Geology topics

R.R. Seal

Publications and source records attributed to R.R. Seal.

21 records · Page 2Linked to original sources

USGS1996

Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: Experimental calibration and theoretical models

Hydrogen isotope fractionation factors were measured for coexisting brines and vapors formed by phase separation of NaCl/H2O fluids at temperatures ranging from 399-450??C and pressures from 277-397 bars. It was found that brines are depleted in D compared to coexisting vapors at all conditions studied. The magnitude of hydrogen isotope fractionation is dependent on the relative amounts of Cl in the two phases and can be empirically correlated to pressure using the following relationship: 1000 ln ??(vap-brine) = 2.54(??0.83) + 2.87(??0.69) x log (??P), where ??(vap-brine) is the fractionation factor and ??P is a pressure term representing distance from the critical curve in the NaCl/H2O system. The effect of phase separation on hydrogen isotope distribution in subseafloor hydrothermal systems depends on a number of factors, including whether phase separation is induced by heating at depth or by decompression of hydrothermal fluids ascending to the seafloor. Phase separation in most subseafloor systems appears to be a simple process driven by heating of seawater to conditions within the two-phase region, followed by segregation and entrainment of brine or vapor into a seawater dominated system. Resulting vent fluids exhibit large ranges in Cl concentration with no measurable effect on ??D. Possible exceptions to this include hydrothermal fluids venting at Axial and 9??N on the East Pacific Rise. High ??D values of low Cl fluids venting at Axial are consistent with phase separation taking place at relatively shallow levels in the oceanic crust while negative ??D values in some low Cl fluids venting at 9??N suggest involvement of a magmatic fluid component or phase separation of D-depleted brines derived during previous hydrothermal activity.

Geochimica et Cosmochimica Acta↗

USGS1993

Stable isotope study of fluid inclusions in fluorite from Idaho: Implications for continental climates during the Eocene

Isotopic studies of fluid inclusions from meteoric water-dominated epithermal ore deposits offer a unique opportunity to study paleoclimates because the fluids can provide direct samples of ancient waters. The oxygen and hydrogen isotope compositions of meteoric waters vary because of changes in climatic variables such as mean annual temperature of precipitation, relative humidity, origin and history of air masses, and the isotope composition of the oceans. Inclusion fluids found in fluorite (CaF 2 ) are especially useful because their host is devoid of oxygen or hydrogen, thus precluding postentrapment isotope exchange. Fluorite-hosted fluid inclusions from the Eocene (51-50 Ma) epithermal deposits of the Bayhorse mining district, northeastern Idaho, have low salinities, most less than 0.6 equivalent wt% NaCl, and low to moderate homogenization temperatures (98 to 146 °C), indicating meteoric origins for the fluids. Oxygen and hydrogen isotope data on inclusion fluids are almost identical to those of modern meteoric waters in the area. The equivalence of the isotope composition of the Eocene inclusion fluids and modern meteoric waters indicates that the Eocene climatic conditions were similar to those today. This conclusion supports the climate modeling of Sloan and Barron, who suggested that the climates of continental interiors do not reflect the magnitude of warming preserved by the deep-ocean paleoclimate record during the Eocene.

Geology↗

USGS1992

Stable isotope study of water-rock interaction and ore formation, Bayhorse base and precious metal district, Idaho

The Bayhorse base and precious metal district is situated east of the Idaho batholith in south-central Idaho. The ores occur near the Nevada Mountain granitic stock as veins cutting the lower Paleozoic Ramshorn Slate and the Garden Creek Phyllite, and as fillings around breccia fragments within the Bayhorse Dolomite. The veins are dominated by siderite and tetrahedrite, with lesser quartz and galena, whereas the breccia ores dominantly comprise only quartz and galena. Mineralization and intrusive activity were contemporaneous during Cretaceous time. Fluid inclusion and stable isotope data indicate that mineralization formed from hot (ca. 375 degrees -225 degrees C), CO 2 -rich (< or =8.3+ or -1.4 mole %) brines (5-20 wt % NaCl equiv) at confining pressures between 1.1 and 1.7 kbars. Fluid cooling and the resulting CO 2 effervescence were the most important causes of ore deposition.Whole-rock delta 18 O and delta D values from the Garden Creek Phyllite define an isotopically depleted zone (60 km 2 ) around the Nevada Mountain stock and are the result of high-temperature interactions with ancient meteoric waters at water/rock ratios (mass fractions) ranging from 0.002 to 0.09. The delta 18 O (sub H 2 O) and delta D (sub H 2 O) values of the ore fluids ranged from 11.2 to 3.9 per rail and from -55 to -146 per mil, respectively. Comparison of the ore fluid delta 18 O (sub H 2 O) and delta D (sub H 2 O) values with hypothetical waters equilibrated with the Garden Creek Phyllite intricates that the hydrothermal fluids must have also interacted with the basal dolomite of Bayhorse Creek, which underlies the phyllite. Early delta 13 C (sub CO 2 ) values calculated in equilibrium with siderite ranged from -9.1 to -7.4 per mil, whereas later delta 13 C (sub CO 2 ) values obtained from quartz-hosted fluid inclusion extracts range from -4.0 to -1.8 per mil. The delta 13 C (sub CO 2 ) values for the hydrothermal fluids thus also record a transition from early water/rock interactions that were dominated by the Garden Creek Phyllite (organic matter delta 13 C = -23.6 to -20.9ppm) to later interactions that were influenced significantly by the basal dolomite of Bayhorse Creek (delta 13 C = -2.3 to -2.2ppm). The delta 34 S values of main-stage hydrothermal sulfides range from 3.0 to 28.3 per mil and display a distinct mode at approximately 10 per mil. The range of values may be interpreted as either a heterogeneous sedimentary source or mixed sedimentary-magmatic sources. On the basis of mass balance considerations, it is likely that all of the fluid salinity and the dissolved metals were derived from the Garden Creek Phyllite. The paragenetic sequence developed in the ores appears to be related to: (1) processes at the site of deposition, most notably cooling and CO 2 effervescence, and (2) water-rock interactions within the Garden Creek Phyllite in the 60-km 2 isotopically altered area surrounding the Nevada Mountain stock.

Economic Geology↗