Roger Hekinian

Massive deep-sea sulphide ore deposits discovered on the East Pacific Rise

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise. Although their presence on the deep ocean-floor had been predicted there was no supporting observational evidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land. They contain at least 29% zinc metal and 6% metallic copper. Their discovery will provide a new focus for deep-sea exploration, leading to new assessments of the concentration of metals in the upper layers of the oceanic crust.

Basaltic liquids and harzburgitic residues in the Garrett Transform: A case study at fast-spreading ridges

Abstract The peridotite-basalt association in the Garrett Transform, ∼ 13°28′S, East Pacific Rise (EPR), provides a prime opportunity for examining mantle melting and melt extraction processes from both melts and residues produced in a common environment beneath fast-spreading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites. Residual spinel, orthopyroxene and clinopyroxene in these harzburgites are extremely depleted in Al2O3, and plot at the most depleted end of the abyssal peridotite array defined by samples from slow-spreading ridges (including samples from hotspot-influenced ridges), suggesting that these harzburgites are residues of very high extents of melting. The residual peridotites from elsewhere at the EPR (i.e., Hess Deep and the Terevaka Transform) also are similarly depleted. This suggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspots (e.g., Azores hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Ocean), and is significantly higher than ≤ 10%, a value that has been advocated to be the average extent of melting beneath global ocean ridges. Many of these harzburgite samples, however, show whole-rock incompatible element abundances higher than expected. These same samples also have various amounts of excess olivine with forsterite contents as low as Fo85. The total olivine modes correlate inversely with olivine forsterite contents, and positively with whole-rock incompatible element abundances. These correlations suggest that the excess olivine and incompatible element enrichment are both the result of melt-solid re-equilibration. The buoyant melts that ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment of these melts leads to whole-rock incompatible element enrichment. These observations contrast with the notion that melts formed at depth experience little low pressure equilibration during ascent.

1 Ma East Pacific Rise oceanic crust and uppermost mantle exposed by rifting in Hess Deep (equatorial Pacific Ocean)

A series of dives with the French submersible Nautile has been conducted in the region of the Galapagos triple junction, at the tip of the Cocos-Nazca propagator. Most of the dives were devoted to the study of Hess Deep, a rift valley produced by rifting of oceanic crust created at the axis of the very fast-spreading East Pacific Rise. The dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra-rift ridge located to the north of the axis of Hess Deep. This shows that this situation is not specific not slow-spreading ridges. A nearly complete and unique log of young (1 Ma) crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered. We propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust. One model stresses symmetry and vertical tectonics through isostasy/diapirism of serpentinized mantle. Another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low-angle detachment faulting. In both models, which differ mainly in the structure at depth, amagmatic extension of East Pacific Rise crust by the Cocos-Nazca propagator tip results in uplift and denudation of lower crust and upper mantle.

PETROLOGY OF THE EAST PACIFIC RISE CRUST AND UPPER MANTLE EXPOSED IN HESS DEEP (EASTERN EQUATORIAL PACIFIC)

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.

Submersible observations at the axis of the ultra-fast-spreading East Pacific Rise (17°30′ to 21°30′S)

Abstract Manned submersible observations conducted with the diving saucer “Cyana” at the axis of the East Pacific Rise at four sites near 21°30′, 21°10′, 18°30′ and 17°30′S demonstrate that the ridge evolves between two end-members. The volcanic end-member is characterized by a non-rifted or only slightly rifted topographic ridge with widespread but diffuse water venting. The tectonic end-member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition. At one site, a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall. The dives illustrate the discontinuous nature of volcanic, tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean.

Dyke complex of the East Pacific Rise exposed in the walls of Hess Deep and the structure of the upper oceanic crust

Abstract Detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the East Pacific Rise during dives conducted with the submersible Nautile on the walls of Hess Deep. At four widely separated locations, the same sequence was found: an upper, thin (100–200 m) unit of extrusives (pillow lavas) underlain by a mixed zone of variable thickness (50–500 m, averaging 200 m) where extrusives are found and intrusives and massive subhorizontal layers which may represent sills. The mixed zone itself is underlain by a sheeted dyke unit, that may be up to 1200 m thick, where dykes are subvertical (i.e. untilted) and parallel to the East Pacific Rise fabric. This sequence may be a general feature of medium to fast-spreading crust because it is similar to that observed in Hole 504B of the Costa Rica Rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the East Pacific Rise. Dyke thickness is a good measure of the depth to the roof of the axial magma chamber. The observed sections provide much needed “ground-truth” for East Pacific Rise crust. The thickness of the lava sequence and the overall thickness of Layer 2 (at most 1400 m) appear to have been overestimated in previous studies of the oceanic crust.

Ultramafic and mafic rocks from the Garret Transform Fault near 13°30′S on the East Pacific Rise: Igneous petrology

Serpentinized peridotites, metagabbros and fresh basaltic rocks were dredged from the Garret Transform Fault near 13°S on the East Pacific Rise. Two dredge hauls taken on the northern wall (from about 3100–3600 m) consisted of aphyric and picritic basalts; while peridotites, gabbros and moderately phyric basalts were recovered in a single dredge located near the deepest part (4616–4820 m) of the transform valley. Well preserved igneous textures and mineralogical assemblages enable us to differentiate between tectonites (including harzburgites and clinopyroxene-bearing harzburgites) and cumulates (consisting of plagioclase-dunite, troctolites, olivine-gabbros and ilmenite-gabbros). The harzburgites are likely to represent deep-seated peridotites left after extraction of basaltic melt during upper mantle partial melting of a lherzolite. The ultramafic cumulates underwent some deformation and show textural and mineralogical evidences which suggest formation at a minimum depth corresponding to the very lower crust. It is also inferred that the composition of the coexisting liquid along with early cumulus crystals has a ratio Mg/(Mg + Fe2+) of 0.7, high CaO/Na2O ratios and a low Ni content (about 150 ppm) when compared to similar rock types from Atlantic fracture zones. Subsequent uplift during rifting of the oceanic lithosphere enhanced plastic deformation, subsolidus recrystallization and retrogressive metamorphism of the gabbros and the ultramafics.

Geochemistry of lavas from the Garrett Transform Fault: insights into mantle heterogeneity beneath the eastern Pacific

Young intra-transform lavas erupted as a result of extension within the Garrett Transform Fault on the southern East Pacific Rise, are more porphyritic, less evolved, have lower concentrations of incompatible trace elements, and lower ratios of more incompatible to less incompatible elements (e.g. low K/Ti and La/Sm) compared to lavas from the adjacent East Pacific Rise ridge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacific mid-ocean ridge basalts, but extend to lower Sr-87/Sr-86 (0.702137), Pb-206/Pb-204 (17.462), Pb-207/Pb-204 (15.331), Pb-208/Pb-204 (36.831), and higher Nd-143/Nd-144 (0.513345) than any lavas previously reported from the Pacific. Peridotites from the Garrett Transform have Nd isotope compositions within the range of the intra-transform lavas. The unusual major and trace element compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in the Pacific. The chemical and isotopic features of the Garrett lavas can be explained by remelting, beneath the transform, a two-component upper mantle which was depleted in incompatible element-enriched heterogeneities during melting beneath the East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotope composition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this component is heterogeneous, both on the scale of a single transform fault, and on the scale of an ocean basin

Inner floor of the Rift Valley: first submersible study

THE Rift Valley of the Mid-Atlantic Ridge1, within which lies a segment of the accreting plate boundary between Africa and North America, is well defined between 36° 40′N and 36° 55′ N. It is about 30 km wide and 1.5 km deep in that area (Fig. 1). This small portion of the rift, WSW of the Azores, was chosen as the primary target of the French–American Mid-Oceanic Undersea Survey (FAMOUS) programme, and many surface ship studies have been conducted already. We report here preliminary results of seven dives into the deepest part of the Rift Valley that were made by the bathyscaphe Archimède during the summer of 1973.

Intense hydrothermal activity at the axis of the east pacific rise near 13°N: Sumbersible witnesses the growth of sulfide chimney

A submersible study of a fast spreading (12 cm yr-1) Mid-Ocean ridge segment (East Pacific Rise near 12°50′ N) led to the discovery of intense hydrothermal activity. Twenty four sites with active vents and sixty inactive hydrothermal deposits were found within a narrow graben averaging about 300 m in width along a 20 km long segment of the ridge crest. The graben is floored with fresh basaltic sheet flows including collapsed pits or lava lake structures. From both deep towed camera stations and manned submersible observations, it is estimated that the average spacing between the ridge axis hydrothermal deposits averaging in size 10–50 m in diameter lies between 100 to 200 m.The hydrothermal deposits found in the central graben are believed to have formed rapidly (on the order of a few decades). Detailed investigations of one active site have enabled us to witness the growth of an active chimney which increased its height by 40 cm in 5 days i.e. 8 cm per day.Extensive hydrothermal deposits were discovered on an off-axis seamount located 6 km East of the ridge axis. The hydrothermal deposits found on both the ridge axis and on the seamount are similar in composition and consist essentially of zinc, copper and iron mineral phases.Chemical studies conducted on the venting fluids (320°C) showed that the concentration of the dissolved major metal ions (Fe, Mn, and Zn) is about 0.10–0.13 gl-1. If we accept an average flow rate of 10ls-1 based on visual observations the mass of metallic products spewing out from an active chimney must be about 100 kg per day.