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, #a_~o 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.
Petrography
The metamorphic rocks described in the present study come from a single dredge-haul taken in the active part of the Vema fracture zone, which offsets the crest of the mid-Atlantic Ridge by more than 300 km near 11 ~ North. The rocks were dredged near 10~ N and 40 ~ 59' W, between depths of about 4,700 and 3,900 m below sea-level (Fig. 1).
The prehnite-pumpellyite facies assemblages were identified in samples of metabasalts and metabasaltir breccias. The breccia consists of angular fragments of basalt (up to 20 cm wide) in a fine-grained green matrix. The rocks, similar to many metabasalts dredged from the ocean floor, are typically greenish as opposed to fresh basalts and show no evidence of deformation; original magmatic textures are prefectly preserved. Numerous veins, up to 10% volume, occur in these samples.
The basalts display quench textures and are evidently pillow fragments, some of them with a variolitic rim.
The magmatic minerals are still identifiable (Table 1). Most of the basalts are porphyritic and contain phenocrysts of plagioclase,