Carbonation of mantle peridotite by CO2-rich fluids: the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada)
Graphical abstract
Introduction
Because mantle peridotites are far from equilibrium under weathering and low temperature metamorphic conditions, they tend to hydrate and carbonate when in contact with aqueous fluids. Oceanic ophicarbonates (e.g., Schwarzenbach et al., 2013) and carbonate travertine in peridotite-hosted alkaline springs (e.g., Kelemen and Matter, 2008) show that serpentinized peridotites have a high capacity to bind dissolved CO2 derived from seawater and the atmosphere. When in contact with CO2-rich hydrothermal fluids, serpentinized peridotite is often pervasively replaced by talc-magnesite and quartz-magnesite rocks (e.g., Beinlich et al., 2012; Falk and Kelemen, 2015; Hansen et al., 2005). Due to the fast kinetics of carbonation reactions in serpentinized peridotites, they are a target for carbon sequestration strategies by mineral carbonation to reduce atmospheric carbon contents (e.g. Andreani et al., 2009; Hövelmann et al., 2011; Kelemen et al., 2011; van Noort et al., 2013). These characteristics make differently carbonated peridotites sensitive markers of carbon fluxes in mantle rocks, providing clues on the transfer of carbon between shallow and deep reservoirs. Understanding the processes involved in carbon transfer and the knowledge of total long-term fluxes between such carbon reservoirs are important for past climate reconstructions. In particular, carbonated peridotite in the cold, leading edge of the mantle wedge above subduction zones may represent a significant, but largely unrecognized carbon reservoir in the deep carbon cycle (Kelemen and Manning, 2015). Thick listvenite occurrences at the basal thrust of the Oman ophiolites (Falk and Kelemen, 2015; Kelemen et al., 2017) and along thrust faults in ophiolites elsewhere (e.g. Qiu and Zhu, 2015; Sofiya et al., 2017; Zhang et al., 2015) demonstrate that carbonation of peridotite can be widespread in similar tectonic settings as those found in the tip of the mantle wedge in cold subduction zones.
Listvenites are fuchsite- (Cr-rich mica)- and Cr-spinel-bearing quartz-magnesite rocks that form by reaction of aqueous, CO2-rich fluids with ultramafic rocks (Halls and Zhao, 1995). Besides CO2, potassium, sulphur and gold are often added during fluid infiltration, leading to the formation of fuchsite and Au-bearing sulphides that can be of economic interest (Emam and Zoheir, 2013; Qiu and Zhu, 2015). Listvenites can form at temperature as low as 80–100 °C (Falk and Kelemen, 2015) and up to of 200–350 °C (Beinlich et al., 2012; Hansen et al., 2005; Schandl and Naldrett, 1992). In high-temperature listvenites, several zones with distinctive assemblages are predicted to record a stepwise (simplified) carbonation reaction of peridotite: olivine + H2O ± CO2 = serpentine ± magnesite; serpentine + CO2 = magnesite + talc; talc + CO2 = magnesite + quartz (Johannes, 1969; Klein and Garrido, 2011). Because the carbonation reaction progress is mainly limited by the availability and transport of CO2-bearing aqueous fluid, natural listvenites often record, to some extent, these arrested reaction steps (e.g., Beinlich et al., 2012; Hansen et al., 2005). Therefore, they are ideal natural laboratories to study the controlling factors and timescales of carbon-bearing fluid flux and carbonation reaction progress of ultramafic rocks at large scale.
Here we report extensively carbonated mantle rocks from the Central Advocate complex, Newfoundland (Canada), that preserve a complete set of arrested progressive carbonation reactions from un-reacted, serpentinized harzburgite protolith to talc-magnesite rocks and listvenites. By means of a detailed petrographic and petrological study, thermodynamic modelling and stable isotope geochemistry, we document the reaction sequence of carbonation of serpentinite by CO2-rich hydrothermal fluids likely derived from metamorphic devolatilization in a forearc setting.
The Baie Verte Peninsula, Newfoundland, comprises the boundary between the Neoproterozoic continental margin of Laurentia and a set of Early Ordovician ophiolites (the Baie Verte Ophiolites; Fig. 1a) and their Early to Middle- Ordovician supra-crustal, sedimentary cover rocks (Bédard and Escayola, 2010; Skulski et al., 2010). The ophiolite cover sequences consist of rather thick volcanic rocks with boninite to tholeiitic and calc-alkaline affinities, alternating with clastic to pelagic sediments derived from the continental margin, island arc material and the ophiolite crust itself, and deposited onto the oceanic crust (e.g. Kidd et al., 1978; Skulski et al., 2010). The Baie Verte ophiolites (490–480 Ma) are thought to have formed by forearc spreading due to incipient east-ward subduction of the Taconic seaway below several micro-continent slices (Castonguay et al., 2014). Closure of the Taconic seaway led to obduction of the Baie-Verte forearc ophiolite and its cover sequences onto the continental basement of the Laurentian margin along the Baie Verte line, a polyphase brittle-ductile shear zone that is part of the Baie Verte – Brompton line (Fig. 1a) (Castonguay et al., 2014; Skulski et al., 2010; Waldron and van Staal, 2001). In the Laurentian continental margin, meta-gabbro and graphitic metapelite of the Birchy complex, in tectonic contact with the ophiolites (Fig. 1b), record eclogite-facies peak conditions (1.0–1.2 GPa, 450–500 °C) with amphibolite-facies overprint (0.70–0.85 GPa, c. 550 °C) (Jamieson, 1990; van Staal et al., 2013). These units have ocean-continent transition (OCT) characteristics, indicating that parts of the hyper-extended Laurentian margin subducted beneath the Baie Verte oceanic crust and were exhumed synchronous with the ophiolite obduction as part of a subduction channel (Castonguay et al., 2014; van Staal et al., 2013). In contrast to the continental margin, rocks associated with the Baie Verte ophiolites only show a weak greenschist facies metamorphic imprint. Both the continental basement and the ophiolitic sequences are intruded by post-collisional, Early Silurian granitoid plutons (Whalen et al., 2006).
Listvenites and talc-magnesite rocks are part of the ultramafic units throughout the Baie Verte ophiolite sequence; major occurrences are in the Point Rousse complex (Escayola et al., 2009), and in the Advocate complex (Bédard and Escayola, 2010; Kidd, 1974; Skulski et al., 2009). At Point Rousse, listvenitization of ultramafic cumulates occurred by hydrothermal alteration at around 200 °C, probably associated with tectonic deformation along the Baie Verte – Brompton Line (Escayola et al., 2009).
The Advocate complex (Fig. 1b) is a highly discontinuous and tectonically dismembered ophiolite sequence that forms the westernmost leading edge of the Baie Verte ophiolites, obducted onto the partially subducted Laurentian continental margin (Bédard and Escayola, 2010; Castonguay et al., 2014). It consists of fault-bounded slivers of serpentinized harzburgite, gabbro, sheeted dykes and rare volcanic mafic rocks (Skulski et al., 2010), which crop out along the Baie Verte line. In large segments along-strike of the Baie Verte line, the crustal sequence of the Advocate complex is absent and only thin tectonically dissected remnants of serpentinized mantle rocks — in parts completely carbonated to listvenite — remain (Kidd, 1974). The primary texture of larger harzburgite slivers and enclosed pyroxenite and dunite layers are oriented oblique to the main regional cleavage that correlates with thrusting along the Baie Verte line (Fig. 1b & c; and Kidd, 1974).
Section snippets
Field relations and sampling
Listvenites, Qtz-Tlc-Mgs rocks1, Atg-Tlc-Mgs rocks, and carbonated serpentinites crop out in the central Advocate complex along a N-S oriented, 40–80 m thick zone of about 1 km length close to the Baie-Verte Road, at the NW corner of Flatwater Pond (Fig. 1b & c, and Table 1). They are situated in the eastern part of the central Advocate mantle rocks surrounded by Atg-serpentinite and, locally, strongly deformed Lz-serpentinite (Fig. 1c). The extent of
Atg-serpentinite
Atg-serpentinites in the central Advocate complex consist of antigorite with interlocked texture, lizardite bastite (orthopyroxene pseudomorphs; 10–25 vol%; Fig. 4a), minor lizardite in mesh centre domains, olivine, magnetite, Cr-spinel, brucite, carbonate (dolomite and magnesite), and rare Fe-Ni-sulphides (pentlandite, pyrite-pyrrhotite, heazlewoodite). Raman spectroscopy reveals that mesh-centre and bastite serpentine occasionally consist of fine intergrowths of lizardite and antigorite
Whole rock chemistry
Cleaned rock chips from homogeneous sample parts were crushed in an agate mill to < 50 μm grain size, excluding weathered rims and thicker veins. Major elements, loss on ignition (LOI), total carbon and sulphur, and ferrous iron were analysed at Geoscience Laboratories (GeoLabs) of the Ontario Geological Service (Canada). Major elements (ISO-accredited) and Co, Cu, Cr, Ni, V, and Zn were measured by X-ray fluorescence (XRF) on fused glass beads of the samples. Several sample duplicates,
Whole rock chemistry
The Advocate serpentinites and related carbonated rocks have highly variable CO2 contents and plot close to the serpentine-CO2 tie line in the MgO-FeO-SiO2-CO2 composition space (Fig. 7a). On a volatile-free basis, all analysed samples have compositions with 44–47 wt% SiO2 and 44–48 wt% MgO (Fig. 7b), low Al2O3 and CaO (0.4–1.2, and 0.1–1.3 wt%, respectively; Table 2), high Cr- and Ni contents (Fig. 7c), and molar MgO/(MgO + FeOtotal) ratio of 0.90–0.93. Listvenites and some Qtz-Tlc-Mgs rocks
Nature of the protolith of the Advocate listvenites
Whole rock major element and textural evidence indicate that the protolith of the Advocate listvenites and related carbonated rocks was serpentinized mantle harzburgite similar to that outcropping elsewhere in the Advocate ophiolite (Bédard and Escayola, 2010). The similarity of the whole rock major element compositions on an anhydrous basis of all lithologies with variable CO2 (Fig. 7b & c) demonstrates that carbonation of serpentinized harzburgite occurred by addition of CO2 and minor removal
Conclusions
The lithological sequence, microstructure and mineral composition of carbonate-bearing Atg-serpentinite, talc-magnesite rich rocks and listvenite in the Advocate complex preserve a remarkable record of natural carbonation of serpentinized harzburgite by CO2-rich fluids. Microstructures and mineral assemblages indicate that carbonation of serpentinized peridotite occurred coeval to the transformation of lizardite/chrysotile to antigorite at greenschist facies conditions (c. 300 °C and likely
Acknowledgments
We acknowledge two anonymous reviewers for helpful comments and suggestions that improved various aspects of the manuscript, and we thank Marguerite Godard and Marco Scambelluri for its editorial handling. Inmaculada Martínez Segura is thanked for her assistance during sample preparation, and Arsenio Granados Torres for his assistance during preparation of stable isotope samples. Manuel J. Román Alpiste is also thanked for his help during SEM operation, and his assistance in the preparation of
Funding
Research leading to these results is part of the Marie Curie Action, Initial Training Network “ABYSS”, funded by the Seventh Framework Programme of the European Union under REA Grant Agreement no. 608001, and is part of M.M.'s Ph.D. project. C.J.G., V.L.S.-V. and C.M. acknowledge funding from the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) grants no. CGL2016-75224-R and CGL2016-81085-R, from the Junta de Andalucía research groups RNM 131 and 145, and grant P12-RNM-3141.
Competing financial interests
The authors declare no competing financial interests.
Data availability
Full electron microprobe data is available on request.
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