High-pressure metamorphism induced porosity in mafic rocks – wet vs. dry

Fluid–rock interaction, and thus fluid flow plays a fundamental role in geological processes from the shallow crust to mantle depths. In the upper crust, fluid flow is predominantly controlled by brittle features and interconnected porosity. In contrast, at lower crustal conditions, elevated lithostatic pressures are commonly assumed to inhibit fracturing and major porosity, leaving unresolved how fluids migrate at depth. To investigate porosity development during high-pressure metamorphism of initially impermeable mafic rocks, we conducted a series of piston-cylinder experiments that varied reaction duration and fluid availability. Dolerite from the Kerforne dyke (Brittany, France) was used as starting material. Cylindrical samples (2.8 mm diameter, ~3.5 mm length) were characterized prior to experimentation using X-ray micro-computed microtomography (µCT; 3 µm Isovoxel), enabling three-dimensional quantification of mineral fabric, modal proportions, and initial porosity. The starting dolerite consists of ~50–65% plagioclase, 20–30% pyroxene, up to 10% ilmenite, and minor quartz, with an initial porosity of ~0.1%. The fabric is near-isotropic and dominated by randomly oriented plagioclase grains up to 300 µm in length.

Experiments were performed under quasi hydrostatic conditions at 700 °C and 2.4 GPa for varying durations. To evaluate the influence of fluid availability on reaction progress and porosity evolution, three experimental setups were employed: (1) nominally dry conditions without added fluid, (2) addition of paragonite as a source of fluid and sodium, and (3) addition of 5 vol% water (“wet” conditions). Wet experiments were conducted for durations of 1, 7, and 21 days to assess the temporal evolution of reactions.

Following experimentation, all samples were re-imaged using µCT, allowing three-dimensional mapping of reaction progress and porosity development. Largely unreactive Fe–Ti oxides served as internal markers, enabling accurate registration of pre- and post-experimental µCT datasets and direct comparison between the protolith and reaction products. Three-dimensional observations were complemented by high-resolution two-dimensional analyses using electron probe microanalysis and scanning electron microscopy.
Reaction progress increases systematically with fluid availability, from dry to paragonite-bearing to water-added conditions. Under nominally dry conditions, reactions are restricted to narrow zones along pyroxene–plagioclase interfaces and plagioclase grain boundaries, producing predominantly fine-grained zoisite needles (<5 µm). In paragonite-bearing experiments, reaction intensity increases within plagioclase, characterized by the growth of zoisite and phengite, while jadeite forms along pyroxene–plagioclase boundaries. In contrast, wet experiments result in complete replacement of plagioclase within 7 days by an assemblage of zoisite, phengite, amphibole, and minor omphacite and quartz. Pyroxene develops narrow reaction rims (<30 µm wide) marked by increasing Al and Na and decreasing Fe and Ca contents, while garnet occurs as idiomorphic grains in the fine-grained matrix or as coronae surrounding oxides.
Porosity development is closely coupled to reaction progress, and three distinct porosity types are identified: (1) micro- to nanopores within plagioclase reaction products, (2) nanopores within pyroxene reaction rims, and (3) microfractures. The first two porosity types are interpreted to result from volume reduction associated with density increases during metamorphic reactions, whereas microfractures likely form in response to stress concentrations and elevated pore-fluid pressures.