Atlantic Mid-ocean Ridge Basalts Research Articles

The Volcanism of the Meseta del Lago Buenos Aires, Patagonia: the Transition from Subduction to Slab Window

Abstract The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts’ composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas.

Tracking the Transition From a Gondwana LIP to the South Atlantic Ocean With Geochronological and Geochemical Indicators

AbstractEvidence of early oceanization during transition from large igneous provinces (LIPs) to new ocean crust is mostly missing in the geological record. Mafic dykes geochemically akin to mid‐ocean ridge basalts (MORBs) occur in late stages of LIPs, but their tectonic role during margin evolution remains poorly constrained. Here we use whole‐rock and high‐resolution clinopyroxene chemistry of two distinct types of mafic dykes from the southeastern Brazilian margin to show that a MORB‐like plumbing system became magmatically active in the central South Atlantic ∼2 Ma after the volcanic peak of the Paraná‐Etendeka Magmatic Province. The LIP basaltic dyke compositions resulted from the mixing between a highly metasomatized lithospheric mantle comprising enriched DMM and Archean contributions. The lack of elemental oscillatory zoning in clinopyroxene crystals suggests that they were produced by a single magma pulse, consistent with the strong magma fluxes in LIPs. In contrast, the MORB‐like dykes, which exhibit strong similarities with South Atlantic MORBs, are interpreted to represent early stages of oceanization deriving from the mixing of enriched asthenospheric melts either with ancient SCLM contributions or small crustal assimilation. The MORB‐like intrusions ascended and cooled rapidly along faults and were fed by numerous magma injections that suggest episodic decompression of the asthenospheric source ∼20 Ma before the birth of central South Atlantic. The combination of subduction‐influenced and MORB‐like dykes is recurrent in ancient and modern rifts globally, suggesting that thinned lithospheric blocks affected by ancient subduction events may become more susceptible to early oceanization in rifted margins.

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts (MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge (MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios. We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as “pipelike channels” and “pancake-like channels”. The “pancake-like channels” plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the “pipe-like channels” plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.

The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites

Young (61 Ma) unaltered picrites from Baffin Island, northeast Canada, possess some of the highest 3He/4He (up to 50 Ra) seen on Earth, and provide a unique opportunity to study primordial mantle that has escaped subsequent chemical modification. These high-degree partial melts also record anomalously high 182W/184W ratios, but their Sr-Nd-Hf-Pb isotopic compositions (including 142Nd) are indistinguishable from those of North Atlantic mid-ocean ridge basalts. New high precision Fe and Zn stable isotope analyses of Baffin Island picrites show limited variability with δ56Fe ranging from −0.03‰ to 0.13‰ and δ66Zn varying from 0.18‰ to 0.28‰. However, a clear inflection is seen in both sets of isotope data around the composition of the parental melt (MgO = 21 wt%; δ56Fe = 0.08 ± 0.04‰; and δ66Zn = 0.24 ± 0.03‰), with two diverging trends interpreted to reflect the crystallisation of olivine and spinel in low-MgO samples and the accumulation of olivine at higher MgO. Olivine mineral separates are significantly isotopically lighter than their corresponding whole rocks (δ56Fe ≥ −0.62‰ and δ66Zn ≥ −0.22‰), with analyses of individual olivine phenocrysts having extremely variable Fe isotope compositions (δ56Fe = −0.01‰ to −0.80‰). By carrying out modelling in three-isotope space, we show that the very negative Fe isotope compositions of olivine phenocryst are the result of kinetic isotope fractionation from disequilibrium diffusional processes. An excellent correlation is observed between δ56Fe and δ66Zn, demonstrating that Zn isotopes are fractionated by the same processes as Fe in simple systems dominated by magmatic olivine. The incompatible behaviour of Cu during magmatic evolution is consistent with the sulfide-undersaturated nature of these melts. Consequently Zn behaves as a purely lithophile element, and estimates of the bulk Earth Zn isotope composition based on Baffin Island should therefore be robust. The ancient undegassed lower mantle sampled at Baffin Island possesses a δ56Fe value that is within error of previous estimates of bulk mantle δ56Fe, however, our estimate of the Baffin mantle δ66Zn (0.20 ± 0.03‰) is significantly lower than some previous estimates. Comparison of our new data with those for Archean and Proterozoic komatiites is consistent with the Fe and Zn isotope composition of the mantle remaining constant from at least 3 Ga to the present day. By focusing on large-degree partial melts (e.g. komatiites and picrites) we are potenitally biasing our record to samples that will inevitably have interacted with, entrained and melted the ambient shallow mantle during ascent. For a major element such as Fe, that will continuosly participate in melting as it rises through the mantle, the final isotopic compositon of the magama will be a weighted average of the complete melting column. Thus it is unsuprising that minimal Fe isotope variations are seen between localities. In contrast, the unique geochemical signatures (e.g. He and W) displayed by the Baffin Island picrites are inferred to solely originate from the lowermost mantle and will be continuously diluted upon magma ascent.

Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5–38.0°S): Constraints on mantle wedge and slab input compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O–Sr–Nd–Hf–Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5°S and 38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300km into the backarc. We also present an equivalent data set for Chile trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc=4.9–5.6‰ and backarc=5.0–5.4‰) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf–Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab.

Influence of magma ascent rate on carbon dioxide degassing at oceanic ridges: Message in a bubble

In order to quantify the magma ascent rate beneath oceanic ridges, we propose a new method based on vesicle size distribution (VSD) and volatiles measurements (CO2 and H2O) on 65 fresh glasses from the global Mid-Ocean Ridge system. Comparisons of VSD between the main oceanic basins reveal that Mid-Ocean-Ridge-Basalts (MORB) from the Pacific Ocean have significantly higher bubble densities (148−124+739 bubbles/cm2) and initial population densities ln(n0)=19.6±2.8cm−4 but lower vesicularities (∼0.1%). We determine the residence time of bubbles in the magma using the linear relationship between the logarithm of the density population and the vesicle diameters. Assuming a constant bubble growth rate (G=1.5×10−7cm/s), we suggest that the transit times of the magmas through the oceanic crust are shorter beneath Pacific ridges (∼2 h) than elsewhere (∼15 h). In addition, the CO2 and H2O contents of the studied glasses allow the carbon saturation pressure to be calculated. Pacific MORB display a significantly lower carbon dioxide saturation pressure (Psat=843−221+300bars) than Atlantic and Indian MORB (Psat=1898−827+1466bars) but identical pressures of eruption (Pe∼310bars). Consequently, the distance traveled by bubbles from the vesiculation depth to the seafloor is shorter for the Pacific (∼1.9km) than for the Atlantic and Indian oceans (∼5.2km): the longer the transit time, the longer the distance traveled. A closer inspection of the data revealed that the decompression rate (dP/dt) varied from 5.0×102 to 2.3×104Pa/s and correlated positively with the measured bubble densities (nb/m3) as expected from experimental data and numerical modeling. At the global scale, most of the Pacific samples are characterized by high ascent rates (0.25−0.13+0.26m/s) relative to those from the Atlantic (0.11−0.06+0.15m/s) and the Indian samples (0.09−0.03+0.05m/s). However, at the local scale some samples from the Mid-Atlantic Ridge at 37 °N (where the reflections of seismic waves from crustal magma chamber have been observed) are not distinguishable from the Pacific data set having high ascent rates and large bubble densities. We suggest that the method presented in this study could be used to identify spots along the Mid-Oceanic-Ridge system where magma chambers are possibly present.

Modelling the composition of melts formed during continental breakup of the Southeast Greenland margin

We have developed a generic dynamic model of extension of the lithosphere, which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading. Stokes equations for non-Newtonian flow are solved and the mantle melts by decompression. Strengthening of the mantle due to dehydration as melting progresses is included. The composition is then empirically related to depletion. Using a crystallisation algorithm, the predicted primary melt composition was compared with mean North Atlantic mid-ocean ridge basalt (MORB). At steady state, using half spreading rates from 10 to 20 mm yr − 1 and mantle potential temperatures of 1300 to 1325 °C we predict a major element composition that is within the variation in the mean of North Atlantic MORB. This model is applied to the Southeast Greenland margin, which has extensive coverage of seismic and ODP core data. These data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11 km. This pattern of melt production can be recreated by introducing an initial hot layer of asthenosphere beneath the continental lithosphere and by having a period of fast spreading during early opening. The hot layer was convected through the melt region giving a pulse of high magnesian and low silica melt during the early rifting process. The predicted major element composition of primary melts generated are in close agreement with primary melts from the Southeast Greenland margin. The observed variations in major element composition are reproduced without a mantle source composition anomaly.

MANTLE MELTING PROCESSES DISCERNED FROM A GEOCHEMICAL STUDY OF THE TINAQUILLO PERIDOTITE MASSIF, VENEZUELA

The Tinaquillo orogenic spinel peridotite massif in north central Venezuela is a sub-horizontal, 3-km thick sheet overlying and in thrust contact with phyllites of the Cordillera de la Costa belt in the north and underlying and in extensional fault contact with the gabbroic to felsic Tinaco complex to the south (Ostos, 1990). While early workers interpreted the complex as a magmatic or crystal-mush intrusion (e.g., MacKenzie, 1960), Seyler and Mattson (1989, 1993) and Seyler et al. (1998) interpreted it as a fragment of the upper mantle possibly having ascended in the form of a diapir, but having been mylonitized during transit through the lower crust. The ultramafic rocks comprise ca. 75% harzburgite locally grading into lherzolite, 20% dunite, and 5% serpentinite. Pyroxenite and hornblendite veins intruding the complex are volumetrically small, but quite conspicuous where present. Minerals in the peridotite have a bimodal grain-size distribution owing to survival of remnant porphyroclasts of olivine, orthopyroxene, and minor clinopyroxene, and the late development of a mylonitic fabric with considerable grain-size reduction probably related to emplacement. The massif is thought to be a fragment of lithospheric mantle emplaced into the Caribbean belt during the late Cretaceous, but not exhumed until the late Eocene to middle Miocene by northwestward-directed thrusting along the Manrique Fault (Pindell et al., 1988; Pindell and Barrett, 1990). Seven peridoitites and three amphibole-rich veins have been investigated for major and trace element concentrations, and Sr-Nd-Pb-Hf isotopic compositions. The peridotites have geochemical characteristics of residues after moderate degrees of partial melting (estimated to be 8 to 16%) assuming a homogeneous primitive mantle source. In addition, they have strong light rare earth element (REE)- depleted to spoon-shaped REE patterns, but flat heavy REE (La/Yb at 0.04-0.55 times primitive mantle). The (La/Yb)N ratios increase with the refractoriness of the peridotites, reflecting secondary metasomatism, which is a common feature of the sub-continental lithospheric mantle xenoliths world-wide. A primitive mantle-normalized element distribution diagram for the peridotites reveals an overall depletion in the highly incompatible elements relative to less incompatible ones, but moderate enrichments in the strongly incompatible elements (Ba, Th, U), without depletion in Nb and Ta relative to La. The peridotites have isotopic compositions of 87Sr/86Sr = 0.70277-0.71044, 143Nd/144Nd = 0.513095-0.514000, 206Pb/204Pb = 19.00-19.15, 208Pb/204Pb = 38.18-38.75, and 176Hf/177Hf = 0.282692-0.284703. The Nd, Pb and Hf isotopic signatures show ranges that overlap with present-day mid-ocean ridge basalts (MORB) but also extending to more depleted compositions. The unusually radiogenic Sr for some samples appears to be the result of pervasive infiltration by seawater at some stage in the massif’s history. The Sr, Nd, Pb and Hf isotopic signatures of the veins (87Sr/86Sr = 0.70231-0.70256, 143Nd/144Nd = 0.513052-0.513289, 206Pb/204Pb = 18.30-18.52, 208Pb/204Pb = 37.66-37.87, 176Hf/177Hf = 0.282988-0.283303) are similar to the Pacific and Atlantic MORB; lack of the highly radiogenic Sr component in the veins indicates its addition to the peridotites prior to vein emplacement. The Lu-Hf data yield an errorchron suggesting a Phanerozoic age for stabilization of this piece of the lithosphere. We attribute the origin of the amphibole-rich veins in the peridotites to melting during subduction of the proto-Caribbean Plate in a back-arc tectonic setting, fairly recently in the massif’s history, possibly during the late Jurassic.

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts.

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting

Dykes of the East Greenland Tertiary dyke swarm can be divided into pre- and syn-break-up tholeiitic dykes, and postbreak-up transitional dykes. Of the pre- and syn-break-up dykes, the most abundant group (Tholeiitic Series; TS) has major element compositions similar to the main part of the East Greenland flood basalts. A group of high-MgO tholeiitic dykes (Picrite---Ankaramite Series; PAS) are much less common, and are equivalent to some of the oldest lavas of the East Greenland flood basalts. Isotopic compositions of the TS and PAS dykes partly overlap with those for Iceland, but Pb isotopic compositions extend to less radiogenic values than those seen in either Iceland or North Atlantic mid-ocean ridge basalt (MORB). The isotopically depleted source required to account for this isotopic variation is interpreted as subcontinental lithospheric mantle with low 87 Sr/ 86 Sr and 206 Pb/ 204 Pb and high eNd. The post-break-up Transitional Series (TRANS) dykes are isotopically distinct from Iceland and MORB, and are interpreted as the products of contamination of Iceland plume melts with continental crust. Comparison of the Nd---Sr---Pb isotopic and trace element compositions of dykes from different segments of the East Greenland margin indicates that there is no systematic compositional change with distance from the presumed protoIcelandic plume centre. This suggests that a northwardincreasing crustal thickness observed offshore may be attributed to active upwelling rather than a systematic rise in temperature towards the plume centre.

Rare gas systematics on Mid Atlantic Ridge (37–40°N)

Rare gas concentrations and isotopic compositions in 19 glasses from the North Atlantic ridge have been analyzed. Sixteen of them are located between 36.8 and 39.9°N on the Azores plateau, three others are located between 21.25 and 30.2°N and may represent normal North Atlantic Mid Ocean Ridge basalts. Helium concentrations decrease by three orders of magnitude between 37° and 38.5°N. These low He concentrations (10 −8 cm 3 STP/g) are attributed to degassing before eruption. The 4He/ 3He ratios decrease from 90 000 at 37°N to 75 000 at 38.5°N and then increase to 100 000 at 40.5°N. The low 4He/ 3He ratio measured on the ridge at 38.5°N indicates a plume–ridge interaction with the Terceira–Pico–São Jorge plume, which has a 4He/ 3He lower than 50 000 ( R/Ra>15). More radiogenic 4He/ 3He ratios (up to 110 000) can also be found in this segment of the ridge and may indicate either in situ production of 4He by radioactive decay or atmospheric contamination of very low helium content samples. The 20Ne/ 22Ne ratios decrease from a mantle-like value (>12.0) between 20 and 37°N to the atmospheric ratio (9.8) at 38.5°N. The 40Ar/ 36Ar ratios show the same tendency as the neon isotopic ratios (atmospheric-like at 38.5°N). This may indicate contamination in shallow degassed magma chambers. However, because the maximum measured 20Ne/ 22Ne ratios are strongly correlated to the 206Pb/ 204Pb, 87Sr/ 86Sr and 143Nd/ 144Nd ratios, an alternative explanation for the atmospheric 20Ne/ 22Ne is the presence of an atmospheric component in the source. However, due to the large He loss (and therefore Ne and Ar), atmospheric contamination is the more probable explanation.

Kistufell: Primitive Melt from the Iceland Mantle Plume

This paper presents new geochemical data from Kistufell (64°48′N, 17°13′W), a monogenetic table mountain situated directly above the inferred locus of the Iceland mantle plume. Kistufell is composed of the most primitive olivine tholeiitic glasses found in central Iceland (MgO 10·56 wt %, olivine Fo89·7). The glasses are interpreted as near-primary, high-degree plume melts derived from a heterogeneous mantle source. Mineral, glass and bulk-rock (glass + minerals) chemistry indicates a low average melting pressure (15 kbar), high initial crystallization pressures and temperatures (10–15 kbar and 1270°C), and eruption temperatures (1240°C) that are among the highest observed in Iceland. The glasses have trace element signatures (Lan/Ybn <1, Ban/Zrn 0·55–0·58) indicative of a trace element depleted source, and the Sr–Nd–Pb isotopic ratios (87Sr/86Sr 0·70304–0·70308, 143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb 18·343–18·361) further suggest a long-term trace element depletion relative to primordial mantle. High He isotopic ratios (15·3–16·8 R/Ra) combined with low 207Pb/204Pb (15·42–15·43) suggest that the mantle source of the magma is different from that of North Atlantic mid-ocean ridge basalt. Negative Pb anomalies, and positive Nb and Ta anomalies indicate that the source includes a recycled, subducted oceanic crustal or mantle component. Positive Sr anomalies (Srn/Ndn = 1·39–1·50) further suggest that this recycled source component involves lower oceanic crustal gabbros. The δ18O values (4·2–4·7‰), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and in situ oceanic gabbros, are consistent with this interpretation. The elevated 3He/4He ratios are primarily attributed to a primitive, relatively undegassed component in the Iceland mantle plume, which dominates the He isotope signature as a result of long-term depletion of U, Th and He in the recycled gabbroic component.

A hafnium isotope and trace element perspective on melting of the depleted mantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform ( 176Hf/ 177Hf≈0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have 176Hf/ 177Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low 176Hf/ 177Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.

Petrogenesis and Stratigraphy of the High-Ti/Y Urubici Magma Type in the Parana Flood Basalt Province and Implications for the Nature of 'Dupal'-Type Mantle in the South Atlantic Region

The high-Ti/Y Urubici (or Khumib) magma type of the Paraná–Etendeka large igneous province has a restricted spatial extent, near the southeast Brazilian coast and in the northern Etendeka (Namibia). Urubici flows are interbedded with low-Ti/Y Gramado flows. Flow correlations indicate that local topographic relief was important in controlling emplacement of flows, and that lavas near the coast have undergone up to 1 km of post-magmatic uplift relative to inland areas. Urubici magmas have undergone extensive fractional crystallization (MgO <5.5 wt %). Stratigraphic variations highlight complexities of mixing and minor crustal assimilation indicative of open-system magmatic plumbing. The least contaminated samples have high La/Nb (∼1.5) and (Tb/Yb)N (∼2.5), Sr–Nd isotopes close to Bulk Earth (87Sr/86Sri ∼0.7050; εNdi −2.7), and Dupal Pb isotopes with unradiogenic 206Pb/204Pb (∼17.6). These features are similar to those of the Walvis Ridge DSDP (Deep Sea Drilling Project) Site 525A basalts that define the EM1 oceanic mantle component, and many are also shared with local Cretaceous alkalic magmas that are inferred to be lithospheric mantle melts. Low 206Pb/204Pb material found in the Urubici and Site 525A basalts is not seen as a mixing end-member within the modern Tristan plume system or in South Atlantic mid-ocean ridge basalt. An origin from lithospheric mantle material, delaminated and dispersed within the asthenosphere following continental break-up, is preferred. Thus the South Atlantic Dupal mantle anomaly cannot be considered as a single entity: Urubici flood basalts and Walvis Ridge Site 525A basalts have a relatively shallow origin within originally lithospheric mantle, whereas the Tristan plume is a deep mantle upwelling.

Recycled ocean crust and sediment in Indian Ocean MORB

Mid-ocean ridge basalts (MORB) from the Indian Ocean are characterized by higher 87Sr/ 86Sr and lower 206Pb/ 204Pb, on average, than Atlantic and Pacific MORB. These distinct isotopic characteristics are thought to be the result of large-scale mantle contamination, either by ancient recycled sediments or delaminated subcontinental lithosphere. To constrain the upper mantle mixing end-members, we have analyzed the concentrations of Nb, U, Th, Pb and 17 other incompatible elements (by HPLC and ID-TIMS methods), as well as Sr, Nd, and Pb isotope ratios in nine hand picked MORB glasses from the Central Indian and Carlsberg Ridge. Nb/U and Ce/Pb ratios are sensitive indicators of crustal contaminants in the mantle, and such contaminations have been identified in EM-type ocean island basalts. Here we show the first case of highly significant correlations of Nb/U, Ba/Nb and Ba/Th with 87Sr/ 86Sr for Indian Ocean MORB and we confirm and strengthen a previously detected correlation with Ce/Pb (or Nd/Pb). These correlations are interpreted as residual heterogeneities that have not been completely rehomogenized by convective stirring of the mantle. The results are explained by a quantitative model involving contamination of an Atlantic-Pacific type upper mantle with a heterogeneous, recycled component consisting of ∼ 1.5 Ga old altered oceanic crust with small, but variable, amounts of ancient pelagic sediment added. This contamination causes higher 87Sr/ 86Sr, lower 143Nd/ 144Nd, generally low but variable 206Pb/ 204Pb and the distinct trace element correlations seen in the Indian Ocean MORB data. Mass balance calculations show that only small amounts of pelagic sediment are necessary to satisfy the criteria of the contamination model, and these can be subducted within as little as 200 Ma. Lead isotopic systematics constrain the recycled sediment component to be older than about 1.25 Ga. Apart from this, the model does not constrain the age of the contamination event since the results are equally valid for (1) “slow” mixing of the recycled component subsequent to subduction at ∼ 1.5 Ga or (2) storage of the material after subduction in a mantle boundary layer followed by relatively recent reinjection into the MORB source and “fast” mixing.

Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean

Lavas from Afanasy-Nikitin Rise, possibly the Late Cretaceous product of the Crozet hotspot, cover a wide range of isotopic compositions that includes the lowest ( 206 Pb 204 Pb) t (to 16.77) and ϵ Nd ( t ) (to −8) values yet found among oceanic islands or spreading centers worldwide, as well as high ( 87 Sr/ 86 Sr) t (to 0.7066). In contrast, young basalts from the Crozet Archipelago exhibit a narrow range of variation around ϵ Nd ∼ +4, 87 Sr/ 86 Sr ∼ 0.7040, and 206 Pb/ 204 Pb ∼ 19.0, closely resembling that of shield lavas of the Reunion hotspot. The Afanasy-Nikitin rocks also have much higher Ba/Nb, Ba/Th, and Pb/Ce than modern oceanic island or ridge lavas, as well as high La/Nb. The data do not obviously support the Crozet plume model but, assuming the model to be plate tectonically correct, would indicate that the plume-source composition either changed dramatically or that Afanasy-Nikitin magmatism involved significant amounts of nonplume mantle. The low 206 Pb/ 204 Pb, low ϵ Nd lavas provide the best evidence to date of the sort of material that, by variably contaminating much of the Indian mid-ocean-ridge basalt (MORB) source asthenosphere, may be responsible for the isotopic difference between most Indian MORB and Pacific or North Atlantic MORB. The combined isotopic and trace element results suggest an ultimate origin in the continental crust or mantle lithosphere for this material, although whether it was cycled through the deep mantle or resided at shallow levels in the convecting mantle cannot currently be determined.

238U/ 204Pb in MORB and open system evolution of the depleted mantle

Concentrations of U, Th and Pb were determined by isotope dilution on 30 mid-ocean ridge basalt (MORB) glasses. The mean values of 238U/ 204Pb (μ) and 232Th/ 238U (κ) in these glasses are 11.20 and 2.67 respectively. There are significant differences in these ratios between ocean basins. Mean κ values increase in the order Atlantic < Pacific < Indian; mean μ is higher in Pacific MORB than in Indian and Atlantic MORB, but differences between the Atlantic and Indian are not significant. μ correlates strongly with U concentration, but not with Pb concentration or with 206Pb/ 204Pb . The correlation of μ with U but not with 206Pb/ 204Pb reflects the effect of partial melting and fractional crystallization on μ. Since the maximum value of U in the depleted mantle must be less than the bulk silicate earth value of 0.018 ppm, the μ-U correlation can be used to estimate the maximum mean value of μ in the depleted mantle ( μ DM ). This yields a maximum μ DM of 4.67. Similarly, the correlation between μ and Rb/Sr suggests a maximum μ DM of 6.3. These results are consistent with values for μ DM derived using mass balance from the crustal and depleted mantle values of κ, the depleted mantle Pb/Ce ratio, and the crustal abundance of Ce. The value of μ DM thus seems to be less than the bulk earth value, estimated to be 8 ± 2. This is consistent with the incompatible-element-depleted character of this reservoir, but inconsistent with Pb isotope ratios, which record time-integrated values for μ of 8–9, and which indicate that μ has increased over time. This inconsistency requires that the depleted mantle be an open system with a residence time for Pb of 1 ± 0.5 Ga. Neither the continental crust nor a primitive lower mantle have the appropriate isotopic composition to be the source of Pb in the depleted mantle. Mantle plumes do have the appropriate isotopic composition, and as little as 10% of the observed plume flux is needed to mix with the depleted mantle to supply the required flux of Pb.

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S. 3He/ 4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R A (R A is the atmospheric ratio of 1.39 × 10 −6 ), encompassing the range of previously reported values for MORB erupted away from high 3He/ 4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in 3He/ 4He , Pb and Sr isotopes, and trace element ratios such as (La/Sm) N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in 3He/ 4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model. MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by 87Sr/ 86Sr of ∼ 0.7022, Pb isotopes close to the geochron and with 206Pb/ 204Pb of ∼ 17.7, and 3He/ 4He of 8.6–8.9 R A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between ∼ 20 and 24°S. The He Pb and He Sr isotope relations along the ridge indicate that the 3He/ 4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of 3He/ 4He ratios in the island lavas. Details of the He Sr Pb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in the He/Pb ratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity. Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.

Isotope geochemistry of Pacific Mid‐Ocean Ridge Basalt

We report Sr, Nd, and Pb isotope ratios of 51 mid‐ocean ridge basalts (MORB) from the Juan de Fuca Ridge, the Gorda Ridge, the East Pacific Rise (EPR) and the Galapagos Spreading Center (GSC). The results confirm the general isotopic homogeneity of Pacific MORB, though high Pb isotope ratios (206pb/204pb up to 19.24) occur in basalts from the Easter microplate, reflecting the influence of the Easter hotspot. There is no evidence in our data of a “Dupal” geochemical anomaly in the mantle beneath the EPR in the southern hemisphere. Sr, Nd, and Pb isotope ratios are all correlated with one another, but correlations involving 87Sr/86Sr are poorer than other correlations, indicating that Rb‐Sr fractionation in the sub‐Pacific mantle has to some degree been decoupled from Sm‐Nd, U‐Pb, and Th‐Pb fractionation. Small but significant isotopic variations occur in MORB on regional scales of 1–100 km. These variations are, however, of lesser amplitude than those observed on larger scales. Small‐scale isotopic variations indicate either that large magma chambers do not exist beneath the EPR or that they are not efficient homogenizers of melts passing through them. On regional scales, Pb isotope ratios are more highly correlated than in Pacific MORB overall. We interpret these correlations as isochrons which record the age of events that created the regional mantle heterogeneities. These ages range from 1.4 to 2.4 Ga. Regional coherence of isotopic compositions indicates that the MORB source region consists of integral volumes which may be related to convection patterns. A comparison of the mean isotopic compositions of Pacific spreading ridges with means of Indian and Atlantic MORB shows that Pacific MORB have generally lower 87Sr/86Sr than MORB from the Mid‐Atlantic Ridge (MAR) and Indian Ocean ridges but have generally lower 143Nd/144Nd and higher 206pb/204pb than the MAR. Presently available data do not support the relationship of isotopic diversity and mean isotopic composition to spreading rate suggested by others. Differences between isotopic compositions and between isotopic diversity appear to be inherent features of the mantle beneath the Pacific, Indian, and Atlantic ocean basins.

The O, Sr, Nd and Pb isotope geochemistry of MORB

We report analyses of O, Sr, Nd and Pb isotope ratios in 52 fresh glasses of mid-ocean ridge basalt (MORB). δ 18O-values in basalts range from +5.35 to +6.05‰ and do not correlate with indices of fractional crystallization or with 87 Sr 86 Sr . Values up to +6.47‰ occur in andesites and dacites from the propagating rift at 95°W on the Galápagos Spreading Center. Neither fractional crystallization nor assimilation of altered material appear to be dominant controls on the δ 18O of basalts. A weak, but statistically significant correlation was found between δ 18O and Nd and Pb isotope ratios in Pacific MORB which warrants further investigation. Otherwise, no statistically significant correlations between δ 18O and radiogenic isotope ratios were found, indicating that any O isotope variations in the depleted upper mantle are largely unrelated to incompatible-element enrichments and depletions. There are important and significant differences in variances of radiogenic isotopes in MORB from different ocean basins, but these differences do not appear to be related to spreading rate. We conclude that they reflect real differences in the homogeneity of the upper mantle beneath ocean ridges and that efficacy of mixing during magmatic processes is not a dominant control of the degree of heterogeneity observed in MORB. Pacific MORB has more uniform isotope ratios than MORB from other ocean basins. Radiogenic isotope ratios are most strongly correlated among Pacific MORB and least correlated among Indian Ocean MORB. Means of radiogenic isotope ratios of MORB from various ridges also vary considerably. However, there is no consistent relationship between means of various isotope ratios: means of both 87 Sr 86 Sr and 143 Nd 144 Nd are lower in Pacific than in Atlantic MORB; Pb isotope ratios in MORB from the two oceans are similar. Indian Ocean MORB has higher 87 Sr 86 Sr and lower 143 Nd 144 Nd than MORB from the Atlantic and Pacific but lower 206 Pb 204 Pb . Our data strongly reinforce the view that the depleted upper mantle consists of a number of distinct subreservoirs produced by a variety of processes. Although the Pb presently in the upper mantle may not have long resided there, it could not have been derived from a primitive reservoir. A positive correlation between 206 Pb 204 Pb and 208 Pb ★ 206 Pb ★ suggests the Pb in the mantle beneath the Pacific and Atlantic is derived from an enriched reservoir. The Pb isotope ratios in Indian Ocean MORB are suggestive of the presence of a recycled component.