Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust's role in the solid Earth oxygen cycle

Abstract

Editor: T.A. MatherKeywords: mantle f O 2 XANES oxygen mantle heterogeneity pyroxenite The cycling of material from Earth's surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge into the deep mantle for long-term storage. Recycled oceanic crust may be entrained into mantle upwellings and melt under ocean islands, creating the potential for basalt chemistry to constrain solid Earth-hydrosphere redox coupling. Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence vary dramatically, with the basalts recording both longand short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe 3+ / Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe 3+ / Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe 3+ / Fe increases by ∼ 0.015 and QFM by ∼ 0.2 log units. We rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal.To determine the ferric iron content of the recycled component ([Fe 2 O 3 ] source ) we project observed liquid compositions to an estimate of Fe 2 O 3 in the pure enriched endmember melt, and then apply simple fractional melting models, considering lherzolitic and pyroxenitic source mineralogies, to estimate [Fe 2 O 3 ] (source) content. Propagating uncertainty through these steps, we obtain a range of [Fe 2 O 3 ] (source) for the enriched melts (0.9-1.4 wt%) that is significantly greater than the ferric iron content of typical upper mantle lherzolites. This range of ferric iron contents is consistent with a hybridised lherzolitebasalt (pyroxenite) mantle component. The oxidised signal in enriched Icelandic basalts is therefore potential evidence for seafloor-hydrosphere interaction having oxidised ancient mid-ocean ridge crust, generating a return flux of oxygen into the deep mantle.

Magmatism in the North Atlantic Igneous Province; mantle temperatures, rifting and geodynamics

“…Hole and Millett (2016) obtained higher TP estimates of ~1475-1500°C for whole-rocks from Icelandic rift zones and 1450°C for rift-flank lavas, but these were generated assuming extremely reducing conditions (Fe 3+ /FeT~0.05), which, according to Shorttle et al (2015), are unlikely at Iceland. For a more realistic oxidation state of Fe 3+ /Fe T~0 .09 (Shorttle et al, 2015;Gaetini, 2016) these T P estimates decrease by ~50°C giving a maximum of 1450°C. We conclude that the maximum TP that can be derived for modern Icelandic lavas is ~1450°C.…”

The temperature of the Icelandic mantle from olivine‐spinel aluminum exchange thermometry

“…We use Putirka et al [2007, equation (4)], and equilibrate with Fo 92 olivine [after Putirka, 2008a] and Fo 91:5 olivine (the most forsteritic olivine analyzed in this study). The equilibration pressure is assumed to be 0.8 GPa [after Winpenny and Maclennan, 2011], the exchange partition coefficient as 0.31 [after Putirka, 2008a], and the ferric/total iron ratio as 0.16 [after Oskarsson et al, 1994;Shorttle et al, 2015]. The resulting estimates are shown as blue circles in Figure 2 and are close to the Al-exchange temperature, though slightly lower.…”

Stable vanadium isotopes as a redox proxy in magmatic systems?

“…Both methods indicate that Mariana arc basalts are generally ~1-2 log units more oxidised compared to Icelandic basalts from Hekla (e.g., de Moor et al, 2005;Moune et al, 2007;Brounce et al, 2014;Shorttle et al, 2015). We chose forty whole rock samples from three well-studied lava suites: 1) primitive arc lavas of the Mariana Central Island Province (CIP; Elliott et al, 1997), 2) co-genetic lavas from Anatahan volcano in the Mariana arc (Wade et al, 2005) and 3) co-genetic lavas from Hekla volcano, Iceland (Savage et al, 2011).…”