Petrology of the East Pacific Rise crust and upper mantle exposed in Hess deep (eastern equatorial Pacific)

Abstract

The Hess Deep, a rifted oval‐shaped depression located east of the Galapagos Triple Junction at the tip of the Cocos‐Nazca ridge (about 101°W, 2°N), was explored in 1988 during 21 submersible dives. A total of 11 dives were devoted to the exploration of the E‐W trending Intrarift ridge (15 km in length, 3000–5400 m in depth) north of the Hess Deep depression. The Intrarift ridge represents an outcrop of recent (1 m.y.) crustal and subcrustal material created at the axis of the East Pacific Rise (EPR), and emplaced during the lithospheric extention responsible for the westward propagation of the Cocos‐Nazca rift (Francheteau et al., 1990). The lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites. The samples are metamorphosed to varying degrees, but their protolith textures are generally well preserved. Their relic mineralogy indicates that they consist of harzburgites, dunites, gabbroic cumulates (gabbronorites and olivine gabbros), isotropic gabbros, dolerites, and basalts. Some samples of refractory harzburgites and most dunitic cumulates (with local accumulation of chromite) have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle‐crust transition zone of the Samail ophiolite complex (Oman). The mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus‐solidus temperature range of 980–1100°C. The dolerites and basalts have been derived from mid‐ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the EPR volcanics near 13°N (Hekinian et al., 1989). Most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies (ultramafics and gabbros) to diagenetic alteration (volcanics). The cumulate gabbronorites, the isotropic gabbros, and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization. Several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks. The gabbroic cumulates and the plagioclase‐rich melt impregnations were variably rodingitized (presence of various Ca‐silicates such as epidote, prehnite, hydrogarnet, and zeolite) in relation to the serpentinization of the peridotites. One dive located on the scarps forming the northern wall of the Hess Deep to the east of the explored area, revealed the presence of in situ outcrops of isotropic gabbros, doleritic dikes, and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros.