from OIB localities have been interpreted as the products of extensive refertilization of the lithosphere by plume‐derived melts (Sen & Leeman, 1991). The relatively oxidized fO2 recorded by these pyroxenite xenoliths provides further evidence that OIB source regions are more oxidized than the MORB source (Figure 3.3d). The opportunity to investigate the Fe3+/∑Fe ratios of submarine and melt inclusion suites from arcs and plumes holds great future promise for reconstructing primitive melt compositions from partially degassed samples.
Crystal Fractionation.
Cottrell and Kelley (2011) and numerous studies since (Birner et al., 2018; O’Neill et al., 2018; Shorttle et al., 2015), have shown how Fe3+/∑Fe ratios increase during low‐pressure crystal fractionation by a few percent, and that the fO2 of MORB glass spans a smaller range of fO2 (e.g., Fig. 3.2a) than indicated by wet‐chemistry. However, extensive crystal fractionation and crustal assimilation are commonly observed in magmas that transit thick arc crust, and this has been invoked to shift magmatic fO2 away from its mantle source (e.g., Chin et al., 2018; Grocke et al., 2016; Lee et al., 2005; Tang et al., 2018). The dearth of fO2 studies on the rare basalts and olivine‐hosted melt inclusions that transit the continental crust poses a challenge to the community. Insights can be gleaned, however, from extensive analytical work on magnetite–ilmenite pairs in more evolved arc rocks. In the arc crust, we observe that magnetite–ilmenite only precipitate once primary magmas have fractionated significant quantities of olivine and clinopyroxene (which remove Fe2+ from the melt), and magnetite–ilmenite pairs in arc lavas record slightly higher fO2s than primitive arc glasses, in accordance with this expectation (see Results, Table 3.1, and Fig. 3.2). Within the BABB suite, silica and fO2 do covary, although this relationship is demonstrably unrelated to crystal fractionation (Brounce et al., 2014). SiO2 and fO2 covary because of two independent phenomena: melting more hydrous mantle yields primary magmas with higher SiO2 concentrations (Kushiro, 1972) and melting mantle with more subduction influence yields primary magmas with higher fO2s and also more water (Kelley & Cottrell, 2009).
Figure 3.6 shows all of the volcanic data we have compiled as a function of both crustal thickness (from 7 to nearly 70 kilometers), SiO2 concentration (a proxy for crystal fractionation, ranging from ~45 to > 75 wt.%), and SiO2/Alkali ratios. We observe that magmas record fO2s in excess of ~QFM +1 only when the crust is thicker than that found in the ridge setting. However, from volcanics erupted through ~25 km of oceanic crust to volcanics erupted through nearly 70 km of continental crust, we observe no global correlation between crustal thickness and fO2. Neither do we observe a correlation between fO2 and silica content or SiO2/alkalis, within or among arcs. Figure 3.6 demonstrates that, to first order and on average, there is no simple relationship amongst the variables of crustal thickness, differentiation, and oxygen fugacity. Thus, while the relative influence of slab characteristics, the mantle wedge, and differentiation within the overlying crust on the geochemistry of arcs in the broadest sense remains an active area of research (Chin et al., 2018; Farner & Lee, 2017; Lee et al., 2013; Tang et al., 2018; Turner & Langmuir, 2015; Turner et al., 2016), thick crust and crystal fractionation are not necessary for the generation of oxidized magmas.
Inferences about Mantle fO2 as a Function of Tectonic Setting.
From the analysis above, we may conclude that neither degassing nor crystal fractionation can generate the increases we observe in Fe3+/∑Fe ratios as we move from the mid‐ocean ridge, to the back‐arc, to the arc‐front environment. The inability of these processes to greatly alter fO2 is evidenced by the fact that magnetite–ilmenite oxybarometry on volcanics, and spinel‐oxybarometry on the source mantle itself, also record increasing fO2 as we move from the ridge, to the back‐arc, to the subduction‐influenced forearc, to the arc‐front setting. We conclude from this evidence that the mantle source itself must become oxidized as the influence of subduction increases. Indeed, in situ work by Kelley and Cottrell (2009) and numerous studies since (Brounce et al., 2014; Brounce et al., 2021; Brounce et al., 2015; Kelley & Cottrell, 2012), have linked the fO2 recorded by submarine glasses and olivine‐hosted melt inclusions to enrichment in slab‐derived fluid‐mobile incompatible trace elements. However, it is critical to consider other lines of evidence, such as trace element proxies, that have been argued to be more robust proxies for mantle fO2 (e.g., Lee et al., 2005).
Figure 3.6 Magmatic oxygen fugacity as recorded by volcanics for samples in our compilation as a function of the crustal thickness (a) and SiO2 (b), and the molar ratio of SiO2/Na2O+K2O (c). We combine oxygen fugacities derived from melt inclusions with those from magnetite‐ilmenite pairs for each tectonic setting in each panel; we do not distinguish results based on method. Crustal thickness taken from (Behn & Grove, 2015; Calvert et al., 2008; Chulick et al., 2013; Darbyshire et al., 2000; Das & Nolet, 1998; Ferrari et al., 2012; Finotello et al., 2011; Janiszewski et al., 2013; Levin et al., 2002; Manalo et al., 2015; McGlashan et al., 2008; Nicolich et al., 2000; Saiga et al., 2010; Sevilla et al., 2010; Spieker et al., 2018; Syuhada et al., 2016; Takahashi et al., 2007; Veenstra et al., 2006; Watts & ten Brink, 1989).
We compare our Fe‐based oxybarometry results to those obtained from trace element partitioning. Several studies based on V/Sc, V/Y, V/Ti, V/Ga, Zn/Fe ratios, or Cu concentrations have concluded that the fO2 recorded by arc volcanics are statistically indistinguishable from those recorded by MORB (Lee et al., 2005; Lee et al., 2010; Lee et al., 2012; Mallmann & O’Neill, 2013); this study, and others, have reached the opposite conclusion (Bucholz & Kelemen, 2019; Laubier et al., 2014; Shervais, 1982). It is beyond the scope of this contribution to translate the V/Yb ratios we have compiled into fO2; however, the conclusion 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