Cracking at the magma–hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus

The nature of the magma–hydrothermal transition in oceanic hydrothermal systems is poorly understood, in part because the geological relations in this critical region have rarely been observed in modern ocean crust. Detailed mapping was conducted in the Troodos Ophiolite, Cyprus, where a gabbronorite sequence intrudes the sheeted dyke complex, which is truncated at its base by a thin contact aureole composed of massive hornfels. Geothermometric data for hornblende and pyroxene hornfels show that hydrated sheeted dykes were recrystallized at amphibolite to granulite facies conditions (778–986°C). Quartz diorite veins and apophyses, and monomineralic amphibole veins cross-cut the contact aureole and show no preferred age relationships. Geothermometric data indicate that quartz diorite was injected at 817–919°C and that fractures were filled with amphibole at 575–750°C. Phase relations of quartz-hosted, halite-bearing fluid inclusions in quartz diorite veins constrain minimum entrapment temperatures of 225–520°C (average 402°C) and minimum pressures that span lithostatic and hydrostatic conditions. We believe that these characteristics are indicative of a conductive boundary layer that separates an active hydrothermal system from the heat source that drives it. Field and petrological data indicate that transient fracturing caused oscillations in temperature and pressure conditions within the conductive boundary layer, and mixing of hydrothermal and magmatic fluids at the magma–hydrothermal interface. Cross-cutting relations between magmatic and hydrothermal vein networks show that fracturing occurred prior to the cessation of magmatic activity. We explore plausible models for the causes and consequences of fracturing that consider the role of dyke injection, thermoelastic stresses, and volatile build-up.