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

Abstract

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.

Applying Phase Equilibria Modelling to Metamorphic and Geological Processes: Recent Developments and Future Potential

“…Each of these methods has advantages and disadvantages, but the result should be a bulk composition that approximates the equilibration volume of the rock. Additional factors that need to be considered in some cases are the concentration of CO 2 and H 2 S in the fluid phase (Tomkins and Evans 2015), the amount of H 2 O in the bulk composition (e.g. White et al 2007), as well as the oxidation state of the system (e.g.…”

“…Metamorphic fluid compositions can be predicted through phase equilibria modelling for a variety of geological settings, which has implications for the geochemical cycles of carbon and sulphur and the generation of greenhouse gases in metamorphic settings (Aarnes et al 2010;Skora et al 2015;Tomkins and Evans 2015). Aarnes et al (2010) evaluated the composition and volumes of metamorphic fluids produced through contact metamorphism of organic-rich metasedimentary rocks.…”

Redistribution of Iron and Titanium in High‐Pressure Ultramafic Rocks

“…This process would increase the mode of Fe 31 -bearing phases during the earliest stages of exhumation. The possibility that SO 2 or SO4 2--bearing fluids are present in subduction zones and that such fluids could increase the redox budget of the subarc mantle has been discussed by other workers (Canil & Fellows, 2017;Evans et al, 2017;Mungall, 2002;Tomkins & Evans, 2015). For example, the presence of heazlewoodite inclusions in magnetite within sheared veins in the Ti-clinohumite-bearing samples, which record the earliest stages of exhumation, could record SO 2 -bearing fluids.…”

Sulfide Formation as a Result of Sulfate Subduction into Silicate Mantle (Experimental Modeling under High P,T-Parameters)

“…Being a redox-sensitive element, sulfur can exist in reduced form (S 2− or S) as sulfides or native sulfur, sulfide melts or reduced fluid, intermediate form (S 4+ ) as an SO 2 -component of erupted melts and in oxidized form (S 6+ ) as sulfates, oxidized fluids, or dissolved in silicate melts [1][2][3][4][5]. According to modern concepts, subduction processes play a key role in the transport of S in deep zones of the Earth and are intimately linked to the global geochemical cycle of sulfur, the genesis of arc-related sulfide ore deposits, and the long-term mantle redox evolution [3,[6][7][8][9]. Existing data demonstrate that only 15-30% of sulfur, transported into the mantle with a subducted slab is released to the atmosphere via magmatic degassing at arcs [7].…”

“…The recent experimental studies at variable oxygen fugacity [8,18] showed that anhydrite in the dehydrated slab below the region of formation of arc magmas may efficiently be recycled into the deep mantle. Thermodynamic calculations [3,9] suggest that anhydrite under high pressures should dominantly dissolve into fluids released across the transition from blueschist to eclogite facies, and enrich those fluids with SO 3 , SO 2 , S, HSO 4 − , or H 2 S. The main process that results in the decomposition of sulfate with S-rich fluid formation under crustal conditions is desulfation which can occur at the interaction of anhydrite or other sulfates with carbon-bearing brines, resulting in the formation of carbonates and elemental sulfur. A recent investigation of S-bearing species in kimberlites demonstrates that the infiltration of brines into kimberlite can dissolve magmatic sulfates and initiate serpentinization accompanied by sulfidation of olivine [5].…”