Controls on the mechanisms of fluid infiltration and front advection during regional metamorphism: a stable isotope and textural study of retrograde Dalradian rocks of the SW Scottish Highlands

Abstract

ABSTRACTVein‐controlled retrograde infiltration of H2O‐CO2 fluids into Dalradian epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite‐rich marble bands to dolomite and spatially associated 18O enrichment of about 10%. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O‐salt mixture with a low CO2 content, and that the temperature of the fluid was about 400d̀ C. Detailed petrographic and textural (backscattered electron imaging) studies at one garnet‐grade locality show that advection of fluid into marbles proceeded by a calcite‐calcite grain edge flow mechanism, while alteration of non‐carbonate wall‐rock is associated with veinlets and microcracks.Stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall‐rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4–28.6 m3/m2 have been computed from measured advection distances. Coincidence of isotope and reaction fronts is considered to result from reaction‐enhanced kinetics of isotope exchange at the reaction front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection distance occurring in nearly pure calcite matrix. This relationship indicates that fluid flow in carbonates is only possible along fluid‐calcite‐calcite grain edges. However, experimental constraints on dihedral angles in calcite‐fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite‐calcite grain junctions rather than to an open calcite pore geometry.The regional extent of the retrograde infiltration event has been documented from the high δ18O of dolomite‐ankerite carbonates from veins and host‐rocks over an area of least 50 × 50 km in the SW Scottish Highlands. Isotopically exotic 18O‐rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral reaction in wall‐rocks only where lithology, pore geometry or mineral solubilities, pressure and temperature have been appropriate for pervasive infiltration to occur.