Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence

Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO^*/FeO^* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.     

Petrogenesis of basalts from the FAMOUS area: Mid-Atlantic Ridge

Fresh basalt glasses most of which have Mg/(Mg + Fe²⁺) of 0.66–0.72 from outcrops within 3 km of one other in the rift valley at the Project FAMOUS locality have been analyzed for major, minor and trace elements in order to determine their petrogenesis.Transition metal abundances of the FAMOUS samples are similar to a wide variety of continental and oceanic basalts with high MgO and Ni, all of which show remarkably little variation, with the exception of Cu, Zn and Ti, on a chondrite-normalized plot. Modelling of these data suggests that the mantle beneath both continents and oceans is systematically fractionated relative to chondrites. This fractionation provides a constraint for models of earth formation and subsequent evolution.The abundances of the rare earth and the incompatible elements, Ba, La, Th, U, and Nb, vary by more than a factor of three and the La/Yb and La/Sm ratios vary by factors of 3.1 and 1.6, respectively, in samples with similar, high Mg/(Mg + Fe²⁺). There is no correlation between the degree of light-REE enrichment and the heavy-REE abundance. Furthermore, the trace element variations do not appear correlated with respect to location in the rift valley or to time of eruption. These trace element features demonstrate that successive eruptions in one small area of the rift valley can show wide variations in trace element chemistry over a short span of time; they preclude the derivation of these basalt glasses from a single magma chamber.Despite the heterogeneities in REE and the variable trace element abundances, a homogeneous mantle source is suggested by the similarities among the samples in the incompatible element ratios of La/Ce, Ba/Th, Zr/Nb and K/Ba and the small range in⁸⁷Sr/⁸⁶Sr isotope ratios observed in other samples from the FAMOUS region (White and Bryan, 1977). Thus, trace element heterogeneities appear to be generated by processes in the mantle during melting. However, processes such as batch partial melting, fractional fusion, fractional crystallization, zone refining, or mixing of magmas or sources acting alone are incapable of explaining the lack of correlation between the light and heavy REE.It is suggested that the observed variations are a consequence of dynamic partial melting of a homogeneous mantle source region. This process includes varying degrees of partial melting of an uprising mantle source with continuous but incomplete removal of melt as melting proceeds, varying extents of batch partial melting, and zone refining. Dynamic melting can produce different melts from a homogeneous source which have different degrees of light-REE enrichment and crossing REE patterns. The variable trace element abundances which may be produced through dynamic melting may be the cause of the apparent decoupling of major and trace elements (Bryan et al., 1976) which previously has been suggested for the FAMOUS region (Bryan and Moore, 1977).     

Oxygen isotope variations in marginal basin and ocean-ridge basalts

Oxygen isotope analyses have been made on 27 tholeiitic basalts from the Lau and Mariana marginal ocean basins and from mid-ocean ridges. The ¹⁸O values are related to the extent of hydration by submarine weathering as indicated by H₂O^? and total water content. Extrapolation to zero H₂O^? content gives a δ¹⁸O value of 5.5‰ on the SMOW scale for unaltered marginal basin basalts, in exact agreement with the oxygen isotope “signature” of ocean-ridge tholeiites. Three alkali basalts from seamount provinces also fit the tholeiite relationship. A Lau Basin gabbro has the tholeiitic ¹⁸O content, but an Indian Ocean gabbro is unusually light (δ¹⁸O = 4.0 for whole rock, plagioclase, and amphibole), and resembles the low -¹⁸O Iceland basalts. The basalt data confirm petrologic and chemical evidence for origin of marginal basins by extensional processes with production of basalts from depleted mantle material isotopically identical to the source of ocean-ridge tholeiites.     

Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge: a comparative study

We have determined the Pb and Sr isotopic compositions in a number of fresh young oceanic basalts from the East Pacific Rise (between 20°N and 21°S latitudes), and from the Mid-Atlantic Ridge (between 65°N and 10°N).The results confirm the PbSr isotopic correlation for mid-ocean ridges basalts obtained by Allègre et al. [1], Cohen et al. [2], Dupréand Allègre [3], and the correlation between isotopic variation and the compatible trace elements ratios variation [1].A comparison between the Atlantic and Pacific results reveals that there is a wider range of values for the Atlantic than for the Pacific. After filtering the short wavelengths, a good correlation is obtained between long-wavelength bathymetric and isotopic variations for the Atlantic.The preferred model proposed to explain these differences involves the constant presence of hot spots under ridges. On slow-spreading ridges like the Atlantic, the hot spots signature is clearly visible in both bathymetry and isotopic ratios. On fast-spreading centers, the hot spot signature in both the bathymetry and isotopic signature may be diluted by the rapid supply of material coming from the asthenosphere.However, an alternative explanation for which no hot spot influence is found on the East Pacific Rise cannot be definitely ruled out.In two occurrences, south of the Hayes fracture zone (Atlantic), large isotopic heterogeneities are observed within a single dredge. This does not contradict the concept of regional isotopic regularities, but suggests that blob injection and source mixing may be observed at very different scales under the ridges.     

Os isotope systematics in ocean island basalts

New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial ¹⁸⁶Os/¹⁸⁷Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial ¹⁸⁷Os/¹⁸⁶Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high ¹⁸⁶Os/¹⁸⁷Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.     

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, neon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion³He/⁴He ratios are 2.2—13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (³He/⁴He=7.5—8.2 Ra) and mid-ocean ridge basalt values (³He/⁴He=6—11 Ra) shows that the variation range of³He/⁴He ratios from sulfide-hosted fluid inclusions is significantly large. Values for²⁰Ne/²²Ne are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion⁴⁰Ar/³⁶Ar ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.     

Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge rift valley

In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 10⁸ cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.     

Lead isotope ratios in DSDP Leg 37 basalts

Analyses of Pb from mid-ocean ridge basalts obtained from DSDP Leg 37 lie on a line of near-zero-age slope. The data seem consistent with an evolutionary model of three stages, the beginning of the second stage being at about 600 m.y. — the model age obtained for the least radiogenic Pb. The beginning of the third stage of evolution at the time of formation of the rocks apparently did not change the average Th/U ratio since measured values are consistent with the ratio deduced from Pb isotope ratios. Ratios of U/Pb are not consistent with Pb isotope ratios, however, thus ruling out a simple two-stage evolutionary model.     

Correlated Nd,Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other.The low ¹⁴³Nd/¹⁴⁴Nd (0.51238), ²⁰⁶Pb/²⁰⁴Pb (17.54), ²⁰⁸Pb/²⁰⁴Pb (15.47), ²⁰⁸Pb/²⁰⁴Pb (38.14) and high⁸⁷Sr/⁸⁶Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher¹⁴³Nd/¹⁴⁴Nd (0.51270), ²⁰⁶Pb/²⁰⁴Pb (18.32), ²⁰⁷Pb/²⁰⁴Pb (15.52), ²⁰⁸Pb/²⁰⁴Pb (38.77) and lower ⁸⁷Sr/⁸⁶Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.     

Mesozoic limestone from the Vema offset zone,Mid-Atlantic Ridge?

A shallow-water, indurated limestone, consisting of fossil benthic foraminifera, calcareous green algae, fragments of gastropods, thalassinid crab coproliths, as well as inorganic ooids and pelloids, and containing detrital grains of quartz, orthoclase, and microcline, was dredged from the summit of a transverse ridge within the Vema offset zone of the Mid-Atlantic Ridge. Subaerial and submarine recrystallization, suggesting past emergence of the limestone, is indicated by oxygen and carbon isotope analysis. The occurrence in the limestone ofParafavreina, an anomuran crab coprolite fossil, suggests a probable Mesozoic age for the rock. The occurrence of this shallow-water Mesozoic limestone would confirm previous suggestions that a non-spreading crustal block exists at the Vema offset zone.     

Gypsum and halite from the Mid-Atlantic Ridge,DSDP Site 395

Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The δ³⁴S value of the gypsum (+19.4‰) indicates a seawater source for the sulfate. The δ¹⁸O values of the saponite (+19.9‰) and phillipsite (+18.1‰) indicate either formation from normal seawater at about 55°C or formation from¹⁸O-depleted seawater at a lower temperature.The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca²⁺ released from basalt and SO₄^2? in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.     

Anisotropy of magnetic susceptibility variations in Icelandic columnar basalts

The anisotropy of magnetic susceptibility (AMS) has been determined for 81 samples from eleven horizontal columnar segments from two Icelandic near-vertical dikes. Near-vertical orientation of short AMS axes and corresponding long-axis horizontal orientations in most columns represents alignments consistent with compactive effects due to vertically directed forces exerted by overlying columns. These data are inconsistent with primary orientations resulting from thermal contractive stresses, but such stresses may be responsible for the secondary orientations observed.     

Geodynamic processes that are involved in the generation of the Northern Mid-Atlantic Ridge

This paper contains a comparative analysis of the theoretical parameters involved in the subsidence of spreading ridges into the asthenosphere: Reykjanes, Kolbeinsey, the Azores segment of the Mid-Atlantic Ridge, as well as the following aseismic ridges: the Ninety East Ridge, Maldives, Hawaiian-Emperor, and Louisville ridges due to the influence of a mantle plume. We conclude that the respective geodynamic processes involved in generating spreading ridges in the North Atlantic and the aseismic ocean ridges due to hotspot action are similar. The main phases in the evolution of the Iceland region are substantiated using geological and geophysical data and computer simulation. We discuss the Cenozoic tectonic evolution of the region, calculated and plotted paleogeodynamic reconstructions of the North Atlantic Ocean in the hotspot system for 60, 50, and 20 Ma.     

The 3He distribution in deep water over the Mid-Atlantic Ridge

³He/⁴He ratios in dissolved helium at GEOSECS stations 115, 117, 30, and 120 provide an east-west section across the Mid-Atlantic Ridge at 30°N. Below 1500 m depth, the³He/⁴He profiles show little structure and have values within a few percent of the atmospheric ratio, indicating that the Mid-Atlantic Ridge is not a significant source of injected³He at this latitude. Mean³He/⁴He ratios calculated for the deep water at each station show that the³He/⁴He ratio in the western Atlantic at 30°N is 2–3% higher than in the eastern basin, probably due to mixing between a³He-rich boundary current in the western basin and low-³He deep water to the east.     

The Mid-Atlantic Ridge at 33°N: the Hayes Fracture Zone

A geophysical study was conducted over the Mid-Atlantic Ridge between 32–39°N and 30–40°W. A particularly deep fracture was observed which offset the ridge crest 110 km in the vicinity of 33°N. A pole of relative motion between the North American and African plates was deduced from this fracture zone as being at 63.1°N, 17°W.     

SrNdPb 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 SrNdPb 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 SrNd 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].     

Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers.The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn (r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg).These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.     

Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22–35°N

New trace element and Hf, Nd, and Pb isotope data are reported for 22 basalts collected between 22°N and 35°N on the Mid-Atlantic Ridge. (La / Sm)_N ratios identify the presence of enriched (E)-MORB in the northernmost part of this area and normal (N)-MORB elsewhere. A negative correlation is observed when ¹⁴³Nd / ¹⁴⁴Nd is plotted against ²⁰⁶Pb / ²⁰⁴Pb, ²⁰⁷Pb / ²⁰⁴Pb, and ²⁰⁸Pb / ²⁰⁴Pb, whereas ¹⁷⁶Hf / ¹⁷⁷Hf appears not to correlate with any of the other isotopic ratios. The E-MORB samples are characterized by high ²⁰⁶Pb / ²⁰⁴Pb, ²⁰⁷Pb / ²⁰⁴Pb, ²⁰⁸Pb / ²⁰⁴Pb, and low ¹⁴³Nd / ¹⁴⁴Nd. Principal Component Analysis (PCA) of Pb isotopes alone identifies three, and only three, significant geochemical end-members (‘components’). Including Nd and Hf isotopic data in the PCA produces spurious components, partly because of curved mixing relationships, and partly because of fractionation during melting. Our preferred interpretation of why ¹⁷⁶Hf / ¹⁷⁷Hf is decoupled from the other isotopic ratios is, as inferred from recent experimental data, that the Hf isotopic compositions of the melt and the residue fail to equilibrate during melting. A strong correlation between (Sr / Nd)_N and (Eu / Eu*)_N indicates that plagioclase is a residual phase of N-MORB, but not of E-MORB melting. The three end-members identified in this study are the depleted mantle, a common-type component, and an enriched plume-type end-member. The common, or ‘C’-type, end-member is characteristic of E-MORB and may itself be a mixture containing recycled oceanic crust (the MORB suite, terrigenous sediments, and/or oceanic plateaus). The plume-type end-member is likely to represent the lower mantle and may involve some primordial material. It is shown that mantle isochrons in general and the Pb–Pb isochron in particular do not characterize a specific geodynamic process acting to create mantle heterogeneities.     

Primordial helium isotope signature from Plio–Quaternary alkaline basalts in Yemen

Abstract Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south-western Arabian Peninsula. Very high ³He/⁴He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial ³He/⁴He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high ³He/⁴He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous-like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.