SUBDUCTION-ZONE METAMORPHIC PATHWAY FOR DEEP CARBON CYCLING: EVIDENCE FROM THE ITALIAN ALPS AND THE TIANSHAN

On a global basis, sediments and altered oceanic crust (AOC) deliver 80-95% of the C currently entering subduction zones, the remainder in carbonated ultramafic rocks. We investigated extents of retention of C in deeply subducted AOC, sediments, and ultramafic rocks represented by HP/UHP meta-ophiolitic and metasedimentary rocks in the Italian Alps and in the Tianshan (Cook-Kollars et al., 2014; Collins et al., in review; both in Chem. Geol.). Study of metapelite devolatilization in the same suite (Bebout et al., 2013; Chem. Geol.) provides a geochemical framework for this study of C behavior along prograde P-T paths similar to those experienced in most modern forearcs. Study of veins in the Tianshan affords examination of C mobility in UHP fluids, in later stages as metabasalts were fragmented in the subduction channel.

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.