Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)

Metabasalts with abundant pumpellyite have been dredged in the Vema fracture zone, Atlantic ocean, and contain prehnite+pumpellyite±epidote+chlorite+white mica. The prehnite — pumpellyite association in these rocks differs from the prehnite-epidote association for most of the prehnite — pumpellyite facies metabasalts from the ocean crust described previously. The occurrence of pumpellyite is discussed in terms of temperature conditions, and oxygen fugacity and the pumpellyiterich metabasalts are believed to be recrystallized by hydrothermal circulation of seawater at about 250° C under a very low pressure (<1 kb).The bulk composition of the rocks demonstrates a strong chemical modification during hydrothermal metamorphism, similar to what is observed under greenschist facies conditions, except for potassium which can be uptaken from seawater by the rocks.