Received: 21 October 2018 | Accepted: 23 February 2019 DOI: 10.1111/jmg.12481 ORIGINAL ARTICLE Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite-serpentinite dehydration (Nevado‐Filábride Complex, Spain) Manuel D. Menzel1 | Carlos J. Garrido1 | Vicente López Sánchez‐Vizcaíno2 | Károly Hidas1,3 | Claudio Marchesi1,4 1Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC – Universidad de Granada, Abstract Armilla, Spain At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly 2Dpto. de Geología (Unidad Asociada al controlled by fluid released by devolatilization reactions such as dehydration of anti- IACT‐CSIC), Universidad de Jaén, Escuela gorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–sili- Politécnica Superior, Linares, Spain 3 cate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Dpto. de Geodinámica, Facultad de Ciencias, Universidad de Granada, Granada, Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Spain Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique oppor- 4 Dpto. de Mineralogía y tunity to study the stability of carbonate during subduction metamorphism at P–T Petrología, Facultad de Ciencias, Universidad de Granada, Granada, conditions before and after the dehydration of Atg‐serpentinite in a warm subduction Spain setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clino- humite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diop- Correspondence Manuel D. Menzel, Instituto Andaluz de side–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, Ciencias de la Tierra (IACT), CSIC – carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assem- Universidad de Granada, Armilla, Spain. blage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐ Email: [email protected]; [email protected] bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO2 contents at nearly constant Mg/ Funding information European Commission, Grant/Award Si ratio and high Ni and Cr contents, indicating that their protoliths were variable Number: REA Grant Agreement n° 608001; mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic European Social Fund; Ministerio de modelling shows that the carbonate–silicate rocks attained peak metamorphic condi- Investigación, Ciencia y Universidades, Agencia Estatal de Investigación (Spain), tions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and Grant/Award Number: FPDI‐2013‐16253, 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). PCIN‐2015‐053, CGL2016‐75224‐R, Microstructures, mineral chemistry and phase relations indicate that the hybrid car- CGL2016‐81085‐R and RYC‐2012‐11314; Junta de Andalucía, Grant/Award Number: bonate–silicate bulk rock compositions formed before prograde metamorphism, P12–RNM–3141, RNM‐131 and RNM‐374 likely during seawater hydrothermal alteration, and subsequently underwent subduc- tion metamorphism. In the CaO–MgO–SiO ternary, these processes resulted in a Handling Editor: Donna Whitney 2 compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2019 The Authors. Journal of Metamorphic Geology Published by John Wiley & Sons Ltd J Metamorph Geol. 2019;37:681–715. wileyonlinelibrary.com/journal/jmg | 681 682 | MENZEL ET AL. modelling using classical models of binary H2O–CO2 fluids shows that the composi- tional variability along this binary determines the temperature of the main devolatili- zation 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 equilib- rium with carbonate–silicate rocks in warm subduction settings. Consequently, ac- counting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by 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 be- yond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle. KEYWORDS carbon cycle, Nevado‐Filábride Complex, ophicarbonate, serpentinite dehydration, subduction fluids 1 | INTRODUCTION Hermann, Garrido, López Sánchez‐Vizcaíno, & Gómez‐ Pugnaire, 2010; Rüpke, Morgan, Hort, & Connolly, 2004; Subduction zones transfer carbon from the Earth′s surface Ulmer & Trommsdorff, 1995). There is mounting evidence to its deep interior and control the long‐term deep carbon that fluids generated during high‐pressure (HP) deserpentini- cycle. Sediments, altered oceanic crust and the hydrated zation are oxidizing—close to the hematite‐magnetite oxygen lithospheric mantle in the subducting slab contain carbon in buffer—and are mildly alkaline (Alt et al., 2012; Debret et al., the form of carbonate minerals and organic carbon. During 2015; Debret & Sverjensky, 2017; Evans, Reddy, Tomkins, subduction of the slab, metamorphic devolatilization reac- Crossley, & Frost, 2017; Galvez et al., 2016). If so, deser- tions of hydrous minerals release fluids that enhance decar- pentinization fluids could mobilize both reduced and inor- – bonation reactions (Connolly, 2005; Gorman, Kerrick, & ganic carbon as CO2(aq) and HCO3 (Facq, Daniel, Montagnac, Connolly,2006 ) and induce partial melting of the overlying Cardon, & Sverjensky, 2014). Antigorite dehydration fluids mantle wedge (Dasgupta, Hirschmann, & Withers, 2004; are rich in non‐volatile components (Scambelluri, Müntener, Poli, 2015; Tumiati, Fumagalli, Tiraboschi, & Poli, 2013). Ottolini, Pettke, & Vannucci, 2004), which may form aqueous + Metamorphic devolatilization reactions thus play a vital complexes such as NaHCO3 and CaHCO3 and increase carbon role in the deep carbon cycle by ultimately modulating the solubility (Facq, Daniel, Montagnac, Cardon, & Sverjensky, amount of the subducted carbon returned to the Earth's sur- 2016; Galvez et al., 2016). Therefore, deserpentinization flu- face via arc volcanism (Dasgupta,2013 ; Kerrick & Connolly, ids might constitute very effective agents for the recycling of 2001). Different estimates of the carbon fluxes at sub‐arc carbon back to arc volcanism in the subduction factory. depths mainly arise from uncertainties about the efficiency of Exhumed metamorphic terranes provide essential in- rock‐buffered devolatilization reactions, the role of congru- sights into the mechanisms of deep carbon recycling and ent carbonate dissolution in fluid‐dominated conditions and mobilization in subduction zones (Bebout & Penniston‐ the pathways of fluids (Connolly & Galvez, 2018; Galvez, Dorland, 2016; Ferrando, Groppo, Frezzotti, Castelli, & Connolly, & Manning, 2016; Kelemen & Manning, 2015). Proyer, 2017; Piccoli et al., 2016; Scambelluri et al., 2016). Because it releases high amounts of H2O‐rich fluids—9 Thermodynamic calculations and studies of eclogite facies wt% H2O at ~660°C—dehydration of Atg‐serpentinite is po- marbles and carbonated eclogites and serpentinites in palaeo‐ tentially the most relevant devolatilization reaction to mobi- subducted metamorphic terranes show that carbonate min- lize carbon at sub‐arc depths into fluids from slab and mantle erals undergo variable extents of decarbonation and can be wedge lithologies (Bromiley & Pawley, 2003; Padrón‐Navarta, stable at high pressure (Collins et al., 2015; Connolly, 2005; MENZEL ET AL. | 683 Cook‐Kollars, Bebout, Collins, Angiboust, & Agard, 2014; serpentinite‐hosted ophicarbonate. Examples of meta‐ophicar- Ferrando et al., 2017; Proyer, Mposkos, Baziotis, & Hoinkes, bonate associated to prograde metamorphism of serpentinite 2008). Studies of natural examples of HP metamorphism of are numerous in the geological record (Figure 1). Many ex- carbonate‐bearing rocks during Atg‐serpentinite dehydration amples are preserved in brucite‐ or olivine‐bearing Atg‐ser- are scarce and limited to assess the mass‐balance of carbon pentinites that underwent metamorphism in intermediate to during dehydration (Alt et al., 2012). Most of the available es- warm subduction settings (cf. orange and pink dotted paths in timates of carbon release in deserpentinization fluids heavily Figure 1; Collins et al., 2015; Scambelluri et al., 2016; Vitale rely on numerical and thermodynamic modelling (Gorman