New applications of radioisotope systems in geologic and hydrologic processes [electronic resource]
Keywords: Geological and Environmental Sciences.

Radioactive isotopic systems are widely used for geochronologic purposes, but as understanding of these systems improves, novel applications become possible. We provide new constraints on the potassium decay series in biotite and phlogopite micas: diffusion of Ca and partitioning of Ar, which open new possibilities for K-Ar and K-Ca geochronology, thermochronology, and geochemistry. We also look at a previously unexplored application for the uranium decay series, characterizing groundwater flow through fractured rock. Chapter 1 focuses on constraining calcium diffusion in mica, a process fundamental to potassium-calcium dating, using natural samples and thermal history modeling. We dated a suite of three-billion-year-old biotite and phlogopite from southwestern Greenland by 40Ar/39Ar and in situ K-Ca methods. The 40Ar/39Ar ages were generally consistent with previous estimates based on U-Pb zircon and field observations, yielding ages clustered near 3 Ga (peak granulite facies metamorphism) and 2.5 Ga (the age of emplacement of the Qôrqut granite, a large-scale suturing event). K-Ca ages were all significantly younger, clustering around 1 Ga. Thus, the thermal history of these samples resulted in more Ca loss than Ar loss. Comparison with diffusion literature suggests a low activation energy for the Arrhenius law for Ca diffusion in phlogopite. Modeling hypothetical thermal histories yields a range of possible diffusion parameters for Ca in phlogopite: Ca diffuses slower than Ar at high temperatures (> 500 °C), but is much faster at low temperature (< 500 °C). We present closure temperatures for a range of cooling rates and domain sizes, and show that K-Ca dating of micas is most appropriate for low-T thermochronology. Chapter 2 focuses on the potassium-argon system, specifically on estimating the partitioning behavior of Ar based on experimental determinations of Ar solubility in fluorophlogopite. Experiments were conducted exposing synthetic fluorophlogopite to Ar gas at pressures of 1 and 10 kbar, and at temperatures from 800 to 1100 °C. Bulk incremental heating analysis of the experiment run products yielded 39Ar release data, which could be modeled to determine the diffusion properties, namely the size distribution of diffusion domains. These domain size distributions could then be used to model the uptake of 40Ar in fluorophlogopite for each experiment. The estimated uptake and the measured 40Ar concentration allow simple calculation of Ar solubility. Experiments indicate five to ten times higher Ar solubility than previous measurements. Partition coefficients between phlogopite and melt are also significantly higher than previous estimates, which has important implications for 40Ar/39Ar chronological methods, in particular how and in what settings some of the fundamental assumptions of Ar dating may not be valid. Chapter 3 uses uranium isotopes in combination with strontium isotopes to explore flow rates, reaction rates, fracture geometry in the groundwater system at the former Homestake Gold Mine in South Dakota. Water samples and field data were collected at various depths from the surface down to 1480 m depth. Major and trace element analyses of water samples reflected the complexity of the site, a product of both compositional and structural heterogeneity. The fluid isotopic ratio 87Sr/86Sr and the activity ratio (234U/238U) tend to equilibrate more slowly and thus showed less scatter, reflecting instead a gradual shift from the initial composition of the fluid (at the surface) to the composition of the host rocks. Modeling efforts require that isotopic evolution is primarily controlled by isotopic exchange with silicate minerals, and suggest that fracture spacing is wider than field observations suggest, implying that many observable fractures may not be carrying significant fluids.