East Pacific Ridge Research Articles

Stochastic melting of the marble cake mantle: evidence from local study of the East Pacific Rise at 12°50′N

Isotopes (Nd, Sr and Pb) and trace elements (REE, Ba, Sr, Rb) have been measured on a set of basaltic glasses from a restricted area (40 × 10 km) at 12°50′N on the East Pacific Rise. The huge variation of incompatible element concentrations (factor 70 for Ba concentrations), and the variable degrees of correlation between element concentrations cannot be explained by usual models of melting and fractional crystallization. A rough correlation between the Ce/Yb ratio and the isotopic ratios favors a “source effect” for the genesis of the glasses. We have developed a model including both partial melting process acting on a heterogeneous mantle source with two components (peridotites and pyroxenites; “marble cake mantle” of Allègre and Turcotte) and fractional crystallization. The purpose of this model is not to obtain values of the four parameters involved (degree of melting in the peridotites, in the pyroxenites, proportion of pyroxenites involved in the melting, degree of fractional crystallization) for each analyzed glass, but to model the whole set of glasses by stochastic genesis and sampling of liquids. We have used the stochastic procedure for the four controlled parameters, currently generating 10, 000 “samples”. Our preferred model for this portion of the East Pacific Ridge is obtained with a degree of melting in the peridotites and in the pyroxenites varying uniformly from 6 to 20%, and from 6 to 50% respectively. The degree of mixing between liquids issued from the two sources varies from 0 to 100%, and the degree of fractional crystallization remains small, without noticeable effect on the concentrations, varying from 0 to 6%.

Physiological constraints upon marine organisms

ABSTRACTWhen compared with sea water, most marine invertebrates are isosmotic but show varying degrees of ionic regulation, from accumulation of potassium and calcium to reductions of magnesium and sulphate. A reduction of sulphate in pelagic animals such as jellyfish and salps, and an accumulation of ammonium ions in arrow-worms (Sagitta) contribute to their near-neutral buoyancy. Marked reductions in salinity exclude mahy marine invertebrates. In the Baltic, polychaetes, bivalve molluscs and decapod crustaceans are reduced from 193, ninety-two and sixty-four species in the Belt Sea (salinity S 10-30‰) to three, four and two in the Gulf of Finland (salinity S 5-9‰). The chelicerateLimuluscan stand a wide range in salinity, a few insect larvae can tolerate concentrations of 200% sea water, while the branchiopod shrimpArtemiacan stand crystallising brine (36-37% NaCl). Very few species can tolerate temperatures of 40°C. One such species is the polychaeteAlvinella pompejana,a hydrothermal vent animal at East Pacific Ridge (to the W of S America). The harpacticid copepodTigriopusliving in evaporating rockpools has a lethal temperature of 42°C at a salinity of S 90‰, but at S 8‰ that temperature is 34°C. Lack of oxygen and presence of hydrogen sulphide limit the distribution of animals in certain areas. Most active animals have respiratory pigments in their principal body fluid. Burrowing invertebrates such asArenicolaandLingulahave respectively haemoglobin in the blood and haemerythrin in the coelomic fluid, with mean oxygen capacities of 6 ml O2per 100 ml in each case, compared to 0·6 and 0·5 ml in sea water at 10° and 20°C.

Coarse-grained volcanic detritus in deep-sea sediments of the northeastern Equatorial Pacific

Coarse-grained volcanic detritus in near-surface sediments of the northeastern equatorial Pacific consists mainly of feldspar (plagioclase with An 35–70Or 0–4) and magnetite, with minor parts of pyroxene (augite, Wo 25–32En 53-32) and volcanic quartz. Except for plagioclase (0.005–0.5% of total sediment) all concentrations range between 0 and 30 grains m −1 of piston-core, where differences exist within the stratigraphic sequence and also from mineral to mineral. The following relationships have been found: • —Plagioclase > magnetite >volcanic quartz >pyroxene. • — Contents are low in Plio-Quaternary sediments and in Lower Miocene pure radiolarian ooze. They increase within the Late Eocene to Early Oligocene sediments and near the Early Oligocene hiatus. The morphology, distribution and alteration of the minerals are described and singlegrain analyses for plagioclase and pyroxene are given. The origin of plagioclase, pyroxene and magnetite is related to the formation of new oceanic crust at the East Pacific Ridge. Transport and distribution by slumping, high-density currents and reworking of older sediments are assumed. The origin of volcanic quartz from potential source areas in northern Central America and Western Mexico and transport and accumulation in deepsea sediments are discussed.

Rayleigh-wave phase velocities in French Polynesia

b Rayleigh-wave phase velocities are investigated in the period range 17–100 s by the two-station method over several paths covering most of French Polynesia. Our results confirm the validity of theoretical models obtained through regionalization of data pertaining to longer paths. They also exhibit a 2–3.5 per cent anisotropy, with the axis of maximum velocity oriented in the direction of spreading of the plate. Part of this anisotropy is, however, due to the presence of the Tuamotu archipelago; when this is removed, the remaining anisotropy (about 1.5 per cent) correlates with the present direction of spreading, indicating that a relaxation of the anisotropy has taken place since the East Pacific ridge jump. Finally, the presence of the Tuamotu Islands explains anomalous waveshapes for surface waves travelling in their vicinity, due to multipathing through their faster structure.

East pacific ridge (2°S–19°S) versus nazca intraplate volcanism: Rare-earth evidence

Basalts from seamounts within the Nazca Plate representing intraplate volcanism, and the East Pacific Ridge between 19°S and 2°N have similar light rare earth depleted abundance patterns. Both intraplate and ridge basalts appear to have been derived from the low-velocity layer apparently depleted in large lithophile elements (DLVL). Nepheline-normative basalts and ferrobasalts occasionally occurring on the East Pacific Rise are shown to have also been derived from the same DLVL source. Furthermore, the rare-earth pattern similarity of nepheline-normative and tholeiitic basalts from the East Pacific Rise is best explained by distinct, pressure induced, conditions of partial melting of the DLVL source; whereas total rare-earth pattern enrichment and relative europium depletion of the ferrobasalts are consistent with shallow depth fractional crystallization during ascent.

Tectonic lineaments on the cocos plate

Mapping of the Siqueiros Fracture Zone shows that it extends at least 850 km east of the crest of the East Pacific Ridge with an overall azimuth of 082°. The bearing of its younger portion is 084°. The relatively constant trend of the fracture zone indicates a constant direction of motion at that plate boundary location for the past 20 my. However, comparison of the observed trend of the fracture zone with a synthetic trend generated from the Cocos-Pacific pole of rotation at 40°N, 110°W shows that the pole has remained in this location for only the past 5–7 my. Prior to that time, the pole must have been close to the Pacific-Farallon pole of rotation at 78°N, 136°W. One minor change in the bearing and character of the Siqueiros Fracture Zone may be interpreted in terms of plate reorganization in the central eastern Pacific, favoring a date of 10 mybp for this episode. A band of east-west oriented fracture zone topography in the Guatemala Basin is interpreted to be the Clipperton Fracture Zone. The change in plate geometry at 10 mybp eliminated this fracture zone offset of the ridge crest. The Tehuantepec Ridge is not the continuation of the Siqueiros Fracture Zone. Instead, the Tehuantepec Ridge may be a hinge fault produced by the Cocos plate underthrusting the North American plate on the northwest and the Caribbean plate on the southeast.

Geological interpretation of the gravity data for a profile across the Pacific Ocean

This paper presents a model of geological interpretation of gravity observations along the profile across the Pacific Ocean from the Japan trench across the Hawaiian ridge to the Middle America trench crossing eleven geological regions (the total length of the profile is about 14,000 km). The interpretation was made by comparing the gravitational attractions of the combined layer of the hydrosphere and the crust and upper mantle from sea level to the depth of isostatic compensation by using the fundamental principles of the theory of isostasy. The density of the crust and mantle beneath the M discontinuity was determined from the results of deep seismic sounding (DSS) and the correlation between compressional velocities and densities of rocks. A statistical analysis was made of the data obtained by many scientists to improve the correlation for different rocks at pressures of 0.001–10 kb. Density distribution curves are presented for different rocks and different velocity intervals. The lateral density inhomogeneities in the lower part of the combined layer from a depth of 50 km to the depth of isostatic compensation are supposed to cause regional gravity anomalies of the first order determined from satellite observations, whereas density inhomogeneities of the combined layer of the hydrosphere and the crust and upper mantle to a depth of 50 km account for residual gravity anomalies obtained from free-air anomalies (the normal gravity formula derived by Helmert (1901–1909)) after the regional first-order anomalies are subtracted from the satellite data. For the profile across the Pacific Ocean we obtained curves of regional gravity anomalies of the first order, residual gravity anomalies, ocean-floor topography, variations of the apparent density of the combined layer of the crust and mantle to a depth of 50 km below sea level, variations of the apparent density of the upper-mantle layer to the same depth, variations of heat flow, and a model of a density section to a depth of 50 km below sea level. The density of the crustal rocks varies from 2.7 to 3.0 g/cm3. The upper-mantle layer has its highest density (3.42 g/cm3) directly beneath the M discontinuity under the Hawaiian ridge (from DSS) and under the Japan trench (under the continental slope). Compact rocks lie beneath the northwest Pacific basin and the mid-Pacific mountains and also in fracture zones. The upper-mantle rocks of the lowest density (3.13 g/cm3) lie beneath the east Pacific ridge, the Hawaiian ridge (at a greater depth), and in the form of isolated horizontal lenses beneath the mountains of the Darwin uplift. Anomalously high values of the heat-flow intensity were found. A suggestion is made that the upper-mantle rocks with a density of 3.13 g/cm3 west of the Hawaiian ridge are genetically related to the rocks of about the same density lying at the base of the third crustal layer (density obtained from DSS data). This interesting region requires geophysical investigation and deep drilling to reach the rocks with a 3.13-g/cm3 density at a depth of about 3 km. These studies will throw light on the processes of transformation of rocks in the crust and upper mantle.

Two Small Crustal Plates in the South Pacific near Easter Island

THE evolution of the South Pacific during the Cainozoic involved several patterns of crustal plates in relative motion for various periods of time1. The present East Pacific Ridge, which now outlines the Pacific-Nasca Plate boundary, has been the only major spreading centre north of 36° S for the past 9 m.y. Before then, a fossil ridge, continuous with the still active Chile Ridge2,3 was a locus of spreading in what is now the middle of the Nasca Plate; between 9 and 20 m.y. ago, both the East Pacific and the Fossil Ridges were active, suggesting that the Antarctic Plate, which now occupies the area south of the Chile Fracture Zone between the Pacific-Antarctic and Chile Ridges, extended almost to the equator only 10 m.y. ago. The apparent ease with which the northern boundary of the Antarctic Plate has moved during the past 20 m.y. suggests the triple junction between the Antarctic, Pacific and Nasca Plates has been unstable.

Sea-Floor Spreading and the Cenozoic History of the East-Central Pacific

The east-central Pacific between 20° N. and 45° S. records several patterns of sea-floor spreading. Three distinct spreading centers are now active in the area, and total spreading rates across one of them, the East Pacific Ridge, are the fastest observed anywhere. The present East Pacific Ridge between 20° N. and 45° S. has maintained its present configuration only during the last 9 m.y. Prior to 10 m.y. ago, a north-northwest-trending ridge was active throughout the east-central Pacific, and the fossil crest of this ridge can be identified in the basin east of the present ridge axis between 10° S. and 30° S. and west of the ridge axis between 20° N. and the equator. Between the equator and 20° N., the north-northwest-trending ridge system has been continuously active since at least the Late Cretaceous, but 5 to 10 m.y. ago the position of the axis jumped to the east. South of the Chile Fracture Zone at 36° S., the north-northwest-trending Chile Ridge appears to be a still active segment of this old ridge system. Spreading on the north-northeast-trending East Pacific Ridge began more than 50 m.y. ago at 55° S. and grew northward so that opening at 35° S. began about 20 m.y. b.p. Between 10° N. and 10° S., spreading about the present axis was initiated only during the last 10 m.y. Analysis of magnetic anomalies associated with north-trending ridges near the equator is of limited value because of the extremely low amplitudes of the anomalies generated by the ridge. However, although identification of individual anomalies is tenuous at best, comparison of relative amplitudes and shapes with computed models offers limited support of the hypothesis of Francheteau and others (1970) that the Pacific Plate has migrated northward since the Late Cretaceous.

Oceanic basalts: Chemistry versus distance from oceanic ridges

The major elements chemistry, as expressed by the normative composition, and the degree of silica saturation of basalts erupted at various distances from the axis of the East Pacific Ridge do not differ systematically from the same parameters of basalts erupted at the axis of the Ridge. These results suggest that the conditions of magma generation are similar beneath Ridges and away from them.

Intraoceanic islands, east pacific ridge, island arcs: Volcanism and upper mantle

Two classes of volcanic rocks can be distinguished with respect to petrochemistry, namely oceanic and continental classes. Lavas of island arcs belong to the continental type. Lavas of oceanic ridges are close not to the oceanic, but to the continental type. On petrochemical and geophysical bases the author has drawn the following conclusions: 1. (1) the foci of volcano feeding lie beyond the limits of the earth's crust—in the upper mantle. 2. (2) the composition of the upper mantle under continents, oceans, island arcs and oceanic ridges is somewhat different. 3. (3) the role of assimilation on the passage of magma to the surface is very limited as a rule. According to these conclusions, volcanism may be considered as a certain indicator of the composition and state of the upper mantle matter. A scheme for the evolution of volcanism is suggested on this basis (as a reflection of the evolution of the upper mantle) which is in an agreement with geological and geophysical facts.

Potential temperature, oxygen and circulation of bottom water in the Southern Ocean

The circulation of bottom and near-bottom water can be determined in a qualitative manner by inspecting parameter distributions. Bottom potential temperature and oxygen (which show a definite relation in the tp-O2 diagram) are very useful for this purpose in the portion of the Southern Ocean dealt with in this paper (20°W westward to 170°W and south of 50°S). Observations used are deeper than 3000 m and within 300 m of the bottom.There are two areas of cold water (<0·0°C): the more extreme emanating from the Weddell Sea and the other from the Ross Sea. The Weddell Sea Bottom Water flows to the east of the South Sandwich Trench into the Argentine Basin, though some water does enter the trench and flows northward The bottom waters of the Scotia Sea come from between the 3000–3200 m level of the Weddell Sea through a passage at 39°W in the South Orkney Ridge. A tongue of this water flows into the southern Drake Passage, confining to the northern Drake Passage the warmer bottom water from the Pacific Ocean. This warmer bottom water forms a rapid easterly current which enters the South Atlantic west of Shag Rocks. The cold Scotia Sea bottom water does not penetrate the Pacific Ocean but is topographically or dynamically restrained. Circulation in the eastern Scotia Sea is weak to moderate.A cold-water tongue in the Bellingshausen Basin is bounded by a band of warm water on its northwest and northeast sides. It is probable that this loop of warm water represents a strong bottom current from the southwest Pacific Basin which overflows the East-Pacific Ridge in the vicinity of a fracture zone. The flow within the cold-water tongue is moderate in its southwestern sections and weak in the southeast.Bottom photographic current evidence is in agreement with the circulation patterns as found from the potential temperature and oxygen distributions. The circulation found by relating the parameter distribution to velocity field is only in relative terms. Direct current measurements are needed to determine volume transport.

Evidence from ocean islands suggesting movement in the Earth

Oceanic islands increase in age from the mid-ocean ridges towards continents and the andesite line reaching a maximum known age of Upper Jurassic. The Seychelles appear to be a continental fragment. Several pairs of lateral aseismic ridges extend from islands on the mid-ocean ridge to adjacent continents. Their continental junctions mark points on opposite coasts which would also fit if the continents were reassembled according to the criteria used by Wegener. As Holmes has shown each pair of ridges tends to have distinctive chemical characteristics. One possible explanation is that convection currents in the mantle rising along the mid-ocean ridges and sinking beneath trenches have carried the crust apart across the Atlantic, India and East Pacific Oceans. The lateral ridges may be approximately streamlines. Although Darwin showed that most volcanic islands sink, a few have been uplifted. Most of these lie a few hundred kilometres in front of deep trenches, suggesting that they may be on the crest of a standing wave in front of the trenches and that the crust is rigid. Of eleven straight chains of young islands in the Pacific ten get older away from the East Pacific Ridge. They could also be streamlines, fed by lava rising from deep within convection cells with stagnant cores. The regularity of ridges suggests non-turbulent flow.

Submarine Fracture Zones, Aseismic Ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge

Submarine Fracture Zones, Aseismic Ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge