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Direct measurements of copper speciation in basaltic glasses: Understanding the relative roles of sulfur and oxygen in copper complexation in melts

December 15, 2019

Micro-analytical determination of copper (Cu) speciation in natural magmatic glasses, equilibrated below the nickel – nickel oxide (NNO) buffer, reveals that two copper species are commonly stabilized in such basaltic melts. X-ray absorption fine structure (XAFS) spectroscopic analysis of basaltic matrix glasses and melt inclusions (MI) from samples of mid-ocean ridge basalt (MORB), and from Nyamuragira, Etna and Kīlauea volcanoes show that both Cu(I)-sulfide and Cu(I)-oxide species are stabilized. The proportion of each species correlates with the measured sulfur (S) abundance of the glass. In glasses with S abundances greater than ~1000 ppm, Cu(I)-sulfide species are dominant, whereas in glasses with S abundances between 500 and 1000 ppm, both species are found to coexist. The Cu(I)-oxide species dominate at S concentrations below 500 ppm. In 1 atm S-free experimental glasses of basaltic composition that we analyzed, only Cu(I)-oxide species are detectable, regardless of the oxygen fugacity (fO2), even at relatively high fO2 values well above the NNO buffer. Our results demonstrate that XAFS techniques are highly sensitive in measuring Cu speciation in reduced (below NNO) basaltic glasses and that both oxide and sulfide complexes can be stabilized. The relative proportion of these two species is highly dependent on the concentration of S in the melt, and thus the Cu speciation in natural melts changes as S is lost from the melt by low pressure degassing.

Citation Information

Publication Year 2019
Title Direct measurements of copper speciation in basaltic glasses: Understanding the relative roles of sulfur and oxygen in copper complexation in melts
DOI 10.1016/j.gca.2019.09.029
Authors Antonio Lanzirotti, R. Lopaka Lee, Elisabet Head, Stephen R Sutton, Matthew Newville, Molly McCanta, Allan Lerner, Paul J. Wallace
Publication Type Article
Publication Subtype Journal Article
Series Title Geochimica et Cosmochimica Acta
Series Number
Index ID 70209359
Record Source USGS Publications Warehouse
USGS Organization Volcano Science Center