Richard Kingsley

Nd-Sr-Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume-Continental Lithosphere Interaction

We report on the rare earth and Nd-Sr-Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800-km-long Nd-Sr-Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil-Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd-Sr-Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end-members which can be interpreted as Pan-African continental lithosphere along the Gulf of Tadjoura (a hybrid EM-l-EM-2), a mantle plume (relatively young HIMU-like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL-like). Combined data from the Gulf of Aden-Red Sea-Afar-Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical-thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan-African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU-like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient stage of either development or abandonment (overlapping with the Afar NW neovolcanic zone), where ocean island alkali volcanism dominates and diffuse lithosphere extension may still operate; (3) the high alkaline field of the Aden Volcanic Series; and (4) the Ethiopian Rift around 8°N in a purely continental setting. The NW Afar neovolcanic zone, which is essentially at a nascent stage of seafloor spreading and is overlapping the ring and the center of the pancakelike flattened mantle plume, is dominated by tholeiites derived from depleted asthenospheric material entrained by the plume during its original ascent. Plate reconstructions further suggest that the original center of the flattened mantle plume head has moved with the lithosphere some 900 km northeastward. The stem feeder of the plume has now been drawn or tilted toward the Afar as a result of the migration of the Gulf of Aden/Red Sea spreading centers which act as sinks of asthenospheric material and the likelihood that the feeder of the mantle plume is encountering with time an African lithosphere increasing in age, thickness, and rigidity.

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.

Petrological and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: Influence of the Tristan da Cunha mantle plume☆

Abstract Basalts from a section of the Mid-Atlantic Ridge close to the active volcanic island of Tristan da Cunha in the South Atlantic have been analysed to investigate the influence of the mantle plume on the geochemistry of basalts being erupted at the spreading center. Although petrographically the rocks show only limited variation, two basaltic types were determined to be erupting in this region based on their major, trace and REE compositions. One group shows depletion in the incompatible and LRE elements, and can be characterised as N-type mid-ocean ridge basalts. The second group shows “enriched” geochemical characteristics and is similar to T-type MORBs. Mixing hyperbolae for the incompatible element and REE ratios suggest that extensive mixing of an end-member, characteristic of a plume region with an end-member of normal depleted MORB, canaccount for the occurrence of the T-type MORBs in this region.Based on the nature and development of the Tristan da Cunha mantle plume over the past 100 Ma, a composite model of evolution is suggested,in which a ridge-centered hotspot progressed to a near ridge hotspot, and finally to a totally intraplate situation. The fact that Tristan da Cunha is highly alkalic now, but that an irregular geochemical anomalyis also present on the Mid-Atlantic Ridge at this latitude would suggest an intermediate stage between the near-ridge and totally intraplate situation. This model leads to the conclusion that, as the Mid-Atlantic Ridge migrated away from the Tristan hotspot, a preferential sublithospheric flow towards the Ridge was established. This discontinuous feature can explain the geochemical variations seen along the Mid-Atlantic Ridge by providing a mechanism for mixing of a depleted N-type MORB component with an enriched component originating through processes active at the Tristan da Cunha mantle plume.

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.

Plume‐ridge interaction in the Easter‐Salas y Gomez seamount chain‐Easter Microplate system: Pb isotope evidence

Lead isotope ratios are reported on fresh basalt glasses from 28 seamounts located along the western end of the Easter-Salas y Gomez seamount chain (ESC), southeast Pacific for the purpose of testing the mantle plume source-migrating ridge sink (MPS-MRS) model for the origin of this extensive intraplate neovolcanic seamount chain. Most of these basalts west of 103°W are less than 1.5 m.y. old. Results reveal a 1100-km-long westward gradient decreasing in radiogenic Pb from Salas y Gomez to the Easter microplate (EMP). The results confirm the previously established four-point average trend based on data from Salas y Gomez, Easter Island, the east rift, and the west rift of the Easter microplate [Hanan and Schilling, 1989]. Tight linear arrays in Pb isotope space of the combined EMP and ESC data sets indicate binary mixing between the radiogenic isotope-rich Salas y Gomez plume and the depleted asthenosphere source is the dominant factor in controlling Pb isotopic variation in the region. Mapping of the plume component mass fraction reveals not only a westward dilution of the plume component from Salas y Gomez to the EMP but also dilution with depleted material across the ESC on both flanks. The mapping is consistent with the MPS-MRS model which calls for a subhorizontal, sublithospheric plume channel connecting the mantle plume conduit near Salas y Gomez with the East Pacific Rise, where plume flow is confined along a thermal groove. Mixing by entrainment of surrounding depleted material takes place progressively downstream. A numerical thermal lithosphere cooling model reveals a rheological boundary layer (i.e., the base of the lithosphere) shoaling by 1.3° to 3.5° from Salas y Gomez to about 100 km east of the EMP. The slope magnitude is consistent with those suggested by laboratory and numerical fluid dynamic models for hot buoyant plume-ridge channeled flow to develop and be maintained.

Carbon in Mid-Atlantic Ridge basalt glasses from 28°N to 63°N: Evidence for a carbon-enriched Azores mantle plume

We report on 91 analyses of CO2 in fresh, ‘macrovesicle-free’ Mid-Atlantic Ridge (MAR) basalt glasses from 48 dredge stations located between 28°N and Iceland. A stepped heating gas chromatographic technique was used for the analyses. Concentrations of carbon as CO2 in the glasses range from 30 to 1014 ppm and always exceed the experimentally determined pressure-dependent equilibrium CO2 solubility curve for tholeiitic basalts at 1200°C determined by Stolper and Holloway [1]. The measured CO2 content mostly represents oversaturation of CO2 in the magma and, to a lesser extent, CO2 trapped in microvesicles during the latest stage of magma emplacement and quenching on the seafloor. The degree of ‘oversaturation’ varies from 1 to 9 times the equilibrium solubility values at any given depth. It is highest over the deeper part of the MAR profile, along the so-called normal ridge segments, and lowest close to Iceland and over the center of the Azores platform, where the oceanic crust is thicker and the mantle hotter than usual. The C/3 He ratio ranges from 1.4 × 109 to 4.3 × 109, as compared to a mean of 2 ± 1 × 109 previously reported for normal MOR segments. The C/3 ratio in MAR glasses over the Azores province correlates positively with 87Sr/86Sr ratio and reaches a maximum over the Azores platform. Based on this correlation, binary mixing modeling indicates minimum enrichments of carbon in the Azores plume ranging from 3.3 to 4.6, using 87Sr/86Sr ratios, and 4.9-;8.8, using 143Nd/144Nd ratios, depending on the enrichment of Sr and Nd estimated in the plume relative to the depleted asthenosphere. For comparison, the H2O content in the Azores plume is enriched by a factor of 2–4. The carbon enrichment over the Azores platform (plume) is in agreement with an independent predictive model which assumes that the likelihood of CO2 outgassing and the percentage fusion along the ridge are proportional to crustal thickness or excess ridge elevation.

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.

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid-oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA-type basalts (H2O/Ce = 215 ± 30, δDSMOW = -45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = -30 to -40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = -57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (-68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = -94 ± 3 ‰). A multi-stage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration and re-equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of <1% carbonated eclogite- ± sediment-derived C-O-H-Cl fluids to depleted mantle at 180 to 280 km (EM) or within the transition zone (PREMA).

Spatial and Temporal Variation in Otolith Chemistry for Tautog (Tautoga onitis) along the US Northeast Coast

Abstract Elemental concentrations and stable (&dgr; 18O, &dgr;13C) isotopic ratios in otoliths of young-of-the year (YOY) Tautoga onitis (Tautog) captured in nurseries in Rhode Island, Connecticut, New Jersey, and Virginia were determined using otolith micro-chemistry. Multi-chemical signatures differed significantly among the distinct nurseries among regions (MANOVA: P < 0.001) and between years (MANOVA: P < 0.001). Classification accuracy for Tautog nurseries among regions ranged from 92% to 96% for each of the two years. Since accurate classification of juvenile Tautog to their nursery sites was achieved, otolith chemistry can potentially be used as a natural habitat tag in assigning adult Tautog to their respective estuarine nurseries, but it is important to consider that the chemical signals may change annually.