GEOCHEMISTRY OF ARSENIC IN MAGMATIC SYSTEMS WITH SOME RESULTS FOR ANTIMONY

Abstract

This thesis reports on the results of high-temperature experiments and detailed chemical analysis of natural samples that assess the oxidation state of As and Sb, and the distribution of As in the solidification products of felsic magmas. In the first study of this thesis, X-ray absorption near edge structure (XANES) determined that As and Sb are predominantly in the trivalent state in basaltic glasses synthesized at 1200°C, 0.1 MPa, and oxygen fugacity (fO2) of FMQ -3.3 to +5.7 (FMQ is the Fayalite-Magnetite-Quartz redox buffer). Additionally, extended X-ray absorption fine structures spectroscopy (EXAFS) revealed that the configuration of both elements is MeIIIO3E trigonal pyramids (Me = metal; E is the lone pair of electrons). As trivalent As and Sb are a poor match for most major elements based on charge and ionic radius, common rock-forming minerals will reject these metalloids during crystallization. The second study of this thesis explores the unexpected enrichment of As in apatite observed in a number of felsic plutonic rock suites. Piston-cylinder experiments were used to measure apatite/melt partitioning and speciation of As in a range of felsic melt compositions at 900-1050°C, 0.75 GPa, and FMQ -0.4 to +7.5. Results indicate that As is incompatible in apatite, except at extraordinarily high fO2. Arsenic-enriched apatite could form from melts after protracted crystallization generates high As levels. Combining the experimental results with crystallization models shows that this process is consistent with the enrichment of incompatible trace elements observed in apatites from the South Mountain Batholith, Nova Scotia. The final study of this thesis determined the geogenic sources of As contamination in groundwater from granitic bedrock terranes in southwestern Nova Scotia. Trace element analysis of the major and accessory minerals from representative bedrock samples and calculation of mineral stability were used to assess the lability of As at near-surface conditions. Pyrite and cordierite are phases with elevated As concentrations and oxidation (pyrite), or dissolution (cordierite) of these phases could release As into the groundwater. This process is consistent with the occurrence of high As concentrations in groundwater associated with cordierite and oxidized pyrite-bearing granitic bedrock. The best predictor of elevated As in groundwater is a combination of the identity and low-temperature stability of the As host minerals and secondary factors such as the permeability of the sample. Collectively, these three studies show that As is likely incompatible in most phases that crystallize from both felsic and mafic magmas over the known range of terrestrial redox conditions. The highest As concentrations will therefore be associated with the most evolved melts and produce the most As-enriched minerals. If these minerals are unstable at low-temperature conditions, then they will become geogenic sources of As contamination after the igneous rocks are brought to the surface by erosion and uplift.