Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

Market area planning strategy: An example of interstate banking markets in the USA

Geographic restrictions exist in the USA banking industry including one which does not permit banks to establish offices outside of their home state. Given the increasing likelihood that this interstate restriction may be removed in the near future, banks need to identify attractive interstate market areas as part of their strategic planning efforts. An approach employing multivariate statistical techniques was used to help a large Southeastern USA bank holding company, the NCNB Corporation, identify attractive interstate market areas.     

Estimating water volume discharge through salt-marsh tidal channels: An aspect of material exchange

Water volume discharge estimates were made at a sample cross-section of the Canary Creek salt marsh (Lewes, Delaware). To account for cross-sectional velocity variations, a dense spatial array of current meters was used. Simultaneous measurements were taken at hourly intervals over three complete tidal cycles. A practical method is presented whereby instantaneous average cross-sectional velocity, instantaneous cross-sectional discharge and tidal cycle discharge can be estimated from a dense current meter array, orideal data set. Based on theseideal estimates, it is shown that a simplified spatial array can be used to estimate water discharges within acceptable error limits. Instantaneous cross-sectional discharges and total tidal cycle discharges were estimated with uncertainties of ±11% and ±7%, respectively. To minimize errors when estimating material exchange between salt marshes and adjacent waters, it is suggested that a comprehensive assessment of cross-sectional lateral current velocity variations be conducted.     

The prediction of mineral solubilities in natural waters: The Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system to high ionic strengths at 25°C

The mineral solubility model of Harvie and Weare (1980) is extended to the eight component system, Na-K-Mg-Ca-H-Cl-SO₄-OH-HCO₃-CO₃-CO₂-H₂O at 25°C to high concentrations. The model is based on the semi-empirical equations of Pitzer (1973) and co-workers for the thermodynamics of aqueous electrolyte solutions. The model is parameterized using many of the available isopiestic, electromotive force, and solubility data available for many of the subsystems. The predictive abilities of the model are demonstrated by comparison to experimental data in systems more complex than those used in parameterization. The essential features of a chemical model for aqueous electrolyte solutions and the relationship between pH and the equilibrium properties of a solution are discussed.     

Volatile contents and ferric-ferrous ratios of basalt,ferrobasalt, andesite and rhyodacite glasses from the Galapagos 95.5°W propagating rift

Volatiles and major elements in abyssal glasses ranging in composition from basalt, ferrobasalt, andesite to rhyodacite from the Galapagos Spreading Center (GSC) near 95°W were analyzed using electron microprobe and high temperature mass spectrometry. Total volatile content ranged from 0.32 wt.% to 2.74 wt.%. Volatile abundances of MORB glasses from the 95.5°W propagating rift are similar to those from the adjacent normal rift (avg. 0.34 wt.%) and lower than those of N-type MORB from the Mid-Atlantic Ridge (avg. 0.49 wt.%). Although both propagating and non-propagating rift glasses contain trace amounts of methane (<0.01 wt.%) and carbon monoxide (0.04 wt.%), significantly higher 100 $\text{Fe}{}_2\text{O}{}_3\text{FeO}\text{+ Fe}{}_2\text{O}{}_3$ ratios are observed for the primitive propagating rift glasses. Water contents of the most primitive GSC glasses are ~0.09 wt.% suggesting a water content for the mantle source of ~0.02 wt.% which indicates that source masses with very low water content can be involved in the generation of MORB.In fractionated ferrobasalt, andesite and rhyodacite glasses from the 95.5°W propagating rift, increasing abundances of H₂O, Cl and F indicate highly incompatible behavior, whereas CO₂ and reduced carbon species appear to decrease in abundance with increasing differentiation. Ferric-ferrous ratios increase from basalt to andesite and reduce to near zero in the rhyodacite. These values are not distinguishable from those previously reported for similar fractionated glasses from the Galapagos 85°W propagating rift, despite the apparent suppression of oxide precipitation in the 85°W suite.     

The Alabama,U.S.A., seismic event and strata collapse of May 7, 1986

On May 7, 1986, the residents of Tuscaloosa, Alabama, felt a seismic event of local magnitude 3.6 that occurred at the same time as a rock burst and roof collapse in an active longwall coal mine. Visual inspection of the seismograms reveals a deficiency in energy at frequencies above 20 Hz compared to tectonic earthquakes or surface blasts. The predominance of energy below 5 Hz may explain reports of body wave magnitudes (m _b ) greater than 4.2. Also, 1.0 Hz surface waves were more strongly excited than body waves and may explain local felt effects more typically associated with greater epicentral distances. All recorded first motions were dilatational. The concentration of stations in the northern hemisphere allows reverse motion on an east-trending near-vertical plane or strike-slip motion on northwest or southeast trending planes. The reverse focal mechanism is preferred, because the area of roof collapse and the area of active longwall mining are located between two east-striking loose vertical fracture zones. The characteristics of the seismic event suggest that it might have been sudden shear failure resulting from accumulated strain energy in overlying strata behind an active longwall. Although an alternate interpretation of the focal mechanism as an implosion or shear failure in the strata above previously mined out areas is also allowed by the first motion data, this alternate intepretation is not supported by geological data.