Geochemistry of a transitional ne-trachybasalt — Q-trachyte lava series from Patmos (Dodecanesos), Greece: further evidence for fractionation,mixing and assimilation

Trace-element and preliminary Sr- and O-isotopic data are reported for a transitional alkaline-sub-alkaline lava series (MVS) from Patmos, Greece. The lava types belonging to this series are ne-trachybasalt, hy-trachybasalt, hy-trachyandesite and Q-trachyte. Rb, Sr and Ba contents, as well as K/Rb ratios, of the ne-trachybasalts differ from those of alkali basalts of oceanic islands and those of K-rich alkaline lavas of continental regions and are consistent with the occurrence of these volcanics in a destructive plate margin environment. Qualitatively, the variations shown by many trace elements throughout the MVS are explicable in terms of magma evolution via fractional crystallization involving removal of the observed phenocryst phases. Cross-cutting REE patterns can be explained by removal of small amounts of apatite. However, certain features of the data cannot be reconciled with the operation of fractional crystallization alone. These are: a) the compatible behavior of Ba throughout the MVS; b) the moderately (as opposed to highly) incompatible behavior of Zr, Rb and Nb relative to Th; and c) the significant decrease of K/Th, Rb/Th, Zr/Th, Zr/Nb, Nb/Th, Yb/Th, Ta/Th, U/Th and Zr/Ta ratios especially (but not exclusively) in the mafic part of the series. Quantitative modeling indicates that the hy-trachybasalts are anomalously enriched in both highly incompatible and highly compatible elements and these lavas are shown to be hybrids formed by mixing of ne-trachybasalt and hy-trachyandesite. Mixing proportions of the end members calculated from incompatible element abundances (19% ne-trachybasalt) differ from those calculated from compatible element abundances (62% ne-trachybasalt) and are inconsistent with proportions calculated from published mineral chemical data. In addition, mixing cannot account for the observed variations in incompatible element ratios and this is taken as evidence for the simultaneous operation of assimilation. Isotopic variations (⁸⁷Sr/ ⁸⁶Sr from 0.7049 to 0.7076 and ¹⁸O/¹⁶O from 4.7 to 8.6) and the positive correlation of isotope ratios with SiO₂ and Th contents provide conclusive proof that assimilation occurred. Calculations show that the isotopic characteristics and the concentrations of many trace elements in the Q-trachytes can be explained by fractional crystallization of ne-trachybasalt combined with assimilation of average continental crust (⁸⁷Sr/⁸⁶Sr-0.710), and that large amounts of assimilation are not necessary (Ma/Mc=0.55). REE data are not well explained by this model and suggest a crustal end-member enriched in LREE relative to the average crust. Zr and Hf data are also not well explained and indicate that the assimilant was depleted in HFSE relative to average crust or that HFSE are held back in relatively insoluble phases such as zircon in the restite during assimilation. Nevertheless, the results of the modeling demonstrate that Ba concentrations may decrease during AFC processes and that high Sr contents (1500 ppm in the MVS ne-trachybasalts) do not render mafic, parental magmas immune to the effects of assimilation in terms of their ⁸⁷Sr/⁸⁶Sr ratios. The results of this study confirm conclusions based upon major-oxide and mineral chemical data for the MVS lavas but, more importantly, show that careful analysis of trace element data allows the various processes involved in magma evolution to be identified and quantified, even in the absence of major oxide and isotopic data. Finally, it is reiterated that magma mixing and assimilation may be coupled processes in the magma chambers beneath many volcanic centers, and recognition of this fact has profound implications for studies of magmas erupted at continental margins and through continental crust.     

Fluid inclusion studies of hydrothermally altered archaean granites around the Witwatersrand Basin

Summary Hydrothermally altered granitic rocks occur along the northern and northwestern edge of what is classically termed the Witwatersrand Basin. Pyrite, chalcopyrite, sphalerite, molybdenite, galena, wurtzite and other sulphides were deposited during this hydrothermal alteration, as were uranium and REE-rich nodules of carbonaceous matter and free gold. Heating and freezing data from secondary fluid inclusions in igneous quartz as well as primary fluid inclusions in vein quartz and carbonate indicate that two main groups of aqueous fluid inclusions exist. The first group has a range of final melting temperatures from 0 °C to –9 °C, corresponding to salinities between 0 and 13 equivalent wt.% NaCl. Homogenization occurred at temperatures between 130 °C and 230 °C. The second group of inclusions generally have final melting temperatures between –14 °C and –26 °C, with salinities ranging between 12 and 30 equivalent wt.% NaCl. Homogenization temperatures range from 120 °C to about 170 °C. The low initial melting temperatures of -60°C to –35°C and SEM-EDX analyses of encrustations formed after evaporation of fluid in opened inclusions indicate as additional components Ca, Cl and S. Rare clathrate melting in both types of fluids indicate the presence of CO₂, CH₄ or some other clathrate compound. The low salinity fluids are interpreted to be of a meteoric, seawater or metamorphic origin, whereas the highly saline fluids are thought to be connate brines or highly evolved formation waters.Zusammenfassung Hydrothermal veränderte granitische Gesteine kommen am nördlichen und nordwest-lichen Rand von dem vor, was man klassisch als Witwatersrand-Becken bezeichnet. Während dieser hydrothermalen Umwandlung wurden Pyrit, Kupferkies, Zinkblende, Molybdänglanz, Bleiglanz, Wurtzit und andere Sulfide abgesetzt, ebenso Uran- und SEE-reiche Knollen aus kohliger Substanz und Freigold. Erhitzungs- und Ausfrierdaten von sekundären Fluideinschlüssen in Gesteinsquarz, ebenso wie von primären Fluideinschlüssen in Gangquarz und Karbonat weisen darauf hin, daß zwei Hauptgruppen von wäßrigen Fluideinschlüssen existieren. Der Bereich der finalen Schmelztemperaturen der ersten Gruppe liegt zwischen 0 °C und –9 °C, was einer Salinität zwischen 0 und 13 äquiv. Gew.-% NaCl entspricht. Homogenisierung erfolgte bei Temperaturen zwischen 130 °C und 230 °C. Die zweite Gruppe von Einschlüssen hat im allgemeinen finale Schmelztemperaturen zwischen –14 °C und –26 °C, mit Salinitäten, die sich zwischen 12 und 30 äquiv. Gew.-% NaCl bewegen. Die Homogenisierungstemperaturen variieren von 120 °C bis ungefähr 170 °C. Die niedrigen initialen Schmelztemperaturen von –60 °C bis –35 °C und SEM-EDX-Analysen von Inkrustationen, die sich nach der Verdunstung der Flüssigkeit in geöffneten Einschlüssen bilden, weisen auf Ca, Cl und S als weitere Bestandteile. Gelegentliches Clathratschmelzen in beiden Typen von Fluiden zeigt die Anwesenheit von CO₂, CH, und einigen anderen Clathratbildnern. Die niedrigsalinaren Fluide werden als von meteorischem, Seewasser oder metamorphem Ursprung gedeutet, während die hochsalinaren Fluide als con.nate brines oder sehr gereifte Formationswässer angesehen werden.

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Studien an Fluideinschlüssen hydrothermal veranderter archaischer Granite um das Witwatersrand-Becken

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With 8 Figures     

Evaluation of combination rules for peak response calculation in three‐component seismic analysis

The responses, r_e, given by several multicomponent combination rules used in seismic codes for determining peak responses to three ground motion components are evaluated for elastic systems and compared with the critical response r_cr; this is defined as the largest response for all possible incident angles of the seismic components and obtained by means of the CQC3‐rule when a principal seismic component is vertical, or the GCQC3‐rule when it departs from the vertical direction. The combination rules examined are the SRSS‐, 30%‐, 40%‐ and IBC‐rules, considering different alternatives for the design horizontal spectrum. Assuming that a principal seismic component is along the vertical direction, the upper and lower bounds of the ratio r_e/r_cr for each combination rule are determined as a function of the spectral intensity ratio of the horizontal seismic components and of the responses to one seismic component acting alternately along each structural axis. Underestimations and overestimations of the critical response are identified for each combination rule and each design spectrum. When a component departs from the vertical direction, the envelopes of the bounds of the ratio r_e/r_cr for each combination rule are calculated, considering all possible values of the spectral intensity ratios. It is shown that the inclination of a principal component with respect to the vertical axis can significantly reduce the values of r_e/r_cr with respect to the case when the component is vertical. Copyright © 2003 John Wiley & Sons, Ltd.     

Influence of acidic atmospheric deposition on soil solution composition in the Daniel Boone National Forest,Kentucky, USA

. Acid atmospheric deposition may enter an environmental ecosystem in a variety of forms and pathways, but the most common components include sulfuric and nitric acids formed when rain water interacts with sulfur (SOx) and nitrogen (NOx) emissions. For many soils and watersheds sensitive to acid deposition, the predominant chronic effect appears to be a low pH, loss of base cations, and a shift in the mineral phase controlling the activity of Al³⁺ and/or SO4^2– in solution. Soil solutions from lysimeters at various depths were taken at two sites in the Daniel Boone National Forest, Kentucky, USA, to evaluate potential impacts caused by acid deposition. The sites chosen were in close proximity to coal-burning power plants near Wolfe and McCreary counties and contained soils from the Rayne and Wernock series, respectively. Physicochemical characteristics of the soils revealed that both sites contained appreciable amounts of exchangeable acidity in the surface horizons, and that their base saturation levels were sufficiently low to be impacted adversely by acidic inputs. Soil solution data indicated that the sites were periodically subjected to relatively high NO3^– and SO4^2– inputs, which may have influenced spatial and temporal variation in Al and pH. As a consequence, the formation of Al-hydroxy-sulfate minerals such as jurbanite, alunite and basaluminite were thermodynamically favored over gibbsite. Given these conditions, long-term changes in soil solution chemistry from acid deposition are acknowledged.     

Systematic analysis of K-feldspar Ar/Ar step heating results II: relevance of laboratory argon diffusion properties to nature

We examine a database containing the results of ⁴⁰Ar/³⁹Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of ⁴⁰Ar/³⁹Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and ³⁹Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield ⁴⁰Ar/³⁹Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar ⁴⁰Ar/³⁹Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms—a requirement implicitly met by those samples exhibiting high degrees of correlation.     

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22°S) show that this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This 110-Ma-long interval of lithospheric thinning ended 160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern Cordillera area.The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau corresponds to a paleotectonic domain of “normal” lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion of the easternmost thinned area.     

3D waveform inversion of downhole microseismic data for transversely isotropic media

3D anisotropic waveform inversion could provide high-resolution velocity models and improved event locations for microseismic surveys. Here we extend our previously developed 2D inversion methodology for microseismic borehole data to 3D transversely isotropic media with a vertical symmetry axis. This extension allows us to invert multicomponent data recorded in multiple boreholes and properly account for vertical and lateral heterogeneity. Synthetic examples illustrate the performance of the algorithm for layer-cake and ‘hydraulically fractured’ (i.e. containing anomalies that simulate hydraulic fractures) models. In both cases, waveform inversion is able to reconstruct the areas which are sufficiently illuminated for the employed source-receiver geometry. In addition, we evaluate the sensitivity of the algorithm to errors in the source locations and to band-limited noise in the input displacements. We also present initial inversion results for a microseismic data set acquired during hydraulic fracturing in a shale reservoir.     

Predicting seasonal ocean variability around New Zealand using a coupled ocean-atmosphere model

We present a first assessment of the Sea Surface Temperature (SST) and 300m Heat Content (HC) ensemble mean skill of the Australian Community Climate and Earth-System Simulator-Seasonal v1.0 (ACCESS-S1) around New Zealand on seasonal timescales, using a set of retrospective ensemble forecasts for 1990-2012. This was verified against Reynolds AVHRR analysis and Bluelink ReANalysis 3.5 (BRAN3.5). For inshore areas with depth <300m, the model shows skill forecasting for summer at a lead time of 0 months, with 66% and 65% of model grid cells having correlation coefficients exceeding 0.6 for SST and 300m HC respectively. Regions of lower skill seem to be associated with the model representation of the complex and variable frontal systems around New Zealand, and an overly-strong response to the El Niño Southern-Oscillation (ENSO). Nevertheless, the skill of ACCESS-S1 around New Zealand suggests there is potential for the development of useful seasonal forecast products for the region.     

Early warning system for rainfall-triggered landslides based on real-time probabilistic hazard assessment

Natural Hazards - A methodology to define rainfall-landslide thresholds, using a probabilistic model in which the accumulated rainfall at any time is treated as a random variable, is proposed. The...