Reaction-induced porosity in an eclogite-facies vein selvage (Monviso Ophiolite, W. Alps): Textural evidence and in situ trace elements and Sr isotopes in apatite


 Porosity generated during fluid-rock reaction can facilitate fluid transport and metasomatism in low permeability high pressure metamorphic rocks. Evidence for reaction-induced porosity is found in an eclogite-facies clinopyroxene + apatite vein in an undeformed eclogitized Fe-Ti metagabbro from the Monviso Ophiolite (W. Alps) with a distinct garnet-rich selvage. Vein-forming fluids were sourced from adjacent metagabbros and reaction with the host rock removed Ca and P from the selvage and added Fe, REE, Pb and Cr. Textures at the selvage-host rock interface and in the host rock record local heterogeneity in reactivity and porosity during metasomatism linked to variable initial lawsonite abundance. These features reflect a hierarchy of pervasive-to-channelized porosity structures that facilitated widespread metasomatism of the host rock. Development of this metasomatic system in response to locally-derived fluids suggests large-scale externally-derived fluid transport is not required to drive extensive fluid-rock exchange. The production of porosity during metasomatic reactions could be important in facilitating further fluid-rock reaction and fluid transport in subducting slabs where permeability is low.