Water and the Oxidation State of Subduction Zone Magmas

Abstract

Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe 3+ /SFe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H 2 O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe 3+ /SFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H 2 O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge. P late tectonics leads to a two-way geochemical exchange between Earth's interior and exterior. This process is driven by the formation of new oceanic crust by mantle melting at mid-ocean ridges, hydration and oxidative alteration of oceanic crust as it transits the seafloor, and the subsequent return of hydrated oxidized oceanic crust to the deep Earth at subduction zones (Fig. 1A) (1, 2). How this exchange has affected the oxygen fugacity of the mantle spatially (3) and over time (2, 4, 5) remains unclear. Many lines of evidence point to oxidizing conditions in arc peridotites and magmas (1, 6), but a quantitative link between oxidation state and the subduction process, although intuitive, has not been established. Here we provide coupled measurements of the redox-sensitive Fe 3+ /SFe ratio and magmatic H 2 O concentrations at the same spatial resolution in a global suite of undegassed basaltic glasses, in order to determine the current oxidation condition of the mantle as a function of tectonic regime.The ratio of oxidized iron to total iron [Fe 3+

Degassing of reduced carbon from planetary basalts

“…These differences should be reflected in how volatiles are dissolved in magmas, but carbon speciation experiments for basaltic systems are limited, particularly at low fO 2 . The developing evidence for volatiles in magmas erupted on the Moon (6, 7) as well as on Earth (3,8), Mars (9)(10)(11)(12), and Mercury (13) is a strong incentive to improve our understanding of C-O-H speciation and solubility in planetary basaltic magmas.…”

“…hydrogen | iron carbonyl | magmatic volatiles | experimental petrology M agmas formed through igneous processes on parent bodies in the early solar system span 12 log units in oxidation state, from 6.3 log units of oxygen fugacity (fO 2 ) below the iron wustite oxygen buffer (IW) to 6 log units above the IW (IW−6.3 to IW+6) (1)(2)(3)(4). Over the wide range of basalt fO 2 observed in these planetary bodies, thermodynamic data for the C-O-H gas system indicate that there are significant changes in gas phase speciation (5).…”

Redox state of early magmas

“…Planetary-scale geodynamics is often cited as a mechanism for bringing oxidized material into the deep, reduced mantle, and water is thought to be responsible for oxidation 7 . This concept is deeply entrenched among geologists, although the details of the redox mechanisms involved remain unknown.…”

The Fina Nagu volcanic complex: Unusual submarine arc volcanism in the rapidly deforming southern Mariana margin

“…This is true of the Fe 31 / P Fe ratios and also of the calculated magmatic fO 2 values, the latter of which takes into consideration the effects of pressure, temperature, and anhydrous major element chemistry on the relationship between the oxidation state of Fe of fO 2 (Figure 6e). Surprisingly, Fina Nagu glasses do not follow the relationship between increasing Ba/La and Fe 31 / P Fe ratios that is defined by >150 submarine glass chips and olivine hosted melt inclusions from Mariana arc volcanoes, the Mariana Trough, and global MORB, including submarine glass chips that record the initiation of subduction along the Izu-Bonin-Mariana convergent margin (Figures 6b and 6e) [Brounce et al, 2014[Brounce et al, , 2015Cottrell and Kelley, 2011;Kelley and Cottrell, 2009].…”