Redox processes and the role of carbon-bearing volatiles from the slab–mantle interface to the mantle wedge

Tumiati, Simone; Malaspina, Nadia

doi:10.1144/jgs2018-046

Cited by 36 publications

(25 citation statements)

References 103 publications

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“…5). At the higher pressure-temperature conditions characteristic of subduction-zone metamorphism, more oxidized conditions are traditionally expected 20 . The results presented in this study, however, show that fluids highly concentrated in such compounds can be formed abiotically down to 40-80 km depth through high-pressure serpentinization, potentially in high amounts, and along thousands of km at convergent plate boundaries (Fig.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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Geological sources of H 2 and abiotic CH 4 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 H 2 and favour the abiotic genesis of CH 4 in shallow settings. However, deeper sources of H 2 and abiotic CH 4 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 H 2 and abiotic CH 4 , as well as H 2 S and NH 3. Our results suggest that subduction, worldwide, hosts large sources of deep H 2 and abiotic CH 4 , potentially providing energy to the overlying subsurface biosphere in the forearc regions of convergent margins.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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“…The volatile composition of carbon-saturated COH fluids, and in particular their XCO 2 [=CO 2 /(H 2 O+CO 2 ) molar ] in relatively oxidized systems, is dependent on the redox state of the system (cf. the review of Tumiati and Malaspina, 2019), which can be controlled indirectly in experiments by fixing the hydrogen fugacity in double capsules (e.g., Eugster and Skippen, 1967).…”

Section: Thermodynamic Modelingmentioning

confidence: 99%

“…The oxidation susceptibility and therefore the dissolution of graphite in aqueous fluids varies as a function of P, T and fO 2 conditions (e.g., Connolly, 1995;Tumiati and Malaspina, 2019). In general, low-temperature and high-pressure conditions characterizing subduction zones are thought to promote the stability of graphite, thus fluids interacting with this mineral should contain very low amounts of carbon and are essentially nearly pure water (Schmidt and Poli, 2013).…”

Section: Implications For Organic Matter Dissolution At Subduction Zonesmentioning

confidence: 99%

Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones

Tumiati

Tiraboschi

Miozzi

et al. 2020

Geochimica et Cosmochimica Acta

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Organic matter, showing variable degrees of crystallinity and thus of graphitization, is an important source of carbon in subducted sediments, as demonstrated by the isotopic signatures of deep and ultra-deep diamonds and volcanic emissions in arc settings. In this experimental study, we investigated the dissolution of sp 2 hybridized carbon in aqueous fluids at 1 and 3 GPa, and 800°C, taking as end-members i) crystalline synthetic graphite and ii) X-ray amorphous glass-like carbon. We chose glass-like carbon as an analogue of natural "disordered" graphitic carbon derived from organic matter, because unlike other forms of poorly ordered carbon it does not undergo any structural modification at the investigated experimental conditions, allowing approach to thermodynamic equilibrium. Textural observations, Raman spectroscopy, synchrotron X-ray diffraction and dissolution susceptibility of char produced by thermal decomposition of glucose (representative of non-transformed organic matter) at the same experimental conditions support this assumption. The redox state of the experiments was buffered at ∆FMQ ≈-0.5 using double capsules and either fayalite-magnetite-quartz (FMQ) or nickel-nickel oxide (NNO) buffers. At the investigated P-T-fO 2 conditions, the dominant aqueous dissolution product is carbon dioxide, formed by oxidation of solid carbon. At 1 GPa and 800°C, oxidative dissolution of glass-like carbon produces 16-19 mol% more carbon dioxide than crystalline graphite. In contrast, fluids interacting with glass-like carbon at the higher pressure of 3 GPa show only a limited increase in CO 2 (fH 2 NNO) or even a lower CO 2 content (fH 2 FMQ) with respect to fluids interacting with crystalline graphite. The measured fluid compositions allowed retrieving the difference in Gibbs free energy (ΔG) between glass-like carbon and graphite, which is +1.7(1) kJ/mol at 1 GPa-800°C and +0.51(1) kJ/mol (fH 2 NNO) at 3 GPa-800°C. Thermodynamic modeling suggests that the decline in dissolution susceptibility at high pressure is related to the higher compressibility of glass-like carbon with respect to crystalline graphite, resulting in G-P curves crossing at about 3.4 GPa at 800°C, close to the graphite-diamond transition. The new experimental data suggest that, in the presence of aqueous fluids that flush subducted sediments, the removal of poorly crystalline "disordered" graphitic carbon is more efficient than that of crystalline graphite especially at shallow levels of subduction zones, where the difference in free energy is higher and the availability of poorly organized metastable carbonaceous matter and of aqueous fluids produced by devolatilization of the downgoing slab is maximized. At depths greater than 110 km, the small differences in ΔG imply that there is minimal energetic drive for transforming "disordered" graphitic carbon to ordered graphite; "disordered" graphitic carbon could even be energetically slightly favored in a narrow P interval.

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“…11; see also Scambelluri & Tonarini, 2012), which commonly display Fe 3+ /Fe tot higher than MORB and OIB basalts because of addition to their mantle sources of fluids carrying oxidised species, like H 2 O, CO 2 , Fe 2 O 3 and sulfate (Arculus, 1994;Kelley & Cottrell, 2009;Malaspina et al, 2009Malaspina et al, , 2010Evans & Tomkins, 2011;Evans & Powell, 2015;Evans et al, 2017). In turn, the speciation of subduction fluids is controlled by the variable oxidation state of the source rocks (Tumiati et al, 2015;Debret & Sverjensky, 2017;Evans et al, 2017;Cannaò & Malaspina, 2018;Tumiati & Malaspina, 2019). Because serpentinite is the rock that primarily controls the release of slab fluids, efforts have been recently done to determine the redox state of serpentinized rocks and fluids by means of (i) direct measurements of the Fe 3+ content of bulk rocks and serpentine (Andreani 420 M. Scambelluri et al et al, 2013;Debret et al, 2014bDebret et al, , 2015Evans et al, 2017) and (ii) theoretical calculations (Debret & Sverjensky, 2017).…”

Section: Redox Statementioning

confidence: 99%

The water and fluid-mobile element cycles during serpentinite subduction. A review

Scambelluri

Cannaò

Gilio

2019

ejm

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The key role of serpentinites in the global cycles of volatiles, halogens and fluid-mobile elements in oceans and in subduction zones is now ascertained by many studies quantifying their element budgets and the composition of fluids they release during subduction. Geochemical tracers (e.g. B, As, Sb; stable B and radiogenic Sr and Pb isotopes) have also been employed to trace the provenance of serpentinites (slab or forearc mantle?) accreted to the plate interface of fossil subduction zones. In turn, this helps defining the tectonic processes, seismicity and mass transfer attending rock burial and exhumation within subduction zones. The results suggest that the sole use of geochemical data is insufficient to track the origin of subduction-zone serpentinites and the timing of serpentinization, whether oceanic or subduction-related. Integrated multidisciplinary studies of ophiolitic serpentinites show that pristine, oceanic, geochemical imprints (e.g. high 11 B, marine Sr isotopes, low As + Sb) become reset towards more radiogenic Sr, lower 11 B, and higher As + Sb via metasomatic exchange with crust-derived fluids during subduction accretion to the plate interface.The dehydration fluids released by serpentinite dehydration at various subduction stages and still preserved in these rocks as inclusions, carry significant amounts of halogens and fluid-mobile elements. The key compositional similarities of antigorite-breakdown fluids from different localities (Betic Cordillera, Spain; Central Alps, Switzerland) indicate that rocks record comparable subduction processes. We individuate the fluid-mediated exchange with sedimentary and/or crustal reservoirs during subduction as the key mechanism for geochemical hybridization of serpentinite. The antigorite dehydration fluids produced by hybrid serpentinites have high Cs, Rb, Ba, B, Pb, As, Sb and Li overlapping those of the arc lavas and representing the mixed serpentinite-sediment (crustal) component released to arcs. This helps discriminating the mass transfer processes responsible for supra-subduction mantle metasomatism and arc magmatism. The studied plate-interface hybrid serpentinites are also proxies of forearc mantle metasomatized by slab fluids. Based on the above observations, we propose that the mass transfer from slabs to plate interface and/or forearc mantle and the subsequent down-drag of this altered mantle to subarc depths potentially is a major process operating in subduction zones.The nominally anhydrous olivine, orhopyroxene, clinopyroxene and garnet produced by serpentinite dehydration host appreciable amounts of halogens and fluid-mobile elements that can be recycled in the deep mantle beyond arcs. Involvement of de-serpentinized residues in lower mantle metasomatism begins to be increasingly recognized by studies of ocean island basalts (OIB) and of B-bearing blue diamonds and by the isotopic serpentinite compositions presented here.

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Redox processes and the role of carbon-bearing volatiles from the slab–mantle interface to the mantle wedge

Cited by 36 publications

References 103 publications

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

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

Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones

The water and fluid-mobile element cycles during serpentinite subduction. A review

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