that V/Yb of MORB < BABB < arc basalts is robust (Figure 3.4). This implies that subduction‐modified mantle is more oxidized than the MORB source mantle, consistent with Fe‐based oxybarometry, for reasons that remain debated (e.g., Andreani et al., 2013; Benard et al., 2018; Canil & Fellows, 2017; Carmichael, 1991; Chin et al., 2018; Debret et al., 2014; Evans, this volume; Farner & Lee, 2017; Foden et al., 2018; Gaillard et al., 2015; Kelley & Cottrell, 2009; Lecuyer & Ricard, 1999; Lee et al., 2005; Mungall, 2002; Nebel et al., 2015; Parkinson & Arculus, 1999; Tang et al., 2018; Tollan & Hermann, 2019; Williams et al., 2004; Wood et al., 1990).
3.5. CONCLUSIONS AND FUTURE DIRECTIONS
Oxygen fugacity varies as a function of tectonic setting. We have shown that all estimators of magmatic fO2 (XANES, magnetite‐ilmentite pairs) and mantle source fO2 (spinel oxybarometry, V/Yb ratio) show independently that the fO2 of ridges < back‐arcs < arcs. Inferences about plume fO2 are strongly model dependent, and our study indicates that plume fO2s range widely, but on average are similar to or higher than mid‐ocean ridges. We also strongly conclude that mantle lithologies record a much greater range in fO2 than volcanics.
The process of subduction generates elevated fO2 in both melts and mantle lithologies, though the mechanism and locus of this shift remains debated. The extent to which arc rocks are oxidized relative to MORB does not correlate with crustal thickness or indices of crystal fractionation. Degassing may oxidize or reduce magmas to small extents (< 0.2 log units) so long as melts are captured at pressures > 500 bar, and the tendency for shallow degassing (< 200 bar) to reduce magmas appears universal across all tectonic settings. Plate recycling may also enable plumes to achieve their elevated fO2 relative to mid‐ocean ridges; however, when attempting to project back to near‐primary compositions, the fO2 of plume lithologies is uncertain. This is because plume volcanics have thus far only constrained fO2 minima, and because plume xenoliths derive from lithospheric mantle that was generated at distal mid‐ocean ridges that have subsequently metamorphosed and are variably overprinted by the passage of transiting plume‐derived melts.
Several additional challenges confront a more complete understanding of oxygen fugacity as a function of tectonic setting. Geographic coverage of fO2 estimates remains poor, with peridotites from ridges, volcanic rocks and xenoliths from plumes, and primitive volcanic rocks from arcs especially so. Further sampling of primitive melts and mantle lithologies from diverse tectonic environments is needed in order to illuminate the geodynamic and compositional origins of variable fO2 across tectonic settings. Analytical challenges must still be overcome. Some of the most promising samples for the elucidation of redox processes – melt inclusions – are difficult to prepare and susceptible to radiation beam damage (Cottrell et al., 2018). Experiments and models are needed to gain insights into processes that may shift the fO2s recorded by melts and residues during partial melting of the source and after melt and residue separate. Additional observations of natural samples and new experiments and models are required to ultimately connect the fO2 recorded by partial melts, peridotites and pyroxenites, and the fO2 of the solid convecting mantle.
ACKNOWLEDGMENTS
We thank Oliver Shorttle for providing his XANES data reprocessed using the updated calibration of Zhang et al. (2018). We are grateful to Paolo Sossi, Frank Spera, and an anonymous reviewer for thoughtful, detailed, and constructive comments that greatly improved this contribution. We thank Roberto Moretti and Daniel Neuville for organizing the monograph and for extending the invitation to submit. EC thanks the Lyda Hill Foundation for support during preparation of this manuscript.
METHODS APPENDIX
OXYGEN FUGACITY CALCULATIONSVolcanics
a. Magnetite‐Ilmenite Pairs
For arc and ocean island volcanics, we collected previously reported compositions of magnetite and ilmenite from 28 studies (Arce et al., 2013; Baggerman & DeBari, 2011; Beier et al., 2006; Browne et al., 2010; Carmichael, 1967, 1964; Coombs & Gardner, 2001; Costa et al., 2004; Crabtree & Lange, 2011; Crabtree & Waters, 2017; Devine et al., 2003; Ferrari et al., 2012; Frey and Lange, 2011; Genske et al., 2012; Grocke et al., 2017; Grove et al., 2005; Gunnarsson et al., 1998; Howe et al., 2014; Izbekov et al., 2002; Larsen, 2006; Muir et al., 2014; Portnyagin et al., 2012; Stelten & Cooper, 2012; Toothill et al., 2007; Wallace & Carmichael, 1994; Waters et al., 2015; Wolfe et al., 1997; Wright, 1972). We applied the equilibrium criteria of Bacon & Hirschman (1988) to all collected oxide compositions and only use those that pass for calculations of temperature and oxygen fugacity. (Equilibrium between ilmenite and magnetite is assessed based on comparison Mn and Mg partitioning between ilmenite and magnetite pairs with a dataset of magnetite‐ilmenite pairs from natural volcanics inferred to be at equilibrium), Oxide compositions were input into the model of Ghiorso and Evans (2008) to obtain temperatures and oxygen fugacities. Ghiorso and Evans (2008) report their error for the parameterization of their model in terms of the residual energies (kJ) associated with the exchange and redox equilibria between magnetite and ilmenite. We evaluate the uncertainty of the model of Ghiorso & Evans (2008) by comparing modeled values of temperature and oxygen fugacity for experimentally grown iron oxide pairs with the reported experimental conditions (see Fig. 10 from Ghiorso & Evans, 2008). We find the uncertainty in oxygen fugacity and temperatures associated with model of Ghiorso and Evans (2008) is ±0.25 log units and ±45°C, respectively. All temperatures and oxygen fugacities obtained from magnetite‐ilmenite pairs are shown in Figure A1.
Figure A1 Temperatures and values of fO2 from the model of Ghiorso and Evans (2008) and all magnetite–ilmenite pairs and used in this study.
Figure A2 XANES spectra of MORB glass VG3385 with spectra of mid‐ocean ridge basalt equilibrated over a range of fO2, modified from Fig. 1 of Cottrell and Kelley (2011). The spectra of published MORB glasses fall between basalts equilibrated at furnace fO2s of QFM‐0.71 to QFM +0.87. XANES spectroscopy strongly indicates a narrow distribution of MORB fO2 around QFM, regardless of the corresponding Fe3+/∑Fe of the glasses.
b. Fe3+/∑Fe ratios from XANES
We calculate magmatic fO2s from measured Fe3+/∑Fe ratios using Kress and Carmichael (1991) and referenced to the QFM oxygen buffer of Frost (1991) at one atmosphere and 1200 °C, using the major element compositions reported by each study. For studies that quantify Fe3+/∑Fe ratios using the standard glasses of Cottrell et al. (2009; see Table 3.1), we have recalculated those Fe3+/∑Fe ratios according to a revision of the Mössbauer‐determined Fe3+/∑Fe ratios of those standard glasses
