Browsing by Subject "Isotopes"

Prior studies of groundwater systems have associated increasing salinity and anoxic conditions with increasing radium (Ra) activities in water due to the decreasing effectiveness of Ra removal processes. However, the components of salinity (e.g. Ca vs. Na and SO₄^2- vs. Cl^--dominated waters), and the relative importance of salinity-sensitive vs. redox-sensitive processes for Ra mobilization, are less well understood. In this research, the response of Ra to hydrochemical change was examined using a multiple tracer approach to obtain detailed information on divalent cation and Ra mobility. A range of salinity and redox conditions was examined in five field-based studies in the United States and Morocco: (1) fresh waters in fractured crystalline rocks in the Piedmont region of North Carolina; (2) the Willcox Basin, an oxic alluvial basin-fill aquifer in southeastern Arizona; (3) the Jordan sandstone aquifer, a carbonate-cemented quartz sandstone in southeastern Minnesota; (4) an unconfined coastal aquifer undergoing salinization in the city of Agadir, Morocco; and (5) the confined, fresh to saline Cretaceous and Pliocene aquifers of the Atlantic Coastal Plain in North Carolina.

In addition to analysis of major element concentrations, trace metal concentrations, and ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra activities, complementary isotope systems were applied to gain insights on the relative stability of chemical processes that remove radium and other alkaline earth metals: (1) strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) trace divalent cation release from sources such as clay and carbonate minerals in the aquifer solids and also indicate conditions in which divalent cation release (rather than uptake) is dominant; (2) boron concentrations and isotopes (δ¹¹B) coincide with the opposite condition in freshening conditions of the Atlantic Coastal Plain, in which divalent cations are removed in exchange for Na; and (3) sulfur and oxygen isotopes (δ³⁴S, δ¹⁸O) of sulfate trace sulfate sources and provide information on sulfate-reducing conditions, which can inhibit barite (BaSO₄) from removing Ra by coprecipitation. In addition, other isotopic and ion measurements trace salinity sources and groundwater residence time, including δ²H, δ¹⁸O, ³H, Br^-/Cl^-, Na/Cl^-, and Ca/Na.

This dissertation documents correlations between salinity and radium in the brackish to saline North Carolina coastal plain aquifer with total dissolved solids (TDS) up to ~18,000 mg L^-1 and to some degree in the Moroccan coastal aquifer, but even the lower-salinity waters (TDS <3000 mg L^-1) exhibit a range of Ra activities spanning approximately 3 orders of magnitude. Among these low-TDS waters, the highest Ra activities were observed in the anoxic Jordan sandstone aquifer and the lowest were observed in the oxic Willcox Basin aquifer. Although the main control on radium activities in fresh groundwater is the U- and Th-series radionuclide content of the aquifer solids, important secondary controls include the stability of redox-sensitive radium adsorption sites (Mn and Fe oxides), the relative dominance of divalent vs. monovalent cations (e.g. the Ca/Na ratio), formation of the uncharged RaSO₄⁰ complex, and/or the saturation state with respect to barite. These processes interact in varied ways in the field-based studies. Increasing radium activities and decreasing ²²²Rn/²²⁶Ra ratios in the North Carolina fractured crystalline rock groundwater system are correlated with increasing Ba, Mn, and Fe concentrations and decreasing dissolved oxygen concentrations, related to weathering and/or organic carbon oxidation. Radium activities in the oxic, neutral to slightly basic Willcox Basin are very low (median ²²⁶Ra activity 2 mBq L^-1), probably due to a combination of effective Ra removal processes including adsorption to Mn and/or Fe oxides and the overall removal of divalent cations during groundwater evolution in this system. These are the same surface charge conditions that release arsenic, of regional water concern, in this pH range. Radium in Jordan aquifer groundwater is dependent on local variations in solid-phase radionuclide levels, probably hosted in the carbonate cement phase. Also, Ra is inefficiently adsorbed to the aquifer solids in the aquifer's anoxic conditions, resulting in the highest radium levels reported in this dissertation (²²⁶Ra up to 420 mBq L^-1) despite apparent barite precipitation that partially removes Ra. Radium-224 activity in the Moroccan coastal aquifer is associated with salinity, but Ra overall is apparently controlled by barite, indicated by conditions near BaSO₄ saturation. Radium activity in the saline waters of the Atlantic Coastal Plain aquifers is associated with TDS concentrations, but the cation exchange properties of the aquifer may provide a major mechanism of Ra removal in the Na-HCO₃^- and Na-Cl^- waters. Overall, the complex interaction between groundwater chemistry and Ra-removing processes implies that in waters with TDS below approximately 3,000 mg L^-1, dissolved solids concentration alone does not fully describe radium's response to hydrochemical conditions, but rather that aquifer-specific examination of Ra removal mechanisms is needed.

The genesis of layered intrusions has been the focus of countless studies. Layered intrusions have historically been viewed as natural laboratories to understand the evolution of a single large magma chamber. Many contain platinum- and palladium-rich reef-type deposits, making layered intrusions particularly important economically. Further, layered intrusions may be a useful analogue for understanding subduction zone hydrothermal systems.This dissertation investigates layered intrusion genesis, specifically in relation to the suggested hydrothermal model of layered intrusion formation, which suggests migrating fluids may have remobilized economically important elements, creating the deposits observed in these intrusions today. The work is divided into eight chapters that explore three layered intrusions: the Bushveld Complex, South Africa, the Stillwater Complex, Montana, and the Skaergaard Intrusion, Greenland. New samples collected from the Stillwater Complex were analyzed for major and trace element compositions and radiogenic and stable isotopes. Investigations into the Bushveld Complex and Skaergaard Intrusion were based on previously published data. The second chapter examines evidence for fluid circulation in the Bushveld Complex, South Africa, as responsible for some of the geochemical and isotopic signatures present in the complex. Previous isotopic studies of Bushveld are combined with numerical modeling of footwall dehydration to suggest that diapir-like structures injected fluids into the Main Zone of the intrusion. This chapter further details the similarities between diapiric structures in the Bushveld Complex and those that have been modeled in subduction zone hydrothermal systems. The third chapter expands upon the Bushveld model, specifically in relation to the formation of iron-rich ultramafic pegmatoids and dunite pipes, which the work presented here suggests to be fluid-related. In the fourth chapter, strontium, neodymium, and lead isotopes are analyzed for rocks from the Stillwater Complex, Montana, to compare with the isotopic mixing model results of the Bushveld Complex. Initial isotopic ratios are used to explore various proposed models of complex formation. Results suggest isotopic heterogeneity during complex formation, whether due to heterogeneous source regions or crustal/fluid contamination. In the fifth chapter, stable isotope analyses (oxygen, hydrogen, and lithium) are used to better understand the formation of the pegmatoidal bodies thought to be related to fluids at Stillwater. Some evidence of fluid circulation may be observed in hydrogen and lithium isotopes. Geothermometry using oxygen isotopes is suggestive of lower cooling temperatures in the pegmatoids, and may provide evidence of mineral-scale disequilibrium attributable to fluid circulation. The sixth and seventh chapters utilize the thermodynamic modeling program MELTS to explore problems of layered intrusion evolution. Understanding the evolution of the liquids that formed various layered intrusions, and identifying magmas parental to layered intrusions, can pose a challenge. Using MELTS, bulk rocks can be synthetically remelted, and the evolution of the complex can be examined through analysis of estimated trapped liquid contents. The sixth chapter applies this method to the Stillwater Complex, while the seventh chapter extends this work to the Skaergaard intrusion in East Greenland. These investigations allow for examination of the magmatic processes operating alongside hydromagmatic processes in layered intrusions.