T018-0014
Carbon and Oxygen Isotope Compositions of the Schistes Lustrés: Closed vs. Open System Behavior and Fluid Sources in Deeply Underplated Sediments

Wednesday, 9 December 2020
Poster
Samuel Marshall, University of Massachusetts Lowell, Environmental, Earth, and Atmospheric Sciences, Lowell, MA, United States, Gabe S Epstein, Lehigh University, Earth and Environmental Sciences, Bethlehem, PA, United States, Samuel Angiboust, Institut de Physique du Globe de Paris, Paris, France and Gray E Bebout, Lehigh University, Dept. Earth and Environmental Sciences, Bethlehem, PA, United States
Abstract:
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 d18OVSMOW 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).

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