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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.