Collaborative Research: Measurement of Copper Speciation in Basaltic Glasses using X-ray Absorption Spectroscopy, a New Window on Metal Solubility and Transport in Magmatic Systems

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The solubility of metals such as copper (Cu) in silicate melts can be strongly influenced by natural properties of the magma such as pressure, temperature, and melt composition. Understanding how such things control metal solubility, particularly for Cu, is important for a number of reasons. It helps researchers understand potential driving mechanisms for core formation in terrestrial planets, or the conditions that lead to the formation of economically important metallogenic ore deposits. It's also key to understanding how volcanic degassing may impact metal emissions to the atmosphere. This study proposes to use synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy of natural and synthetic magmatic glasses with the goal of providing the first direct, experimental constraints on the speciation of Cu in terrestrial melts. XAFS is currently the only methodology available for directly measuring Cu speciation in magmatic glasses at natural concentrations with high spatial resolution. In addition to the scientific goal of constraining which Cu species exist in melts, this work will broaden the participation of minority students in geoscience, and train students in state-of-the-art experimental and analytical techniques.This will be achieved through three overlapping projects. Project 1 will include copper speciation measurements of synthetic basaltic glasses that are produced at controlled sulfur and oxygen conditions. This suite of synthetic glasses will have both oxygen and sulfur fugacity carefully controlled and will then be analyzed for changes in Cu speciation using XAFS. Project 2 includes Cu speciation measurements of natural glasses sampled from volcanic systems. This project targets natural samples from a sulfide-dominated system with H2O-poor melts (using samples collected from the Kilauea summit vent between 2008 and present), samples from a sulfide-dominated system with higher-H2O melts (from a basaltic cinder cone in the southern Cascade Range), and from sulfate dominated volcanics from both the southern Cascades Range and the Colima Graben in western Mexico. Project 3 will focus on developing advanced, synchrotron-based spectroscopic techniques for Cu K-edge XAFS of magmatic glasses, which will be applied to samples available from Projects 1 and 2, with a focus on developing High Energy-Resolution Fluorescence Detected XAFS (HERFD-XAFS) techniques for the analysis of Cu in geologic materials and magmatic glasses specifically. Such new HERFD-XAFS techniques hold the promise of providing superior spectroscopic sensitivity and resolution compared to conventional XAFS methods for uniquely constraining the copper ligands are present in these samples. These proposed measurements will be the first application of these methods to magmatic glasses and will establish a new database of spectroscopic measurements that will be available to the petrology community. Together, this suite of studies will significantly contribute to our understanding of metal solubility and transport in magmatic systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.