The interplay between fluid mobility and deformation along subduction zone interfaces, and its influence on subduction C flux and other chemical cycling, can be evaluated through study of exposed paleo-subduction suites. Along traverses of three interface exposures in the Western Alps (Switzerland and Italy), lowering of carbonate δ18O values toward the fault surfaces is consistent with infiltration by H2O-rich fluid with δ18OVSMOW of +8.5 to +10.5‰ calculated using temperatures of 400–460 °C from Raman spectroscopy of carbonaceous matter. The lowering of δ18O occurs in rocks showing pervasive mylonitization and flattening parallel to the paleo-interface, and containing abundant deformed carbonate ± quartz veins, consistent with the enhancement of fluid infiltration by this deformation. These δ18O values could reflect mixtures of far-traveled fluids emanating from metabasaltic, meta-ultramafic, and metasedimentary sources at greater depths in the subducting slab (perhaps including hydrated mantle) and/or along the interface.

Thermal gradients in the uppermost parts of subducting sections and inter­faces (particularly at depths of >80 km) could result in flow paths that are initially up-T, thus conceivably promoting carbonate dissolution. This flow would presumably be followed by flow down-T and down-P, and thus down the solubility gradient for calcite in H2O (potentially precipitating carbonate), as the fluids then move toward the surface along the interface. At all three of the Western Alps localities, the carbonate (and quartz) precipitated in veins could reflect focusing of fluid flow along these zones of enhanced deformation. These observations, and other recently published accounts of carbonation in ultramafic rocks from similar structural settings, indicate precipitation and storage of carbonate in forearcs of a magnitude potentially important for whole-margin C cycling.

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