Fluid-flux and reaction rate from advective-diffusive carbonation of mafic sill margins in the Dalradian, southwest Scottish Highlands

Abstract

Greenschist facies mafic sills in the Dalradian of the southwest Highlands, Scotland, have been carbonated by infiltration of a CO 2-bearing hydrous fluid from adjacent calc-phyllites. The primary amphibole-epidote bearing assemblage is preserved in the cores of many sills in which the margins were altered to calcite, chlorite and quartz. The asymmetric widths of the carbonated margins allow determination of flow direction and magnitude, and indicate that sill margins were buffered to nearly constant fluid composition by copious flow in the more permeable phyllites. Partially carbonated amphibole-epi-dote assemblages within the reaction fronts preserve evidence for sluggish reaction kinetics. Downstream margins developed by diffusion against the flow direction and this allows calculation of reaction front broadening due to both diffusion and reaction kinetics. An advective-diffusive transport model with linear reaction kinetics has been fitted to the reaction progress profile for a sill at Port Cill Maluaig, Knapdale. This implies a cross-layer time-integrated fluid flux of 62.1 ± 1.3 m 3/m 2, a Damköhler Number of 22.4 ± 4.2 and a Peclet Number of 66.2 ± 20.3 (1 σ errors). The Peclet Number and time-integrated fluid flux imply that the flow event lasted between 0.02 and 20 Ma for plausible porosities in the range 10 −3 to 10 −6. The inferred rate constant for the reaction kinetics is two or more orders of magnitude slower than that calculated by extrapolation of experimentally determined surface reaction rates. Either the kinetic dispersion arose from factors additional to reaction kinetics or fluid-solid reaction was controlled by a slower mechamism such as diffusion away from flow channels. The latter conclusion implicates deformation as an important control on the rates of fluid infiltration and fluid-rock reaction.