Origin of the Oceanic Lithosphere

In a global examination of mid-ocean ridge basalt (MORB) glasscompositions, we find that Na₈–Fe₈–depth variationsdo not support modeling of MORBs as aggregates of melt compositionsgenerated over a large range of temperature and pressure. However,the Na₈–Fe₈ variations are consistent with the compositionalsystematics of solidus melts in the plagioclase–spinellherzolite transition in the CaO–MgO–Al₂O₃–SiO₂–Na₂O–FeO(CMASNF) system. For natural compositions, the P–T rangefor melt extraction is estimated to be 1·2–1·5GPa and 1250–1280°C. This P–T range is a closematch with the maximum P–T conditions for explosive pressure-releasevaporization of carbonate-bearing melts. It is proposed thatfracturing of the lithosphere induces explosive formation andescape of CO₂ vapor. This provides the vehicle for extractionof MORBs at a relatively uniform T and P. The upper portionof the CO₂-bearing and slightly melted seismic low-velocityzone flows toward the ridge, rises at the ridge axis to lower-lithospheredepths, melts much more extensively during this rise, and releasesMORB melts to the surface driven by explosively escaping CO₂vapor. The residue and overlying crust produced by this meltingthen migrate away from the ridge axis as new oceanic lithosphere.The entire process of oceanic lithosphere creation involvesonly the upper 140 km. When lithospheric stresses shift fractureformation to other localities, escape of CO₂ ceases, the vehiclefor transporting melt to the surface disappears, and ridgesdie. Inverse correlations of Na₈ vs Fe₈ for MORB glasses areexplained by mantle heterogeneity, and positive variations superimposedon the inverse variations are consistent with progressive extractionof melts from short, ascending melting columns. The uniformlylow temperatures of MORB extraction are not consistent withthe existence of hot plumes on or close to ocean ridges. Inthis modeling, the southern Atlantic mantle from Bouvet to about26°N is relatively homogeneous, whereas the Atlantic mantlenorth of about 26°N shows significant long-range heterogeneity.The mantle between the Charlie Gibbs and Jan Mayen fracturezones is strongly enriched in FeO/MgO, perhaps by a trappedfragment of basaltic crust. Iceland is explained as the productof this enrichment, not a hot plume. The East Pacific Rise,Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge samplemantle that is heterogeneous over short distances. The mantlebeneath the Red Sea is enriched in FeO/MgO relative to the mantlebeneath the northern Indian Ocean.     

The occurrence and geochemistry of arsenic in groundwaters of the Newark basin of Pennsylvania

Elevated As concentrations in groundwater in the eastern United States have been recognized predominantly in the accretionary geologic terranes of northern New England. A retrospective examination of more than 18,000 existing groundwater samples from the Pennsylvania Department of Environmental Protection (PA DEP) Drinking Water and Sampling Information System database indicates that elevated groundwater As concentrations occur throughout the northern half of the Piedmont Province of Pennsylvania. Chemical analyses of 53 samples collected in 2005 from drinking water wells in this area all had detectable As, and 23% of these samples contained elevated (>133 nmol/L or >10 μg/L) concentrations of As. Elevated concentrations of As in the groundwater samples were most common in the Mesozoic sedimentary strata composed of sandstone and red mudstone with interbedded gray shale, and gray to black siltstone and shale. Arsenic was typically not elevated in groundwater of diabase intrusions of the Newark Basin or in crystalline and calcareous aquifers to the north of the Newark Basin. Geochemical parameters such as pH and oxidation–reduction potential can indicate mobility mechanisms of As in some regions. In this area, measured groundwater conditions were predominantly oxidizing (Eh > +50 mV), and more than 85% of samples contained arsenate as the dominant As species. Variations in pH were strongly correlated to the As concentration, with highest As concentrations observed at pH values greater than 6.4. The original source of As is most likely the black and gray shales that contain some arsenian pyrite with groundwater concentrations likely to be controlled by adsorption/desorption reactions with Fe oxides in the red mudstone aquifer materials.     

The presence of anomalous trace element levels in present day Jamaican soils and the geochemistry of Late-Miocene or Pliocene phosphorites

High levels of Cd and Zn in Jamaican soils observed in geochemical surveys are related to the presence of phosphorites of possible Late-Miocene or Pliocene age. The trace element and REE geochemistry of the phosphorites, together with SEM studies, indicate a guano origin for the phosphorites. No specific host minerals for Cd could be identified in the fossiliferous phosphorite which is characterized by uniquely high levels of Cd, Zn, Ag, Be, U and Y. However, in the soil Cd is present in lithiophorite and a complex history of pedological development is preserved in the aluminous–goethite present in the soil. The unique guano signature is preserved in the soil despite the fact that guanos themselves have either not been observed or have been destroyed by continuing karst and soil development. The phosphorite geochemical signature can be traced in the data of a 1988 island-wide soil geochemical survey, identifying areas where the Palaeo-environment that supported bird ‘rookeries’ existed in the Late-Miocene or Pliocene.     

Leaching mechanisms of oxyanionic metalloid and metal species in alkaline solid wastes: A review

An overview is presented on possible mechanisms that control the leaching behaviour of the oxyanion forming elements As, Cr, Mo, Sb, Se, V and W in cementituous systems and alkaline solid wastes, such as municipal solid waste incinerator bottom ash, fly ash and air pollution control residues, coal fly ash and metallurgical slags. Although the leachability of these elements generally depends on their redox state, speciation measurements are not common. Therefore, experimental observations available in the literature are combined with a summary of the thermal behaviour of these elements to assess possible redox states in freshly produced alkaline wastes, given their origin at high temperature. Possible redox reactions occurring at room temperature, on the other hand, are reviewed because these may alter the initial redox state in alkaline wastes and their leachates. In many cases, precipitation of oxyanions as a pure metalate cannot provide a satisfactory explanation for their leaching behaviour. It is therefore highly likely that adsorption and solid solution formation with common minerals in alkaline waste and cement reduce the leachate concentration of oxyanions below pure-phase solubility.     

First U–Pb SHRIMP age of the Hauterivian stage, Neuquén Basin, Argentina

A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.