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.

Petrologic and Stable Isotope Constraints on Hydrothermal Alteration and Serpentinization of the EPR Shallow Mantle at Hess Deep (Site 895)

“…The results of these dive programs showed that a complete, relatively fresh, crustal section (consisting of volcanics, sheeted dikes, and minor gabbros) is exposed along the scarps that bound the rift valley. Similar; but more dismembered, sections of the EPR oceanic crust (including lavas, dikes, gabbros, and peridotites) are exposed on the floor of the Hess Deep Rift Valley and along the intra-rift ridge (Francheteau et al, 1990;Hekinian et al, 1993). Two alternative rifting models have been proposed for the formation of the Hess Deep Rift Valley and the emplacement of deep crustal rocks at shallow levels in the oceanic crust (see fig.…”

Mineralogy of MARK peridotites: replacement through magma channeling examined from Hole 920D, MARK area

“…In addition, melt impregnation and melt/rock interaction appear to occur as pervasive processes in mantle peridotite (Kelemen et al, 1992(Kelemen et al, ,1995Boudier and Nicolas, 1995). Recent discussions about impregnated peridotites from mid-ocean ridge areas suggest that melt impregnation may enrich wall peridotites in melt components Girardeau and Mercier, 1992;Girardeau and Francheteau, 1993;Hekinian et al, 1993).…”

“…Recent reports on impregnated peridotites dredged from the East Pacific Rise Hekinian et al, 1993;Girardeau and Francheteau, 1993) and drilled peridotites from Site 895 of ODP Leg 147 to Hess Deep (Arai and Matsukage, 1996) suggest the role of melt per-ite. Primary minerals are olivine, orthopyroxene, clinopyroxene, and spinel.…”

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

“…Melt impregnation and refertilization of the lithospheric mantle is caused by the interstitial crystallization of percolating melts. This process leads to the formation of plagioclase-(±px) rich peridotites, and it has been widely documented in abyssal (Hebert et al 1983;Dick 1989;Cannat et al 1990;Hekinian et al 1993;Constantin et al 1995;Seyler et al 2001;Tartarotti et al 2002;Chazot et al 2005;von der Handt et al 2006) and ophiolitic peridotites (e.g. Menzies 1973;Menzies and Allen 1974;Dick 1977;Jackson and Ohnenstetter 1981;Boudier and Nicolas 1977;Nicolas 1986Nicolas , 1989Tartarotti and Vaggelli 1995;Rampone et al 1997;Dijkstra et al 2001;Piccardo 2003;Piccardo et al 2004Müntener et al 2004;Piccardo and Vissers 2007;).…”