR data for the middle-aged radio pulsar PSR 0656+14 obtained on January 20-21, 1996 at the 6-m telescope of the Special Astrophysical Observatory are presented. The brightness of a star-like object coincident with the position of the VLA radio source in the Cousins B filter is B 25.1, with eff = 4448Å, adjacent to the HST F130LP long-pass filter. The relatively large V and R fluxes (3 or > 10-30 ergs cm-2 s-2 Hz-1) provide evidence that the optical emission of this pulsar is non-thermal up to 6600 Å. Most probably, in the UV-optical (BVR) spectral range, a power-law spectrum is super-imposed on the thermal-like emission of the neutron star surface, which could be related to the mechanism of the pulsar activity itself.     

Observation of OI 1300 Å radiation in the Martian atmosphere

Measurements from the 1225 to 1340 Å region by the ultraviolet detectors on Mars-3 are presented. Model calculations of the intensity of the OI triplet lines at 1304 Å are compared with the measurements made on December 27, 1971, and February 17, 1972. Agreement is found between experimental data and a model in which the neutral oxygen density at 100 km is 2–8 × 10⁹ cm^?3.