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doi: 10.7185/geochemlet.1823 The redox state of mantle lithologies, based on xenoliths from continental lithospheric mantle, has been shown to decrease with depth and reach oxygen fugacities ( fO2) at which graphite/diamond will be the stable form of carbon at pressures greater than about 3-4 GPa (e.g., Frost and McCammon, 2008). On the other hand, the depth-fO2 profile of the convecting mantle remains poorly known. We compare the CO2-Ba and CO2-Nb systematics of natural oceanic basalts to the CO2-trace element concentrations that can be generated via contributions from depleted peridotite partial melts and graphite-saturated partial melts of subducted lithologies. Results suggest that to produce the CO2 enrichments relative to the depleted end member observed in natural oceanic basalts, subducted lithologies cannot be graphite-saturated at the onset of melting or must undergo oxidative transformation below the respective volatile-free solidi. Therefore, the oxygen fugacity profile of the continental lithospheric mantle may not be applicable to the deep convecting upper mantle, with the convecting upper mantle to at least 150 km depth being more oxidised than the carbonate vs. graphite/diamond buffer. Received 20 April 2018 | Accepted 1 August 2018 | Published 18 September 2018 1. Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005 * Corresponding author (email: james.eguchi@rice.edu) Introduction The oxidation state of the mantle plays a critical role in dictating the geochemical and geodynamic evolution of Earth and its atmosphere (e.g., Frost and McCammon, 2008). Current understandings of mantle redox are largely based on measurements of Fe3+ and Fe2+ in mantle-derived basalts, which suggest that the oxygen fugacity of Earth’s shallow mantle to about 1.5 GPa is close to the FMQ buffer (e.g., Cottrell and Kelley, 2011; Hartley et al., 2017). The fO2 of the deeper portions of Earth’s mantle is chiefly known from mantle xenoliths recovered from continental lithospheric mantle (CLM), which suggest fO2 decreases with increasing depth (e.g., Woodland and Koch, 2003). This trend has been attributed to the increasing stability of the Fe3+-bearing skiagite component of garnet with increasing depth (Gudmundsson and Wood, 1995). The fO2 decrease with increasing depth observed in CLM has been proposed to apply to the convecting mantle (e.g., Frost and McCammon, 2008). However, several observations offer up the possibility that the fO2 trend estimated for CLM may not be applicable to the convecting mantle. (1) Kimberlites (oxidised, CO2-rich magmas) are generated deep in the mantle, then migrate through overlying cratons, often picking up diamonds, a reduced form of carbon, implying there are regions in the mantle deeper than CLM depths where diamond resides that are more oxidised (Yaxley et al., 2017; Dasgupta, 2018). (2) Carbonate inclusions have been found in diamonds from depths in the mantle that should be in the graphite/diamond stability field if the CLM fO2 profile is applied to the convecting upper mantle (Brenker et al., 2007). (3) Kiseeva et al. (2018) argue that fO2 measurements of garnet inclusions in diamonds from the mantle transition zone become more oxidised with depth, and are more oxidised than predicted by the Fe-controlled fO2 profile inferred from CLMs. (4) Komatiites, which are generated deeper in the mantle than basalts have similar redox states to MORB (Gaillard et al., 2015). While these natural observations may be due to special circumstances, they raise the possibility that the CLM fO2 profile may not be applicable to the convecting mantle. CO2 Enrichment of Oceanic Basalt – Graphite/Diamond or Carbonatepresent Melting? Many oceanic basalts are argued to contain partial melt contributions of subducted lithologies such as sediments, MORB-eclogite, and pyroxenite (Hofmann and White, 1982), which begin to melt deeper than peridotite due to their lower solidi (e.g., Spandler et al., 2008). If there are redox sensitive geochemical proxies that track deep melting processes, they may provide constraints on the redox state of deeper portions of the convecting upper mantle, where the recycled lithologies undergo melting. Many of these basalts have elevated CO2 concentrations relative to normal mid-ocean ridge basalts (Aubaud et al., 2006), which may be due to a CO2 contribution Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2018) 8, 17-21 | doi: 10.7185/geochemlet.1823 18 from subducted lithologies, which have higher CO2 concentrations relative to primitive mantle (Anderson and Poland, 2017; Hauri et al., 2017). At sub-solidus conditions, the stable form of carbon depends on fO2; at low fO2 C exists as graphite/ diamond, at high fO2 C exists as carbonate. Graphite-bearing lithologies melt at the lithology’s nominally volatile-free solidus and generate a silicate melt with fO2-dependent CO2 concentrations (e.g., Eguchi and Dasgupta, 2017, 2018). In addition, the presence of graphite during partial melting has negligible effect on the composition of generated melts due to the refractory nature of graphite. On the other hand, carbonate-bearing lithologies melt at the lithology’s carbonated solidus and generate a carbonated melt with up to ~45 wt. % CO2 (e.g., Dasgupta and Hirschmann, 2006). Therefore, oceanic basalts receiving contributions from crustal/mafic lithologies will have different CO2 concentrations depending on whether partial melting of these easily fusible lithologies occurred under graphite/diamond-saturated versus carbonated conditions. In the following, we test whether CO2 and other trace element enrichments observed in natural basalts can be produced by graphite-saturated melting of recycled lithologies. If CO2 concentrations of natural samples are too high to be explained by recycled lithology contributions via graphitesaturated melting, it would suggest that the fO2 of the subducted lithologies at or below their nominally volatile-free solidi must be high enough for carbon to exist as carbonate. CO2-Nb and CO2-Ba Systematics of Oceanic Basalts Here we use CO2-Nb and CO2-Ba systematics of oceanic basalts argued to be receiving contributions from recycled lithologies (see Table S-2) to investigate the redox state of the deeper portions of the convective upper mantle. We use a new, compositionally dependent CO2 solubility model for silicate melts (Eguchi and Dasgupta, 2018) applicable to graphite/diamond-saturated conditions to investigate a wide range of lithologies. We compiled partial melting experiments conducted on lithologies which may contribute to the generation of oceanic basalts. We only consider partial melting studies conducted at pressure-temperature conditions below the peridotite solidus to investigate processes which occur deeper than the onset of peridotite partial melting, as previous studies have already demonstrated that at shallow depths where major peridotite melting occurs, fO2 is around the FMQ buffer (e.g., Cottrell and Kelley, 2011). Figure 1 shows compiled experiments in P-T space with colours corresponding to wt. % SiO2 in the melt, which is a major controller of CO2 solubility, with more silica-depleted melts dissolving more CO2. In Figure 2 we show CO2 concentrations calculated for experimental partial melts at the CCO buffer (maximum amount of CO2 dissolved with graphite present in the source) compared to the highest CO2 concentrations measured in enriched oceanic basalts from several locations. CO2 concentrations from basalts at Siqueiros are measured in undegassed melt inclusions and may be widely representative of CO2 concentrations for basalts from partial melts of normal primitive mantle (Saal et al., 2002). Therefore, if the elevated CO2 levels observed in enriched, natural oceanic basalts are due to contributions from subducted lithologies at graphite-saturation, then the CO2 concentrations of the natural oceanic basalts must lie between CO2 concentrations of Siqueiros and those calculated for the graphite-saturated experimental partial melts of subducted lithologies. Figure 2 shows that it is unlikely that a graphite/diamond-saturated melt derived from any recycled lithology can dissolve enough CO2 to generate the high CO2 concentrations observed at the North Arch of Hawaii and popping rocks from the North Atlantic (Dixon et al., 1997; Cartigny et al., 2008). For locations with lower CO2 concentrations in basalts, such as those from Laki and Loihi, even many of the silica-rich melts may dissolve sufficient CO2 under graphite/diamond-saturated conditions. The question, however, is whether realistic proportions of subducted lithology-derived melt can yield the CO2-trace element systematics of these basalts. Figure 1 Pressure-temperature conditions for generation of partial melts from crustal/mafic lithologies in the mantle with colour map for wt. % SiO2 in the melt. Also plotted for reference are mantle adiabats for mantle potential temperatures of 1350 and 1650 °C (dashed lines), solidi of various volatile-free lithologies (1-metapelite, 2-MORB-eclogite, 3-silica-deficient garnet pyroxenite, 4-peridotite, references given in footnote of Table S-3 – the table of compiled experiments), and graphite-diamond transition (dotted line). Figure 2 CO2 concentrations calculated at the CCO buffer for each experiment in Figure 1. Vertical blue lines are highest CO2 concentrations measured in natural oceanic basalts from different locations (Table S-2) that are thought to receive contributions from subducted lithologies. Red band is for CO2 contents expected in depleted peridotite-derived partial melts based on undegassed melt inclusions from Siqueiros (Saal et al., 2002) and calculations using an experimental bulk partition coefficient for CO2 and source CO2 estimate of ~75 ppm at F = 10 % (Rosenthal et al., 2015).

Redox state of the convective mantle from CO2-trace element systematics of oceanic basalts