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Subduction influences the composition of mantle melts and arc volcanics, generating continental crust in the process (e.g., Elliott et al., 1997; Gill, 1981; Grove et al., 2012; Kelemen et al., 2003; Kelemen et al., 2007; Osborn, 1959; Plank & Langmuir, 1988; Stolper & Newman, 1994; Turner & Langmuir, 2015; Zimmer et al., 2010). Both melts and mantle lithologies offer opportunities for oxybarometry. We begin with the volcanics.

Seminal contributions by Carmichael (1991) and Frost and Lindsley (1992) surveyed the fO₂s recorded by arc rocks using wet‐chemistry and magnetite‐ilmenite pairs, respectively, and found that arc rocks record fO₂s up to several orders of magnitude higher than MORBs. Our compilation of 5 XANES spectroscopic studies (n=119 samples, Figure 3.2c) of olivine‐hosted melt inclusions and submarine pillow glasses shows that arc basalts record, on average, QFM +0.96 (±0.39). One set of outliers from Cerro Negro record QFM +4.75 (±0.40) (Gaetani et al., 2012), but spectra from these hydrous samples have suffered from radiation‐induced beam damage (Cottrell et al., 2018, Gaetani, pers. comm.) and are not included in our statistical analysis. Nearly 90% of samples with XANES measurements erupted through the thin crust (~25 km, Takahashi et al., 2007) of the active Mariana arc front (Fig. 3.1a), and thus there is significant location bias in this dataset.

Globally, magnetite–ilmenite pairs, from 114 arc lavas sampling 11 different arcs, record QFM +1.28 (±0.64) (Figure 3.2d; see methods appendix and the online data library associated with this contribution, Cottrell et al., 2021, for citations). These samples contain Fe–Ti oxides with compositions that record a range of temperatures (700–1085 °C), span a wide range of compositions (basaltic andesite to rhyolite) but are predominantly dacitic, and erupt through crust ranging from 25 to 66km thick.

The mean fO₂ recorded by olivine‐hosted melt inclusions and submarine arc‐front glasses (ΔQFM = 0.96±0.39, n=119) is slightly lower than that recorded by magnetite‐ilmenite pairs (ΔQFM = 1.28±0.64 n=114) at the 95% confidence level (t_statistic = 4.6, t_critical = 2.0, degrees of freedom [DF] = 186, p‐value < 0.001). (When we compare distribution means in this contribution, we will always apply a two‐sample student’s t‐test with α= 0.05 [Krzywinski & Altman, 2013] for samples of unequal variance.) We caution that the datasets are not directly comparable because of the limited geographic distribution of the melt inclusion and submarine glass dataset; because there are no samples in common between the two distributions; and because the melt inclusions may reflect magma compositions that precede magnetite and ilmenite saturation. Thus, to first order, our global result is not inconsistent with the results of Waters and Lange (2016) and Crabtree and Lange (2011) who found congruence when they compared magnetite‐ilmenite oxybarometry to wet‐chemical titration on the same suite of very fresh aphyric lavas.

A more direct comparison can be made between the olivine‐hosted melt inclusions and submarine glasses erupted along the active Mariana Arc, and back arc basin (BAB) glasses erupted at depth along the associated back arc spreading center: the Mariana Trough. Both datasets apply the same method (XANES) to arrive at fO₂ estimates, and both sample suites comprise basaltic to basaltic andesite glasses representing similar stages of differentiation in thin crust (similar MgO). The Mariana arc front samples record fO₂s that are on average 0.73 log units higher than the Mariana trough samples (Mariana arc: QFM+0.95±0.36 for n = 107 vs Mariana trough: QFM+0.22±0.30 for n=37, respectively) (Brounce et al., 2014; Kelley & Cottrell, 2009).

Another direct comparison can be made between submarine basalts in ridge (MORB) and BAB tectonic settings. Here we find that BAB from the Mariana trough (n=37) record significantly higher fO₂s than MORB globally (n=160) by 0.4 log units (t_statistic = 7.8, t_critical = 2.0, degrees of freedom [df] = 41, p‐value << 0.001). This comparison is particularly germane for inferring the effect of subduction on mantle fO₂ because submarine back‐arc ridges and mid‐ocean ridges are tectonically similar and differences in their melt chemistry can be largely attributed to the influence of subduction (Stolper & Newman, 1994). As indices of subduction influence in Mariana trough lavas go from negligible to significant (e.g., as H₂O contents and the ratios of fluid mobile to fluid immobile incompatible trace elements increase), Fe³⁺/∑Fe ratios (and fO₂s) also increase (Brounce et al., 2014; Kelley & Cottrell, 2009, 2012). In the Marianas, volcanics erupted over the course of the arc’s maturation also record increasing fO₂ with increasing subduction influence. Modern arc tholeiites record similar fO₂s to the boninites that erupted during the early stages of slab influence on the mantle wedge; and both lithologies are more oxidized than the forearc basalts that tapped the mantle prior to slab influence (Brounce et al., 2021; Brounce et al., 2015). The volatile and trace element signals of subduction appear intimately tied to elevated fO₂s in space and in time.

Mantle lithologies recovered from arc settings comprise primarily forearc and arc peridotites. Forearc peridotites are exposed on trench walls and may sample ancient lithospheric mantle (Parkinson & Pearce, 1998), mantle wedge metamorphosed by the subducting slab (Fryer et al., 1985), or processes associated with subduction initiation (Birner et al., 2017). In comparison, arc peridotites rapidly ascend to the surface as xenoliths encased within their basaltic hosts at arc front volcanoes. The mean fO₂ recorded by forearc peridotites is statistically indistinguishable from the mean fO₂ recorded by ridge peridotites (t_statistic = 0.73, t_critical = 2, df = 131, p‐value = 0.47) (Fig. 3.3). As discussed by Birner et al. (2017), this result contrasts with Parkinson and Pearce (1998)’s study of forearc peridotites from the Izu‐Bonin subduction zone, primarily because we apply the spinel activity model of Sack and Ghiorso (1991) instead of Nell and Wood (1991). Yet, consistent with Parkinson and Pearce (1998), Birner et al. (2017) show that peridotites that have interacted with slab‐influenced melts do yield elevated fO₂. This influence is additionally evident in the distribution of fO₂ recorded by arc xenoliths from five studies (Table 3.1), which lies significantly higher, by 0.65 log units, than ridge peridotites (t_stat = 4.4, t_crit = 2.0, df = 90, p‐value << 0.001) or forearc peridotites. Another unique characteristic of sub‐arc peridotites is the extended range of melt extraction they record. Spinel Cr#, commonly taken as a proxy for melt extraction, extends to much higher values (> 60) in sub‐arc peridotites than in ridge peridotites. This extended range of melt extraction may provide an opportunity to investigate the relationship between extent of melting and fO₂. For example, Benard et al. (2018b) found a weak positive correlation (p‐value > 0.06) between fO₂ and modal orthopyroxene, which they interpreted as evidence of fO₂ falling with melt extraction; however, the positive correlation between spinel Cr# and fO₂ in these same samples suggests the relationship between fO₂ and melt extraction may be more complicated. No correlation exists between fO₂ and orthopyroxene mode or spinel Cr# in the Tonga peridotites of Birner et al. (2017). More work is needed to better constrain the effects on fO₂ of extracting melt from the mantle.