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Transition metals (Fe, Co, Ni, Cu, Zn, Ti, Cr, and Mn) forming sulfides and oxides are found in a large variety of lava flows erupted on the sea floor of the South Pacific at the Pitcairn Hotspot and the Pacific Antarctic Ridge. These volcanic rocks consist of an enriched alkaline series (basanite - alkali basalt - trachybasalt - trachyandesite) and a depleted tholeiitic series (MORB - basaltic andesite - andesite - dacite). The distribution of the sulfides and oxides found in globules and as disseminated (<1-5%) crystals in the rocks is controlled by the degree of fractional crystallization. The formation of these constituents reflects the decrease of the bulk transition metal content in the rock. Sulfide globules of pentlandite, pyrrhotite and cubanite are prominent in the intermediate and evolved rocks. Ti-bearing chromite and titanomagnetite are mainly concentrated in the least evolved (picritic alkali basalt, basanite, alkali basalt) and in the silica-rich (trachyte, andesite, dacite) lavas, respectively. The paragenesis of sulfide globules and oxides also correlates with the magma evolution during crystal-liquid fractionation.

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.

The St. Paul F.Z. is a large structural domain made up of multiple transform faults interrupted by several Intra-Transform Ridge (ITR) spreading segments. Two regions were studied in details by submersible: (1) The ITR short (<20 km in length) segment near 0° 37′N–25° 27′W and 1° N–27° 42′W and (2) The St. Peter and St. Paul's Rocks (SPPR) massif located at 29° 25′W (¡3700 m depth). (1) The short ITR segments consist of a magma starved rift valley with recent volcanic activities at 4700 m depth. A geological profile made along the rift valley wall showed localized volcanics (basalts and dykes) which are believed to overlay and intrude the ultramafics. The geological setting and the high ultramafic/volcanic ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Paul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active transform zone. The SPPR is divided into two different geological domains called the North and the South Ridges. The North Ridge consists of strongly tectonized fault scarps composed of banded and mylonitized peridotite, sporadic gabbros (3900–2500 m) and metabasalts (2700–1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000–1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Ridge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MORBs collected from the eastern part of the St. Paul F.Z. (25° 27′ W IRT) are similar to the volcanics from the southern segments of the MAR. The peridotites exposed in these provinces (SPPR and ITR) are similar in their REE and trace element distribution. Different degrees (3–15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bearing lherzolite veined with 5–40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.

Submersible observations and sampling were carried out in the rift valley of the Mid-Atlantic Ridge (MAR) near 34°40′N–35°N. The 4-km-wide rift valley consists of a Neo Volcanic Zone (NVZ) (<1km wide) bounded at the west by a Median Ridge (MR) (5km wide and 20km long) and at the east by the first scarps of the eastern wall. The MR and the eastern wall are characterized by volcanic cones about 200–300m height culminating at depths of 1500–1900m which are made up of volcaniclastic deposits (pyroclasts and hyaloclasts) suggestive of explosive volcanism. Based on their surface morphology, degree of vesicularity, and composition, the erupted deposits are classified into four groups: (1) poorly vesicular (<15% vesicles) N- and T-MORBs (K/Ti <0.25, Na2O+K2O<2.9%) consisting of sheet flows and pillows formed during fissure eruptions in the NVZ at 2000–2300m depths; (2) vesicular (15–30% vesicles) E-MORBs (K/Ti=0.25−0.45,Na2O+K2O>2.8−3.2%)and alkali basalts (K/Ti=0.45−0.70,Na2O+K2O>3.3−4)made up mainly of pillows; (3) highly vesicular (>35% vesicles) pillow lava and pyroclastic (scoria-like) alkali basalts (K/Ti>0.45−0.80,Na2O+K2O>3−4%); and (4) hyaloclastites consisting of glassy shards of alkali basalt composition. The total water and carbon contents of the deposits increase with the incompatible element concentrations. The estimated initial H2O content for the N- and T-MORBs is less than 3500ppm, whereas for the E-MORBs and alkali basalts the H2O content is near 4000 and 7000ppm, respectively. While the H2O is mainly in the melt, the carbon is in the form of CO2filling vesicles. The vesicles are formed from magma with an initial carbon content of 1000–3000 for the N- and T-MORBs, 3000–6500ppm for the E-MORBs and higher than 1wt% for the alkali basalts.

Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters.

Summary The Pitcairn hotspot, located about 60 km east of Pitcairn Island (South Pacific), consists of several active volcanoes < 500 m below sea level. The volcanic rocks from these seamounts are classified in four main rock-types: (1) picritic basalt containing Ti-bearing chromite (8–10 wt.% TiO2); (2) alkali basalt (Ti-bearing chromite with 4–6 wt.% TiO2); (3) trachyandesite containing titanomagnetite (18–22 wt.% TiO2); and sulfides, and (4) trachyte (titanomagnetite with 19–23 wt.% TiO2); The metallic oxides are zoned with decreasing Tîl02 contents from core to rim. Crystal fractionation (> 60%) is the main process responsible for differentiating these rock-types from an enriched source.

Abstract: Still photographs and video images collected along the Neovolcanic Zone of the East Pacific Rise from 1015′N to 1153′N show that recent volcanic sheet flows, possibly less than 100 years old, are superimposed on an older sediment-laden pillow terrane. This recent activity is restricted to a narrow zone that crosses two topographic highs at 1055′N and 1126′N and diminishes along-axis away from these highs. The association of recent sheet flows with older flows and collapse structures on the overlapping spreading centers at 1145′N supports the evolutionary model for the occurrence and evolution of overlapping spreading centers by MacDonald and others (1986, 1988).

Abstract: Magmatic gases extracted and analysed from basaltic rocks collected in the FAMOUS area near 3650′ N in the Atlantic ocean show that the total amount of gas included in the samples varies between about 500 ppm to 1600 ppm. The main gaseous phases included in the various types of basalts consist of CO2 (270–700 ppm), CO (150–800 ppm), HCl (100–1000 ppm), H2 (0–50 ppm), SO2 (up to 175 ppm), N2 (up to about 213 ppm) and traces of hydrocarbons (up to about 24 ppm). The relative amount of CO, CO2 and SO2 varies with both the degree of crystallinity of the rock and with fractional crystallization and/or fractional melting. The glassy margin of pillow lavas have a higher CO/CO2 ratio than the more crystalline interior. The most fractionated rocks of the series rich in clinopyroxene are depleted in the CO/CO2 ratio and have a higher SO2 content than do the most mafic end members rich in olivine. Early-formed olivine was crystallized in a reducing environment rich in CO and H2 with respect to later formed mineral associations. It is likely that the carbon and sulfur oxidation is taking place at a relatively shallow depth during magmatic ascent or during volcanism. The ocean floor volcanics when compared to subaerial basalts are depleted in SO2 and have on the average ten times more H2.

The Offset Spreading Center located between 12°52' and 12°54' N on the East Pacific Rise (Macdonald and Fox, 1983) has been studied in 1982 and 1984 with submersible Cyana and in 1983 with the deep towed vehicle Seamarc I. The two O.S.C. segments, about 1.5 km apart and 4 km in length, separated by a ‘depression’ (about 100 m in depth) show different volcano-tectonic settings. The Western Spreading Center (WSC) segment is characterised mainly by recent volcanic constructional features, while the Eastern Spreading Center (ESC) is highly fissured and consists essentially of older pillow-lava terrain. The intervening ‘depression’ located between the two segments is floored by small constructional mounds (<10 m in height) of pillow lava. The crust of both segments becomes older along strike towards their respective tips. However, the W.S.C. comprises generally younger flows than does the E.S.C. A small central volcano (80 m in height and 1 km in diameter) located near 12°51' N near the Southern tip of the W.S.C. contains a different type of volcanics than that found on both spreading centers. The volcanics collected along the O.S.C. ridges are depleted tholeiites, with low K2O (<0.15%), Na2O (<3%) and TiO2 (<1.76%) contents, comparable to other MORB from the axial graben of the E.P.R. south of the area of overlap. Instead the specimen from the small volcano is enriched in K2O (>0.2%), Na2O (>3%) and TiO2 (2%).

The Teahitia-Mehetia hot spot region located in the southeastern extension of the Society Islands chain, near 18° S–148° W consists of several active volcanoes. The distribution of recent volcanic activity correlates with seismic epicenters, and covers an area of more than 1000 km2. Intermittent volcanic activity has given rise to large (>1000 m high) and small (<500 m high) edifices composed of various types of flows. Several recent volcanic events have produced a suite of alkalic rocks ranging from ankaramites, through alkali basalts to trachy-phonolites. The presence of altered MORB-like tholeiites on one small seamount suggests that a different mantle source material was involved in forming some of the crust in this hot spot region.

An investigation of glassy volcanics erupted within the last ten-million years along various segments of the mid-Atlantic Ridge and the East Pacific Rise has revealed major crustal compositional changes. The available data from the mid-Atlantic Ridge shows the existence of two petrological provinces: One, located between latitudes 33° and 53° N, is characterized by volcanics which have a tendency to be oversaturated ocean ridge basalts (OSORB) with respect to normative quartz; the second group of rocks, found between 25° S and 33° N, is generally composed of saturated ocean ridge basalts (SORB). In addition, the SORB volcanics have higher TiO2 (1.7±0.3%), higher Na2O (2.8±0.2%) and higher FeO*/MgO (1.36±0.2) values than do the OSORB types (with 1.1±0.2%, 2.2±0.2% and 1.22±0.2 for the TiO2, Na2O, and FeO*/MgO respectively), There is a correlation between the rate of crustal spreading and the compositional changes observed on the volcanics erupted along various segments of oceanic ridges. Slow-accreting plate boundaries having a total spreading rate of 2–3 cm/year are characterized by a low TiO2 content (1.1±0.2%), low FeO*/ MgO ratio (1.22±0.2) and a high an/an+ab ratio (0.62±0.05). Segments of fast-spreading ridges (total rate 11–13 cm/year) show a higher range of TiO2 (2.1±0.4%) and FeO*/MgO (1.6±0.4) and a lower range of the an/an + ab ratio (0.5±0.07). Ridge segments with a total spreading rate of 5–9 cm/year con sist of volcanics having intermediate values for the above parameters. Different degrees of partial melting of rising mantle material are suggested as a possible mechanism for explaining the compositional diversities encountered along oceanic ridge systems.

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.

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.

DURING the course of the French–American Mid-Oceanic Undersea Study (FAMOUS), many samples of basaltic pillow lava were collected from the axial region of the Mid-Atlantic Ridge near 37°N. The vertical axes and the tops of nine of these samples have been determined fairly accurately, allowing calculations of magnetic inclination at the time of the cooling of the lavas to be made with reasonable confidence.

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.

DURING the 1972 Midlante cruise of the RV Jean Charcot to survey a limited area of the rift valley south of the Azores, a dredge (CH-DR11) was taken at a depth of 1,360 fathoms in a depression near the foot of the eastern wall at latitude 36° 49.3′ N and longitude 33° 15′ W. Two types of rocks were recognized from their structure and their degree of weathering. One type is “slab-like” rock, 2 to 13 cm in diameter and 1 to 2 cm thick (Fig. 1a and b). The other type of rock consists of oval glassy pebbles (1 to 5 cm in diameter, Fig. 1c) which were observed to jump and explode, one or two at a time, for 3 d after the dredge haul was emptied on to the deck of the ship.