Residence time of S-type anatectic magmas beneath the Neogene Volcanic Province of SE Spain: a zircon and monazite SHRIMP study

Zircon and monazite from three restitic enclaves and one host dacite have been dated by ion microprobe (SHRIMP), with the aim of characterising their Miocene history and defining the timing relationships between crustal melting and eruption in the high-K calc-alkaline volcanics of the Neogene Volcanic Province of SE Spain. The studied samples are from the volcanic edifices of El Joyazo (Cerro del Hoyazo) and Mazarrón. Zircons in the enclaves are characterized by a thin euhedral rim overgrowing a detrital core. The core-rim boundary is marked by tiny glass inclusions of S-type granitic composition, which attest to the growth of zircon rims during a crustal melting event. At El Joyazo, where lavas erupted at 6.3 Ma (Zeck and Williams 2002), U-Pb ages of zircon overgrowths define an age of anatexis of 9.63±0.26 Ma (95% c.l.), in agreement with monazite ages of 9.74±0.21 Ma (95% c.l.). At Mazarrón, the age of anatexis provided by monazite at 9.13±0.18 Ma (95% c.l.) overlaps with that of melt-precipitated zircon in the host dacite, dated at 9.06±0.53 Ma (95% c.l.). These results indicate that after partial melting, the enclaves and the syngenetic S-type melts resided at depth for >3 m.y. at El Joyazo. Compared with the results from Mazarrón, the long residence time obtained at El Joyazo is probably due to the greater depth of melting (c. 25 km vs. c. 15 km). At such depth, corresponding to the Miocene palaeo-Moho, the more ductile regime of the crust is likely to have favoured magma ponding. The thermal anomaly beneath the Neogene Volcanic Province, which generated the S-type crustal melts, is today visible from geophysical data and can be traced back to the Lower Miocene. As a consequence, residence times longer than determined in this work may be expected.Editorial responsibility: J. Hoefs     

Fate of polycyclic aromatic hydrocarbon (PAH) contamination in a mangrove swamp in Hong Kong following an oil spill

The fate of polycyclic aromatic hydrocarbon (PAH) contamination in a mangrove swamp (Yi O) in Hong Kong after an oil spill accident was investigated. The concentrations and profiles of PAHs in surface sediments collected from five quadrats (each of 10 m×10 m) covering different degrees of oil contamination and the most contaminated mangrove leaves were examined in December 2000 (30 days after the accident) and March 2001 (126 days later). The concentrations of total PAHs in surface sediments ranged from 138 to 2135 ng g⁻¹, and PAHs concentrations decreased with time. In the most contaminated sediments, total PAHs dropped from 2135 (30 days) to 1196 ng g⁻¹ (120 days), and the decrease was smaller in less contaminated sediments. The percentage reduction in sediment PAHs over three months (44%) was less significant than that in contaminated leaves (85%), indicating PAH in or on leaves disappeared more rapidly. The PAH profiles were very similar in sediments collected from quadrats Q1 and Q2 with benzo[a]anthracene and pyrene being the most abundant PAH compounds, but were different in the other three quadrats. The proportion of the light molecular weight PAHs to total PAHs increased after three months, especially phenanthrene. Results suggest that physical and photo-chemical weathering (tidal washing and photo-oxidation) of crude oil in surface sediments and on plant leaves were important processes in the first few months after the oil spill. The PAH contamination in Yi O swamp came from both petrogenic and pyrolytic sources. The petrogenic characteristic in the most contaminated sediment was confirmed with high values of phenanthrene to anthracene ratio (>10) and low values of fluoranthene to pyrene ratio (0.3–0.4).     

Effects of industrial by-product amendments on As,Cd and Tl retention/release in an element-spiked acidic soil

To asses the efficiency of two by-products (phosphogypsum (PG) and sugar foam (SF), rich in gypsum and calcium carbonate, respectively) in the immobilization of three toxic elements (As, Cd and Tl) in an acidic soil, batch-scale sorption and desorption experiments were conducted after 18 months of in situ amendment application. The Langmuir isotherms applied for sorption studies showed that the estimated maximum sorption capacity of the elements was highest in the SF-treated samples. The amount of element retained and the percentage of extraction after TCLP tests indicated that those samples amended with sugar foam (SF and PG + SF) had the potential to immobilize As, Cd and Tl in an acidic soil with low sorptive capacity. In addition to sorption and desorption experiments, scanning electron microscopy in back-scattered electron mode (SEM-BSE) showed the formation of Al-hydroxy polymers which provides the soil with additional sorption capacity. The three target elements were associated with the Al-hydroxy polymers, probably through direct coordination or the formation of ternary complexes. By means of statistical analysis it has been found that sorption processes of As, Cd and Tl in this soil mainly depend on the treatment, whereas desorption is an element-dependent process.     

Simulated thermal alteration of sterols and keto—Alcohols in marine sediments of the Okinawa Trough

Compositional data on the sterol and alcohol fractions isolated from deep-sea marine sediments from the Okinawa Trough were obtained to determine the relative contribution from marine and terrestrial inputs. Following extraction, the sterol plus alcohol fraction was isolated by layer chromatography, derivatized with BSTFA and then analysed by capillary GC and GC—MS. A suite of C₂₆−C₂₉ stenols and stanols and C₃₀−C₃₂ keto—alcohols were identified in the sediments. The thermal stability of the compounds in these sediments was studied by heating portions of the surface sediment in glass tubes for 16 hours at temperatures from 50°C to 200°C. The C₂₇ stanol/stenol ratio increased when temperatures went up to 175°C, but the distribution of C₃₀−C₃₂ Keto—alcohols remained unaffected. At 200°C most of the sterols and Keto—alcohols were destroyed. Supported by the National Natural Foundation of China.     

The geochemistry of Jurassic dolerites from Portal Peak,Antarctica

Geochemical and isotopic analyses have been performed on a suite of samples from a Jurassic quartz tholeiite sill of the Ferrar Group at Portal Peak, Queen Alexandra Range, near the Beardmore Glacier in Antarctica. The data include major and trace element (XRF and INAA) concentrations, as well as Sr and Nd isotopic compositions, and are combined with the results of other studies on samples from Antarctica. It is demonstrated that despite differences in the pre-intrusion (or eruption) evolution of the Ferrar Group magmas, the similarity in isotopic and chemical compositions for these rocks supports the existence of a remarkably uniform mantle source with unusual signature over a distance of thousands of kilometres. The favoured origin of this source involves the subduction of terrestrial sedimentary material into a depleted mantle reservoir.     

Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H2O-CO2-NaCl on phase relations in geologic systems: Equation of state for H2O-CO2-NaCl fluids at high pressures and temperatures

Fluid inclusion analyses leave little doubt that solutions containing large concentrations of H₂O, CO₂, and electrolytes are involved in a wide range of geologic processes. Although the miscibility gap in the system H₂O-CO₂ occurs only at low temperatures, experimental data reported by Takenouchi and Kennedy (1965) and Gehrig (1980) indicate that the addition of 6 weight percent NaCl relative to H₂O + Nacl extends the region of immiscibility in the system H₂O-CO₂-NaC] to ≥700°C at 500 bars and mole fractions of CO₂ (X_CO2) ? 0.1. In contrast, addition of 20 weight percent NaCl relative to H₂O + NaCl at 700°C and 500 bars expands the miscibility gap to X_CO2 ? 0.2. At 2000 bars, addition of 20 and 35 weight percent NaCl relative to H₂O + NaCl causes the miscibility gap to extend to ~500° and ~700°C, respectively, at . The existence of the immiscible region in this high-pressure/temperature environment has a profound effect on temperatures of equilibration for metamorphic mineral assemblages (Bowers and Helgeson, 1983). To determine the extent to which nonideality in the ternary system affects these equilibria, the modified Redlich-Kwong (MRK.) equation of state was fit to pressure-volume-temperature data taken from Gehrig (1980) along pseudobinaries for which is constant. Fugacity coefficients of the components were then generated from the fugacity coefficient analog of the MRK equation of state and these coefficients were used together with solubility data to determine the compositions of the coexisting immiscible phases. The tie lines connecting the coexisting phases shift in orientation from nearly parallel to the H₂O-CO₂ binary at low temperatures to almost perpendicular to this binary at high temperatures.