Microstructure evolution of aqueous fluid‐bearing wehrlites: Implications for the fluid distribution in polymineralic rocks

The microstructure of aqueous fluid‐bearing wehrlites was experimentally studied in order to investigate the fluid distribution in polymineralic rocks. Wehrlites of various forsterite/diopside ratios were synthesized with 1.0–1.5 wt % water at 1200°C and 1.2 GPa for 1.5–668 hours. Conspicuous grain growth, drag, and coagulation of pore fluids were observed in the time series experiments. The pore fluids were classified into two types: those surrounded only by a single phase (G‐type) and those surrounded by both forsterite and diopside (I‐type). In the short time runs the relative volume fraction of I‐type pores in all the pores, F I , agrees well with the value calculated from an ideal pore distribution model in which all the grains are randomly distributed. The F I of wehrlites with various forsterite/diopside ratios increased with average grain size and reached a steady state value at 80–90% in about 160–163 hours. The increase of F I is explained by “interphase boundary trapping”: G‐type pore fluids move faster than I‐type, and the number of G‐type becoming I‐type by being trapped into the interphase boundary per unit time is larger than that of I‐type pores transforming to G‐type. The experimental results suggest that large fractions of pore fluids might be distributed at the grain junctions surrounded by more than two mineral phases in natural rocks as a result of grain growth. The connectivity of the fluid network in polymineralic rocks may be strongly influenced by modal composition of the rocks as well as by the dihedral angles among the fluid and two mineral phases.