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Results for “American Journal of Science”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 73 records · Page 4Linked to original sources
USGS1986

The Center Pond pluton: The restite of the story (phase separation and melt evolution in granitoid genesis)

The Center Pond pluton is a calc-alkaline post-tectonic Acadian body intruding chlorite grade Silurian metasediments of E-central Maine. Mapping and petrographic work reveal that the pluton contains 5 igneous rock types; quartz diorite, hornblende-biotite granodiorite, biotite granite, porphyritic granite, and aplitic granite. The linearity of major and trace element trends and Rb-Sr isotope systematics indicate that partial melting and unmixing of a single mafic or intermediate metaigneous rock to restite (residual phases in equilibrium with the melting event at depth) and near-eutectic minimum melt may account for much of the observed geochemical variation within the Center Pond pluton. Major and trace element trends rule out fractional crystallization as a model for Center Pond. -from Authors

Maine↗
USGS1987

Phase separation in seafloor geothermal systems: An experimental study of the effects on metal transport

Experimental investigations were carried out by reacting natural and evolved sea-water with crystalline basalt from the Juan de Fuca Ridge in order to study the effects of vapour-liquid phase separation on metal mobilization in sea-floor geothermal systems in which phase separation occurs between the top of a magma chamber (300-500 bar) and the sea-floor (230-250 bar). The geochemical processes produced by a combination of closed-system phase separation and adiabatic expansion under the conditions characterizing the sea-floor geothermal systems resulted in a significant increase in the acidity and concentration of heavy metals in the liquid phase and strikingly high concentrations of metals in the vapour.

American Journal of Science↗
USGS1987

Diagenesis and fluid flow in the San Juan Basin, New Mexico - regional zonation in the mineralogy and stable isotope composition of clay minerals in sandstone.

The Westwater Canyon Member of the Upper Jurassic Morrison Formation is a relatively homogeneous, hydrologically continuous 100-m-thick sequence of massive fluvial sandstone, bounded above and below by relatively heterogeneous, hydrologically discontinuous units and has served as a primary conduit for fluids within this stratigraphic interval. Patterns of mineral-fluid reactions suggest a basinwide hydrologic regime in which warm, evolved fluids migrated up-dip from the center of the basin under the influence of a regional hydraulic head. -from Authors

American Journal of Science↗
USGS1987

Alleghanian deformation, metamorphism, and granite emplacement in the central Piedmont of the southern Appalachians

Evidence of late Paleozoic (Alleghanian) penetrative deformation, amphilbolite-facies, metamorphism, and syntectonic granite emplacement has been found in an area of the central Piedmont in the southern Appalachians. The High Shoals Granite batholith in the Kings Mountain belt of south-central North Carolina consists of coarse-grained, megacrystic biotite granite with a strong, nearly vertical, gneissoid, foliation, defined by parallel feldspar megacrysts and biotite. U-Ph isotopic data on zircons from the granite yield a concordant age of 317 Ma (Pennsylvanian). This pervasively deformed late Paleozoic granite in the central Piedmont is far west of those known in the Kiokee and Raleigh belts. Field relationships indicate that the High Shoals Granite was emplaced during the late stages of regional F2 folding, close in time to the thermal peak of amphibolite-facies metamorphism. Mineral assemblages typical of Barrovian metamorphism are found in zones of decreasing grade away from the High Shoals contact. These zones, delimited by a kyanite(out)-sillimanite (in) isograd which completely surrounds the batholith and by part of a regional chloritoid (out)-staurolite (in) isograd west and southwest of it, conform to the shape of the batholith and cut across F2 folds. Furthermore, hornblendes from epidoteamphibolite facies to upper amphibolite facies metamorphic rocks in the Kings Mountain belt of North Carolina and South Carolina yield 40Ar/39Ar plateau ages of 318 to 323 Ma. These dates are analytically the same as the crystallization age of High Shoals Granite as established of zireon using the U-Ph method and demonstrate the synchroneity of intrusion of the High Shoals with the dominant regional metamorphism in this part of the Kings Mountain belt.

Southern Appalachians↗
USGS1988

Pyrite formation in the Lower Cretaceous Mowry Shale: Effect of organic matter type and reactive iron content

Geochemical analysis of the Lower Cretaceous Mowry Shale in Wyoming and Montana shows that organic matter type and reactive iron content limited pyrite formation during early diagenesis. Consequently, direct interpretation of paleosalinity cannot be made from a carbon/sulfur plot. Mowry Shale samples with hydrogen index (HI) less than 150 (mg hydrocarbon/ organic carbon) contain low amounts of sulfur. In these samples, bacterial reduction of sulfate may have been inhabited by low nutritive content of the organic matter. For Mowry samples with HI greater than 150 there is a range of sulfur values and a positive correlation between iron and sulfur. The iron sulfur regression is approximately parallel to the stoichiometric pyrite line and intersects the iron axis at 0.5 percent. On average, 60 percent of the iron present was nonreactive; the remaining iron reacted with sulfide to form pyrite. For samples with HI greater than 150, pyrite formation was limited by sulfide or organic carbon content but by reactive iron content. In one modern setting, where salinity is greater than one-half normal seawater, sulfate content does not limit pyrite formation (Berner and Raiswell, 1984). From this analysis then, it can be estimated that the Mowry Sea was not a fresh water body but was either of normal marine salinity or contained brackish water with greater than one-half normal salinity.

Wyoming↗
USGS1989

Liquid-vapor relations for the system NaCl-H2O: Summary of the P-T- x surface from 300° to 500°C

Experimental data on the vapor-liquid equilibrium relations for the system NaCl-H 2 O were compiled and compared in order to provide an improved estimate of the P-T-x surface between 300 ° to 500 ° C, a range for which the system changes from subcritical to critical behavior. Data for the three-phase curve (halite + liquid + vapor) and the NaCl-H 2 O critical curve were evaluated, and the best fits for these extrema then were used to guide selection of best fit for isothermal plots for the vapor-liquid region in-between. Smoothing was carried out in an iterative procedure by replotting the best-fit data as isobars and then as isopleths, until an internally consistent set of data was obtained. The results are presented in table form that will have application to theoretical modelling and to the understanding of two-phase behavior in saline geothermal systems.

American Journal of Science↗
USGS1990

Solid-solution aqueous-solution equilibria: Thermodynamic theory and representation

Thorstenson and Plummer's (1977) "stoichiometric saturation' model is reviewed, and a general relation between stoichiometric saturation Kss constants and excess free energies of mixing is derived for a binary solid-solution B1-xCxA: GE = RT[ln Kss - xln(xKCA) - (l-x)ln((l-x)KBA)]. This equation allows a suitable excess free energy function, such as Guggenheim's (1937) sub-regular function, to be fitted from experimentally determined Kss constants. Solid-phase free energies and component activity-coefficients can then be determined from one or two fitted parameters and from the endmember solubility products KBA and KCA. A general form of Lippmann's (1977,1980) "solutus equation is derived from an examination of Lippmann's (1977,1980) "total solubility product' model. Lippmann's II or "total solubility product' variable is used to represent graphically not only thermodynamic equilibrium states and primary saturation states but also stoichiometric saturation and pure phase saturation states.

American Journal of Science↗