Highly oxidising fluids generated during serpentinite breakdown in subduction zones

Mobilization of isotopically heavy sulfur during serpentinite subduction

Primitive arc magmas are more oxidized and enriched in sulfur-34 (34S) compared to mid-ocean ridge basalts. These findings have been linked to the addition of slab-derived volatiles, particularly sulfate, to arc magmas. However, the oxidation state of sulfur in slab fluids and the mechanisms of sulfur transfer in the slab remain inconclusive. Juxtaposed serpentinite and eclogitic metagabbro from the Voltri Massif (Italy) provide evidence for sulfur mobilization and associated redox processes during infiltration of fluids. Using bulk rock and in situ δ34S measurements, combined with thermodynamic calculations, we document the transfer of bisulfide-dominated, 34S-enriched fluids in equilibrium with serpentinite into adjacent metagabbro. We argue that the process documented in this study is pervasive along the subduction interface and infer that subsequent melting of these reacted slab-mantle interface rocks could produce melts that display the characteristic oxygen fugacity and sulfur isotope signatures of arc magmas worldwide.

Garnet crystallization does not drive oxidation at arcs

Arc magmas, the building blocks of continental crust, are depleted in total iron (Fe), have higher ratios of oxidized Fe to total Fe (Fe³⁺/∑Fe), and record higher oxygen fugacities (fO₂'s) compared with magmas erupted at mid-ocean ridges. Garnet crystallization could explain these observations if garnet removes substantial amounts of Fe²⁺, but not Fe³⁺, from magma, yet this model for continental crust generation has never been tested experimentally. Analysis of garnets and melts in laboratory experiments show that the compatibilities of Fe²⁺ and Fe³⁺ in garnet are of similar magnitudes. Our results indicate that fractional crystallization of garnet-bearing cumulates will remove 20% of total Fe from primary arc basalts but negligibly alter the Fe³⁺/∑Fe ratio and fO₂ of the melt. Garnet crystallization is unlikely to be responsible for the relatively oxidized nature of basaltic arc magmas or the Fe-depletion trend observed in continental crust.

Decoupling of inorganic and organic carbon during slab mantle devolatilisation

Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

Hydrated mantle rocks store a significant amount of organic and inorganic carbon, impacting the geological cycle. During subduction the carbonate return to the upper plate while organic carbon remains trapped to be recycled in the deep earth.

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

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.

Insights on the deep carbon cycle from the electrical conductivity of carbon-bearing aqueous fluids

The dehydration and decarbonation in the subducting slab are intricately related and the knowledge of the physical properties of the resulting C-H-O fluid is crucial to interpret the petrological, geochemical, and geophysical processes associated with subduction zones. In this study, we investigate the C-H-O fluid released during the progressive devolatilization of carbonate-bearing serpentine-polymorph chrysotile, with in situ electrical conductivity measurements at high pressures and temperatures. The C-H-O fluid produced by carbonated chrysotile exhibits high electrical conductivity compared to carbon-free aqueous fluids and can be an excellent indicator of the migration of carbon in subduction zones. The crystallization of diamond and graphite indicates that the oxidized C-H-O fluids are responsible for the recycling of carbon in the wedge mantle. The carbonate and chrysotile bearing assemblages stabilize dolomite during the devolatilization process. This unique dolomite forming mechanism in chrysotile in subduction slabs may facilitate the transport of carbon into the deep mantle.

Diamond formation in an electric field under deep Earth conditions

Most natural diamonds are formed in Earth's lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth's mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral-forming processes, carbon isotope fractionation, and the global carbon cycle.

Subduction hides high-pressure sources of energy that may feed the deep subsurface biosphere

Geological sources of H₂ and abiotic CH₄ have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere. Yet the origins of these sources are largely unconstrained. Hydration of mantle rocks, or serpentinization, is widely recognized to produce H₂ and favour the abiotic genesis of CH₄ in shallow settings. However, deeper sources of H₂ and abiotic CH₄ are missing from current models, which mainly invoke more oxidized fluids at convergent margins. Here we combine data from exhumed subduction zone high-pressure rocks and thermodynamic modelling to show that deep serpentinization (40–80 km) generates significant amounts of H₂ and abiotic CH₄, as well as H₂S and NH₃. Our results suggest that subduction, worldwide, hosts large sources of deep H₂ and abiotic CH₄, potentially providing energy to the overlying subsurface biosphere in the forearc regions of convergent margins.

Geological sources of H₂ and abiotic CH₄ have had a critical role in the evolution of life and sustainability of the deep subsurface biosphere, yet the origins of these sources remain largely unconstrained. Here the authors show that deep serpentinization (40–80 km) during subduction generates significant amounts of H₂ and abiotic CH₄, potentially providing energy to the overlying subsurface biosphere.

Subducting serpentinites release reduced, not oxidized, aqueous fluids

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO₂ and CO₂ that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO₂) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO₂ conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H₂S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

Molybdenum systematics of subducted crust record reactive fluid flow from underlying slab serpentine dehydration

Fluids liberated from subducting slabs are critical in global geochemical cycles. We investigate the behaviour of Mo during slab dehydration using two suites of exhumed fragments of subducted, oceanic lithosphere. Our samples display a positive correlation of δ^98/95Mo_{NIST 3134} with Mo/Ce, from compositions close to typical mantle (−0.2‰ and 0.03, respectively) to very low values of both δ^98/95Mo_{NIST 3134} (−1‰) and Mo/Ce (0.002). Together with new, experimental data, we show that molybdenum isotopic fractionation is driven by preference of heavier Mo isotopes for a fluid phase over rutile, the dominant mineral host of Mo in eclogites. Moreover, the strongly perturbed δ^98/95Mo_{NIST 3134} and Mo/Ce of our samples requires that they experienced a large flux of oxidised fluid. This is consistent with channelised, reactive fluid flow through the subducted crust, following dehydration of the underlying, serpentinised slab mantle. The high δ^98/95Mo_{NIST 3134} of some arc lavas is the complement to this process.

Fluid liberation and migration from subducted oceanic slabs play a critical role in arc magmatism but the volume and origin of the released fluids is unclear and difficult to trace. Here, the authors use Molybdenum isotope ratios to tackle these problems.

Origin of Short-Chain Organic Acids in Serpentinite Mud Volcanoes of the Mariana Convergent Margin

Serpentinitic systems are potential habitats for microbial life due to frequently high concentrations of microbial energy substrates, such as hydrogen (H₂), methane (CH₄), and short-chain organic acids (SCOAs). Yet, many serpentinitic systems are also physiologically challenging environments due to highly alkaline conditions (pH > 10) and elevated temperatures (>80°C). To elucidate the possibility of microbial life in deep serpentinitic crustal environments, International Ocean Discovery Program (IODP) Expedition 366 drilled into the Yinazao, Fantangisña, and Asùt Tesoru serpentinite mud volcanoes on the Mariana Forearc. These mud volcanoes differ in temperature (80, 150, 250°C, respectively) of the underlying subducting slab, and in the porewater pH (11.0, 11.2, 12.5, respectively) of the serpentinite mud. Increases in formate and acetate concentrations across the three mud volcanoes, which are positively correlated with temperature in the subducting slab and coincide with strong increases in H₂ concentrations, indicate a serpentinization-related origin. Thermodynamic calculations suggest that formate is produced by equilibrium reactions with dissolved inorganic carbon (DIC) + H₂, and that equilibration continues during fluid ascent at temperatures below 80°C. By contrast, the mechanism(s) of acetate production are not clear. Besides formate, acetate, and H₂ data, we present concentrations of other SCOAs, methane, carbon monoxide, and sulfate, δ¹³C-data on bulk carbon pools, and microbial cell counts. Even though calculations indicate a wide range of microbial catabolic reactions to be thermodynamically favorable, concentration profiles of potential energy substrates, and very low cell numbers suggest that microbial life is scarce or absent. We discuss the potential roles of temperature, pH, pressure, and dispersal in limiting the occurrence of microbial life in deep serpentinitic environments.

Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava

Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ³⁴S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ³³S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ³⁴S ~ + 3‰), which supports the presence of younger ³⁴S-rich recycled oceanic material in the Canary Island mantle plume.

Mantle-sourced magmas erupted at hotspot volcanoes can provide clues to the sulphur cycle over geological timescales. In this work, sulphur isotopes were analysed in crystal-hosted inclusions entrapped at depth, and reveal the presence of post-Archaean recycled material in the Canary Island mantle.

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

Changes in the oxygen fugacity (fO₂) of the Earth's mantle have been proposed to control the spatial and temporal distribution of arc-related ore deposits, and possibly reflect the evolution of the atmosphere over billions of years. Thermodynamic calculations and natural evidence indicate that fluids released from subducting slabs can oxidise the mantle, but whether their oxidation potential varied in space and time remains controversial. Here, we use garnet peridotites from western Norway to show that there is a linear decrease in maximum fO₂ with increasing depth in the mantle wedge. We ascribe this relation to changes in the speciation of sulfur released in slab fluids, with sulfate, controlling maximum oxidation, preferentially released at shallow depths. Even though the amount of sulfate in the Precambrian oceans, and thus in subducted lithologies, is thought to have been dramatically lower than during the Phanerozoic, garnet peridotites metasomatised during these two periods have a comparable fO₂ range. This opens to the possibility that an oxidised mantle with fO₂ similar to modern-day values has existed since the Proterozoic and possibly earlier. Consequently, early magmas derived from partial melting of metasomatised mantle may have had suitable fO₂ to generate porphyry Cu-Au and iron-oxide Cu-Au deposits.