Speciation of arsenic and antimony in basaltic magmas

Abstract

This study applies X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structures (EXAFS) spectroscopy at the K-edge to determine the speciation of arsenic and antimony in a suite of basaltic glasses synthesized over a range of oxygen fugacity (fO2) at 1200 °C and 0.1 MPa. Experiments were executed in evacuated fused silica ampoules using a variety of solid metal metal-oxide buffers to achieve fO2’s ranging from FMQ −3.3 to FMQ +5.7 (where FMQ is the fayalite magnetite quartz buffer). The oxidation state was calculated using linear combination fitting (LCF) to spectral reference material with known oxidation states using the XANES spectra. Speciation results were corrected for the quench effect of iron. Trivalent arsenic and antimony were determined to be the dominant oxidation state in the samples, with pentavalent arsenic contributing to less than 10% of the budget of arsenic unless the fO2 is greater than FMQ +5.3 ± 0.9, while pentavalent antimony was not observed. Additionally, no reduced oxidation states of arsenic or antimony were found in the glasses even at the lowest fO2 investigated. Structural parameters such as the coordination number and bond length were determined by fitting theoretical electron scattering paths to the EXAFS spectra. Arsenic is coordinated by three oxygens at 1.78 ± 0.01 Å forming AsIIIO3E (where E is the lone pair of electrons) trigonal pyramids. Antimony is coordinated by three oxygens at 1.98 ± 0.01 Å, interpreted to be in a trigonal pyramid structure similar to As3+. As both metalloids are primarily present in the trivalent state over the range of terrestrial fO2 (FMQ −3 to FMQ +5) it is expected that each will behave incompatibly during basaltic melting or crystallization, owing to (1) the tendency for these elements to form oxyanions, and (2) their poor match in ionic radius and charge for the major cation sites in mafic minerals.