Carbon gradients as tracers of structurally controlled fluid flow in homogeneous metabasaltic sills (Loch Stornoway, Scottish Highlands).

In the past decades, the extent of fluid-induced reaction halos in metabasaltic sills within the Argyll Group of the Dalradian Supergroup in the SW Scottish Highlands has been intensively used to constrain metamorphic fluid flow velocities (Skelton, 2011). However, recent findings revealed that reaction front propagation within numerous sills was primarily controlled by preferred fabric alignment at the margins during deformation events, as well as by mineralogical and chemical heterogeneities across the sills. Here, we revisit hydration and carbonation fronts in metabasaltic sills in the vicinity of major fluid pathways, i.e., the Loch Awe Syncline and Ardrishaig Anticline, to reevaluate fluid-induced reaction front propagation and constrain metamorphic fluid flow velocities.
This study integrates field observations, detailed petrological-textural analyses, and whole-rock geochemistry, including carbon and water contents as well as trace element data, along a transect across compositionally homogeneous metabasaltic sills. The aim is to constrain the mechanisms controlling fluid-induced reaction progress at the contact between metasedimentary rocks and metabasaltic sills.
The selected basaltic sills were metamorphosed under greenschist- and epidote-amphibolite-facies conditions and record at least four deformation events. In the sill margins, the rocks show increased calcite and chlorite contents and replacement of garnet, amphibole, and dark mica, reflecting localized retrogression. This retrograde overprint is also characterized by mobilization of large-ion lithophile elements (LILE; e.g., K, Na, Sr). In contrast, the sill interior preserves textural and mineralogical equilibrium among amphibole, dark mica, epidote, garnet, and titanite. Textural variations indicate a progressive decrease in hydration and carbonation toward the sill interior.
Carbon contents decrease systematically from 1.22-1.16 wt.% in the sill margins to 0.07-0.02 wt.% toward the sill interior. Similarly, water contents are highest in the sill margins (up to 1.95 wt.%) and lowest in the sill interior (0.52 wt.%). Petrographic observations further suggest that fluid infiltration and reaction are controlled by structural anisotropy inherited from earlier deformation during retrogression, rather than mineralogical heterogeneity. Fluid flow is preferentially channelized along lithological contacts and deformation-related weaknesses, such as foliation and mineral lineation, which are most developed near the sill margins.
The new dataset enables a recalculation of the true spatial extent of metamorphic fluid infiltration and allows time-integrated estimates of fluid fluxes based on carbonation and hydration reaction front geometries, as well as their relationships with trace element redistribution. Understanding the rates of CO₂ release and sequestration during orogenic processes provides new insights into the role of structural anisotropies and brittle-ductile processes in controlling the volume, pathways, and metal-enriching potential of metamorphic fluid flow.