Serpentinite dehydration as a multiscale process

Abstract

Fluid release from hydrated oceanic lithosphere in subduction zones is a key process in the deep earth water cycle. During prograde metamorphism a channelized vein network forms in the dehydrating rock that allows efficient fluid release from the slab into the mantle wedge. The formation of such a vein network is a multiscale process that occurs over a wide range of time and length scales. Previous studies as well as field observations of exhumed meta-serpentinites suggest that the processes governing rock dehydration shift from chemical to mechanical processes going from small to large scales.To investigate the behavior of a dehydrating slab over this wide range of scales we present a multiscale dataset that includes field-based observations from m to sub-µm scale of a representative serpentinite from the Mirdita ophiolite (Albania). This ophiolite has experienced seafloor alteration, but has not been metamorphosed at conditions that would cause any dehydration. We use these data as input for thermodynamic equilibrium calculations to investigate the effect of chemical heterogeneities in the bulk rock composition while the PT-conditions will be increased following a typical subduction zone geothermal gradient.For upscaling we perform the calculations at various effective thermodynamic domain sizes (20-100 µm), showing that anisotropic chemical heterogeneities lead to heterogeneous porosity formation on all scales. Bloch et al. (2018) found that for vein-like porosity structures the percolation threshold of an effective bulk media may be reached at a porosity as low as 10-3. Therefore, the anisotropic porosity structure formed by chemical heterogeneities leads to a high connectivity even at low porosities and thus allows efficient fluid flow. Accordingly, going to even larger scales we can use these findings to describe the lithologies found in the field as effective bulk media with an effective fluid flow. This allows us to investigate fluid release from the dehydrating slab on the km-scale by reactive porosity waves using a numerical model.Bloch, W., John, T., Kummerow, J., Salazar, P., Krüger, O. S., & Shapiro, S. A. (2018). Watching Dehydration: Seismic Indication for Transient Fluid Pathways in the Oceanic Mantle of the Subducting Nazca Slab. Geochemistry, Geophysics, Geosystems, 19(9), 3189–3207. https://doi.org/10.1029/2018GC007703