Carbon and Oxygen Isotope Compositions of the Schistes Lustrés: Closed vs. Open System Behavior and Fluid Sources in Deeply Underplated Sediments

The Schistes Lustrés, located in the W. Alps, is a subduction-zone metamorphosed marine complex consisting of interlayered calc-schist and metapelite, with mafic/ultramafic lenses, in km-scale sub-units thought to reflect underplating in a paleo-forearc. Ranging from blueschist to eclogite grade, the regional-scale Schistes Lustrés complex was subducted to depths of 40-70 km and experienced only minor overprinting during exhumation. We present d¹⁸O and d¹³C values for 65 samples gathered along three traverses across major ductile shear zones separating sub-units located in the French Alps near Val d'Isere. This allows evaluation of the extents and scales of fluid-rock interaction during HP metamorphism, including along the shear zones.

The samples from across the three transects have average carbonate d¹⁸O_VSMOW shifted from protolithic values near +28‰ to +21.9‰, +19.6‰, and +18.8‰, with no obvious deflection in either d¹⁸O or d¹³C at/nearest the shear zone boundaries. We postulate two explanations for the isotope systematics observed throughout this part of the Schistes Lustrés: (1) decreases in d¹⁸O ascribed to local(μm to cm)-scale exchange of carbonate with silicate minerals in the same sample; and/or (2) interaction with externally-derived H₂O-rich fluids leading to larger-magnitude shift to lower d¹⁸O. The fluid infiltrating these rocks appears to have been H₂O-rich and produced in metapelites within the Schistes Lustrés. Although the carbonate in all of the samples, across the three transects, has undergone significant shift to lower d¹⁸O, from protolith values, the shear zones separating the sub-units appear not to have afforded the infiltration by the lower-d¹⁸O, mafic/ultramafic-derived fluids documented by Jaeckel et al. (2018; Geosphere) for larger-scale transient paleo-subduction interfaces.

This study builds on prior work by our group investigating degrees of fluid-rock interaction, and related C mobility, over a range of scales (µm to 10's of km), and significance for margin-scale C cycling. Together, these studies have produced a large C and O isotope dataset for meta-oceanic rocks in the W. Alps that can be regarded as analogs for deeply underplated rocks in modern margins in which similar lithologies are being subducted (carbonate-rich sediments, Atlantic-type oceanic crust).