SUBDUCTION-ZONE METAMORPHIC PATHWAY FOR DEEP CARBON CYCLING: EVIDENCE FROM THE ITALIAN ALPS AND THE TIANSHAN
Our results for sediments and AOC indicate impressive retention of oxidized and reduced C to depths approaching those beneath arc volcanic fronts, in metasedimentary rocks but with extensive isotopic exchange between the two C reservoirs. Much of the carbonate in metabasalts has δ13C overlapping with that for carbonate in seafloor-altered basalt, with some HP/UHP metamorphic veins showing greater influence of organic C signatures from metasedimentary rocks. Calculation of prograde devolatilization histories for sediments and AOC, using Perple-X, demonstrates that little decarbonation occurs in such sections in forearc regions unless they are flushed by H2O-rich fluids from an external source such as the hydrated ultramafic section of subducting slabs (cf. Gorman et al., 2006; G3).
Comparison of recent thermal modeling for modern margins (van Keken et al., 2011; JGR) with calculated phase relations indicates that significant C loss during devolatilization (and partial melting) should occur as subducting sections traverse the depths beneath arc volcanic fronts. On a global basis, imbalance between subducted C input and C return flux by magmatism (~40±20% of subducted C return via arcs and perhaps ~80±20% by all magmatism; modified after Bebout, 2014, Treat. Geochem.) indicates net ingassing of C into the mantle. Global C cycle models predict that a relatively small change in the subduction/volcanic C flux could significantly affect atmospheric CO2 levels and thus global climate.