Evidence of sub-arc mantle oxidation by sulphur and carbon

Abstract

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...

Oxidation State of Arc Mantle Revealed by Partitioning of V, Sc, and Ti Between Mantle Minerals and Basaltic Melts

“…A generally positive correlation between fO 2 and fluid input has been observed in primitive arc basalts in previous studies (Bénard et al, ; Brounce et al, , ; Debret et al, ; Evans, ; Evans & Tomkins, ; Kelley & Cottrell, ; Pons et al, ; Rielli et al, ), and thus, fluid influx was proposed as the cause of mantle wedge oxidization. V/Ti ratios in ophiolites (Shervais, ) and basalts (Woodhead et al, and this study) can be used to record fO 2 in their mantle sources.…”

Garnet peridotites reveal spatial and temporal changes in the oxidation potential of subduction

“…Results are reported in Table 1 and graphically in Fig. 3 , together with literature data for the Bardane and Ugelvik peridotites 13 , 14 and other garnet peridotites metasomatised by subduction at similar depths during the Phanerozoic 22 . In Fig.…”

“…Blue diamonds represent mantle metasomatised by Phanerozoic subductions, new data from this work is integrated with values from ref. 13 , the Ulten amphibole-bearing garnet peridotite, the magnesite-phlogopite garnet peridotite from Sulu 22 , and a garnet-websterite from Bardane 14 . The latter, unlike our samples, records the M3-3 metasomatic event in veins crosscutting the earlier assemblages.…”

“…Because these peridotites were mechanically emplaced into subducting continental crust they derive from the near-bottom end of the mantle wedge 12 . Based on previously published f O 2 data for the Ugelvik and Bardane peridotites 13 , 14 , we selected five new samples to obtain the widest range of depths possible.

Figure 1 Samples localities.

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Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

“…Among the factors explaining the occurrence of metallogenic provinces dominated by one or more ore metals, widely contemplated possibilities include heterogeneously distributed metal preconcentration, favorable redox conditions, or other parameters somewhere above the subducted slab, between the mantle wedge and upper crust (Sillitoe, 2008). We argue that the formation of restricted blocks of crust highly endowed with gold deposits requires, in addition to hydrothermal processes in the upper crust, the efficient extraction of Au from refertilized and oxidized domains of SCLM (Rielli et al, 2017). Evidence presented here suggests that redox gradients between ascending magmas and the surrounding mantle can trigger the destabilization of mantle sulfides and sulfide melts, resulting in the release of their Au cargo into the silicate melt fraction.…”