Isotope geochemistry of minerals and fluids from Newberry volcano, Oregon

Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The δ¹⁵O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The δ¹⁸O values of quartz and calcite from the andesite and basalt flows (700–932 m) have isotopic values which require that the equilibrated water δ¹⁸O values increase slightly (− 11.3 to −9.2‰) with increasing measured temperatures (150–265°C). The lithic tuffs and brecciated lava flows (300–700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76–100°C). The δ¹⁸O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water (− 10.5 ‰) with meteoric water (− 15.5 ‰) recharged within the caldera. The δ¹³C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO₂ of about −8 ‰.The δ¹⁸O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100°C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The δ¹⁸O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water.The calculated δ¹⁸O and δD values of bottom hole water determined from the fluid produced during the 20 hour flow test are −10.2 and −109‰, respectively. The δD value of the hydrothermal water indicates recharge from outside the caldera.     

Towards the use of catchment geomorphology in flood frequency predictions

A further development of a topography-based model of catchment hydrology (TOPMODEL) is described and applied to the problem of predicting flood frequency characteristics. The model can simulate infiltration excess, saturation excess, and subsurface runoff contributions to peak flows. Catchment geomorphology plays a central role in predicting the nature of the hydrological response. Using stochastic rainfall and initial condition inputs based on measured data, the model satisfactorily reproduces the mean hourly flow flood frequency growth curve for the Wye catchment, but not the mean number of peaks greater than 3mm h^?1 each year. Suggestions for further improvements are made.     

Geochemistry of tertiary alkaline rocks of the Eastern Trans-Pecos Magmatic Province,Texas

The Trans-Pecos Magmatic Province (TPMP) is an alkalic field that was active between 48-17 Ma. Rocks of two subprovinces in the eastern alkalic belt of the TPMP, the Big Bend region and the Davis Mountains, have been analyzed for major and trace element concentrations in order to determine what magmatic processes operated to influence the compositional evolution of the magmas, and to explore what relationship existed between the silica-oversaturated evolved rocks and the silica-undersaturated mafic rocks. Similar compositional trends exist in both subprovinces, implying that the evolved rocks are genetically related to the mafic rocks, and that the differentiation processes were broadly reproducible spatially and temporally.Four stages of evolution have been identified: Stage I, alkali basalt to trachyte; stage II, trachyte to quartz trachyte; stage III, quartz trachyte to rhyolite/comendite; and stage IV, rhyolite/comendite to high-silica rhyolite/comendite. These stages were identified by discontinuities in trends on variation diagrams; within stages I and II, more than one subtrend exist.Stage I can best be modeled as the result of simple crystal fractionation with minor magma replenishment. Two subtrends within this stage indicate that variations in the processes or their rates may have occurred. Stage II exhibits both closed- and open-system behavior. The open system behavior consists of combined fractionation-assimilation and episodic mixing of stage II and stage I magmas. Stages III and IV evolved under open system processes of combined fractionation-assimilation, with the assimilant having compositional characteristics of a shale-dominated sedimentary assemblage.The four stages and subtrends within the stages occur in both subprovinces. Further, some subtrends comprise rocks that differ in age by as much as 10 m.y. In each subprovince, the stratigraphy indicates a random interlaying of rocks of the different stages, generally erupted from more than one center. That contemporaneous magmas of different stages existed in a given subprovince is indicated by the interfingering of their erupted products. These constraints argue against a single magma production-evolution scheme. Rather, the data suggest that magmas of the different stages were produced more than once during the evolution of the eastern TPMP, and that during any given time, production and evolution of magma of all stages were occurring.     

Tidal flushing of an estuarine embayment subject to recurrent dinoflagellate blooms

A rhodamine dye tracer study was conducted over eight tidal cycles to investigate mixing and tidal exchange processes in Perch Pond, a Cape Cod embayment subject to recurrent blooms of the toxic dinoflagellate, Gonyaulax tamarensis. Dye injected at the inlet to Perch Pond during flood tide became well-mixed within the pond in one day and was removed at an effective first order rate of 0.36 d^?1, equivalent to a 70% utilization of the maximum possible tidal exchange. This relatively high flushing efficiency can be attributed to a density-driven circulation within the pond, consisting of a subsurface inflow of high salinity dense water on the flood tide followed by removal of lighter surface layers through the shallow inlet during ebb tide. The formation of a frontal convergence near the inlet on flood tide is consistent with the observed distribution of G. tamarensis cysts and shelifish toxicity. It is also clear that phytoplankton like G. tamarensis, whose maximum growth rates approximate the rate of tidal flushing, can only bloom within the embayment by avoiding the outflowing surface waters. Mixing within the pond is probably less efficient and population losses greater during dry periods when the pond salinity is higher and the stratification weaker.     

The relationship of soil Po and Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

Complexation and reduction as factors in the link between metal ion concentrations and organic matter in the Indian River

Complexation of metal ions by organic matter is frequently considered to play a part in metal ion dissolution in natural waters. A field study of a relatively unperturbed stream, high in organics, associates this with the fraction related to soil organic acids (humic acids). The association might have two origins. The first is complexation. However, well known sequences of complexing tendency do not predict the behaviour. A better theory uses the additional factor of the reducing capacity of dissolved organic matter toward Fe(III) and Mn(IV).     

Stereochemistry of amino acids in surface samples of a marine sediment

In two surface samples of marine sediment, the percentages of d-alanine and d-aspartic acid are significantly higher than the other d-amino acids and are similar to the range found in soils. The percentage of d-glutamic acid is also higher than the other amino acids but less than d-alanine and d-aspartic acid. These d-amino acids may come mainly from bacteria.     

Hillslope hydrographs by the finite element method

Simulation models may be used to explore the implications of making specific assumptions about the nature of a real world system, and then to make predictions of the behaviour of that system under a set of naturally occurring conditions. It is important that understanding generated by the former should be gained before predictive use of the system model. This paper describes and uses a finite-element model of transient, partially saturated water flow within a hillslope soil mantle overlying an impermeable bedrock, to make an investigation into the effects of parameter variations and initial conditions on the hillslope hydrograph. The results clearly demonstrate that the response of the hillslope system to rainfall is highly non-linear and that the initial conditions, particularly in the unsaturated zone, are of paramount importance in governing the timing and magnitude of the hydrograph peak. Hillslope convergence appears as the dominant topographic parameter but the non-linearity of the response and the complex interdependence between the soil and topographic parameters restrict the possibility of further definite conclusions about the relative sensitivity of the simulated hillslope hydrograph to changes in these parameters.     

Subsidence analysis on the Sardinian margin and the central Tyrrhenian Basin: Thermal modelling and heat flow control; deep structure implications

The subsidence of the Tyrrhenian Basin is analysed based on numerous MCS reflection data and related seismic velocities, and calibrated using subsidence curves versus time at ODP Leg 107 sites 654 and 652 on the Sardinian margin. Two main phases of rifting are clearly defined for the margin-basin evolution: the first, beginning at 7 Ma, split the Sardinian-Calabrian margins, the second, beginning at 5 Ma formed the Central oceanic subbasins.Thermal models, based on thinned crust geometry from reflection-refraction data, are used which consider both vertical and horizontal conduction over time along a NW-SE transect across the entire Tyrrhenian Basin from Sardinia to Calabria. In general, there is a close agrrement between the predicted and the observed surface heat flow and the rate of subsidence over time. However, within the oceanic domain an initial subsidence must be chosen about 1000 m greater than observed on typical spreading centers, i.e., at about 3500 m as postulated in numerous back-arc basins.Thermal models, constrained by subsidence analysis and gravity observations, imply a large difference for the Moho depth beneath the Sardinian margin as determined after refraction seismic experiments. The upper mantle, hot but with high density, would be situated much higher than is postulated by seismic data and probably includes 7.3–7.4 km/s velocity layers previously interpreted as deep crustal layers.