Composition and processes of the mantle lithosphere in northeastern Brazil and Fernando de Noronha: evidence from mantle xenoliths

Spinel–peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State, Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cabuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 ± 116 and 1248 ± 19 °C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer). Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er, Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes. HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 ± 80 °C) and the pressure is 1–1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 × PM and are smoothly fractionated from LREE to HREE, with deep, negative, Zr and Ti anomalies. The geochemical heterogeneities of the xenoliths from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower part of a mantle column (the head of a mantle diapir, at the transition conductive–adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column, chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (¹⁴³Nd/¹⁴⁴Nd= 0.51339–0.51255, ⁸⁷Sr/⁸⁶Sr=0.70275–0.70319) and of Fernando de Noronha (¹⁴³Nd/¹⁴⁴Nd=0.51323–0.51285, ⁸⁷Sr/⁸⁶Sr=0.70323–0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd–low Sr (EMI) and low Nd–high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components enriched a lithosphere isotopically similar to DMM (depleted MORB mantle) at both localities. At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the ∼ 8 Ma old lavas of the Remedios Formation, suggesting that this is the age of metasomatism. At Pico Cabuji, the mantle xenoliths do not record the high ⁸⁷Sr/⁸⁶Sr component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume. Received: 12 June 1999 / Accepted: 13 December 1999     

Carbonate metasomatism of the oceanic mantle beneath Fernando de Noronha Island, Brazil

Evidence is presented for a carbonate-immiscibility phenomena in the upper mantle based on data from a set of strongly metasomatized xenoliths in a basanitic lava flow from Fernando de Noronha Island (southwest Atlantic). A petrological and geochemical study of lherzolitic and harzburgitic xenoliths reveals that the oceanic mantle of this region has been affected by very strong carbonate metasomatism. The metasomatism led to wehrlitization of the primary mantle mineral assemblage (ol, opx, sp). The wehrlitization was the result of interaction between a possibly ephemeral sodic dolomitic melt or fluid with the mantle peridotite according to the following reactions, which include sodic components: 4MgSiO₃+CaMg(CO₃)₂=2Mg₂SiO₄+CaMgSi₂O₆+2CO2 3CaMg(CO₃)₂+CaMgSi₂O₆=4CaCO₃+2Mg₂SiO₄+2CO₂ The olivine has abundant micro-inclusions consisting of Na-Al-Si-rich glass, Fe, Ni and Cu-monosulfide, Ca-rich carbonate and dense CO₂. The interrelationships between the glass, sulfide and carbonate inclusions permit speculation that silicate, sulfide, and Ca-rich carbonatite melts were in equilibrium with each other and originated from partial melting of metasomatized and wehrlitized peridotite underneath Fernando de Noronha Island. These results support a two-stage model of Ca-rich carbonatite formation: first stage - metasomatic wehrlitization and carbonatization of mantle rocks; second stage - partial melting of the carbonate-bearing wehrlitic rock resulting in the formation of immiscible silicate, sodic carbonate and sulfide liquids and the ultimately generation of calciocarbonatites.     

The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.     

新疆西天山巴音沟与蛇绿岩伴生的洋岛玄武岩——主微量元素证据

对巴音沟蛇绿混杂岩内发育的两套玄武岩进行了主、微量元素研究,结果表明,其中一套TiO2的含量较低,在0.81%~1.00%之间,REE含量低,LREE相对于HREE具轻微亏损,稀土元素配分图和微量元素的原始地幔标准化图上呈平坦分布型式,Nb、Ta略亏损,为大洋中脊玄武岩。结合其他微量元素特征,初步认为该玄武岩源于大洋形成的初期,源区是还没有经历大规模岩浆提取阶段的洋中脊玄武岩。另一套玄武岩则具有典型的洋岛玄武岩特征,具有较高的TiO2含量(1.89%~3.14%),富碱质,同时富集LREE和HFSE元素,高Nb、Ta含量,在微量元素原始地幔标准化图上显示明显的Nb、Ta正异常,微量地球化学元素具有EMⅡ型OIB特征。     

Pericontinental petroliferous basins of the South Atlantic Ocean

Sedimentary basins located at Atlantic margins of South America and Africa (Brazil, Congo, and Angola) are composed of thick Cretaceous-Pleistocene rocks (7–9 km). The principal oil-bearing complexes of both regions are dominated by terrigenous rocks. Their accumulation took place at the rifting stage, ended with precipitation of evaporites in the Aptian at the oceanic stage of evolution. Starting with Coniacian and Campanian, the accumulation of terrigenous clastic formations in the Santos and Campos basins was governed by main phases of orogenesis in the Peruvian Andes. In the Angola and Lower Congo basins, the sedimentation was governed by movements related to the development of the East African Rift System. The formation of carbonate and carbonate-clayey sediments prevailed here at early (Albian-Late Eocene) stages of the South Atlantic opening.     

Length scales of mantle heterogeneities and their relationship to ocean island basalt geochemistry

The upper mantle is widely considered to be heterogeneous, possibly comprising a “marble-cake” mixture of heterogeneous domains in a relatively well-mixed matrix. The extent to which such domains are capable of producing and expelling melts with characteristic geochemical signatures upon partial melting, rather than equilibrating diffusively with surrounding peridotite, is a critical question for the origin of ocean island basalts (OIB) and mantle heterogeneity, but is poorly constrained. Central to this problem is the characteristic length scale of heterogeneous domains. If radiogenic osmium signatures in OIB are derived from discrete domains, then sub-linear correlations between Os isotopes and other geochemical indices, suggesting melt-melt mixing, may be used to constrain the length scales of these domains. These constraints arise because partial melts of geochemically distinct domains must segregate from their sources without significant equilibration with surrounding peridotite. Segregation of partial melts from such domains in upwelling mantle is promoted by compaction of the domain mineral matrix, and must occur faster than diffusive equilibration between the domain and its surroundings. Our calculations show that the diffusive equilibration time depends on the ratios of partition and diffusion coefficients of the partial melt and surrounding peridotite. Comparison of time scales between diffusion and melt segregation shows that segregation is more rapid than diffusive equilibration for Os, Sr, Pb, and Nd isotopes if the body widths are greater than tens of centimeter to several meters, depending on the aspect ratio of the bodies, on the melt fraction at which melt becomes interconnected in the bodies, and on the diffusivity in the solid. However, because Fe-Mg exchange occurs significantly more rapidly than equilibration of these isotopes under solid-state and partially molten conditions, it is possible that some domains can produce melts with Fe/Mg ratios reflecting that of the surrounding mantle but retaining isotopic signatures of heterogeneous domains. Although more refined estimates on the rates of, and controls on, Os mobility are needed, our preliminary analysis shows that heterogeneous domains large enough to remain compositionally distinct in the mantle (as solids) for ∼10⁹ yr in a marble-cake mantle, can produce and expel partial melts faster than they equilibrate with surrounding peridotite.     

Predicting bathymetry from Geosat-ERM and shipborne profiles in the South Atlantic Ocean

Portions of two Geosat-ERM altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the South Atlantic Ocean were analyzed: one across the Walvis Ridge (about 1100 km long) and the other in the Brazil Basin (about 2300 km long). Together, these profiles sample those types of sea-floor topography which dominate the gravity signature at wavelengths of 20 to 300 km. The Walvis Ridge is a massive aseismic ridge and the Brazil Basin profile crosses both an old seamount (emplaced at the time the crust was young) and a very young mid-plate volcano. Both profiles cross fracture zones. After the gravity and bathymetry profiles were split into subprofiles, various cross-spectral characteristics could be determined by FFT techniques. Analysis showed that observed admittance is not well constrained by either an Airy-type or flexural compensation models across the Walvis Ridge, but those over the Brazil Basin can be readily explained by an Airy-type model with a mean crustal thickness of about 20 km.

A theoretical filter was then designed, based on a priori geological knowledge, and used to predict bathymetry from the high-passed gravity/geoid anomalies.

Not surprisingly, the predicted bathymetry shows more detailed and correct short-wavelength (20–300 km) features than those predicted from the historical data base, as represented e.g., by the DBDB5 gridded bathymetric model. For areas where historical shipboard bathymetry measurements are widely spaced (longer than about 10 km for single-beam data) but where some regional geologic information is available (such as the relative ages of mid-plate volcanoes and crust), bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases, on which regional geologic/geophysical understanding depends.     

Geochemistry and petrology of dredged basalts from the Bouvet triple junction,South Atlantic

Basalts dredged from the Bouvet triple junction (South Atlantic), from the Mid-Atlantic Ridge near the triple junction, and from a spreading center east of Bouvet Island differ from normal mid-ocean ridge tholeiites by having higher concentrations of K and other large-ion-lithophile elements, higher ${}^{87}\text{Sr}\text{:}{}^{86}\text{Sr}$ ratios, and rare earth element distributions which show relative enrichment in the lighter rare earths. The basalts are more fractionated than typical oceanic tholeiites, however, fractional crystallization does not fully account for their chemical characteristics, and it appears that they were derived from special source materials, contaminated perhaps by a mantle plume rising beneath Bouvet Island.     

Geochemistry and genesis of Fe-Mn mineralization in island arcs in the west Pacific Ocean

This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kurile, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxi-hydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxi-hydroxides are todorokite and “Ca-birnessite.” The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxi-hydroxides.     

A new starting point for the South and Equatorial Atlantic Ocean

The opening of the Equatorial and South Atlantic Oceans is still a matter of debate, particularly as concerns the locations of the intraplate deformation. We propose here a critical review of the kinematic models published since Bullard et al., 1965, based on a series of constraints: new interpretation of the magnetic anomalies, seafloor isochrons, flow lines, fracture zones, continental and oceanic homologous structures and radiometric dating of igneous rocks. All of these models present numerous unexplained misfits (gaps, overlaps and misalignments). We present here a new evolution of the Equatorial and South Atlantic Ocean from the tightest reconstruction to Chron C34. This new model confirms the hypothesis of a northward propagation of the South American deformation proposed by Eagles, but rejuvenates slightly the ages for this propagation and refines the plate reconstructions. In particular, we highlight the role of the kinematic «buffer» Santos block, located between the salty Aptian Central segment in the North and the Volcanic Hauterivian Austral segment in the South. The new initial fit presented in this study represents the tightest reconstruction that could be obtained and constitutes the base canvas on which the problem of the continental margin genesis should be addressed.     

Petrogenesis of the Back-arc East Scotia Ridge, South Atlantic Ocean

The East Scotia Ridge is an active back-arc spreading centrelocated to the west of the South Sandwich island arc in theSouth Atlantic Ocean, consisting of nine main segments, E1 (north)to E9 (south). Major and trace element and Sr–Nd–Pbisotope compositions are presented, together with water contents,for lavas sampled along the active ridge axis. Magmatism alongthe East Scotia Ridge is chemically heterogeneous, but thereis a common mid-ocean ridge basalt (MORB)-type source componentfor all the magmas. An almost unmodified MORB-source mantleappears to underlie the central part of the back-arc. Subductioncomponents are found at the northern and southern ends of theridge, and there is a marked sediment melt input of up to 2%in segment E4. Enriched (plume) mantle is present beneath segmentE2 at the northern end of the ridge, suggesting that plume mantleis flowing westward around the edges of the subducting slab.The southern part of segment E8 is unique in that its magmasource is similar to sub-arc depleted mantle. KEY WORDS: geochemistry; petrogenesis; volcanism; back-arc; subduction     

Geochemistry of Rare Earth Elements and Thorium in Sediments and Ferromanganese Nodules of the Atlantic Ocean

The behavior of rare earth elements (REE) and Th is studied along the west–east transect at 22°N across the Atlantic Ocean. It is shown that both REE and Th contents, relative to Al (the most lithogenic element), increase toward the pelagic region. The increasing trend becomes more complicated due to variations in the content of biogenic carbonate that serves as a diluting component in sediments. The REE composition varies symmetrically relative to the Mid-Atlantic Ridge (MAR) emphasizing a weak hydrothermal influence on sediments of the ridge axis, although the well-known criteria for hydrothermal contribution, such as Al/(Al + Mn + Fe) and (Fe + Mn)/Ti, do not reach critical values. Variations in the REE content and composition allowed us to distinguish the following five sediment zones in the transect: (I) terrigenous sediments of the Nares abyssal plain; (II) pelagic sediments of the North American Basin; (III) carbonate ooze of the MAR axis; (IV) pelagic sediments of the Canary Basin; and (V) terrigenous clay and calcareous mud of the African continental slope and slope base. Ferromanganese nodules of the hydrogenetic type with extremely high Ce (up to 1801 ppm) and Th (up to 138 ppm) contents occur in pelagic sediments. It is ascertained that P, REE, and Th concentrations depend on Fe content in Atlantic sediments. Therefore, one can suggest that only a minor amount of phosphorus is bound in bone debris. The low concentration of bone debris phosphorus is a result of relatively high sedimentation rates in the Atlantic, as compared with those in pelagic regions of the Pacific.     

Geochemistry, Mineralogy and Magmatic Evolution of the Basaltic and Trachytic Lavas from Gough Island, South Atlantic

The Gough Island lavas range from picrite basalt through tosodalite-bearing aegirine-augite trachyte. The basaltic lavasare predominantly nepheline normative alkali basalts, althougha group of hypersthene normative tholeiitic basalts does occur.The oldest lavas on the island, represented by the Lower Basaltseries, are approximately 1?0 m.y. old and the youngest arethe Upper Basalts with an age of {small tilde} 0?13 m.y. Relatively coherent variations are described by the basalticand trachytic lavas with respect to both bulk rock major andtrace element geochemistry and mineral chemistry, and quantitativepetrogenetic modelling suggests that most of the variation canbe attributed to crystal fractionation/accumulation processesacting on a number of geochemically distinct parental magmas.The Upper Basalts and Lower Basalts have (within the limitsof sampling) a relatively restricted composition compared tothe Middle Basalt series lavas, with the latter ranging frompicrite basalt through to trachyandesite. The picrite basaltsand coarsely pyroxene-olivine phyric basalts represent partialcumulates with varying proportions (up to 40 wt. per cent) ofaccumulated olivine and clinopyroxene. In contrast, the moderatelyphyric and aphyric/finely porphyritic lavas represent the productsof crystal fractionation with the most evolved lavas havingexperienced at least 40 per cent fractional crystallizationof clinopyroxene, olivine, plagioclase and minor Fe-Ti oxidesand apatite. The detailed abundance variations in these lavasindicate that a number of parental magma compositions have fractionatedto produce the overall variations in basalt geochemistry, andsome of the magmas have interacted through mixing processes. The trachytic lavas show a large range in trace element abundance,but have only a limited major element variation. Most of thisvariation can be attributed to extensive (up to 70 per cent)fractional crystallization of predominantly alkali feldsparwith minor clinopyroxene, olivine, biotite, titano-magnetiteand apatite. A number of genetically distinct trachytes canbe recognized which are probably not related to each other byany simple fractional crystallization process. The compositionof the least evolved trachytes can be adequately accounted forby relatively extensive (up to 60 per cent) fractionation ofthe more evolved Middle Basalt series lavas. The trace element and isotopic characteristics of primitiveGough Island basalts support the concept that the source region(s)giving rise to these lavas is extremely enriched in highly incompatibleelements relative to primordial or ‘undepleted’mantle of bulk earth composition. It is unlikely that the lavashave sampled undepleted mantle as might be suggested by thesimilarity of the Sr and Nd isotopic ratios to ‘bulk earth’values. Rather, a model is favoured whereby the lavas are derivedfrom previously enriched sub-oceanic mantle which was subsequentlyinvaded and further enriched, at some time prior to partialmelting, by melts or fluids highly enriched in incompatibleelements. The enrichment could have occurred as veining by smalldegree partial melts or by infiltration of metasomatic fluids.     

Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher,West Germany

Among the spinel peridotite nodules from Dreiser Weiher. West Germany which represent fragments of the earth's upper mantle two series may be distinguished. One group (Ib) is anhydrous while the second one (Ia) is characterized by the presence of amphibole and/or breakdown-products of amphibole. Both suites display a wide range in modal composition. Pyroxene geothermometry yields equilibration temperatures of ~ 1150°C for group Ib and ~950°C for group Ia.Rare earth element (REE) patterns reveal marked differences between both groups: nearly unfractionated, light REE depleted or slightly light REE enriched chondrite normalized patterns in nodules from group Ib, but a high relative light REE enrichment in the amphibole containing suite Ia. An inverse correlation between the clinopyroxene content (or CaO) and relative light REE enrichment is observed in group Ia only. Two sources of light REE enrichment of the nodules can be distinguished: The first is a contaminant on mineral surfaces and can be removed by acid leaching. The second is an integral part of the constituent minerals of the nodules. Trace element abundances and modelling show that nodules of group Ib cannot be derived from upper mantle pyrolite by a one-stage partial melting process. Nodules of type Ia are interpreted as being the result of a reaction between Ib-type mantle and a fluid or liquid which provides H₂O and incompatible elements. At Dreiser Weiher this ‘open system’ upper mantle metasomatism may be related to the young uplift of the Rhenish Shield in a similar way as previously proposed by Lloyd and Bailey (1975).A model for the lithospheric mantle below the Westeifel is different from the San Carlos model proposed by Frey and Prinz (1978) and more complex.     

Tristan da Cunha, South Atlantic: Geochemistry and Petrogenesis of a Basanite-Phonolite Lava Series

The island of Tristan da Cunha, located at 37?S, 12?W in theSouth Atlantic, is the largest of a group of three islands,the others being Nightingale and Inaccessible. Tristan da Cunhacomprises a continuous series of alkaline lavas ranging in compositionfrom ankaramitic basanite through phonotephnte and tephnphonoliteto phonolite. Moderately porphyritic basanite is the dominantrock type ({small tilde}on the island. Major and trace element variations in the lavas describe well-definedtrends with increasing differentiation which are generally consistentwith control by fractional crystallization of phenocryst phases.None of the lavas can be considered to be primary in composition,mg-numbers range from 40 to 62, and covariation of certain minorand trace elements (e.g., Sr, Ba, P) suggests the presence ofat least two distinct fractionation trends. Sr, Nd, and Pb isotopicanalyses of a subset of the lavas confirm previously publisheddata for the island, but show a slightly greater range: ⁸⁷Sr/⁸⁶Sr= 0{dot}70495–0{dot}70517; ¹⁴³Nd/¹⁴⁴Nd = 0{dot}51259–0{dot}51247;²⁰⁶Pb/²⁰⁴Pb = 18{dot}47–18-{dot}74. Quantitative modelling of the compositional variations suggeststhat the ankaramitic basanites are partial olivine + clinopyroxene+ titanomagnetite (?minor plagioclase) cumulates, with {smalltilde}40% crystal accumulation being required to account forthe most porphyritic varieties. The range in composition frombasanite to phonotephrite can be accounted for by up to 50%fractional crystallization of clinopyroxene, olivine, titanomagnetite,and plagioclase, with minor apatite and, in some models, amphibole.Average proportions of these phases in the fractionate are Cpx40, TiMgt 20, Plag 30, Oliv 10. As much as 20% amphibole fractionationis required in models involving the phonotephrites. The compositionsof the evolved tephriphonolites and phonolites are consistentwith extensive (up to 80%) fractional crystallization of aninitial basanitic magma with clinopyroxenc (1–4%), amphibole(17–23%), plagioclase (6–20%) alkali feldspar (0–13%),and titanomagnetite (4–6%)?minor apatite and sphene beingthe dominant fractionating phases. Inferred trace element and isotopic characteristics of the sourceregions of the Tristan lavas are distinct from those givingrise to the Walvis Ridge, Gough Island, or Discovery Seamountbasalts. Normalized trace element abundances of the Tristanlavas are more similar to those of Marion Island, whereas Sr-,Nd-, dnd Pb-isotopic ratios are most similar to nearby InaccessibleIsland lavas. If Tristan da Cunha is the present-day surfaceexpression of the upwelling mantle plume that previously gaverise to the Walvis Ridge, then the source material tapped bythe lavas is distinctly heterogeneous (or has changed with time)within the limits imposed by maintaining the general characteristicsof DUPAL-type mantle.     

Geochemistry of rare earth elements in bottom sediments of the Brazil Basin, Atlantic Ocean

The sedimentation and ore formation were studied in sediments from nine stations located in the 24°W profile in the Brazil Basin of the Atlantic Ocean. The sediments are represented by mio- and hemipelagic muds, which are variably enriched in hydrothermal iron and manganese oxyhydroxides. As compared to the sediments from other basins of the Atlantic Ocean, these rocks are marked by extremely high manganese contents (up to 1.33%) and maximal enrichment in Ce. It was shown that the positive Ce anomaly is related to the REE accumulation on iron oxyhydroxides. Influence of hydrothermal source leads to the decrease of Ce anomaly and LREE/HREE ratio. In the reduced sediments, preservation of positive Ce anomaly and/or its disappearance was observed after iron and manganese reduction. The REE contents were determined for the first time in the Ethmodiscus oozes of the Brazil Basin. Ore deposits of the Brazil Basin are represented by ferromanganese crust and ferromanganese nodules. Judging from the contents of iron, manganese, rare, and trace elements, these formations are ascribed to the sedimentation (hydrogenic) deposits. They are characterized by a notable positive Ce anomaly in the REE pattern. The extremely high Ce content (up to 96% of total REE) was discovered for the first time in the buried nodules (Mn/Fe = 0.88).