Abundance, Textures, and O-C Isotope Compositions of Carbonate in HP-Metamorphosed Pillow-Basalt Breccias in the Western Alps: Implications for Deep-Earth Carbon Cycling

Abstract:

A large fraction of the carbonate in oceanic crustal sections resides in pillow-basalt breccias, thus HP/UHP-metamorphosed pillow breccias are targets for investigation of deep subduction-zone C cycling. In this study, we examine W. Alps blueschist-grade pillow breccias that experienced a range of peak P-T (300-450˚C at 1.0 to 1.5 GPa) similar to that experienced in modern subduction margins and we evaluate the extent to which metamorphic fluid-rock interactions resulted in significant C release.

In these rocks, carbonate resides in breccia clasts and cements, vein generations showing a range of textures, and interpillow regions, with the cements in general constituting the largest carbonate reservoir. We have in particular examined for evidence of decarbonation reaction between breccia clasts and adjacent carbonate cements to produce Ca-rich phases such as lawsonite/epidote, pyroxene, or garnet. For most of these rocks, the infiltration (by H₂O-rich fluids) producing shifts in δ¹⁸O_VSMOW to as low as +12‰ (protolith values +20‰ and higher), with little δ¹³C shift, did not drive significant amounts of decarbonation. Some veins and clasts at Lago Nero have carbonate O-C isotope compositions overlapping those of seafloor altered oceanic crustal equivalents, perhaps indicating the impermeability of parts of that section during the HP metamorphism. In some basaltic clasts, modest amounts of decarbonation stabilized phases such as lawsonite and epidote but without downward shift in δ¹³C expected for more significant reaction. δ¹³C values of the carbonate in general fall in the range observed for unsubducted seafloor equivalents, with the only exception being a subset of carbonate textural types at Ubaye Valley showing shifts to much higher δ¹⁸O up to +23‰ and lower δ¹³C, consistent with exchange with nearby calc-schists. The lowered δ¹⁸O of most of the samples suggests exchange with fluids generated in mafic-ultramafic sources as postulated by Jaeckel et al. (2018; Geosphere) to produce low-δ¹⁸O carbonate along large-scale subduction interface structures.

This study integrating field, petrographic, and C-O isotope compositions demonstrates that pillow basaltic breccias can potentially retain carbonate into deep forearcs thus making this C available for contribution to arcs and the deeper-mantle C budget.