Oxygen Fugacity Across Tectonic Settings

Cottrell, Elizabeth; Birner, Suzanne; Brounce, Maryjo; Davis, Fred A.; Waters, L. E.; Kelley, K. A.

doi:10.1002/9781119473206.ch3

Cited by 69 publications

(49 citation statements)

References 205 publications

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“…Our conclusions are based on the average estimates for entire arc segments. The f O2 can be highly heterogeneous within a single arc (e.g., from FMQ to FMQ+2) ( 50 ). High f O2 lavas (e.g., ~FMQ+2) have been observed in some continental arcs, such as Mexico ( 3 ) and Cascades ( 51 ).…”

Section: Resultsmentioning

confidence: 99%

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

et al. 2021

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Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system's thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones.

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Section: Resultsmentioning

confidence: 99%

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

et al. 2021

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“…Our model calculations place the SS min for this Hawaiian basalt composition at 1200 °C at SSmin fO2  FMQ+1.2. For comparison, this is slightly more oxidised than Hawaiian basalts (FMQ-0.5 to +1.0; Moussallam et al 2016;Brounce et al 2017;Lerner et al 2021) and more oxidised than mid-ocean ridge basalts (MORB), but within the range measured in arc and ocean island basalts (OIB) (e.g., Cottrell et al, 2021). The position of SSmin fO2 will depend on T, composition of the silicate component, and additional volatile components (e.g., H2O, CO2).…”

Section: Implications Of Ss Min and Ss Max For Magmatic And Volcanic Processesmentioning

confidence: 84%

“…However, OIB and arc volcanic glasses can have [S 6+ /ST] m up to 1 (e.g., Jugo et al 2010;de Moor et al 2013;Labidi et al 2015;Muth and Wallace 2021), and therefore the effects of the SS min on the contribution of partial pressures from S-bearing species to the total P at vapour-saturation (e.g., S2 and SO2 in the S-O system, but potentially H2S and OCS as well in C-O-H-S system) could be non-negligible (e.g., in the green band of Figure 2d). Melt inclusions from arc and OIB magmas reach w m ST ~ 5000 ppm (Wallace 2005) and are FMQ-3 to +3 (Cottrell et al 2021), which could lead to underestimating P v sat by up to 650 bars for melts at FMQ = +1 to +2 (Figure 9). The exact value of the underestimate would depend on the fO2, CS6+, and CS2, and hence T and silicate melt composition, and including the H and C species (e.g., Burgisser et al 2015;Lesne et al 2015;Hughes et al 2021).…”

Section: Magma Ascent and Degassingmentioning

confidence: 99%

“…Although this is much higher than the typical total dissolved S contents of common magmas, S 6+ CAS decreases significantly with increasing dissolved H2O and decreasing T: e.g., S 6+ CAS = ~3000 ppm at 5 wt% H2O and 1200 °C for a basaltic melt (Chowdhury and Dasgupta 2019). Melt inclusions from arcs and ocean islands can be S-rich (up to ~5000 ppm ST), hydrous (up to ~6 wt% H2O for arcs and ~3 wt% H2O for ocean islands), and relatively oxidised (up to +3 FMQ) (e.g., Wallace 2005;Moussallam et al 2019;Cottrell et al 2021) and may therefore provide useful fO2 estimates based on this technique. However, this will require accurate knowledge of CS2-, CS6+, S 2-CSS, and S 6+ CSS at the relevant conditions (especially the effects of T and silicate melt compositionincluding the influence of H2O), as these parameters strongly influence the fO2 of the transition from S 2--to S 6+ -dominated silicate melt (e.g., O'Neill andMavrogenes 2002, 2019;Li and Ripley 2005;Moretti and Ottonello 2005;Baker and Moretti 2011;Chowdhury and Dasgupta 2019;Nash et al 2019;Zajacz and Tsay 2019;O'Neill 2020;Boulliung and Wood 2021).…”

Section: Volcanic Emissionsmentioning

confidence: 99%

“…The grey curve indicates the boundary above which silicate melt + vapour is metastable, which is solid when sulfide melt would saturate, dashed when anhydrite would saturate, and the grey star indicates the silicate melt is multiply-saturated with vapour, sulfide melt, and anhydrite. The fO2 for different tectonic settings from Cottrell et al (2021) is shown by horizonal blue bars (extent of the bar shows the range and black vertical line shows the mode) for mid-ocean ridge (light blue), ocean island (intermediate blue), and arc (dark blue) basalts. Abbreviations: sil m , silicate melt; v, vapour; sulf m , sulfide melt; anh, anhydrite; MORB, mid-ocean ridge basalts; OIB, ocean island basalts; and Arc, arc basalts.…”

Section: Figure Captionsmentioning

confidence: 99%

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The sulfur solubility minimum and maximum in silicate melt

Hughes¹,

Saper²,

Liggins³

et al. 2021

Preprint

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The behaviour of sulfur in magmas is complex because it dissolves as both sulfide (S2-) and sulfate (S6+) in silicate melt. An interesting aspect in the behaviour of sulfur is the solubility minima (SSmin) and maxima (SSmax) with varying oxygen fugacity (fO2). We use a simple ternary model (silicate–S2–O2) to explore the varying fO2 paths where these phenomena occur. Both SSmin and SSmax occur when S2- and S6+ are present in the silicate melt in similar quantities due to the differing solubility mechanism of these species. At constant T, a minimum in dissolved total S content (wmST) in vapour-saturated silicate melt occurs along paths of increasing fO2 and either constant fS2 or P; for paths on which wmST is held constant with increasing fO2, the SSmin is expressed as a maximum in P. However, the SSmin is not encountered during closed-system depressurisation in the simple system we modelled. The SSmax occurs when the silicate melt is multiply-saturated with vapour, sulfide melt, and anhydrite. The SSmin and SSmax influence processes throughout the magmatic system, such as mantle melting, magma mixing and degassing, and SO2 emissions; and calculations of the pressures of vapour-saturation, fO2, and SO2 emissions using melt inclusions.

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The Role of Redox Processes in Determining the Iron Isotope Compositions of Minerals, Melts, and Fluids

Sossi¹,

Debret²

2021

Geophysical Monograph Series

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Stable isotope fractionation is a response to the minimisation of free energy associated with differences in vibrational frequencies during substitution of isotopic masses between two phases. Site properties are dictated by the bonding environment of iron, and influence its 'force constant', a descriptor for bond strength. Because iron readily transitions between its ferrous (Fe 2+ ) and ferric (Fe 3+ ) state over the oxygen fugacities (fO 2 ) of terrestrial magmas, redox processes are presumed to control iron isotope fractionation. In fact, both co-ordination and redox state are key to interpreting iron isotope variations in high-temperature, high-pressure geological systems. Determinations of iron force constants by experimental, theoretical and spectroscopic means in minerals, melts and fluids are reviewed, emphasising the effect of composition in influencing isotopic fractionation. Within this framework, the application of numerical models of partial melting to iron isotope variations in oceanic basalts and subduction-zone (or arc) magmas has shed light on their genesis. The influence of melt composition, mineral mode and fluid exsolution in producing iron isotope fractionation in evolving magmas is explored. Iron isotopic signatures in magmatic rocks are linked to that of their sources, and are influenced by the iron isotopic evolution of subducting slabs and transfer of fluids to the mantle wedge. 2.0. Principles and nomenclature Ratios of two isotopes, n and d of an element, E, are generally expressed relative to their ratio in a standard in delta notation, which is given by:(1) 𝛿 𝑛/𝑑 𝐸 = ( ( 𝑛 𝐸/ 𝑑 𝐸) 𝑠𝑎𝑚𝑝𝑙𝑒

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Oxygen Fugacity Across Tectonic Settings

Cited by 69 publications

References 205 publications

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

The sulfur solubility minimum and maximum in silicate melt

The Role of Redox Processes in Determining the Iron Isotope Compositions of Minerals, Melts, and Fluids

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