Sub-sea-floor metamorphism,heat and mass transfer

The ophiolitic rocks of E. Liguria, Italy contain a spilitic metamorphic assemblage sequence, cross-cut by hydrothermal veins, which developed in the oceanic environment. Metamorphic parageneses indicate that temperatures as high as 400°C were realised at depths as shallow as 300 m below the original rock/water interface. The inferred temperature interval was equivalent to a geothermal gradient of 1300°C/km.It is suggested that metamorphism took place in a sub-sea-floor geothermal system, and that such systems are an integral part of the sea-floor spreading process. Modern evidence is provided to support this hypothesis, and to suggest that heavy metal rich solutions discharged from such systems are responsible for the formation of a metal enriched sedimentary component. A unified model of sub-sea-floor metamorphism and mass transfer is proposed, and possible differences between sub-sea-floor and terrestial geothermal systems are discussed. In the light of the model, the origins of certain aspects of bedded cherts found associated with ophiolitic rocks, of ophiolitic massive sulphide deposits and of certain trace element patterns are considered.