Rapid fluid flow along fractures at greenschist facies conditions on Syros, Greece

Abstract

Brittle fractures cut through greenschist facies metavolcanic rocks at Delfini on Syros, Greece. An equation for one-dimensional transport by advection along a single fracture and transverse diffusion outwards from this fracture was used to calculate time-averaged fluid velocities and the duration of fluid flow along the brittle fractures. These quartz and carbonate filled fractures are surrounded by symmetrical dark reaction halos. These reaction halos were formed by diffusion of CO₂ outwards from the fracture in a hydrous fluid which caused carbonation of the country rock. Changes in concentration of relatively mobile elements (for example K, Na, Cs, Ba, Pb and Sr) occurred. However, little to no changes in most of the major elements and less mobile trace elements were observed. This implies that carbonation was largely isochemical with respect to most non-volatile components. The Sr/Ca ratio was used to model time-averaged fluid velocities and the duration of fluid flow along the fractures. Fluid flow along narrower fractures with discernibly tapering haloes was found to be rapid (10⁻⁶ – 10⁻⁵ ms⁻¹) and short lived (0.1–400 years). These are time-averaged values and can therefore alternatively record a series of even shorter and faster pulses, perhaps associated with fracture propagation and associated seismicity. Within the widest fractures with constant halo widths (ca. 60 cm) fluid flow was slower (10⁻⁸ to 10⁻⁶ ms⁻¹) and longer lived (100–15000 years). We suspect that the constant width of these haloes reflects a steady state having been reached at which halo width was controlled by the relative rates of fluid flow along the fracture and in the surrounding rock.