Search

Infiltration of a metabasite sill from Islay, Scotland by an [H.sub.2]O-C[O.sub.2] fluid caused (1) modification of [delta][sup.18]O and (2) carbonation at the sill margins. Maps of [delta][sup.18]O and reaction progress were constructed from a 20 x 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both [delta][sup.18]O and reaction progress. The [delta][sup.18]O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite ('fizz-o-meter', Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic 'fluid pathways' whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995).

The oxygen isotopes (O) move to a greater distance in the silicate than in the marble rocks of the Lizzies Basin, East Humboldt Range, Nevada. The strontium isotope (Sr) moves about the same distance in both types of rocks. This is probably due to a difference in the porosity of the silicate rocks or different diffusion rates of the isotopes. Details of the isotope profiles across the marble to calc-silicate contacts are given.

A model was formulated to illustrate the mineralogical consequences of fluid infiltration on rocks experiencing metamorphism. The model developed is an extension of past published work since it permits calculation of fluid fluxes for different T-Xco2 paths within a system undergoing changes in the angle of flow within time, multivariant reactions and space and lateral fluxes from adjacent more carbon dioxide or water-rich layers. The model entails computation of mineral assemblages, modes and composition.