INTERACTIONS BETWEEN MAGMA CHAMBERS AND HYDROTHERMAL SYSTEMS - OCEANIC AND OPHIOLITIC CONSTRAINTS

A compilation of published exit temperatures and salinities for seafloor hydrothermal fluids, along with fluid inclusion data on oceanic and ophiolitic rocks, shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300-degrees-360-degrees-C, thus clearly being in the one-phase field. They overlie a lower system marked by the generation of highly saline brines at high temperatures. The brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers. In ophiolites, the brines typically occur within the transition zone between the sheeted-dike complex and the plutonic sequence, i.e., on top of magma chambers. This transition zone, relatively well known from ophiolite studies (Oman, Troodos, Bay of Islands, Josephine), is characterized by mutually intrusive relationships between gabbro, plagiogranite, and dikes and xenoliths of altered diabase within plagiogranite and gabbro. In fast to medium spreading ophiolites (Oman) it is the locus of a major downward decrease in the density of hydrothermal veins and alteration. The vein system is strongly anisotropic and shows a well-marked preferred vertical along-strike orientation. The underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins, while the overlying sheeted-dike complex is densely veined and pervasively altered. This zone is marked by major gradients in the physical parameters over short distances: temperature (several degrees Celsius per meter), pressure (several hundred kilopascals), transition from brittle to ductile. In steady state systems, direct interactions between the magma chamber and the hydrothermal system are restricted to this zone, characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks. Complications to this steady state model resulting from a non-steady state magma chamber (e.g., Josephine ophiolite) are mainly the development of late stage faulting and more pronounced retrogressive alteration.