Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA

Zhang, Shuang; Ague, Jay J.; Brovarone, Alberto Vitale

doi:10.1016/j.chemgeo.2018.05.003

Cited by 14 publications

(12 citation statements)

References 72 publications

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“…the Mariana and the Aegean OC subduction hot spots) could reach the diamond stability field before completion of the carbonization and graphitization process. The graphitization process itself is irreversible, and field observations indicate that slabs may not lose much OC beyond 350-400 C. 48,49 The low reactivity of compact graphitic materials and their intricate weaving within poorly soluble mineral matrices explain why a graphitic fraction dislodged from rocks during erosion and weathering (18-104 MtC/yr 33 ) is continuously exported to the ocean (Figure 10.1). How much of this fossil petrogenic material subducts is not yet known, but it may represent 50% of OC in the trenches of South America.…”

Section: Hot Spot Of Organic Carbon Subduction In the Sub-arctic Pacific Rim (Soft Tissue Pump)mentioning

confidence: 99%

How Do Subduction Zones Regulate the Carbon Cycle?

Gálvez¹,

Pubellier²

2019

Deep Carbon

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Section: Hot Spot Of Organic Carbon Subduction In the Sub-arctic Pacific Rim (Soft Tissue Pump)mentioning

confidence: 99%

How Do Subduction Zones Regulate the Carbon Cycle?

Gálvez¹,

Pubellier²

2019

Deep Carbon

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“…Although C-3(b) is a very small fraction of the total OC present in shale, its survival during combustion at temperature > 1000 • C, imply that it can be preserved even in the hottest subduction zones. Transfer of carbon to the deep Earth locked as graphite at temperature of up to 1200 • C with negligible fluid flux, correspond to depths of ∼80-250 km depending on the geothermal gradient (Stern, 2002;Galvez et al, 2013;Zhang et al, 2018). However, given the low density of graphite (2.3 gm/cc) as compared to that of mantle peridotite (3.1-3.4 gm/cc), it can seggregate as a refractory phase in the shallow mantle wedge with restricted deep subduction, while the associated metasediments and carbonates from the slab can form CO 2rich melt (Kelemen and Manning, 2015).…”

Section: >800 • Cmentioning

confidence: 99%

“…Carbonates reduced to graphitized form, at very high to lower temperature of 430 • C, can be transported to deeper regions resulting in long term (Gyrs) removal of reduced, light carbon from surficial reservoirs of the Earth (Galvez et al, 2013;Duncan and Dasgupta, 2017). Degassing of graphitic OC can be substantial up to the chlorite zone but restricted at higher metamorphic grades (Zhang et al, 2018). A highly stable OC component in the graphitized form can be retained as a refractory phase under mantle conditions during subduction (Duncan and Dasgupta, 2017).…”

Section: Introductionmentioning

confidence: 99%

Stability of Organic Carbon Components in Shale: Implications for Carbon Cycle

et al. 2019

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Stability and mobility of organic matter in shale is significant from the perspective of carbon cycle. Shale can only be an effective sink provided that the organic carbon present is stable and immobile from the host sites and, not released easily during geological processes such as low pressure-temperature burial diagenesis and higher pressure-temperature subduction. To examine this, three Jurassic shale samples of known mineralogy and total organic carbon content, with dominantly continental source of organic matter, belonging to the Haynesville-Bossier Formation were combusted by incremental heating from temperature of 200 to 1400 • C. The samples were analyzed for their carbon and nitrogen release profiles, bulk δ 13 C composition and C/N atomic ratio, based on which, at least four organic carbon components are identified associated with different minerals such as clay, carbonate, and silicate. They have different stability depending on their host sites and occurrences relative to the mineral phases and consequently, released at different temperature during combustion. The components identified are denoted as, C-1 (organic carbon occurring as free accumulates at the edge or mouth of pore spaces), C-2 (associated with clay minerals, adsorbed or as organomineral nanocomposites; with carbonate minerals, biomineralized and/or occluded), C-3(a) (occurring with silicate minerals, biomineralized and/or occluded) and C-3(b) (graphitized carbon). They show an increasing stability and decreasing mobility from C-1 to C-3(b). Based on the stability of the different OC components, shale is clearly an efficient sink for the long term C cycle as, except for C-1 which forms a very small fraction of the total and is released at temperature of ∼200 • C, OC can be efficiently locked in shale surviving conditions of burial diagenesis and, subduction at fore arc regions in absence of infiltrating fluids. Under low fluid flux, C-3(b) can be efficiently retained as a refractory phase in the mantle when subducted. It is evident that the association and interaction of the organic matter with the different minerals play an important role in its retention in the shale.

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“…The carbon isotopic signature of the upper mantle, transition zone and lower mantle (Stachel et al, 2002;Cartigny et al, 2004;Palot et al, 2014), and of gaseous CO 2 emitted from arc volcanoes (Mason et al, 2017) suggests that organic matter subducted within sediments displays a major role in the deep carbon cycle (Hayes and Waldbauer, 2006). The dissolution of graphitic carbon in aqueous fluids due to oxidation or reduction processes (Connolly and Cesare, 1993;Connolly, 1995;Zhang et al, 2018;Tumiati and Malaspina, 2019b) is of primary importance as it governs the removal of organic matter from the sediments flushed by fluids released from the dehydrating subducted plate (Schmidt and Poli, 2013). In contrast to carbonates (e.g., Kelemen and Manning, 2015), graphite has long been considered to represent a refractory sink of carbon in the subducting slab (Plank and Manning, 2019), showing low solubility in metamorphic fluids (Connolly and Cesare, 1993) and silicate melts (Duncan and Dasgupta, 2017).…”

Section: Introductionmentioning

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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Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA

Cited by 14 publications

References 72 publications

How Do Subduction Zones Regulate the Carbon Cycle?

How Do Subduction Zones Regulate the Carbon Cycle?

Stability of Organic Carbon Components in Shale: Implications for Carbon Cycle

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

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