Subduction zone metamorphic pathway for deep carbon cycling: II. Evidence from HP/UHP metabasaltic rocks and ophicarbonates

Abstract

Exposures of low-grade metabasalts and ophicarbonates in the Northern Apennines, and their high- and ultrahigh-pressure metamorphic equivalents in the Western and Ligurian Alps and Tianshan (representing an overall peak P- T range of ~ 0.2-3.0. GPa, 200-610. °C), allow investigation of the effects of prograde metamorphic devolatilization, and other fluid-rock interactions, on degrees of retention and isotopic evolution of C in subducting oceanic crust and associated mantle rocks. Such work can inform models of C cycling at convergent margins, helping to constrain the efficiency of return of initially subducted C via arc volcanism and the fraction of this subducted C entering the deeper mantle beyond arcs. In the metabasaltic rocks, the preservation of finely disseminated carbonate with δ13C overlapping that of seafloor-altered protoliths, and the minimal mineralogical evidence of decarbonation, indicates large degrees of carbonate retention in this suite extending to UHP conditions similar to those beneath modern volcanic fronts. For many of the metabasalts, the δ18O of this carbonate can be explained by closed-system equilibration with silicate phases (e.g., garnet, clinopyroxene) during HP/UHP metamorphism. Larger volumes of carbonate preserved in interpillow regions and as breccia-filling largely escaped decarbonation, showing little or no evidence for reaction with adjacent metabasalt. Calculated devolatilization histories demonstrate that, in a closed-system model, carbonate in metabasaltic rocks can largely be preserved to depths approaching those beneath volcanic fronts (80-90km). Modeling of open-system behavior indicates that episodic infiltration of such rocks by H2O-rich fluids would have greatly enhanced decarbonation. Trends in O-C isotope composition of carbonate in some metabasaltic suites likely reflect effects of infiltration by externally-derived fluid with or without resulting decarbonation. Most carbonated ultramafic rocks similarly show little mineralogical evidence for decarbonation, consistent with calculated reaction histories, and have δ13C largely overlapping that of seafloor equivalents. However, the high-grade ophicarbonates show more restricted ranges in δ18O consistent with some control by infiltrating fluids, likely during subduction. This combination of field, petrographic, and isotopic evidence, together with calculated decarbonation histories, is consistent with minimal loss of CO2 from these rocks via decarbonation during forearc metamorphism. Combining our results with those of Cook-Kollars et al. (2014; Chemical Geology) for associated W. Alps metasedimentary rocks, we suggest that the majority of the CO2 (perhaps 80-90%, considering the full range of rock types) could be retained through forearcs in more intact volumes of subducting sediment, basalt, and ophicarbonate experiencing closed- or limited open-system conditions. Deep in forearcs and beneath arcs, decarbonation (and also carbonate dissolution) could be enhanced in shear zones and highly fractured volumes experiencin...

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

“…Comparison of δ 13 C and δ 18 O of calcite veins and marble/calcsilicates from the ASZ (see data in Table S3) with values for calcite in other HP lithologies in the Penninic domain, including the Zermatt‐Saas unit ( ZSU ) and the Monviso massif ( MV ; data from Cook‐Kollars et al ., ; Collins et al ., ). The grey arrow indicates the trajectory in O and C isotope compositions followed by a rock undergoing decarbonation by a Rayleigh process.…”

“…Collins et al . () suggested that the low δ 18 O values for some particularly highly‐veined ZSU (and Monviso) metabasalts and ophicarbonates could reflect infiltration of the rocks by an H 2 O‐rich fluid with δ 18 O as low as +6.0‰ sourced in the slab mafic/ultramafic section. An H 2 O‐rich fluid equilibrated with a clinopyroxene/garnet source lithology (i.e.…”

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

“…Exhumed metamorphic terranes provide essential insights into the mechanisms of deep carbon recycling and mobilization in subduction zones (Bebout & Penniston‐Dorland, ; Ferrando, Groppo, Frezzotti, Castelli, & Proyer, ; Piccoli et al., ; Scambelluri et al., ). Thermodynamic calculations and studies of eclogite facies marbles and carbonated eclogites and serpentinites in palaeo‐subducted metamorphic terranes show that carbonate minerals undergo variable extents of decarbonation and can be stable at high pressure (Collins et al., ; Connolly, ; Cook‐Kollars, Bebout, Collins, Angiboust, & Agard, ; Ferrando et al., ; Proyer, Mposkos, Baziotis, & Hoinkes, ). Studies of natural examples of HP metamorphism of carbonate‐bearing rocks during Atg‐serpentinite dehydration are scarce and limited to assess the mass‐balance of carbon during dehydration (Alt et al., ).…”

“…Compilation of peak pressure–temperature (P–T) conditions of meta‐ophicarbonates (white fields) superposed on the pseudosection of a Ca‐bearing serpentinite from the Almirez massif. VM : Voltri massif (Scambelluri et al., ); ZS : Zermatt‐Saas; B: Bracco Unit, Internal Ligurides; UV : Ubaye Valley, Western Alps (all from Collins et al., ). L, L*: Lanzo massif peak conditions and PT of graphite‐formation in ophicarbonate, respectively (Vitale Brovarone et al., ); BA : contact‐metamorphic ophicarbonates, Bergell aureole (Pozzorini & Früh‐Green, ; Trommsdorff & Connolly, ); BD : Bellinzona‐Dascio unit, Central Alps (Stucki, ).…”

Magnesium Isotope Composition of Subduction Zone Fluids as Constrained by Jadeitites From Myanmar

“…Carbon transfer in subduction zone fluids is induced by decarbonation or dissolution processes (Ague & Nicolescu, ; Gorman et al, ; Kelemen & Manning, ; Kerrick & Connolly, , ). However, carbonates can remain stable to very high pressures and temperatures in subduction zones, resulting in inefficient decarbonation in forearcs (Collins et al, ; Dasgupta & Hirschmann, ; Kerrick & Connolly, ; Molina & Poli, ; Poli et al, ; Thomsen & Schmidt, ). Furthermore, the formation temperatures of the Myanmar jadeitites ( T < 500 °C) are significantly lower than the decarbonation conditions (Goffé et al, ; Shi et al, , ).…”