COH-fluid induced metasomatism of peridotites in the forearc mantle

Contributions to Mineralogy and Petrology (2022) 177:44 https://doi.org/10.1007/s00410-022-01905-w ORIGINAL PAPER COH‑fluid induced metasomatism of peridotites in the forearc mantle Melanie J. Sieber1,2 · Gregory M. Yaxley1 · Jörg Hermann3 Received: 24 January 2022 / Accepted: 28 February 2022 © The Author(s) 2022 Abstract Devolatilization of subducting lithologies liberates COH-fluids. These may become partially sequestered in peridotites in the slab and the overlying forearc mantle, affecting the cycling of volatiles and fluid mobile elements in subduction zones. Sr 2+ Here we assess the magnitudes, timescales and mechanism of channelized injection of COH-fluids doped with Ca2+ aq , aq 2+ and Baaq into the dry forearc mantle by performing piston cylinder experiments between 1–2.5 GPa and 600–700 °C. Cylindrical cores of natural spinel-bearing harzburgites were used as starting materials. Based on mineral assemblage and composition three reaction zones are distinguishable from the rim towards the core of primary olivine and orthopyroxene grains. Zone 1 contains carbonates + quartz ± kyanite and zone 2 contains carbonates + talc ± chlorite. Olivine is further replaced in zone 3 by either antigorite + magnesite or magnesite + talc within or above antigorite stability, respectively. Orthopyroxene is replaced in zone 3 by talc + chlorite. Mineral assemblages and the compositions of secondary minerals depend on fluid composition and the replaced primary silicate. The extent of alteration depends on fluid C O2 content and fluid/rock-ratio, and is further promoted by fluid permeable reaction zones and reaction driven cracking. Our results show that COH-fluid Sr 2+ Ba2+ induced metasomatism of the forearc mantle is self-perpetuating and efficient at sequestering Ca2+ aq and CO2aq aq , aq , into newly formed carbonates. This process is fast with 90% of the available C sequestered and nearly 50% of the initial minerals altered at 650 °C, 2 GPa within 55 h. The dissolution of primary silicates under high COH-fluid/rock-ratios, as in channelized fluid flow, enriches SiO2aq in the fluid, while CO2aq is sequestered into carbonates. In an open system, the remaining CO2-depleted, Si-enriched aqueous fluid may cause Si-metasomatism in the forearc further away from the injection of the COH-fluid into peridotite. Keywords Carbonation · Deep carbon cycle · COH-fluid · Forearc · HP-experiments Introduction Volumetrically significant metasomatism takes place in subduction zones because of the juxtaposition and interaction among disparate lithologies. Metasomatism is, for instance, driven by activity gradients when fluids, released from the subducting slab, move along variable pressure/ Communicated by Othmar Müntener. * Melanie J. Sieber 1 Research School of Earth Science, The Australian National University, Canberra 2601, Australia 2 Present Address: Institute of Geosciences‑Mineralogy, University Potsdam, 14476 Potsdam‑Golm, Germany 3 Institute of Geological Science, University of Bern, 3012 Bern, Switzerland temperature-trajectories and are injected into lithologies other than their source region (Bebout and Barton 1989; Bebout 2013; Stern 2002). Such fluid injection and related mass transfer and metasomatism are particularly relevant at the slab–mantle interface (e.g., mélange zones; Breeding et al. (2004)) and when fluids are injected into the hanging wall of the overlying, ultramafic mantle. Subducting (meta-) sediments and altered oceanic crust are a source of C O2aq and volatile elements for metasomatising fluids particularly when they are infiltrated by externally derived aqueous fluids (Gorman et al. 2006). The flux and CO2-concentrations of COH-fluids released from the subducting slab are poorly known, which is partially related to different approaches used for constraining CO2 concentrations in fluids liberated from the subducting slab. For instance, Kelemen and Manning (2015) re-evaluated and compiled C-fluxes and calculated fluid compositions expelled from the subducting plate by estimating the 13 Vol.:(0123456789) 44 Page 2 of 22 amount of aqueous fluids released and modelling the solubility of carbon in aqueous fluids. According to these authors, the C-fluxes depend on the amount of released water and the solubility of calcite and aragonite into aqueous fluid. They supposed that the molar proportion of C O2 in a C O2 and f luid H2O fluid ( XCO ) is ≤ 0.001 between 0.5 and 2 GPa. As the 2 solubility of carbonates in aqueous fluids increases with increasing pressure and temperature (Caciagli and Manning 2003; Dolejs and Manning 2010) fluids liberated between 2 f luid and 6 GPa have higher XCO ranging from around 0.008 to 2 0.012 if dissolution of carbonation into aqueous fluids is assumed to be the single process contributing carbon to fluids (Kelemen and Manning 2015). Note that mineral decarbonation reactions may also release carbon into hydrous melts (Martin and Hermann 2018; Poli 2015) and must also be considered when estimating total C-fluxes (Kelemen and Manning 2015). Phase equilibria studies report a ~ 700 times higher CO2 concentration in fluids liberated under the forearc. Despite the increase in solubility, the experimental approach of Molina and Poli (2000) and Poli et al. (2009) showed that progressive decomposition of hydrated phases with continuing subduction is coupled with increasing stability of carbonates. Thus, fluids released under the forearc f luid contain more C O 2 (0.04 < XCO <0.4; ~ 1–2 GPa 2 and ~ 650–750 °C) compared to fluids released at higher f luid presssures ( XCO <0.2; 2.2–5 GPa and 680–800 °C) (Molina 2 and Poli 2000; Poli et al. 2009). Thermodynamic models of phase equilibria in closed and open systems support those experimental results (Connolly 2005; Gorman et al. 2006; Kerrick and Connolly 2001a, b). Peridotites are suitable for sequestering some of the released CO2aq as carbonate minerals, because of their abundance of divalent metal ions, required for carbonate mineral formation (Kelemen et al. 2011). Seawater alteration of peridotites typically forms Ca-rich carbonates (calcite and aragonite) (Grozeva et al. (2017), while alteration of peridotites by metamorphic fluids released during subduction forms Mg-rich carbonates (dolomite and magnesite) (Menzel et al. 2018; Sieber et al. 2018). The latter is accompanied by the formation of talc, quartz, and fuchsite (Cr-muscovite) along with Fe-oxide, -hydroxide, and/or sulfide phases (Falk and Kelemen 2015; Menzel et al. 2018). Ophiolites record the importance of such COH-fluid/ peridotite interaction in the forearc. For instance, fully carbonated peridotites have been reported from ophiolite complexes as soapstones (carbonate + talc rocks) and listvenites (carbonate + quartz rocks) formed under pressures and temperatures of 0.2–1.5 GPa and 80–350 °C (e.g., Samail ophiolite: Falk and Kelemen (2015); Linajavri and Leika ophiolite: Beinlich et al. (2012); Bjerga et al. (...truncated)