Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, ⁸⁷Sr/⁸⁶Sr=0.70307–0.70371, _Nd=+7.7-+4.8, ¹⁸O=+5.6±6.1, and ²⁰⁶Pb/²⁰⁴Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low ⁸⁷Sr/⁸⁶Sr, high _Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like ¹⁸O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher ⁸⁷Sr/ ⁸⁶Sr and ¹⁸O, lower _Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K₂O/TiO₂, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased     

Plate boundary deformation and man-made subsidence around geothermal fields on the Reykjanes Peninsula,Iceland

We present Interferometric Synthetic Aperture Radar (InSAR) data from 1992–1999 and 2003–2008 as well as GPS data from 2000–2009 for the active plate boundary on the Reykjanes Peninsula, southwest Iceland. The geodetic data reveal deformation mainly due to plate spreading, anthropogenic subsidence caused by geothermal fluid extraction and, possibly, increasing pressure in a geothermal system. Subsidence of around 10 cm is observed during the first 2 years of production at the Reykjanes geothermal power plant, which started operating in May 2006. We model the surface subsidence around the new power plant using point and ellipsoidal pressure sources in an elastic halfspace. Short-lived swarms of micro-earthquakes as well as aseismic fault movement are observed near the geothermal field following the start of production, possibly triggered by the stresses induced by geothermal fluid extraction.     

The small-scale structure of VHF mesospheric summer echo layers observed at mid-latitudes

The VHF radar system at Aberystwyth (52.4° N, 4.1° E) has been used to make high-time-resolution, multi-beam observations of mesospheric summer echo layers. These show that the altitude and the sense of vertical movement of the layers can vary over time-scales of minutes and horizontal scales of kilometres. In general, the altitude profiles of signal-to-noise ratio provide evidence of a bifurcated structure with sharp changes in the horizontal wind vector and vertical velocity, and enhanced spectral width occurring at the bifurcation level. The implications of the small-scale structure for studies of the aspect sensitivity of radar returns are discussed, and the changes in wind-field at the bifurcation level are compared with wind corners observed in rocket studies of the mesosphere at polar latitudes.     

High energy neutrinos from the TeV Blazar 1ES 1959 + 650

The AMANDA neutrino telescope has recently reported the detection of high-energy neutrinos spatially and temporally coincident with the flaring of the TeV blazar 1ES 1959 + 650. If high-energy neutrinos are in fact generated by this blazar, it would be the first strong evidence for the hadronic acceleration of cosmic rays. At present, the statistical significance of this observation cannot be reliably assessed, however. In this letter, we investigate whether circumstances exist where the source can produce the flux implied by the coincident events. We show that if the TeV gamma-ray emission observed from 1ES 1959 + 650 or other nearby TeV blazars is the result of accelerated protons interacting with nucleons, it is reasonable that AMANDA could detect several events during a flaring period. Such rates require that the spectral index of the source be rather high (for instance 2.8 for 1ES 1959 + 650) and that the Lorentz factor of the jet be fairly small (Γ  1).     

Socioeconomic dynamics of the Ghanaian tuna industry: a value-chain approach to understanding aspects of global fisheries

This study investigated how an industrial tuna fishery functions in terms of procedures, practices, governance and finance in the context of Ghana, West Africa. Tuna is Ghana’s biggest seafood export, contributing significantly to the domestic fisheries sector. A case-study approach was used to analyse relevant social and economic factors at the local scale to better understand how the global seafood industry operates in a low-income country. A value-chain framework was adopted to assess market structures, sales pathways and revenue distribution. We also investigated the role of actors engaged in the industry using secondary data, interviews, questionnaires and participant observations. The results revealed a changing organisational structure in tuna production, moving from bait-boat fishing with smaller companies to large-scale purse-seine fishing backed by consolidated Asian seafood companies. Production was found to depend significantly on local female intermediaries for access to funds through prefinancing arrangements. Considerable illegalities were identified within the value chain, highlighting the need for improved partnership and licensing negotiations, and for low-cost marine control and surveillance tools. As vertical integration increases within the industry, the influence of corporations as keystone actors becomes evident for the future social and ecological sustainability of the industry.     

Trends in the chemistry of precipitation and surface water in a national network of small watersheds

Trends in precipitation and surface water chemistry at a network of 15 small watersheds (< 10 km²) in the USA were evaluated using a statistical test for monotonic trends (the seasonal Kendall test) and a graphical smoothing technique for the visual identification of trends. Composite precipitation samples were collected weekly and surface water samples were collected at least monthly. Concentrations were adjusted before trend analysis, by volume for precipitation samples and by flow for surface water samples. A relation between precipitation and surface water trends was not evident either for individual inorganic solutes or for solute combinations, such as ionic strength, at most sites. The only exception was chloride, for which there was a similar trend at 60% of the sites. The smoothing technique indicated that short-term patterns in precipitation chemistry were not reflected in surface waters. The magnitude of the short-term variations in surface water concentration was generally larger than the overall long-term trend, possibly because flow adjustment did not adequately correct for climatic variability. Detecting the relation between precipitation and surface water chemistry trends may be improved by using a more powerful sampling strategy and by developing better methods of concentration adjustment to remove the effects of natural variation in surface waters.     

The Madinah eruption,Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types

During a 52-day eruption in 1256 A.D., 0.5 km³ of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two.Three mineral assemblages occur out of equilibrium in all three chemical types.Assemblage 1 contains resorbed olivine and clinopyroxene megacrysts and ultramafic microxenoliths (Fo₉₀ + Cr spinel + Cr endiopside) which fractionated within the spinel zone of the mantle.Assemblage 2 contains resorbed plagioclase megacrysts (An₆₀) with olivine inclusions (Fo₇₈) which fractionated in the crust.Assemblage 3 contains microphenocrysts of plagioclase and olivine in a groundmass of the same minerals with late-crystallizing titansalite and titanomagnetite; assemblage 3 crystallized at the surface and/or in the upper crust. Each assemblage represents a distinct range in PTX environment, suggesting that their coexistence in each chemical type may be a function of magma mixing. Such a process is confirmed by variable ratios of incompatible element pairs in a range of analyses.All three chemical types are products of mixing. Some of the hybrid types may have developed from surface mixing of the low-K and high-K lavas; however, the occurrence of all three types at the vent system suggests that subsurface mixing was the dominant process. We suggest that the Madinah flow was extruded from a heterogeneous magma chamber containing vertically stacked sections equivalent to the six eruptive pulses. This chamber may have developed contemporaneously with magma mixing when a crustal reservoir containing a magma in equilibrium with assemblage 2 was invaded by a more primitive magma containing cognate microxenoliths and megacrysts of assemblage 1.     

A case of simple magma mixing in the Columbia River Basalt Group: The Wilbur Creek,Lapwai, and Asotin flows,Saddle Mountains Formation

Three small sequential Saddle Mountains flows, occupying similar areas on the Columbia Plateau, were erupted over a short interval of time. In the Lewiston Basin area the middle flow of the trio (Lapwai) has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty-five elements analyzed. Systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting. The Lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous. This implies efficient mixing between equal amounts of Asotin and Wilbur Creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption. The Wilbur Creek and Asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source (Carlson 1984). The mixing processes described here cannot be related to that earlier mantle/crust mixing process.     

Importance of seismic design accidental torsion requirements for building collapse capacity

Seismic ground motions induce torsional responses in buildings that can be difficult to predict. To compensate for this, most modern building codes require the consideration of accidental torsion when computing design earthquake forces. This study evaluates the influence of ASCE/SEI 7 accidental torsion seismic design requirements on the performance of 230 archetypical buildings that are designed with and without accidental torsion design provisions, taking building collapse capacity as the performance metric. The test case archetypes include a broad range of heights, gravity load levels, and plan configurations. Results show that the ASCE/SEI 7 accidental torsion provisions lead to significant changes in collapse capacity for buildings that are very torsionally flexible or asymmetric. However, only inconsequential changes in collapse capacity are observed in the buildings that are both torsionally stiff and regular in plan. Therefore, the study concludes that accidental torsion provisions are not necessary for seismic design of buildings without excessive torsional flexibility or asymmetry. Copyright © 2013 John Wiley & Sons, Ltd.     

The geography of mercury and PCBs in North Carolina's local seafood

Mercury and PCBs are used by non-governmental organizations and federal agencies to inform seafood safety recommendations. Pollution dynamics suggest recommendations on the national scale may be too large to be accurate. We tested softshell and hardshell blue crab, white and pink shrimp, oysters, clams, spot, and mullet from fishers in each of the three North Carolina fishery districts. We measured mercury using EPA method 7473 and PCBs using a commercially available ELISA kit. Over 97% of samples were below the Environmental Protection Agency levels of concern for both mercury and PCBs. Mercury and PCBs have different spatial dynamics, but both differ significantly by water body, suggesting that seafood safety recommendations should occur by water body instead of at the national scale. This finding supports previous research suggesting that differences in water chemistry, terrestrial influence, and flushing time in a particular water body control the contaminant load in locally resident species.