Denis Fontignie

Plume‐ridge interactions of the Discovery and Shona mantle plumes with the southern Mid‐Atlantic Ridge (40°‐55°S)

We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the Mid-Atlantic Ridge (MAR) from 40° to 55°S. The results strongly indicate interaction and mixing between the off-ridge Discovery and ridge-centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al [1987]. The Discovery and Shona plumes have enriched mantle and high-μ(μ = 238U/204Pb) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning ∼13 Ma. The presence of a component with unusually low 206Pb/204Pb accompanied by high 87Sr/86Sr and low 208Pb/204Pb and 143Nd/144Nd in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three-component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low-μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the “C” component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.

Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He‐Pb‐Nd‐Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges

He-Pb-Nd-Sr isotope systematics in basalts from the Mid-Atlantic Ridge (MAR) from 65°N to 78°N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He-Pb-Nd-Sr isotope space clearly indicate that the boundary between the zone of influence of the “low 3He/4He” Jan Mayen plume and the “high 3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb-Nd-Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He-Pb-Nd-Sr isotopic systematics and incompatible element variations, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume by Mertz et al. [1991] which was based on more limited isotopic data, and the fluid dynamic models of Ito et al. [1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb-Nd-Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjornes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb-Nd-Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear-dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.

Influence of the Sierra Leone mantle plume on the equatorial Mid‐Atlantic Ridge: A Nd‐Sr‐Pb isotopic study

We report on a Pb-Nd-Sr isotope and rare earth study of Mid-Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600-km-long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3–2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peters Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb-rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400-km-long MAR segment between the Charcot and Ascension fracture zones is also apparent in the data.

Mantle heterogeneities beneath the South Atlantic: a NdSrPb 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.87Sr86Sr ratios range from 0.70212 to 0.70410 and143Nd144Nd from 0.51285 to 0.51331. The along-ridge long wavelength87Sr86Sr variation delineated by light-REE depleted basalts increases progressively southward toward Tristan da Cunha. Short wavelength, spike-like, positive87Sr86Sr 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 wavelength143Nd144Nd variations anti-correlate with those of87Sr86Sr. 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 NdSrPb 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 NdSr isotopic subspace the Circe and the St. Helena mixing vector are not distinguishable. The short wavelength NdSrPbHe 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 NdSrPb 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.

87Sr86Sr and REE variations along the Easter Microplate boundaries (south Pacific): Application of multivariate statistical analyses to ridge segmentation

Abstract We report 87 Sr 86 Sr , REE, Sc, V, Cr and Co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the Easter Microplate (EMP). The latitudinal 87 Sr 86 Sr variation along the East and West Rifts closely parallels that of the Pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the East Pacific Rise (EPR). The data also further support the binary mantle mixing model previously proposed. We evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses. The data base includes major and trace elements ranging from compatible (e.g., Cr) to highly incompatible (e.g., La), and the Pb and Sr isotope ratios. Multivariate classification of these geochemical variables in Q-mode (sample space) allows to clearly distinguish major-element grouping from those of moderate to highly incompatible elements, and the isotope ratios reflecting mantle source heterogeneities present in the region. The power of this multivariate geochemical classification is best represented in two-dimensional space by combining principal components analysis (PCA) with cluster analysis (CA) illustrated in the form of a dendrogram. The method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities. Using the variance-covariance matrix and correlation matrix in R-mode, we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation. Both discriminant analysis and PCA further indicate a close relationship between the geochemical anomaly located on the East Rift of the EMP and Sala y Gomez and Easter Island, to a lesser extent the Tuamotu Chain, whereas there is no close affinity between the EMP and the more distant Juan Fernandez, San Felix or Society Islands in the isotopic space considered. This provides further support that the geochemical anomaly located on the East Rift of the EMP may be related to the influence of a plume located either beneath the East Rift, or in the vicinity of Sala y Gomez, or Easter Island. In the last two models the plume would be laterally deflected at shallow depth towards the EPR-EMP spreading complex. An analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle. The flow between the plume and the East Rift of the EMP must be more confined or channelled. The possibility that the plume would be located directly beneath the East Rift cannot be ruled out but appears less likely.

Mantle plume-ridge interactions in the Central North Atlantic: A Nd isotope study of Mid-Atlantic Ridge basalts from 30°N to 50°N

We report on Nd isotopes and Sm and Nd contents of 46 fresh Mid-Atlantic Ridge basalts over the Azores swell from 30°N to 50°N. The latitudinal 143Nd/144Nd and Sm/Nd profiles anti-correlate with the 87Sr/86Sr profile previously established [1]. Sharp minima in 143Nd/144Nd, identical in value, are found at the 46°N, 43°N, 39°N, and 35°N short-wavelength geochemical anomalies previously recognized, as well as the 1100 km long gradient south of the Azores platform. A single binary mixing line is observed in 143Nd/144Nd vs. 87Sr/86Sr space, suggesting that the incompatible element-rich mantle sources underlying these four short-wavelength anomalies have a similar long-term origin. However, at a given 143Nd/144Nd (or 87Sr/86Sr), the MORB population from 40.5°N to 50°N is systematically lower in incompatible elements and their ratios, such as La/Sm, Nd/Sm, Cl/F and Rb/Sr, than the 40.5°N-30°N population. The required fractionation must be relatively recent. But the bulk chemistry of these MORB rules out that it was generated directly under the ridge by large varying degrees of partial melting of passive heterogeneities embedded in the depleted asthenosphere. An earlier, but not too distant, metasomatic enrichment or depletion event is required. A small 1–2% melt fraction removal by fractional decompression melting of a wet garnet lherzolite, at its wet solidus, can readily account for the depletion of the 46°N and 43°N relative to the 39°N mantle sources. We speculate that the short-wavelength 46°N, 43°N and 39°N anomalies were caused either by: (1) a family of plumes with the same long-term origin and thermal boundary layer source; or (2) by the detachment of blobs from a single bending plume converging with the MAR axis at 39°N and preferentially discharging southwestward along the spreading axis. In either case, the rising plumes, or the detached blobs, at 46°N and 43°N occurred on the east flank of the MAR below the thick lithosphere. The plume-derived material reaching the MAR axis at these latitudes would be residual in nature (mantle restite), as a result of an early removal in intraplate settings of small melt fractions, directly over the points where the plumes are rising, or over where the blobs detached from the bending plume. The previously proposed model invoking a separate off-ridge plume (Great Meteor) captured by the westward migrating MAR is retained for explaining the 35°N MAR anomaly and the related Great Meteor-Corner Seamount-New England Seamount hotspot track [2–4].

The Northernmost CAMP: 40Ar/39Ar Age, Petrology and Sr‐Nd‐Pb Isotope Geochemistry of the Kerforne Dike, Brittany, France

The Central Atlantic Magmatic Province (CAMP) is defined by tholeiitic basaltic flows and dikes associated with the initial break-up of Pangea at 200 Ma, preceding the opening of the Atlantic Ocean. These tholeiites occur in once-contiguous parts of North America, Africa, South America and Europe over a total area of about 7 million square kilometers. The Kerforne dike, located in Brittany (NW France), represents the northernmost outcrop of this province. Due to its orientation and location, more than 1500 Km from the Early Jurassic Atlantic rift, like other CAMP dikes, it can not be considered as the magmatic expression of the Central Atlantic rifting. Despite its distal position, this dike has an Ar/Ar age (193 ± 3 Ma, obtained on plagioclase separates) similar to the CAMP tholeiites. Kerforne dolerites are characterized by augite and minor plagioclase phenocrysts. According to petrographic observation and microprobe analyses, some of these plagioclases contain resorbed high-An (An 85 ) possibly xenocrystic cores which may be evidence of interaction with a mafic lower crust. The low-TiO 2 (1.0 wt%) tholeiitic Kerforne basalts are characterized by negative Nb anomalies, by a positive correlation between . Sr and . Nd , by high radiogenic 207 Pb/ 204 Pb in comparison to relatively unradiogenic 206 Pb/ 204 Pb, and by an enrichment in LREE relative to HREE. These chemical features, along with the mineralogic observations, are indicative of a minor contamination with mafic lower crust, like that represented by granulitic xenoliths of the Massif Central, France. By contrast, contamination with the silicic upper crust (e.g., with the granitic basement) was negligible. The isotopic compositions of the little contaminated Kerforne basalts are similar to those of most other CAMP low-TiO 2 basalts, and are different from those of most oceanic basalts. It is suggested that this high 87 Sr/ 86 Sr and 207 Pb/ 204 Pb isotopic signature was inherited by interaction of primitive mantle with metasomatized portions of the continental lithospheric mantle, similar to the sources of Variscan lamproites of Brittany. Moreover, The contribution of an OIB mantle component may be ruled out as the Kerforne mantle source was isotopically different from those of the oceanic islands which have been suggested to represent the present-day expression of the hypothetic CAMP mantle plume.

Migration of sharks into freshwater systems during the Miocene and implications for Alpine paleoelevation

Trace-element and isotopic compositions of fossilized shark teeth sampled from Miocene marine sediments of the north Alpine Molasse Basin, the Vienna Basin, and the Pannonian Basin generally show evidence of formation in a marine environment under conditions geochemically equivalent to the open ocean. In contrast, two of eight shark teeth from the Swiss Upper Marine Molasse locality of La Moliere have extremely low ?18O values (10.3‰ and 11.3‰) and low 87Sr/86Sr ratios (0.707840 and 0.707812) compared to other teeth from this locality (21.1‰–22.4‰ and 0.708421–0.708630). The rare earth element (REE) abundances and patterns from La Moliere not only differ between dentine and enameloid of the same tooth, but also between different teeth, supporting variable conditions of diagenesis at this site. However, the REE patterns of enameloid from the “exotic” teeth analyzed for O and Sr isotopic compositions are similar to those of teeth that have O and Sr isotopic compositions typical of a marine setting at this site. Collectively, this suggests that the two “exotic” teeth were formed while the sharks frequented a freshwater environment with very low 18O-content and Sr isotopic composition controlled by Mesozoic calcareous rocks. This is consistent with a paleogeography of high-elevation (2300 m) Miocene Alps adjacent to a marginal sea.

Hafnium, neodymium, and strontium isotope and parent-daughter element systematics in basalts from the plume-ridge interaction system of the Salas y Gomez Seamount Chain and Easter Microplate

We present a comprehensive data set with Hf, Nd, and Sr isotope ratios and parent-daughter trace element concentrations in 111 basalts and glasses from seamounts of the western Easter–Salas y Gomez Seamount Chain (ESC), the Easter Microplate (EMP) spreading centers, and the East Pacific Rise (EPR). Sr and Pb radiogenic isotope ratios and related ratios of highly incompatible parent to lesser incompatible daughter elements grade from high values near the Salas y Gomez (SyG) hot spot location to low values 1000 km west. Here the west rift of the EMP is dominated by typical depleted mid-ocean ridge basalt (MORB). Hf and Nd radiogenic isotope ratios show the opposite gradients, which also reflect the long-term enriched nature of the hot spot source and mixing of the hot spot with the depleted upper mantle. Gradients of these parameters occur north and south along the EMP boundaries. These observations confirm the plume-ridge interaction model proposed for this region by Schilling et al. and further characterize the SyG hot spot. The binary mixing relationship evident in the isotope variations of the basalts is somewhat compromised when the trace elements are considered. This complexity can be explained by modification of trace element abundances during the process of partial melting of the two end-member components (enriched hot spot and depleted upper mantle). In addition, the melting variability is evident in the basalt bulk compositions, which range from tholeiitic (EMP to 108°W) to alkali basalts (108°W to SyG). A single highly correlated data array in Pb-isotope space and a linear Pb-Sr isotope relation indicate that the SyG hot spot is homogeneous. The similarity of the Pb isotope ratios of the SyG hot spot to other long-term high-U/Pb mantle domains suggests an origin in subduction-modified altered oceanic crust. The SyG end-member isotope composition appears to be pervasive in the south central Pacific mantle, evoking a widespread mantle contamination event by the SyG hot spot in the past.

Fluorite 87Sr/86Sr and REE constraints on fluid–melt relations, crystallization time span and bulk DSr of evolved high-silica granites. Tabuleiro granites, Santa Catarina, Brazil

Abstract The evolved high-silica Tabuleiro granites within the Early Paleozoic Santa Catarina Composite Massif, Pelotas Batholith, southern Brazil are characterized by the presence of euhedral to subeuhedral accessory fluorite and geochemical features typical of topaz–rhyolites and related A-type granites. Sr isotopes and REE data of the Tabuleiro granites and their accessory fluorite are used to constrain fluid–melt relations, crystallization time span and bulk crystal–melt Sr partition coefficient D Sr .Correlations involving REE, Eu/Eu*, Rb/Sr, Sr and 87 Sr / 86 Sr in fluorite and fluorite-host granites show that fluorite records the differentiation trend of the host Tabuleiro granites. REE-normalized patterns and Eu/Eu* relations in fluorite-bearing granites indicate that fluorite forms after the crystallization of the quartzo-feldspathic framework in residual melts. The Tabuleiro accessory fluorites yield high and variable 87 Sr / 86 Sr ratios between 0.72334 and 0.8192. These ratios are neither the result of fluorite precipitation from a fluid nor Sr isotopic resetting. They result from 87 Rb decay in fluorine-rich high-Rb/Sr melts evolved by fractional crystallization in a magmatic system with a long crystallization time-span. Melt residence times of 300 to 700 ka and D Sr of 4.7 to 6.0 are necessary to yield the high fluorite 87 Sr / 86 Sr ratios. These results are compatible with those deduced elsewhere from high-silica rhyolitic volcanic equivalents.