The role of peridotite and serpentinite in the deep carbon cycleconstraints from ophiolites and exhumed metamorphic terranes

Abstract

Subduction of oceanic lithosphere is the only active flux of carbon from Earth’s atmosphere-hydrosphere-crustal system reservoirs into the deep convecting mantle. Serpentinites in the mantle wedge of the forearc can sequester and store carbon derived from the subducting slab and exert an important control on the carbon mass balance in subduction zones. At subarc depths of subduction zones, devolatilization reactions and fluid-rock interaction mostly regulate the balance between the carbon returned by arc volcanism and that retained and transported by subducting slabs into the deep mantle. Dehydration of antigorite serpentinite at these depths releases a large amount of fluids that react with other slab lithologies, playing a decisive role in the modulation of the deep carbon fluxes. The main aim of the present Ph.D. thesis is to improve our understanding of carbonation processes of serpentinized peridotite and the fate of serpentinite-hosted carbon during high-pressure metamorphism, in order to better constrain their role in the deep carbon cycle of subduction zones. To advance in this overall research goal, this Ph.D. thesis presents the results of two natural case studies of key processes of carbon transfer during fluid-rock interaction in subduction zones: (i) listvenites from the Baie Verte ophiolite (Newfoundland, Canada), a natural analogue for carbon storage in the forearc mantle of subduction zones by carbonation of peridotites; and (ii) meta-ophicarbonates from the Nevado-Filábride Complex (Spain), which provide unprecedented insights into the stability of carbonates during antigorite-serpentinite dehydration in a paleo-subduction terrane. The combination of field, (micro) structural, petrological and geochemical data with thermodynamic modelling sheds new light on the interaction of fluids with the hydrated mantle, which regulates the carbon mobility at forearc and subarc depths of subduction zones. By means of thermodynamic models of prograde and high-pressure infiltration-driven devolatilization reactions in serpentinite-hosted meta-carbonate rocks, this thesis further investigates the effects of electrolytic fluids and the role of open-system flux of serpentinite dehydration fluids on the fluid-mediated release of carbon in subduction zones. In a first chapter, this thesis presents the microstructural and petrological record of carbonation of peridotite by CO2-rich fluids in a forearc setting, as evidenced by the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada). The mantle section of the Advocate ophiolite contains unique outcrops of listvenite (magnesite-quartz), antigorite- and quartz-bearing talc-magnesite rock, and carbonated antigorite-serpentinite. This lithological sequence records the sequential carbonation of serpentinite by CO2-rich hydrothermal fluids. High Cr and Ni contents and preservation of Cr-spinel with a composition similar to that of Atg-serpentinite (molar Mg/Mg+Fe = 0.50 – 0.65; Cr/Cr+Al = 0.50 – 0.70), show that the Advocate listvenite and talc-magnesite rocks formed by carbonation of variably serpentinized mantle harzburgite. Replacement of lizardite by magnesite coeval with the breakdown of lizardite to antigorite + brucite and the lack of prograde olivine and magnetite in antigorite serpentinite and talc-magnesite rocks constrain the temperature of carbonation between c. 280 °C and 420 °C. Thermodynamic modelling of carbonation of serpentinite at 300 ºC and 0.2 – 0.5 GPa accounts for the sequence of carbonated rocks in the Advocate complex. Phase relations and petrological observations indicate that the aqueous aSiO2 and aCO2 of the infiltrating CO2-rich fluid were buffered at the Atg-Tlc-Mgs and Qtz-Tlc-Mgs pseudo-invariant points, forming dominantly three-phase rocks by variable extents of carbonation at these pseudo-invariant points. Listvenites formed at large fluid-rock ratio when quartz became saturated in the fluid and precipitated along magnesite grain boundaries and in variably sized tensional veins. The whole rock Fe3+/Fetotal ratio of the Advocate carbonate-bearing sequence decreases with increasing whole rock carbon content, from 0.65 – 0.80 in brucite-bearing antigorite serpentinite to 0.10 – 0.30 in talc-magnesite rocks and listvenite. The whole rock iron reduction is associated with an increase in the ferrous iron content of magnesite and the formation of hematite and goethite, indicating a concomitant increase of the fluid oxygen fugacity. The sequence of carbonation reactions is uniquely preserved in three main growth zones characteristic of listvenite magnesite: (i) an inner zone of magnetite-bearing, Fe-poor, Mn-bearing magnesite formed by carbonation of lizardite, brucite and olivine from Atg-serpentinite; (ii) an outer zone of Fe-rich magnesite formed by carbonation of antigorite and in equilibrium with Fe-poor talc; and (iii) an outermost rim of Fe-poor magnesite formed by carbonation of talc. Carbonation of the Advocate serpentinized mantle harzburgite likely occurred in a supra-subduction upper plate ophiolite by fluxing of slab-derived, CO2-rich fluids channelled along deep faults at the onset of accretion of the forearc basin (c. 300 ºC, < 0.5 GPa). The rather constant δ18O (11.0 – 14.4 ‰ V-SMOW) and relatively low δ13C (–8.9 to –5.0 ‰ V-PDB) of magnesite throughout the sequence of carbonated rocks in the Advocate complex is consistent with CO2-rich fluids derived from decarbonation or dissolution of organic carbon- and carbonate-bearing meta-sediments, such as those occurring in the underlying Birchy complex —the partially subducted continental margin of Laurentia. Carbonation of serpentinized oceanic or continental mantle lithosphere by reactive percolation of CO2-rich fluids derived from the slab in forearc settings may represent a significant carbon reservoir for the deep carbon cycle. In a second chapter, this thesis provides new insights into the petrology and phase relations of meta-ophicarbonates during subduction metamorphism. At subarc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg-) serpentinite to prograde peridotite. This chapter investigates carbonate-silicate rocks hosted in Atg-serpentinite and prograde chlorite (Chl-) harzburgite in the Milagrosa and Almirez ultramafic massifs in the paleo-subducted Nevado-Filábride Complex (NFC, Betic Cordillera, S Spain). These massifs present a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg-serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti-clinohumite-diopside-calcite marbles, diopside-dolomite marbles, and antigorite-diopside-dolomite rocks hosted in clinopyroxene-bearing Atg-serpentinite. In Almirez, carbonate-silicate rocks are hosted in Chl-harzburgite and show a high-grade assemblage composed of olivine, Ti-clinohumite, diopside, chlorite, dolomite, calcite, Cr-bearing magnetite, pentlandite, and rare aragonite inclusions. These NFC carbonate-silicate rocks have variable CaO and CO2 contents at nearly constant Mg/Si ratio, and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca-carbonate (i.e. ophicarbonates). Thermodynamic modelling shows that the carbonate-silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550 – 600 °C and 1.0 – 1.4 GPa) and Chl-harzburgite (Almirez massif; 1.7 – 1.9 GPa and 680 °C). Microstructures, mineral chemistry, and phase relations indicate that the hybrid carbonate-silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO2 ternary, these processes resulted in a compositional variability of NFC serpentinite-hosted carbonate-silicate rocks along the serpentinite–calcite mixing trend, similar to that observed in serpentinite-hosted carbonate-rocks in other paleo-subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2O–CO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition, and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid species and charged carbon-bearing species occur only in minor amounts in equilibrium with carbonate-silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions based on classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite-hosted carbonate-rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)-serpentinite-hosted carbonate-silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration-driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite-hosted carbonate-silicate rocks beyond the Atg-serpentinite breakdown at subarc depths, demonstrating that carbon can be recycled into the deep mantle. In a third chapter, this thesis investigates the effects of electrolytic fluid compositions and open-system flux of serpentinite dehydration fluids on fluid-mediated carbon release in subduction zones by means of thermodynamic models. Serpentinite-hosted carbonate rocks formed during the oceanic stage of subducting oceanic lithosphere (ophicalcite) and metasomatism at the subduction plate interface (hybrid carbonate–talc rocks) can be important sources of carbon at subarc depths. If hosted in the hydrated mantle lithosphere of the incoming slab, these carbonate-rocks may be preserved from the infiltration of fluids produced by prograde metamorphism of the overlying meta-sediments and the hydrated oceanic crust to subarc depths, where they are fluxed by fluids derived from dehydration of their host serpentinite. Because the dissolution of carbonate in aqueous fluids is enhanced at high P and T, fluid-mediated carbon release at subarc depth is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. This section of the thesis presents thermodynamic modelling — using the implementation of the DEW aqueous database in Perple_X — of prograde devolatilization reactions (solids = fluid + solids) and infiltration-driven devolatilization reactions (external fluid + solids = fluid + solids) of serpentinite-hosted carbonate rocks. In line with previous studies of prograde devolatilization of carbon-bearing oceanic crust, our models show that, in warm and cold subductions zones, the solubility of carbon in fluids produced by subsolidus devolatilization reactions of meta-ophicalcite and carbonate-talc rocks is limited even considering metal ion complexes of electrolytic fluids. Therefore, serpentinite-hosted meta-carbonate rocks are likely to be preserved to subarc depths where they undergo infiltration-driven devolatilization by Atg-serpentinite dehydration fluids. At the P–T conditions of Atg-breakdown of cold to warm subduction zones, models of infiltration-driven devolatilization of meta-ophicalcite and carbonate-talc rocks indicate that carbon release is accompanied by significant Ca loss, particularly at high pressures (P > 3.5 GPa) due to the high activity of CaHCO3+ in the fluid. The carbon solubility in fluids equilibrated with meta-ophicalcite and carbonate–talc rocks is markedly higher than for pure aragonite due to open-system buffer reactions between carbonates, silicates, and fluid. Mass balance considerations combined with the parameterization of the P–T conditions of Atg-serpentinite dehydration as a function of the thermal parameter of subduction zones allow us to investigate the carbon loss caused by infiltration of serpentinite dehydration fluids. Unlike the dissolution of CaCO3, the carbonate dissolution of serpentinite-hosted meta-carbonate rocks is highest at the slab surface of warm subduction zones and is lowest at Moho depths of cold subduction zones where Atg-serpentinite dehydration takes place at greater depths. The subduction of meta-ophicalcite —with thicknesses of up to 70 m observed in oceanic ophicalcite— will preserve carbonate beyond subarc depths, and will recycle carbonate-garnet-clinopyroxene-olivine rocks into the deep mantle even in hot subduction zones. On the other hand, infiltration-driven devolatilization of subducted carbonate-talc rocks is an efficient source of carbon at subarc depths where they readily transform to orthopyroxenite in most subduction zone regimes. Publications related to this Ph.D. Thesis: Menzel, M.D., Garrido, C.J., López Sánchez‐Vizcaíno, V., Hidas, K. and Marchesi, C. (2019) Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain). Journal of Metamorphic Geology (in press). DOI: 10.1111/jmg.12481 Menzel, M.D., Garrido, C.J., López Sánchez-Vizcaíno, V., Marchesi, C., Hidas, K., Escayola, M.P. and Delgado Huertas, A. (2018) Carbonation of mantle peridotite by CO2-rich fluids: the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada). Lithos 323, 238-261. DOI: 10.1016/j.lithos.2018.06.001