Axis Of Mid-Atlantic Ridge Research Articles

Geology of ascension Island, South Atlantic Ocean

Ascension Island is the exposed part of a large volcano located about 80 km west of the Mid-Atlantic Ridge axis in the notrhern South Atlantic Ocean. The volcanic rocks of the island form an alkaline suite with a compositional range of basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite. Trachyte and rhyolite compose approximately 14% of surface exposures. Petrochemical studies by other investigators have shown that the igneous suite was formed by fractional cyrstallization. The abundance of felsic volcanic rocks and the presence of granite and syenite blocks in pyroclastic deposits suggest that felsic magma chambers have formed and could provide the heat for a convective hydrothermal system. The felsic volcanic rocks form two eruptive centers that may be contemporaneous. The oldest rocks exposed on the surface are rhyolite with KAr ages of approximately one million years. Faults mapped on the island have four principal trends and appear to be related to regional structures that are apparent on the bathymetry of the island pedestal.

Mantle heterogeneities beneath the South Atlantic: a Nd [sbnd]Sr [sbnd]Pb isotope study along the Mid-Atlantic Ridge (3°S–46°S)

We report on 55 Nd and Sr isotope analyses of Mid-Atlantic Ridge (MAR) basalt glasses from 3°S to 46°S, using the same samples on which Pb and He isotope ratios were reported earlier (Hanan et al. [1] and Graham et al. [2]). Eighteen new Pb, Sr, and Nd isotope analyses are also reported on basalt glasses from 17 stations from the same region. 87Sr 86Sr ratios range from 0.70212 to 0.70410 and 143Nd 144Nd from 0.51285 to 0.51331. The along-ridge long wavelength 87Sr 86Sr variation delineated by light-REE depleted basalts increases progressively southward toward Tristan da Cunha. Short wavelength, spike-like, positive 87Sr 86Sr anomalies composed of light-REE enriched basalts are found opposite the Tristan, St. Helena, and Circe (Ascension) hotspots (as for the Pb isotopes). The short and long wavelength 143Nd 144Nd variations anti-correlate with those of 87Sr 86Sr . The 17 new Pb isotope analyses confirm both the short and long wavelength trends previously reported by Hanan et al. [1]. These spatial variations, as well as the variations in Nd Sr Pb isotopic space fully confirm the mantle plume-ridge interaction model and upper mantle mixing conditions beneath the South Atlantic inferred previously on the basis of Pb isotopic data only (Hanan et al. [1]). However, in Nd Sr isotopic subspace the Circe and the St. Helena mixing vector are not distinguishable. The short wavelength Nd Sr Pb He anomalies suggest recent lateral sublithospheric channeled flows from these off-ridge plumes to the migrating MAR axis. The long wavelength variations reflect a broad pollution of the asthenosphere by Pb and Sr radiogenic, isotope-rich, mantle material, which has been partly depleted of incompatible elements relatively recently. This broad pollution may be related to the partial melting and dispersion of the Tristan and St. Helena plume heads into the subcontinental asthenosphere, prior to the opening of the South Atlantic. In this two-stage model, the mixing relations in Nd Sr Pb space further require that the incompatible element depletion and dispersion of the St. Helena plume into the asthenosphere occurred before that of the Tristan plume head. We emphasize that this two-stage model is based purely on isotope systematics. The implied thermal and dynamic aspects of this model remain to be evaluated and tested.

Along‐axis variability in crustal accretion at the Mid‐Atlantic Ridge: Results from the OCEAN study

The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid‐Atlantic Ridge axis. We describe the results of a 150‐km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low‐amplitude basement highs at ∼15°; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment there is a zone at the base of the crust within which both high‐ and low‐velocity materials are present. This zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge.

Vema Fracture Zone (central Atlantic): Tectonic and magmatic evolution of the median ridge and the eastern ridge‐Transform intersection domain

The eastern Vema fracture zone intersection with the Mid‐Atlantic Ridge (MAR) axis was surveyed with the French submersible Nautile during the Vemanaute cruise in 1989. At this ridge‐transform intersection (RTI), an elongated, E‐W ridge, more than 50 km long, is present in the transform valley. This median ridge rises up to 1000 m above the surrounding seafloor. The crest of the median ridge is lower and presents an arcuate shape at the tip of the MAR axis. Dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation. This detrital cover consists of polymictic sedimentary breccias, sandstones and siltstones, composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle. Most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation. Disaggregation, mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls. This event could be related to the uplift of the southern wall of the fracture zone (the “transverse ridge”) which took place probably between 10 and 3 Ma ago. Since this uplift episode, the transverse ridge (which is now undergoing subsidence) and the detritus covered transform valley floor, separated by the transform fault zone, have migrated westward and eastward respectively. Vertical tectonics of the median ridge at the approach of the RTI can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors. A possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western RTI, is more than 1 km out of isostatic equilibrium. Recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the MAR axis have been recorded on the southern flank of the median ridge. Several stages in the very recent tectonic‐volcanic history of the eastern RTI, that is, roughly during the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shape of the median ridge east of 40°57′W are inferred to have resulted from tectonic extension during the creation of the nodal basin.

Upper mantle heterogeneity below the Mid‐Atlantic Ridge, 0°–15°N

Small‐scale variations in composition of mantle‐derived peridotites have been investigated in the 0°–15°N portion of the Mid‐Atlantic Ridge (MAR), thanks to a relatively close‐spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle‐equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter‐Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid‐ocean ridge basalt showing a “hot spot”‐type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero‐age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O‐rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.

Evidence for plagioclase-lherzolite intrusion in the Mid-Atlantic Ridge, DSDP Leg 37

Abstract From studies of ophiolites, it is thought that plagioclase-peridotites represent a non-negligible constituent of the oceanic lithosphere that reflects magmatism at oceanic ridges. To date, very few of them have been described from the abundant peridotites recovered from oceanic domains at present day oceanic ridges, except from transform faults. However, some plagioclase-lherzolites, interpreted as cumulate rocks, were drilled at Site 334 of DSDP Leg 37 on the Mid-Atlantic Ridge. These plagioclase-lherzolites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine crystals and subidiomorphic spinels. Petrofabric data for olivine point to a plastic deformation stage for these rocks as they display weak lattice fabrics which may have resulted from high-temperature plastic flow. Mineral compositions bear a strong resemblance to ophiolitic and oceanic rocks interpreted as residues from partial melting, except for clinopyroxenes which show a slight titanium enrichment. It is considered that these plagioclase-lherzolites represent residual harzburgites which have been pervasively impregnated by a melt from which clinopyroxene and plagioclase crystallized. Due to this impregnation episode, the primary structure and chemical composition of olivines were partly modified. It is suggested that these peridotites were locally so sufficiently lubricated by interstitial melt that they behaved as a crystal mush that had intruded the high-level crustal gabbros of the Mid-Atlantic Ridge. Their formation reflects pervasive magma percolation in the upper-most residual upper mantle at the Mid-Atlantic Ridge axis.

Bottom roughness of the Mid-Atlantic Ridge flank observed in SeaMARC II sidescan sonar and sea beam multibeam bathymetry.

Existing sea beam bathymetry along the Mid-Atlantic Ridge (MAR) axis, and SeaMARC II sidescan-sonar images along Kane Fracture Zone, have been analyzed to characterize patterns of bottom roughness that may be expected in the ONR acoustic reverberation corridor. Fault patterns along the MAR axis were determined in slope maps calculated from gridded beam-point data, and along Kane Fracture Zone they were mapped from sidescan-sonar images geometrically corrected for seafloor depth variations. Faults occur primarily in three orientations within well-defined provinces. MAR-parallel faults are restricted to the interior of MAR spreading segments, dip toward the spreading axis, and commonly have large throw (≳100’s m) and steep dips (up to 50°). Smaller, oblique to curvilinear faults occur at boundaries between spreading segments. MAR-orthogonal faults are restricted to fracture-valley crust generated in the large-offset Kane Transform Zone. Hummocky volcanic edifices and volcanic cones appear intermittently within spreading segments.

Sr [sbnd]Nd [sbnd]Pb geochemical morphology between 10° and 17°N on the Mid-Atlantic Ridge: A new MORB isotope signature

Basalts dredged along the Mid-Atlantic Ridge axis between 10°N and 17°N have been studied for their trace element characteristics [1]. To give complementary information on mantle source history and magma genesis, these samples have been analysed for their Sr Nd Pb isotopic compositions. There is a good correlation between the structure of the ridge axis which shows a topographic anomaly centered around 14°N and hygromagmaphile element ratios such as Rb/Sr, (Nb/Zr) N or Sm/Nd as well as isotopic ratios plotted as a function of latitude. The samples coming from the 14°N topographic high show new MORB Sr Nd isotopic characteristics which pictured in a classical mantle array diagram, put their representative points close to HIMU sources of ocean islands such as St. Helena, Tubuaïand Mangaia. The 14°N mantle source presents geochemical characteristics which indicate mantle differentiation processes and a mantle history that are more distinct than so far envisaged from typical MORB data. Pb data indicates that the 14°N mantle source cannot be the result of binary mixing between a depleted mantle and a HIMU-type source. Rather, the enriched endmember could itself be a mixture of Walvis-like and HIMU-like materials. The geochimical observations presented favour the model of an incipient ridge-centered plume, in agreement with [1].

Reflections of P'P' from upper mantle discontinuities beneath the Mid-Atlantic Ridge

SUMMARY Precursors to P'P’ observed in Japan from a large deep earthquake (mb= 6.4) in the Fiji Islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the Mid-Atlantic Ridge (MAR) in the equatorial Atlantic Ocean. Traveltime analysis indicates that the precursors are interpreted as underside reflections of P'P’ from discontinuities at depths of 655 and 415 km in the mantle. The latter reflection is much weaker and more intermittent than the former. Slant-stacking and beam-forming are attempted to measure the slowness and incidence azimuth of P'P’ and its precursors. The obtained slownesses are 3.4 and 2.8 s deg-1 for P'P’ and P'P', respectively. Little deviations of the ray incidence off the diametral azimuth are found for both phases. Amplitude ratio P'dP'/P'P’ (here d= 655 or 415) is studied as a function of the wave period and the distance of reflection point of P'P’ to the axis of MAR. The overall trend of the spectral amplitude ratio for P'655P’ is consistent with the theoretical value for the 670 km discontinuity of the PREM model (Dziewonski & Anderson 1981). P'415P’ signals were very weak and it was difficult to obtain an unbiased estimate of P'415P'/P'P'. The amplitude ratios P'655P'/P'P’ of filtered seismograms with a passband of 0.5-4.0 s have a mean value of 0.13 and a standard deviation of 0.03. This amplitude ratio is in good agreement with those of previous studies on the reflections from the 650-km discontinuity beneath Antarctica and surrounding seas. When the reflection points of P'P’ are calculated under the assumption of a spherically symmetric earth, the observed P'655P'/P'P’ seems to decrease as the points become close to the axis of MAR. Thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center. Another interpretation may also be possible. The low velocity beneath the MAR region would cause focussing of P'P’ rays.

Tectonic pattern of the Azores spreading centre and triple junction

The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance. The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N. Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate. The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.

An experimental investigation of Iceland and Reykjanes Ridge tholeiites: I. Phase relations

Tholeiite basalts from 60° N to 65° N on the Mid-Atlantic Ridge were melted and recrystallized at atmospheric pressure in a CO2-H2 gas mixture. Seven basalts are from the Langjokull-Thingvellir volcanic zone and the Reykjanes Peninsula of Iceland and nine are from the Reykjanes Ridge. The crystallization sequence in both Iceland and Reykjanes Ridge basalts with (Total Fe as FeO)/(Total Fe as FeO+ MgO) [F/F + M] less than 0.6 is olivine, plagioclase, clinopyroxene. Chromian spinel crystallizes before plagioclase in one Iceland and one Reykjanes Ridge basalt with F/F+M less than 0.57. Chemical differences of the two groups of basalts (lower SiO2 and higher alkalis in Iceland basalts) can not simply be a result of low pressure fractional crystallization. Liquidus temperatures of the seven Iceland basalts decreases from 1,230° C to 1,170° C as the F/F+M of the rock increases from 0.52 to 0.70. The liquidus temperatures of the Reykjanes Ridge basalts are about 10° C lower than those of the Iceland basalts for the same F/F+M value. The profile of measured liquidus temperatures from 65° N on Iceland to 60° N on the Reykjanes Ridge has a minimum value at 63.2° N on the Reykjanes Ridge just south of Iceland. Model calculations of the pressure of phenocryst crystallization indicate that olivine and plagioclase in Langjokull basalts could have equilibrated between 2.0 and 6.2 kb (200 to 620 MPa). Phenocryst assemblages in Reykjanes Ridge basalts at 60° N could have crystallized together at greater than 2 kb (200 MPa) and probably less than 8 kb (800 MPa). A minimum in the equilibrium pressure of phenocryst crystallization occurs between 62.9° and 64° N and coincides with the minimum in the experimentally determined liquidus temperatures. The more extensive fractionation at low pressure in this area could be related to the shift of the Mid-Atlantic Ridge axis along the leaky transform fault from the Reykjanes Ridge to the Thingvellir volcanic zone.

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing.

Atlantic ocean floor: Geochemistry and petrology of basalts from legs 2 and 3 of the Deep-Sea Drilling Project

Basalts cored on legs 2 and 3 of the Deep-Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr. Although many of the basalts are highly altered, fresh glass is usually present. Except for site 2–10 the fresh glasses are petrographically and geochemically similar to mid-Atlantic ridge (MAR) axial basalts. There are no systematic compositional differences as a function of distance from the MAR axis. Two sites contain basalts with olivine (Fo90) phenocrysts, high Mg/Mg + ΣFe, high Ni and Cr abundances, and very low large ion lithophile (LIL) element abundances. These basalts are the best candidates for primary magma recovered from the sea floor; fractional crystallization of such basalt may yield the more evolved basalts typical of the MAR. More fractionated basalts with clinopyroxene phenocrysts occur at twp other sites, but they retain low LIL element abundances. Site 2-10 contains titaniferous augite and is relatively enriched in LIL elements. It is unlikely that this basalt was derived by fractional crystallization from LIL element depleted tholeiites; instead, the site 2-10 basalt requires a different mantle source. These results imply that the upper Atlantic Ocean basement is dominantly LIL element depleted tholeiite.

FAMOUS: A Plate Tectonics Study of the Genesis of the Lithosphere

In a joint French-American program, the axis of the Mid-Atlantic Ridge is undergoing an intensive study. Broad regional mapping has been followed with surveys on a detailed scale. Twenty cruises have been made in a 100-km section of the Rift Valley, located about 400 km south of the Azores. In mid-1974, three submersibles will undertake a coordinated program of mapping, sampling, and observing the Rift Valley floor.

Mid-Atlantic Ridge: Age and Spreading Rates

Fission-track dating of basaltic glass from the Mid-Atlantic Ridge gives results which are consistent with the proposal of ocean-floor spreading. Solidification ages from ~10,000 years to ~300,000 years were measured. Correlation is also possible between the magnetic anomaly patterns over the Crest Mountains at 45 degrees N and the geochronology of the outcropping basalts. Renewed volcanic activity well removed from the axis of the Mid-Atlantic Ridge has been demonstrated to have taken place in recent times.

Comments on mantle convection and mid-ocean ridges

In a recent article Ewing, LePichon, and Ewing cite the proximity of a number of pre-Pleistocene cores to the axis of the mid-Atlantic ridge as evidence that continental rifting ceased 20 m.y.b.p. at the latest and probably did not occur at all. It is here suggested that these data do not necessarily preclude crustal spreading at a constant rate. Further, a schematic model of mantle convection is suggested which fits the sediment core data, provides for nonuniform rates of crustal spreading, while allowing steady-state mantle convection, and qualitatively explains many features observed on mid-ocean ridges.