Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km³) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H₂O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO₂ with 0·8–3·3wt % K₂O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (¹⁸O 4·4–4·9,⁸⁷Sr/ ⁸⁶Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(²³⁰Th/ ²³⁸U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc     

Changing patterns in rainfall extremes in South Australia

Theoretical and Applied Climatology - Daily rainfall records from seven stations in South Australia, with record lengths from 50 to 137 years and a common period of 36 years, are...     

Regional magma plumbing and emplacement mechanisms of the Faroe‐Shetland Sill Complex: implications for magma transport and petroleum systems within sedimentary basins

The movement of magma through the shallow crust and the impact of subsurface sill complexes on the hydrocarbon systems of prospective sedimentary basins has long been an area of interest and debate. Based on 3D seismic reflection and well data, we present a regional analysis of the emplacement and magmatic plumbing system of the Palaeogene Faroe‐Shetland Sill Complex (FSSC), which is intruded into the Mesozoic and Cenozoic sequences of the Faroe‐Shetland Basin (FSB). Identification of magma flow directions through detailed seismic interpretation of approximately 100 sills indicates that the main magma input zones into the FSB were controlled primarily by the NE–SW basin structure that compartmentalise the FSB into its constituent sub‐basins. An analysis of well data shows that potentially up to 88% of sills in the FSSC are <40 m in thickness, and thus below the vertical resolution limit of seismic data at depths at which most sills occur. This resolution limitation suggests that caution needs to be exercised when interpreting magmatic systems from seismic data alone, as a large amount of intrusive material could potentially be missed. The interaction of the FSSC with the petroleum systems of the FSB is not well understood. Given the close association between the FSSC and potential petroleum migration routes into some of the oil/gas fields (e.g. Tormore), the role the intrusions may have played in compartmentalisation of basin fill needs to be taken fully into account to further unlock the future petroleum potential of the FSB.     

A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials

Microanalytical trace element techniques (such as ion probe or laser ablation ICP-MS) are hampered by a lack of well characterized, homogeneous standards. Two silicate glass reference materials produced by National Institute of Standards and Technology (NIST), NIST SRM 610 and NIST SRM 612, have been shown to be homogeneous and are spiked with up to sixty one trace elements at nominal concentrations of 500 μg g-1 and 50 μg g-1 respectively. These samples (supplied as 3 mm wafers) are equivalent to NIST SRM 611 and NIST SRM 613 respectively (which are supplied as 1 mm wafers) and are becoming more widely used as potential microanalytical reference materials. NIST however, only certifies up to eight elements in these glasses. Here we have compiled concentration data from approximately sixty published works for both glasses, and have produced new analyses from our laboratories. Compilations are presented for the matrix composition of these glasses and for fifty eight trace elements. The trace element data includes all available new and published data, and summaries present the overall average and standard deviation, the range, median, geometric mean and a preferred average (which excludes all data outside ± one standard deviation of the overall average). For the elements which have been certified, there is a good agreement between the compiled averages and the NIST data. This compilation is designed to provide useful new working values for these reference materials.     

Deep-crustal break-back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin, Grenville orogen, Canada

Granulite to upper amphibolite facies ductile thrusting in the Central Gneiss Belt, Grenville orogen, Ontario, represents the tectonic shortening of a continental-scale footwall beneath the thrust-emplaced Central Metasedimentary Belt, during the closure of a postulated back-arc basin (ca. 1.19-1.18 Ga). Break-back stacking in the footwall occurred at mid- to deep-crustal depths (ca. 35 km) within tectonically thickened (ca. 70 km) continental crust, and culminated with renewed thrusting at the base of the overlying Central Metasedimentary Belt (ca. 1.08-1.05 Ga).

The individual mylonite belts which constitute the ductile thrust zones, and the scale of penetrative deformation of the intervening crystalline thrust sheets, are comparable with the largest known examples of high-grade thrust belts elsewhere. They reflect the large-scale thermal and Theological boundary conditions of the deformation. Flow within individual thrust zones may reflect local boundary conditions, such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack.

Restoration of the thickness of erosionally removed crustal overburden by break-back thrusting may retard the rates of exhumation and cooling of a mid- to deep-crustal thrust stack.     

Temperature-dependence of ultraviolet absorption cross-sections of alternative chlorofluoroethanes: 2. The 2-chloro-1,1,1,2-tetrafluoro ethane -HCFC-124

The temperature-dependent ultraviolet absorption cross-sections of CF₃-CHFCI (HCFC-124) have been measured between 170 and 230 nm for temperatures ranging from 295 to 210 K, with uncertainties between 2 and 4%. These results are compared with other available sets of determinations. Temperature effects are discussed and the photodissociation coefficients, presented with their temperature dependence, are calculated. Implication of the temperature dependences on the stratospheric chemistry is also discussed. Parametrical formulae are proposed to compute absorption crosssection values for wavelengths and temperatures useful in modelling calculations.     

Trace elements in the shallow ground water of The Netherlands. A geochemical and statistical interpretation of the national monitoring network data

Concentration levels of the trace elements Al, As, B, Ba, Be, Cd, Co, Cr, Cu, Li, Mo, Ni, Pb, Sr, V, Y, and Zn in the shallow Dutch ground water were studied. Data were extracted from the data base of the Dutch National Ground Water Quality Monitoring Network, a network set up to monitor the diffuse contamination. The network contains over 350 sites at a low density of about 1 per 100 km². The sites are sampled once a year at two depths (approx. 10 and 25 m below surface). A two-step multivariate statistical approach was used, in which the major element chemistry was used to define water types. Within each water type, trace element behavior could be coupled to distinct geochemical processes: dilution, acidification and weathering, carbonate dissolution, oxidation/reduction, and ion exchange. In recently infiltrated acid rain water in low buffering capacity sands, the anthropogenic influence indirectly caused mobilization of Al (median 430 g/l), Cd (0.6 g/l), Co (14 g/l), Cu (2.7 g/l), Ni (16 g/l), Y (11 g/l), and Zn (50 mg/l). In carbonate bearing sediments the acidification is neutralized, and the mentioned trace elements remain immobile. Arsenic and Cr have higher concentrations levels in ground waters with a slightly reducing character and are possibly governed by the dissolution of iron-manganese hydroxides. Boron, Li, and Sr have high concentrations (respectively 875, 80, 2700 g/l) in the water type related to a seawater source. Strontium is related to carbonate dissolution in all other water types (medians ranging from 100 to 1000 g/l). Barium shows a complex behaviour. It is concluded that the high Al, Cd, Co, Cr, Ni, and Zn concentrations are anthropogenically induced. High Ba and Cr concentrations are inferred to be due to natural processes.     

Multiple step ahead river flow modelling for south east tasmanian aquaculture

We develop multiple step ahead prediction models of river flow for locations in Tasmania (Australia) for decision support in aquaculture. In predicting river flows for multiple days ahead, we first statistically determine the maximum input lags of rainfall and river flow. We then use machine learning techniques in building models. In multiple step ahead prediction, we consider both static and dynamic approaches. In dynamic approach, one day prediction is served as input to two days ahead prediction. The experimental results demonstrate that, in general, a dynamic approach provides better accuracy in multiple day’s ahead prediction. For Duck Bay location using dynamic approach, support vector regression performs best over linear regression, M5P and multilayer perceptron. However, at Montagu Bay location, we find that M5P performs best over methods. We find that multiple step ahead prediction of river flow for each location requires modelling of lags with associated machine learning techniques.