The relationship between subduction zone redox budget and arc magma fertility

Evans, Katy; Tomkins, Andrew G.

doi:10.1016/j.epsl.2011.06.009

Cited by 109 publications

(76 citation statements)

References 54 publications

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“…There is some evidence that the sub-arc mantle has been oxidised by fluids escaping from the subducted slab (McInnes et al, 2001), and sulfate is thought to be an important oxidant (Evans and Tomkins, 2011).…”

Section: On the Stability Of Anhydritementioning

confidence: 99%

“…Although there is considerable discussion at present, some evidence favours the hypothesis that magma oxidation state is inherited from processes that occur in the mantle wedge above the subducting oceanic slab (e.g., Evans et al, 2012;McInnes et al, 2001, and references therein). A significant proportion of this mantle wedge oxidation may evolve through addition of slab-derived sulfate (Evans and Tomkins, 2011) via fluids that also carry other elements that are typically enriched in arc magmas.…”

Section: Introductionmentioning

confidence: 99%

“…Basaltic oceanic crust provides the biggest contribution to the subduction zone sulfur cycle (Evans et al, 2012;Evans and Tomkins, 2011), so this paper focuses on sulfur-bearing mineral stability during subduction zone metamorphism of oceanic basalts. We firstly review the current understanding of the distribution of sulfur within the basaltic oceanic crust prior to subduction, before examining the metamorphic stability of two key minerals, anhydrite and pyrite.…”

Section: Introductionmentioning

confidence: 99%

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Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Tomkins

Evans

2015

Earth and Planetary Science Letters

104

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Available online xxxx Editor: M. Bickle Keywords: sulfur sulfate sulfur cycle subduction arc magmatism porphyry copperLiberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO 4 ) and pyrite (FeS 2 ) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450-650 • C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 • C. This higher temperature stability is indicated by the preservation of pyrite-bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 • C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO 2 , HSO 4 − and H 2 S are released via anhydrite breakdown at the blueschist-eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H 2 S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist-eclogite transition given the association between oxidised magmas and porphyry Cu(-Au-Mo) deposits. We speculate that this zoned sulfur liberation might be one of the factors that lead to the apparently redox-influenced zoned distribution of ore deposit types in the Andean arc. Furthermore, given the lack of sulfate-associated sea floor oxidation prior to the second great oxidation event, the pattern of sulfur transfer from the slab to the sub-arc mantle likely changed over time, becoming shallower and more oxidised from the Neoproterozoic onwards.

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Section: On the Stability Of Anhydritementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Tomkins

Evans

2015

Earth and Planetary Science Letters

104

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“…Plate tectonics drives recycling of surface material into the Earth's interior, introducing hydrated and oxidised oceanic lithosphere into the mantle at subduction zones (Wood et al, 1990;Evans, 2012). Fluids liberated during dehydration of subducted crust trigger partial melting of the overlying mantle leading to the formation of volcanic arcs, dominated by oxidised rocks (Ballhaus, 1993;Kelley and Cottrell, 2009).…”

Section: Lettermentioning

confidence: 99%

“…Given that sulphur and carbon are the major redox-sensitive elements in aqueous slab fluids (Evans, 2012), the extent of oxidation achievable by mantle metasomatism is likely to be limited to the sulphide-sulphate transition.…”

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confidence: 99%

Evidence of sub-arc mantle oxidation by sulphur and carbon

Rielli¹,

Tomkins²,

Nebel³

et al. 2017

Geochem. Persp. Let.

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The oxygen fugacity ( fO 2 ) of the Earth's mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle fO 2 such as the introduction of oxidised material by subduction are still unresolved. Here, we present evidence for the reduction of oxidised fluid-borne sulphur and carbon during alteration of depleted mantle by slab fluids at ultra-high pressure in the Bardane peridotite (Western Gneiss Region, Norway). Elevated ferric iron in metasomatic garnet, determined using synchrotron X-ray absorption near edge structure (XANES) spectroscopy, indicates that this process drove oxidation of the silicate assemblage. Our finding indicates that subduction oxidises the Earth's mantle by cycling of sulphur and carbon. LetterPlate tectonics drives recycling of surface material into the Earth's interior, introducing hydrated and oxidised oceanic lithosphere into the mantle at subduction zones (Wood et al., 1990;Evans, 2012). Fluids liberated during dehydration of subducted crust trigger partial melting of the overlying mantle leading to the formation of volcanic arcs, dominated by oxidised rocks (Ballhaus, 1993;Kelley and Cottrell, 2009). Correlation between oxidation of arc lavas and addition of slab material by aqueous fluids to their mantle source has been quantitatively established (Kelley and Cottrell, 2009; Brounce et al., 2014) fluids are somehow responsible for the observed oxidation. However, although calculations suggest that fluid-borne oxidised sulphur may be important (Evans and Tomkins, 2011), the process has not been resolved with physical evidence. Determination of redox processes associated with fluid-mantle interaction cannot be indisputably resolved by studying the product of the arc system (i.e. arc lavas); it is best approached by examining rocks from the deep portion of mantle wedge close to the slab-mantle interface that have been altered by slab fluids, which are preserved as orogenic peridotites.The Western Gneiss Region of Norway hosts several orogenic peridotite bodies that represent a portion of transition zone mantle that upwelled, melted (Stage M1) and accreted to cratonic lithosphere (Stage M2;Spengler et al., 2006). Subsequently, during the Caledonian orogeny, fluids derived from subducting slab locally promoted metasomatism (Stage M3), as recorded by the Bardane peridotite on Fjørtoft island (Brueckner et al., 2002;Van Roermund et al., 2002;Scambelluri et al., 2008). Other portions have not been infiltrated by slab fluids and preserve the original mantle assemblage (M1-M2), exemplified by the Ugelvik peridotite on the island of Otrøy (Brueckner et al., 2002;Spengler et al., 2006). Although there is debate on the origin of metasomatism at Bardane, the trace element and isotopic signature of the metasomatic assemblage are comparable to the effects of typical subduction zone fluids sensu lato (cf. Brueckner et al., 2002;Scambelluri et...

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Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

Giacometti

Evans

Rebay

et al. 2014

Geochem. Geophys. Geosyst.

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Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with d 34 S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

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The relationship between subduction zone redox budget and arc magma fertility

Cited by 109 publications

References 54 publications

Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Evidence of sub-arc mantle oxidation by sulphur and carbon

Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

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