Origin of silicic volcanic rocks in Central Costa Rica: a study of a chemically variable ash-flow sheet in the Tiribí Tuff

Chemical heterogeneities of pumice clasts in an ash-flow sheet can be used to determine processes that occur in the magma chamber because they represent samples of magma that were erupted at the same time. The dominant ash-flow sheet in the Tiribí Tuff contains pumice clasts that range in composition from 55.1 to 69.2 wt% SiO₂. It covers about 820 km² and has a volume of about 25 km³ dense-rock equivalent (DRE). Based on pumice clast compositions, the sheet can be divided into three distinct chemical groupings: a low-silica group (55.1-65.6 wt% SiO₂), a silicic group (66.2-69.2 wt% SiO₂), and a mingled group (58.6-67.7 wt% SiO₂; all compositions calculated 100% anhydrous). Major and trace element modeling indicates that the low-silica magma represents a mantle melt that has undergone fractional crystallization, creating a continuous range of silica content from 55.1-65.6 wt% SiO₂. Eu/Eu*, MREE, and HREE differences between the two groups are not consistent with crystal fractionation of the low-silica magma to produce the silicic magma. The low-silica group and the silicic group represent two distinct magmas, which did not evolve in the same magma chamber. We suggest that the silicic melts resulted from partial melting of relatively hot, evolved calc-alkaline rocks that were previously emplaced and ponded at the base of an over-thickened basaltic crust. The mingled group represents mingling of the two magmas shortly before eruption. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-001-0188-8.     

Overexploitation and Deterioration of Aquifers in Venezuela

Approximately 50 percent of the potable, industrial, and agricultural water supply in Venezuela comes from ground water, mined through some 40,000 wells scattered around the country. The majority of the productive aquifers consist of quaternary deposits in mountain valleys and coastal lowlands, all of which are situated in areas with limited or low precipitation. Intensive exploitation of several of these aquifers for the last 30 years has resulted in overdraft conditions and ground water contamination, causing serious socio-economic consequences in various regions of Venezuela. This paper reviews the quality and quantity aspects of overdrafted aquifers in Venezuela, which can he classified into three major categories: (1) aquifers in the coastal lowlands in the stales of Falcon and Zulia of northern Venezuela: (2) the intermontane Carora and Quibor Valley Aquifers: and (3) aquifers in the Lake Valencia Watershed. If current trends of aquifer overexploitation continue, further water deficits and irreversible environmental damage are predicted.     

Kinetics of the Gas-Phase Reactions of OH and NO3 Radicals and O3 with the Monoterpene Reaction Products Pinonaldehyde, Caronaldehyde, and Sabinaketone

Using a relative rate method, rate constants have been measured for the gas-phase reactions of OH and NO3 radicals with pinonaldehyde, caronaldehyde and sabinaketone at 296 ± 2 K. The OH radical reaction rate constants obtained are (in units of 10–12 cm3 molecule–1 s–1): pinonaldehyde, 48 ± 8; caronaldehyde, 48 ± 8; and sabinaketone, 5.1 ± 1.4, and the NO3 radical reaction rate constants are (in units of 10–14 cm3 molecule–1 s–1): pinonaldehyde, 2.0 ± 0.9; caronaldehyde, 2.5 ± 1.1; and sabinaketone, 0.036 ± 0.023, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Upper limits to the O3 reaction rate constants were also obtained, of <2 × 10–20 cm3 molecule–1 s–1 for pinonaldehyde and caronaldehyde, and <5 × 10–20 cm3 molecule–1 s–1 for sabinaketone. These reaction rate constants are combined with estimated ambient tropospheric concentrations of OH radicals, NO3 radicals and O3 to calculate tropospheric lifetimes and dominant transformation process(es) of these and other monoterpene reaction products.     

Ensemble Spatial Interpolation: A New Approach to Natural or Anthropogenic Variable Assessment

Natural Resources Research - This paper presents a novel and versatile framework for building ensemble spatial interpolation functions. As with all ensemble methods, the central idea is to assemble...     

Distribution,Population Structure, ­Population Biomass and Morphological Characteristics of the Tunicate Pyura stoloniferain the Bay of Antofagasta,Chile

Abstract. The present study analyzes the distribution, population structure, population biomass and morphological characteristics of the intertidal tunicate Pyura stolonifera (Heller, 1878) in the Bay of Antofagasta, Chile, where the species shows a reduced geographical range extending around 60 – 70 km. Three sites along its distribution were studied. At the central site the tunicate presents the widest intertidal belts, reaching nearly 11 m (the mid‐intertidal‐Pyura‐zone shows the higher percent cover). Population dry biomass here averaged up to 20.45 kg·m^–2, representing one of the highest intertidal specific biomasses reported in the literature. Lower values occur towards the borders of its distribution. Population densities were over 1800 indiv.·m^–2 at mid‐intertidal‐Pyura‐belts. Crowding and competition potentially affect morphological characteristics and the energy/tissue allocation along the intertidal and geographical gradient of P. stolonifera.     

Geophysical investigation of the Colônia structure,Brazil

The origin of the nearly circular Colônia structure, located at the southwestern edge of the city of São Paulo, Brazil, has been the subject of a long‐standing debate, ever since the 1960s when the structure was first investigated by geophysical methods. The structure still raises interest for geological research, as its sedimentary infill holds important paleoclimatic information about the evolution of the tropical rainforest, as well as the interplay between the South American summer monsoon, the Intertropical Convergence Zone, and the southern Westerly wind belt—for possibly as long as several million years. In addition, the search for evidence to conclusively establish the origin of this structure continues, and the answer most likely lies in the lower portions of the basin's sedimentary infill, which also holds a significant potential for underground water resources. Here, we present the results from recent seismic (reflection and HVSR), gravimetric, and geoelectrical surveys. They have provided a reliable image of the sedimentary infill, and the maximum depth to basement within the structure has been constrained consistently by the different methods to approximately ?400 m. The geophysical data have also allowed to map the lateral contact between the crystalline basement rocks and the sedimentary infill, which indicates a diameter of approximately 2.8 km for the sedimentary basin, with 3.6 km being the diameter of the outermost limit of the structure. A total of six seismic stratigraphic boundaries were identified within the sedimentary infill, providing a framework for the planning of a deep drilling campaign and subsequent sampling program aimed at geological and paleoclimatology studies.     

The metamorphic aureole of the Nisa-Alburquerque batholith (SW Iberia): implications for deep structure and emplacement mode

The Nisa-Alburquerque granitic batholith (southern Variscan Belt, Iberian Peninsula) has been studied by petrological, structural and geophysical approaches, obtaining contrasting models for its deep structure and emplacement sequence. In order to test these models and gain knowledge on the thermal increase induced by the intrusion, we have studied its contact aureole, which was developed in similar country rock lithologies (mica schists alternating with metasandstones and feldespatic schists) all along the northern external contact of the batholith. Our results indicate no change in metamorphic grade and some variations in aureole width, which narrows toward the western sectors of the batholith. Cordierite is the only contact metamorphic mineral developed together with a high temperature biotite probably related to the granite thermal input. By considering these new data, together with zircon saturation temperatures within the granite and previous petrological and geophysical studies, we propose a model in which the feeder zones of the granitic magmas were an eastern main one and a western secondary one. We have also made comparisons of the metamorphic grade in the country rocks and the xenoliths within the granite. Most of the xenoliths have the same metamorphic facies as the country rocks (Crd-zone), though some of them contain slightly different assemblages (And + Crd), which could be explained in different ways: (1) differences in the primary schist compositions, (2) increased time-span of xenoliths in contact with the melt and (3) xenolith incorporation at slightly higher depths during final granite ascent.