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.     

Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh,India, Vietnam,and Nepal

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.     

An Attempt to Predict the Failure Time of Abandoned Mine Pillars

Summary  Left in place pillars of abandoned mines are subject to weathering (e.g action of water, bacteria) that degrades their mechanical strength and eventually leads to collapse. A simple weathering model is proposed, that is governed by two parameters: the rate of progression of weathered front and the rate of degradation of the compressive strength with time. Both plane strain and axisymmetric analyses are performed and closed form solutions of the variation with time of the bearing capacity of the pillar are given. Experimental data of the tests conducted on gypsum and anhydrite specimens attacked by water are presented. It is shown that in order to fit the experimental data a third parameter must be introduced. New closed form solutions are given and the data are used for estimating the time to failure of abandoned gypsum mines in Northern Italy. Authors’ address: Dr. Riccardo Castellanza, Research Assistant, Department of Structural Engineering, Milan University of Technology (Politecnico), Piazza Leonardo da Vinci 32, 20133 Milan, Italy     

Origins of high pH mineral waters from ultramafic rocks, Central Portugal

This paper reviews the geochemical, isotopic (²H, ¹⁸O, ¹³C, ³H and ¹⁴C) and numerical modelling approaches to evaluate possible geological sources of the high pH (11.5)/Na–Cl/Ca–OH mineral waters from the Cabeço de Vide region (Central-Portugal). Water–rock interaction studies have greatly contributed to a conceptual hydrogeological circulation model of the Cabeço de Vide mineral waters, which was corroborated by numerical modelling approaches. The local shallow groundwaters belong to the Mg–HCO₃ type, and are derived by interaction with the local serpentinized rocks. At depth, these type waters evolve into the high pH/Na–Cl/Ca–OH mineral waters of Cabeço de Vide spas, issuing from the intrusive contact between mafic/ultramafic rocks and an older carbonate sequence. The Cabeço de Vide mineral waters are supersaturated with respect to serpentine indicating that they may cause serpentinization. Magnesium silicate phases (brucite and serpentine) seem to control Mg concentrations in Cabeço de Vide mineral waters. Similar δ²H and δ¹⁸O suggest a common meteoric origin and that the Mg–HCO₃ type waters have evolved towards Cabeço de Vide mineral waters. The reaction path simulations show that the progressive evolution of the Ca–HCO₃ to Mg–HCO₃ waters can be attributed to the interaction of meteoric waters with serpentinites. The sequential dissolution at CO₂ (g) closed system conditions leads to the precipitation of calcite, magnesite, amorphous silica, chrysotile and brucite, indicating that the waters would be responsible for the serpentinization of fresh ultramafic rocks (dunites) present at depth. The apparent age of Cabeço de Vide mineral waters was determined as 2790 ± 40 a BP, on the basis of ¹⁴C and ¹³C values, which is in agreement with the ³H concentrations being below the detection limit.     

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.