Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

Abstract

At sub‐arc 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. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide 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 serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐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 by classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral c...

Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones

“…The minimum temperature of crystallization of Chlharzburgites is constrained by the limit of antigorite stability field, between 660 and 680 °C at 1.6 and 2.5 GPa (e.g., Ulmer and Trommsdorff, 1995;Wunder and Schreyer, 1997;Padrón-Navarta et al, 2010b), In addition, the occurrence of tremolite in the Chl-harzburgites limits the maximum temperature from 680 to 710 °C for 1.9 and 1.6 GPa, respectively (Padrón-Navarta et al, 2010b). These P-T conditions compare well with independent estimates for metarodingite (1.6-1.9 GPa and 660-684 °C; Laborda-López et al, 2018) and ophicarbonate lenses hosted within the Chl-harzburgites (18 kbar, 650-670 °C; Menzel et al, 2019).…”

“…Furthermore, these geochemical characteristics (i.e., LILE, Sr and B enrichments) are also shared with fluid inclusions trapped in metamorphic olivine and orthopyroxene from Chl-harzburgites (Scambelluri et al, 2004a, b), providing strong evidence for the equilibration of the Chl-harzburgites with a fluid phase enriched in these elements. Taken together, these observations show that dehydration must have occurred in an open system involving external fluids that equilibrated with other lithologies such as metasedimentary rocks (Jabaloy et al, 2015), meta-rodingites (Laborda-López et al, 2018), meta-ophicarbonates (Menzel et al, 2019), and/or an exotic slab-derived component (Marchesi et al, 2013;Harvey et al, 2014). Interestingly, the Cerro del Almirez samples display broad positive arrays between δ 66 Zn and Sr/Y, Ba/Ce or Rb/Ce ratios with the Chlharzburgites displaying high δ 66 Zn, Sr/Y, Ba/Ce and Rb/Ce ratios relative to the other lithologies as well as higher and more variable U/Yb ratios (Fig.…”

“…The contact between Atg-serpentinites and transitional lithologies developed obliquely to the foliation of the Atg-serpentinite and shows no signs of tectonic activity or offset. In addition to the Atg-serpentinite, Chl-harzburgite and transitional lithologies, the massif also contains minor metarodingite boudins and ophicarbonates lenses that, together with the Atg-serpentinites, preserve a textural and geochemical record of seafloor serpentinization and carbonation processes (Alt et al, 2012;Laborda-López et al, 2018;Menzel et al, 2019), which likely took place during the opening of the western branch of the Alpine Tethys Ocean in the Mesozoic (Puga et al, 2011).…”

Calcium Stable Isotopes of Tonga and Mariana Arc Lavas: Implications for Slab Fluid‐Mediated Carbonate Transfer in Cold Subduction Zones

“…This result is consistent with our model that the Tonga and Mariana arc volcanos release significant CO 2 but the Ca content of C‐bearing slab fluids is limited. Slab fluids in cold subduction zones are insufficient to dissolve all AOL carbonates (Gorman et al., 2006; Menzel et al., 2019; Molina & Poli, 2000; Thomson et al., 2016). In this case, a fraction of C and more Ca from the AOL could remain in subducted slabs and enter the deeper convective mantle, reducing the influence of slab‐fluid mediated carbonates on the Ca isotopes of mantle wedge and related arc lavas in cold subduction zones (Figure 7).…”

“…On the other hand, thermodynamic calculation indicates a strong dependence of carbonate dissolution on the fluid flux, fluid property, and thermal parameters of subduction zones (Connolly & Galvez, 2018; Gorce et al., 2019; Huang & Sverjensky, 2019; Menzel et al., 2020). A fraction of carbonates in the AOL should be preserved at sub‐arc depths and recycled into the deeper mantle (Connolly, 2005; Frezzotti et al., 2011; Gorman et al., 2006; Menzel et al., 2019; Poli et al., 2009). Appropriate independent constraints are necessary to better understand the behavior of fluid‐mediated migration of carbonates in subduction zones.…”