David I. Norman

The evolution of volcano-hosted geothermal systems based on deep wells from Karaha-Telaga Bodas, Indonesia

In late Mesozoic time, the southern Cordilleran foreland basin was bounded on the west by the Sevier thrust belt and on the south by the Mogollon highlands. Paleocurrent indicators in fluvial and fluviodeltaic strata imply sediment delivery into the basin from both tectonic features. Ages of detrital zircons in sandstones of the basin provide insights into the nature of the sediment sources. Upper Jurassic and Lower Cretaceous fluvial strata were deposited as sediment blankets across the width of the basin but Upper Cretaceous marginal-marine facies were restricted to the basin margin, with marine facies in the basin interior. Most Upper Jurassic and Lower Cretaceous fluvial sandstones contain heterogeneous age populations of Precambrian and Paleozoic detrital zircons largely recycled from Jurassic eolianites uplifted within the Sevier thrust belt or antecedent highlands, and exposed as sedimentary cover over the Mogollon highlands, with only minor contributions of Mesozoic zircon grains from the Cordilleran magmatic arc along the continental margin. Sources in Yavapai-Mazatzal Proterozoic basement intruded by anorogenic Mesoproterozoic plutons along the Mogollon highlands were significant for the Westwater Canyon Member of the Upper Jurassic Morrison Formation and for early Upper Cretaceous (Turonian) fluviodeltaic depositional systems, in which arc-derived Cordilleran zircon grains are more abundant than in older and younger units composed dominantly of recycled detritus. Detrital zircons confirm that the Salt Wash and Westwater Canyon Members of the Morrison Formation formed separate foreland megafans of different provenance. Late Upper Cretaceous (Campanian) fluvial sandstones include units containing mostly recycled sand lacking arc-derived grains in the Sevier foredeep adjacent to the Sevier thrust front, and units derived from both Yavapai-Mazatzal basement and the Cordilleran arc farther east, with some mingling of sand from both sources at selected horizons within the Sevier foredeep. Evidence for longitudinal as well as transverse delivery of sediment to the foreland basin shows that paleogeographic and isostatic analyses of thrust-belt erosion, sediment loads, and basin subsidence in foreland systems need to allow for derivation of foreland sediment in significant volumes from sources lying outside adjacent thrust belts.

Arsenic sorption onto laterite iron concretions: Temperature effect

We investigated arsenate and arsenite sorption onto laterite iron concretions (LIC) to test its suitability for use in the low-tech treatment of arsenic-bearing drinking water. Batch experiments on crushed LIC from Prestea, Ghana were conducted at a series of temperatures, ionic strengths, and pHs. The point of zero net charge on laterite iron concretion was determined by potentiometric titrations yielding an average pHp(ZNC) around 8.64. Experiments show that sorption capacity for both arsenite and arsenate increase with temperature. The equilibrium sorption capacity for arsenite was larger than that for arsenate over the 25 to 60 degrees C temperature range. A Langmuir model satisfactorily fits the arsenite and arsenate sorption isotherm data. Both arsenite and arsenate sorbed over the pH range of natural waters. Arsenite sorption increases with increasing solution pH to a maximum at pH 7, then decreases with further increase in solution pH. Arsenate sorption, on the other hand, shows little change with increasing solution pH. Increasing solution ionic strength 10-fold results in a slight increase in sorption. Ionic strength experiments show that an inner-sphere sorption mechanism is responsible for As (V) sorption on LIC, while As (III) sorption is by an outer-sphere mechanism. Gibbs free energy (DeltaG degrees) for arsenite and arsenate sorption onto LIC was calculated from Langmuir isotherms; the negative values agree with reaction spontaneity. The positive values of the standard enthalpy (DeltaH degrees) show the endothermic nature of arsenite and arsenate sorption onto LIC. Positive entropy (DeltaS degrees) values suggest the affinity of LIC for the arsenic species in solution. Analysis of the arsenic sorption data suggests that LIC can be used for low-tech natural-materials arsenic water treatment. Laterite iron concretions have a number of advantages for this use over commercial materials, including the ability to remove arsenic from waters with a wide range in pH, the ability to sorb both common arsenic aqueous species equally well, and cost less. Laterite iron concretions positive sorption temperature dependence will enhance sorption in tropical climates, and more especially in areas where groundwater sources are related to geothermal springs.

N2-Ar-He compositions in fluid inclusions: Indicators of fluid source

Abstract A quadrupole mass spectrometer was used to measure bulk samples of conservative gas species N 2 , Ar, and He in fluid inclusions from a variety of hydrothermal systems. Analyses of these tracer elements help determine 1. (1) if gases extracted by bulk inclusion analyses can provide accurate measurement of N 2 -Ar-He in active and fossil geothermal systems, 2. (2) if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow N 2 ArHe systematics similar to th the western Pacific Rim active geothermal systems, specifically New Zealand, and 3. (3) whether different deposit types systematically vary with regard to N 2 ArHe. The N 2 ArHe ratios of fluid inclusion volatiles released from recently deposited minerals from the Valles system are similar to those of present day Valles thermal waters. Those inclusion samples from deep within the Valles system, below a regional aquitard, increase in N 2 . Compositions for inclusions from the Questa and Copper Flat-porphyry deposits are N 2 -rich, similar to those of arc-related volcanic gases, whereas those from Taylor Creek Sn deposit appear to be mixtures of magmatic and crustal components. N 2 -Ar-He ratios of the Precambrian Tribag deposit suggest a basalt source, but significant levels of self-generated He from U and Th in the inclusion fluids are also possible. Inclusions from two epithermal deposits with low-salinity inclusions have N 2 -Ar-He ratios trending towards air-saturated meteoric waters (ASW), and those inclusions with higher salinities indicate minor to no ASW component. The N 2 -Ar-He ratios in Fresnillo and Cochiti inclusions, which have magmatic helium isotopic ratios, indicate additions of magmatic gases to meteoric fluids. Inclusions from sediment-hosted deposits that contain hydrocarbon-bearing brines are He-rich, as are meteoric waters with a long residence time in the crust. At relevant pressure-temperature-composition conditions, Henrys Law constants of N 2 , Ar, and He are similar, and thus, the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility-controlled amounts in coexisting liquid. Furthermore, there is no evidence of He loss or gain in inclusions by diffusion process. Data from the various systems that were examined indicate that fluid inclusion N 2 -Ar-He compositions may be related to magmatic fluid, sedimentary brine, deep circulating meteoric fluid, and shallow circulating meteoric fluid sources, which we modified from those proposed by Giggenbach (1986). Relationships between the N 2 -Ar-He tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems.

Fluid inclusion gas compositions from an active magmatic-hydrothermal system: a case study of The Geysers geothermal field, USA

Abstract Hydrothermal alteration and the active vapor-dominated geothermal system at The Geysers, CA are related to a composite hypabyssal granitic pluton emplaced beneath the field 1.1 to 1.2 million years ago. Deep drill holes provide a complete transect across the thermal system and samples of the modern-day steam. The hydrothermal system was liquid-dominated prior to formation of the modern vapor-dominated regime at 0.25 to 0.28 Ma. Maximum temperatures and salinities ranged from 440°C and 44 wt.% NaCl equivalent in the biotite hornfels adjacent to the pluton to 305°C and 5 wt.% NaCl equivalent at distances of 1730 m from the intrusive contact. The major, minor, and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling. Major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation; noble gases were released by crushing. The samples were analyzed by mass spectrometry. The analyses document the presence of magmatic, crustal, and meteoric components in the trapped fluids. Hydrothermal fluids present during the liquid-dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources. At The Geysers, N 2 /Ar ratios greater than 525 and 3 He/ 4 He ratios of 6–10.7 Ra are diagnostic of a magmatic component. Crustal gas has CO 2 /CH 4 ratios less than 4, N 2 /Ar ratios between 45 and 525, and low 3 He/ 4 He ratios (0.5 Ra). Meteoric fluids have CO 2 /CH 4 ratios greater than 4 and N 2 /Ar ratios between 38 (air-saturated water) and 84 (air). However, N 2 /Ar ratios between 15 and 110 can result from boiling. Ratios less than 15 reflect the incorporation of N 2 into NH 3 -bearing clay minerals. In the central Geysers, the incursion of meteoric fluids occurred during the transition from the liquid- to vapor-dominated regime. Variations in the relative CH 4 , CO 2 , and H 2 contents of the gas analyses demonstrate that boiling took place under open-system conditions. The gas data indicate that the inclusions have remained closed to the diffusion of He and H 2 since their formation.

Analysis of volatiles in fluid inclusions by mass spectrometry

Abstract Means of extracting and quantitatively analyzing volatiles in fluid inclusions have been studied and details of the methods developed are presented. Thermal decrepitation is the preferred method to open inclusions because experiments indicate that this method yields more accurate measurements of the sulphur-species gases and water. Two assemblages of inclusion species have repeatedly been detected in inclusions from mineral deposits. A CO 2 -dominated assemblage typically has been measured in inclusions from porphyry-type deposits. A CO 2 N 2 C n H n assemblage is typical of inclusion volatiles from sediment-hosted and epithermal deposits. The probable source of N 2 and organic compounds is biologic material which is consistent with a crustal source for the ore solutions. Inclusion volatiles from most ore deposits have 1–3 mole% gaseous species. However, levels of 5–15% have been measured in assemblages of vapor-dominant and liquid dominant inclusions from epithermal deposits. The interpretation is that vapor-dominant inclusions are rich in gaseous species. This implies that the indicated “boiling” of the ore solutions was not strictly aqueous boiling but more an unmixing of the gaseous components. Analytical data on volatiles in fluid inclusions have been used to calculate f O 2 , f S 2 , f CO 2 and pH of ore fluids, and these values agree with values calculated from other data. Data on gases in fluid inclusions have also been used to estimate trapping pressure and study paragenesis.

A comment on arsenic species separation using ion exchange

The Ficklin anion exchange method for separation of arsenate and arsenite is a solution to the problem of preservation of arsenic species. Ion exchange separations, in the field, of arsenite and arsenate are necessary because there are not suitable sample preservation methods that will maintain the arsenate/arsenite ratio, or suitable field separation techniques other than ion exchange. Ion exchange separations were performed on the four arsenic species common in drinking water sources using the Ficklin method. The common organic arsenic species elute with arsenite. This implies that the use of the Ficklin method to speciate waters bearing organic arsenic will have the undesirable consequence of falsely indicating higher concentrations of the most toxic form of arsenic.

Spatial variability of magnetic soil properties

The presence of magnetic iron oxides in the soil can seriously hamper the performance of electromagnetic sensors for the detection of buried land mines and unexploded ordnance (UXO). Previous work has shown that spatial variability in soil water content and texture affects the performance of ground penetrating radar and thermal sensors for land mine detection. In this paper we aim to study the spatial variability of iron oxides in tropical soils and the possible effect on electromagnetic induction sensors for buried low-metal land mine and UXO detection. We selected field sites in Panama, Hawaii, and Ghana. Along several horizontal transects in Panama and Hawaii we took closely spaced magnetic susceptibility readings using Bartington MS2D and MS2F sensors. In addition to the field measurements, we took soil samples from the selected sites for laboratory measurements of dual frequency magnetic susceptibility and textural characteristics of the material. The magnetic susceptibility values show a significant spatial variation in susceptibility and are comparable to values reported to hamper the operation of metal detectors in parts of Africa and Asia. The absolute values of susceptibility do not correlate with both frequency dependence and total iron content, which is an indication of the presence of different types of iron oxides in the studied material.

N2-Ar-He systematics and source of ore-forming fluid in Changkeng Au-Ag deposit, central Guangdong, China*

Changkeng Au-Ag deposit is a newly-discovered new type precious metal deposit. N2-Ar-He systematics studies and3He/4He and δD- δ18O composition analyses show that the ore-forming fluid of the deposit is composed mainly of formation water (sedimentary brine) but not of meteoric water, which was thought to be source of the ore-forming fluid by most previous researchers. The content of mantle-derived magmatic water in the ore-forming fluid is quite low, usually lower than 10%. According to the source of the ore-forming fluid, the Changkeng Au-Ag deposit should belong to sedimentary brine transformed deposits. From the Late Jurassic to the Early Cretaceous Period, with deposition and accumulation of thick sediments in Sanzhou Basin, the formation water in the sedimentary layers was expelled from the basin because of overburden pressure and increasing temperature. The expelled fluid moved laterally along sedimentary layers to the margin of the basin, and finally moved upward along a gently-dipping interlayer fault. Because of a decline in pressure and temperature, ore minerals were deposited in the fault.

Understanding the genesis of nonmarine calcite deposits through quadrupole mass spectrometric analysis of fluid inclusion gases

Abstract Quadrupole mass spectroscopy was used to analyze fluid inclusion volatiles in different types of nonmarine calcites including travertines, phreatic calcretes, hydrothermal vein calcites, and pedogenic calcites. The objectives of the study were to determine if there were any diagnostic or characteristic differences between the inclusion volatiles from the different calcites, and to see if the quadrupole results could be used to understand the processes and environmental conditions that controlled calcite precipitation. The calcites formed under saturated conditions have water-dominated inclusions with well-constrained N 2 /Ar ratios and either CO 2 or N 2 as the dominant inclusion gas. Some calcretes and travertines were exceptions to the above characteristics, but it is likely that these were subjected to secondary precipitation in an environment different from the primary precipitation phase. Results for the pedogenic calcites are quite different from calcites formed in water-saturated systems. Pedogenic calcites have gas-dominated (water-poor) inclusions, unexpectedly large CH 4 contents, low O 2 contents, and highly variable N 2 /Ar ratios. The dominance of CH 4 and low O 2 contents in the pedogenic calcites suggest that anaerobic conditions occurred during at least part of the precipitation process. Scanning electron microscope (SEM) examination of the pedogenic calcites shows fungal remains, bacteria-like forms, and gels from fungi or plant roots. The quadrupole and SEM results suggest pedogenic calcite precipitation occurred as a result of aerobic and, ultimately, anaerobic microbial decay processes.

Magnetic soil properties in Ghana

In this paper we present the results of a study of some soil magnetic properties in Ghana. The soils sampled formed in different parent materials: Granites, Birimian rocks, and Voltaian sandstones. We discuss the role of environmental controls such as parent material, soil drainage, and precipitation on the magnetic properties. The main conclusion of this reconnaissance study is that the eight different soil types sampled have their own unique magnetic signature. Future research will have to confirm whether this conclusion holds for other soils in Ghana. If it does, the measurement of magnetic soil properties may become a viable complement for the investigation of soil erosion, land degeneration, and pedogenesis. The magnetic soil properties measured would probably not pose any limitations for the use of electromagnetic sensors for the detection of land mines and UXO.