Jean-Guy Schilling

Evolution of the Ratio of Strontium-87 to Strontium-86 in Seawater from Cretaceous to Present

A detailed record of the strontium-87 to strontium-86 ratio in seawater during the last 100 million years was determined by measuring this ratio in 137 well-preserved and well-dated fossil foraminifera samples. Sample preservation was evaluated from scanning electron microscopy studies, measured strontium-calcium ratios, and pore water strontium isotope ratios. The evolution of the strontium isotopic ratio in seawater offers a means to evaluate long-term changes in the global strontium isotope mass balance. Results show that the marine strontium isotope composition can be used for correlating and dating well-preserved authigenic marine sediments throughout much of the Cenozoic to a precision of �1 million years. The strontium-87 to strontium-86 ratio in seawater increased sharply across the Cretaceous/Tertiary boundary, but this feature is not readily explained as strontium input from a bolide impact on land.

Mantle Plume Mixing Along the Reykjanes Ridge Axis: Lead Isotopic Evidence

Gradients of lead isotopic ratios from basalts erupted along the Reykjanes Ridge and Median Neovolcanic Zone ofIceland confirm mantle plume mixing with the depleted asthenosphere along the ridge axis. Geochemical studies of basalts erupted along the Reykjanes Ridge and its extension over the Median Neovolcanic Zone of Iceland have revealed striking gradients in minor and trace element concentrations (1). These studies and other geophysical and morphological evidence in the area have given strong support to the mantle plume hypothesis (2, 3) and led Schilling (1) to suggest a model consisting of two mantle sources, a mantle plume rising beneath Iceland and a depleted low velocity layer beneath the ridge. Mixing of these two sources at mantle depth was assumed to be the cause of the gradients, particularly in the La/Sm ratio. A subsequent 87Sr/ 86Sr test of the model (4) supported the concept presented by Schilling, but also suggested a less regular mixing along the transitional zone than was apparent from the gradient in the La/Sm ratio. Although challenged on various and debatable grounds (5-8), the binary mantle mixing model remains, we believe, the most important process in producing the gradients of large ionic lithophile (LIL) trace element ratios thus far observed along the Reykjanes Ridge-Iceland profile (1, 9). As further evidence in support of this model, we now present Pb iotopic compositions for 16 Reykjanes Ridge and 2 Iceland basalts, as well as Th, U, and Pb concentrations for 7 of these samples, whose locations are shown in Fig. 1. Results. Table I shows the Pb isotopic data obtained in this study and Table 2 the U, Th, and Pb concentrations (10, 11). All basalts analyzed are very fresh, tholeiitic in composition, and vary from slightly quartz normative to olivine normative (12). Figure 2 shows the 206Pb/204Pb and 208Pb/ 204Pb ratios of these basalts with respect to their distance from the southern tip of Iceland. Data for three samples from Sun and Jahn (13) and one from Welke et al (14), all from the Median Neovolcanic Zone of Iceland, are also plotted in Fig. 2. For comparison, 87Sr/86Sr and La/Sm ratios previously reported by Hart et al. (4) and Schilling (1), respectively, are also shown in Fig. 2. The 206Pb/204Pb profile shows high values over Iceland and low values south of 61°N to the Gibbs fracture zone. In be-

The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for 87Sr86Sr and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have 87Sr86Sr ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the 87Sr86Sr profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N. These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N. Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (⪢ 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed.

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.

Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components

Suites of depleted MORB glasses from the fast-spreading Pacific-Nazca Ridge at 28°S and 32°S and the slow-spreading eastern boundary of the Juan Fernandez microplate were analyzed for chlorine by electron microprobe. Cl contents of primitive MORB are about 20–50 ppm, similar to values reported previously for primitive MORB from the Mid-Atlantic Ridge (MAR). Cl increases steadily with decreasing MgO to 1100 ppm in evolved MORB (FeTi basalts). FeTi basalts can be related to primitive magmas by a maximum of 67% fractional crystallization based on major element modelling. The Cl concentrations in FeTi basalts exceed by a factor of 5 to 10 the amounts that can be generated by fractional crystallization of the primitive magmas. An additional process besides crystallization must be contributing the excess Cl. FeTi basalts also contain more H2O than can be produced by fractional crystallization of a primitive parent. The H2OCl ratio of the hypothetical additional component that is necessary to account for the excess Cl and H2O in FeTi basalts is 1–6 and rules out direct addition of seawater to the magma chamber. Assimilation of hydrothermally altered wall rocks of the magma chamber most likely provides the extra Cl and H2O. Selective melting or breakdown of amphibole and incorporation of Cl-rich brine contained in the wall rocks may be important processes. Bulk assimilation is less likely because the Cl content of altered crust is too low to generate the excess Cl unless unrealistically large amounts of assimilation are invoked. A magmatic source for the additional Cl and H2O cannot be ruled out on geochemical grounds but is physically unrealistic because it requires that large volumes of magma have crystallized and exsolved a Cl-rich vapor phase that has somehow migrated to a small magma chamber. Excess Cl in evolved magmas (i.e., Cl overenrichment) is best developed in evolved MORB from propagating or overlapping spreading centers such as the Galapagos Spreading Center at 85°W and 95°W and the west ridge of the Juan Fernandez microplate. Cl overenrichment has not been observed on slow-spreading ridges including the eastern ridge of the Juan Fernandez microplate, the Southwest Indian Ridge, and the mid-Atlantic Ridge. The existence of high-Cl magmas implies that some of the Cl-rich mineralization observed in deep crustal sections and ophiolites could be due to exsolved magmatic volatiles. The assimilation of hydrothermally altered material could influence the concentration and isotopic ratios of other elements which have low abundances in MORB relative to seawater.

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.

SrNdPb geochemical morphology between 10° and 17°N on the Mid-Atlantic Ridge: A new MORB isotope signature

Basalts dredged along the Mid-Atlantic Ridge axis between 10°N and 17°N have been studied for their trace element characteristics [1]. To give complementary information on mantle source history and magma genesis, these samples have been analysed for their SrNdPb isotopic compositions. There is a good correlation between the structure of the ridge axis which shows a topographic anomaly centered around 14°N and hygromagmaphile element ratios such as Rb/Sr, (Nb/Zr)N or Sm/Nd as well as isotopic ratios plotted as a function of latitude. The samples coming from the 14°N topographic high show new MORB SrNd isotopic characteristics which pictured in a classical mantle array diagram, put their representative points close to HIMU sources of ocean islands such as St. Helena, Tubuaiand Mangaia. The 14°N mantle source presents geochemical characteristics which indicate mantle differentiation processes and a mantle history that are more distinct than so far envisaged from typical MORB data. Pb data indicates that the 14°N mantle source cannot be the result of binary mixing between a depleted mantle and a HIMU-type source. Rather, the enriched endmember could itself be a mixture of Walvis-like and HIMU-like materials. The geochimical observations presented favour the model of an incipient ridge-centered plume, in agreement with [1].

A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51–52°S)

We report on He and Ne isotopes in basaltic glasses from eight dredge stations occupied over the Shona gravity and topographic anomaly high on the Mid-Atlantic Ridge around 51–52°S. The results indicate the presence of a primitive, little degassed, 3He-rich mantle component. 3He4He ratios correlate positively with the bathymetric and gravity anomaly, with values ranging from 12.5 to 6.4 times the atmospheric ratio of 1.38 × 10−6. The highest value is almost identical to that of the Bouvet and Reunion hotspots. Neon isotopic ratios in 20Ne22Ne vs. 21Ne22Ne isotope space indicate recent mixing between a primitive mantle plume component with near-solar Ne and the upper mantle MORB source; and again, the most solar-like Ne found over the Shona ridge-anomaly is similar to that found at the Reunion hotspot. For the first time, a negative trend pointing toward solar values is bridging the commonly observed MORB-air and the Hawaiian L-K-air mixing lines of positive slope. These results suggest that the source of the Shona ridge-anomaly has a similar origin to that of the 3He-rich Bouvet and Reunion plume sources in the deep mantle, which could define a particular noble gas signature in the South Atlantic and the Indian Ocean. It is also evident that the source of the Shona plume is distinct from the 4He-rich source of the Tristan and Gough family of plumes located further north.

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid-ocean ridge basalt glasses from the Mid-Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid-Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal-related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along-strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal-type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

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.