HIGH ENERGY SYNCHROTRON X-RAY MICROSPECTROSCOPY FOR GEOCHEMICAL CHARACTERIZATION OF TERRESTRIAL AND EXTRATERRESTRIAL SAMPLES

High energy synchrotron X-ray microprobes are powerful tools for a wide variety of spatially-resolved geochemical research. The primary techniques, microfocused X-ray fluorescence (µXRF) analysis, absorption fine structure (µXAFS) spectroscopy, and diffraction (µXRD), are used to determine properties of earth, environmental and planetary materials including chemical and mineralogical compositions, crystallographic and other physical structures, and elemental oxidation states. Of these techniques, µXAFS is arguably the most unique, allowing geochemists to probe the local atomic and chemical environment of a selected atomic species with spatial resolutions better than 1µm at ppm concentrations. By analyzing the modulations in the absorption coefficient at energies at or just above the X-ray absorption edge threshold, µXAFS measurements give quantitative information about coordination species, number and distance, and the valence state of the probe atom. In igneous petrology µXAFS of multivalent elements such as Fe, Cr, Ti, V, and S can be used to quantify the redox evolution of terrestrial and extraterrestrial magmas and their source regions. For example, µXAFS analysis of the valence state of V in igneous phenocrysts and entrapped melt inclusions can provide an in-situ, accurate proxy for fO₂, covering at least six orders of magnitude in buffer-relative oxygen fugacity. These studies are useful for understanding magma chamber evolution and determining links between magma redox heterogeneity and plate tectonic cycling. In sedimentary petrology, the ability to produce µXAFS oxidation state maps has opened new opportunities for understanding the biogeochemical cycling of sulfur in the oceans over geologic time. For example, imaging of the S species present in fossilized bryozoans to constrain sulfate reduction, disproportionation, and sulfide oxidation. In environmental geochemistry µXAFS has become an indispensible tool for quantifying hazardous element mobility, both in surface and subsurface environments. For example, to determine the speciation of elements such as Mo and As and phase associations in flowback and produced wastewaters from shale gas extraction. This presentation will describe instrumental capabilites at the Advanced Photon Source and examples of recent experiments.