Megan Holycross: Probing the deep Earth oxygen cycle with XANES

The oxidation state of the solid Earth influences, to a first order, the structure of the planet and the chemistry of rocks, ores and volcanic gases; mass transfer of oxygen between terrestrial reservoirs enables a habitable world. Fe K-edge XANES spectroscopy shows magmas erupted from continental crust, formed at subduction zones, are more oxidized than magmas erupted from oceanic crust, formed at mid-ocean ridges (e.g., Kelley and Cottrell 2009; Science). However, there is long-standing debate over the timing and mechanism of the processes that produce oxidized continental crust. The high-pressure mineral phase garnet is a key reservoir of iron in the crust and garnet Fe3+/ƩFe ratios record the transfer of oxygen throughout the subduction process. Here, I’ll use garnet Fe-XANES to quantify the oxidation state of two suites of subduction-related rocks to examine changes in the deep Earth oxygen cycle over space (scale of ~100 km) and time (scale of ~2.5 billion years).

References:

1. K. A.  Kelley and E. Cottrell, Water and the Oxidation State of Subduction Zone Magmas Science  325, pp 605  (2009)   https://doi.org/10.1126/science.1174156 
2. M. D. Dyar, et al,  Accurate determination of ferric iron in garnets by bulk Mössbauer spectroscopy and synchrotron micro-XAS American Mineralogist 97 pp 1726–1740 (2012) https://doi.org/10.2138/am.2012.4107