Source Of Oceanic Basalts Research Articles

Evidence of ghost plagioclase signature induced by kinetic fractionation of europium in the Earth’s mantle

Crustal recycling in the Earth’s mantle is fingerprinted by trace-element and isotopic proxies in oceanic basalts. Positive Eu and Sr anomalies in primitive lavas and melt inclusions that are not otherwise enriched in Al2O3 are often interpreted as reflecting the presence of recycled, plagioclase-rich oceanic crust in their mantle source – referred to as “ghost plagioclase” signatures. Here, we report natural evidence of Eu anomalies and extreme crystal-scale heterogeneity developed kinetically in mantle peridotite clinopyroxene. Numerical modelling shows that diffusional fractionation between clinopyroxene and melts can account for this intra-crystal heterogeneity and generate Eu anomalies without requiring plagioclase. We demonstrate that kinetically induced Eu anomalies are likely to develop at temperatures, redox conditions and transport timescales compatible with the genesis of mid-ocean ridge and ocean island basalts. Our results show that, in the absence of converging lines of evidence such as radiogenic isotope data, ghost plagioclase signatures are not an unequivocal proxy for the presence of recycled crust in oceanic basalt sources.

Conditions of Rock Formation and Magma Sources of the Late Cenozoic Udokan Volcanic Plateau

The Udokan volcanic plateau (UVP), like other areas of the Late Cenozoic volcanic province of Central Asia, formed in the interval from the Middle Miocene to the Pleistocene. Its products have high alkalinity and vary in composition from alkaline picrobasalts and basanites to alkaline trachytes. Compositional changes were controlled by two differentiation trends, which corresponded to different conditions of the original magmas generation. Rocks with low SiO2 contents (45 wt. %) were formed from melts of low melting degrees that arose under conditions of elevated pressures and temperatures. Formation of rocks corresponding to the composition range 45–61 wt. %. % SiO2 was associated with the differentiation of basalt melts, which arose at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of a moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province of Central Asia, which indicates the involvement of the lower mantle plume in the formation of these regions.

Fe3+ Distribution and Fe3+/ΣFe-Oxygen Fugacity Variations in Kimberlite-Borne Eclogite Xenoliths, with Comments on Clinopyroxene-Garnet Oxy-Thermobarometry

Abstract The valence state of iron in mantle-derived melts, such as mid-ocean ridge basalt, is a useful proxy for oxygen fugacity (ƒO2). On subduction, oceanic crust undergoes metamorphic reactions that alter its initial ƒO2, generating compositional and redox heterogeneity in the supra-subduction zone and in the convecting mantle source of oceanic basalts. Kimberlite-borne eclogite xenoliths with ancient oceanic crustal protoliths represent an important archive to trace these processes in deep time. We determined, by Mössbauer spectroscopy, Fe3+/ΣFe for garnet (0.03–0.15, average 0.08; n = 13) and clinopyroxene (0.05–0.37, average 0.23; n = 11) in bimineralic eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), combined the results with high-quality literature data to estimate bulk Fe3+/ΣFe and ƒO2, assessed associated uncertainties and discussed petrological implications. The incorporation of Fe3+ in, and distribution between, eclogite minerals is controlled by chemical composition (X), temperature (T), pressure (P) and ƒO2. Therefore, Fe3+/ΣFe in garnet alone is an imperfect proxy for bulk Fe3+/ΣFe. Values for the distribution of Fe3+/ΣFe between clinopyroxene and garnet (D(Fe3+/ΣFe)cpx-grt) range from 1.4 ± 0.1 to 20 ± 13 and correlate strongly with the abundance distribution of another polyvalent element, vanadium (r2 = 0.80, n = 28), probably because both vary as a function of X-T–P-ƒO2. This allows to estimate Fe3+/ΣFe in clinopyroxene and bulk rocks (0.01–0.32, average 0.06, n = 159) for the majority of literature data where only garnet Fe3+/ΣFe is known. Low Fe3+/ΣFe is retained in many high-Al2O3 eclogites with plagioclase-rich cumulate protoliths (Eu/Eu*> > 1), despite extended residence in the cratonic lithosphere. Bulk Fe3+/ΣFe increases during mantle metasomatism but is not particularly linked to enrichment in highly incompatible elements. Low grossular content in garnet ensuing from mantle metasomatism, especially at low temperature, limits uptake of Fe3+, which is accommodated in clinopyroxene instead, leading to very high D(Fe3+/ΣFe)cpx-grt. Eclogite oxybarometry requires that Fe3+/ΣFe of only garnet be known, whereby the input temperatures should be calculated with all Fe as Fe2+. For temperatures projected onto regional steady-state geothermal gradients, ƒO2 values relative to the fayalite-magnetite-quartz buffer (∆logƒO2(FMQ)) range from −5.9 to −0.2 (average − 3.1, n = 174), and are too low to stabilise oxidised sulphur in all and oxidised carbon in the vast majority of samples. Thus, ancient oceanic crust was a sink rather than a source of oxygen. In particular CaO-rich eclogite xenoliths occasionally are corundum-bearing and SiO2-undersaturated, typically with silica-deficient clinopyroxene with cations per formula unit <1.97 for 6 O anions, resulting in overestimated ƒO2. For the remaining samples, cryptic metasomatism, with enrichment in highly incompatible elements, is clearly focused in the ƒO2 interval of FMQ−4 to FMQ−3, indicating that mantle metasomatism can have both an oxidising and a reducing effect on eclogite. Unmetasomatised bimineralic eclogites have lower ƒO2 than coesite-bearing ones, reflecting reduction during metamorphism and silica-consuming partial melting of ancient subducted oceanic crust.

Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts

Pyroxenites are diffuse in fertile mantle peridotites and considered an important component in the mantle source of oceanic basalts. They are rarely documented in abyssal and ophiolitic peridotites representing residual mantle after melt generation, and few studies defining their origin are to date available. We present a field-based microstructural and geochemical investigation of the pyroxenite layers associated with depleted peridotites from the Mt. Maggiore ophiolitic body (Corsica, France). Field and petrographic evidence indicate that pyroxenite formation preceded the melt–rock interaction history that affected this mantle sector during Jurassic exhumation, namely (1) spinel-facies reactive porous flow leading to partial dissolution of the pyroxenites, and (2) plagioclase-facies melt impregnation leading to [plagioclase + orthopyroxene] interstitial crystallization. Pyroxenes show major element compositions similar to abyssal pyroxenites from slow-spreading ridges, indicative of magmatic segregation at pressures higher than 7 kbar. Both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by Na2O-poor, LREE-depleted compositions, consistent with unaggregated melt increments. This implies that they represent the continuous evolution of similarly depleted melts leading to different processes (pyroxenite segregation and later melt–rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt–rock interaction history, we modeled all the documented processes in sequence, i.e.: (1) formation of single-melt increments after 6% mantle decompressional fractional melting; (2) high-pressure segregation of pyroxenites; (3) spinel-facies reactive porous flow, (4) plagioclase-facies melt impregnation. The early fractionation of pyroxenites leads to a decrease in pyroxene saturation that is necessary for the subsequent reactive porous flow process, without any significant change in the melt REE composition.

Ubiquitous ultra-depleted domains in Earth’s mantle

Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle. Depleted mantle is a volumetrically dominant component of the Azores plume and possibly of oceanic basalt sources more generally, according to neodymium isotope compositions of olivine-hosted melt inclusions from lavas of the Azores mantle plume.

GENERAL PB‐ISOTOPE SYSTEMATICS OF SOURCES FOR VOLCANIC ROCKS OF THE LATEST GEODYNAMIC STAGE IN ASIA

The modern theory of the evolving Earth is based on integrated isotopic data obtained for the accessible part of the planet and cosmic bodies, in which the U–Pb isotope system plays a key role. The theory is tested by the isotope systematics of oceanic basalt sources. The origin of continental volcanic rocks is often interpreted in terms of the isotopic systematics of oceanic basalts. However, such interpretations, as a rule, reveal contradictions arising from differences in the history and current mantle dynamics of oceans and continents. Under the oceans, a mantle material has long lost connection with the accessible Earth tectonic units; under the continents such a connection is often established. The nature of the evolution of deep‐seated processes under the continents remains uncertain and, by analogy with the oceans, requires deciphering in terms of the components of the mantle sources for volcanic rocks. In modern lithospheric plates of the Earth, there are regions ranging in width from hundreds to thousands of kilometers, which are characterized by high strain rates and, consequently, at least one to two orders of magnitude lower viscosity relative to that of the internal stable parts of the plates. This gives them a special structural status of “dispersed plate boundaries”. The isotope‐geochemical studies of volcanic rocks from regions of the unstable Asia revealed the different nature of components in sources, for which particular interpretations have been proposed. In this paper, a general systematics of sources is defined for volcanic rocks of the latest geodynamic stage in Asia through estimating the incubation time on the 207Pb/204Pb versus 206Pb/204Pb diagram. Two domains are designated: (1) low 238U/204Pb (LOMU) derived from the viscous protomantle (VIPMA), and (2) elevated 238U/204Pb (ELMU). The mantle domains evolved from the Earth's primary material between 4.51 and 4.36 Gyr ago, 4.0 and 3.7 Gyr ago, 2.9 and 2.6 Gyr ago, 2.0 and 1.8 Gyr ago, about 0.66 Gyr ago and <0.09 Gyr ago. Melting anomalies of ELMU sources characterize the unstable mantle of Southern Asia, and those of LOMU sources belong to the Japan‐Baikal geodynamic corridor of the transitional region between the unstable mantle of Asia and its stable core. The Late Cenozoic evolution of the Japan‐Baikal geodynamic corridor resulted in cutting the LOMU domain by the Jeju‐Vitim ELMU source line.

Marine Carbonates in the Mantle Source of Oceanic Basalts: Pb Isotopic Constraints

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high 206Pb/204Pb coming from one of the end-components with a long time-integrated high 238U/204Pb or μ (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high 206Pb/204Pb, indicating a HIMU-like source. Moreover, measured 232Th/238U (κ) for most MORB are lower than those deduced from their 208Pb/204Pb and 206Pb/204Pb. Such high μ and low κ features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

CO2 content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550 °C, and fO2 of FMQ −3.2 to FMQ −2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln(K0) = −21.79 ± 0.04, ΔV0 = 32.91 ± 0.65 cm3mol−1, ΔH0 = 107 ± 21 kJ mol−1, and dissolution of CO2 as CO32−: ln(K0) = −21.38 ± 0.08, ΔV0 = 30.66 ± 1.33 cm3 mol−1, ΔH0 = 42 ± 37 kJ mol−1, where K0 is the equilibrium constant at some reference pressure and temperature, ΔV0 is the volume change of reaction, and ΔH0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.

Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust

Knowledge of the 187 Os/ 188 Os ratio as well as the inventories of rhenium and platinum group elements (PGE) in oceanic crust allows quantification of the proportion of recycled oceanic crust in oceanic basalt sources. Our knowledge is limited by the availability of well-characterized sections of oceanic crust, specifically of the plutonic, lower portion that has not been drilled in situ to the Moho. Here we report new data for plutonic rocks that compose the bottom 4680 m of an ocean crust section from the Oman ophiolite. Major and trace element data as well as mineral analyses indicate that Oman gabbros are primitive cumulates from melts similar to typical mid-oceanic ridge basalt. The mean weighted composition of this section (Re: 427 pg/g; Os: 55 pg/g; Ir: 182 pg/g; Pd: 2846 pg/g; Pt: 4151 pg/g; initial 187 Os/ 188 Os: 0.142) indicates significantly higher Os and lower Re concentrations than previously analyzed partial sections of ocean crust that lack cumulate lower crust [Deep Sea Drilling Project–Ocean Drilling Program (DSDP-ODP) Hole 504B, ODP Hole 735B], emphasizing that the lower, cumulate oceanic crust dominates the Os budget of oceanic crust. Analyses of mineral grain size fractions indicate that rhenium, PGE, and lead are enriched in the sulfur-rich, fine fraction. This corroborates the notion that small accessory phases, and the melt migration processes affecting them, control these elements’ budgets, distributions, and susceptibilities to alteration. The Re-Os-PGE inventories of a hypothetical 6.5-km-thick composite section that consists of 1825 m of DSDP Hole 504B−like upper oceanic crust and 4680 m of Oman-like lower ocean crust (Re: 736 pg/g; Os: 45 pg/g; Ir: 133 pg/g; Pd: 2122 pg/g; Pt: 2072 pg/g; initial 187 Os/ 188 Os: 0.146) provide a new comprehensive assessment of oceanic crust composition. Upon recycling and mixing with reasonable proportions of mantle peridotite, this composite requires at least 2 G.y. to develop sufficiently radiogenic 187 Os/ 188 Os to generate high μ (HIMU: μ = 238 U/ 204 Pb) basalts.

Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle array

The isotopic composition of oceanic basalts suggests that they are composed of true recycled oceanic crust and sediments, which are mixed with the depleted mantle. Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle1. Most2,3, but not all authors4 believe that recycled crust is incorporated into the plume source of oceanic basalts. The hafnium (Hf) and neodymium (Nd) isotopic compositions of basalts from oceanic islands and mid-ocean ridges exhibit a linear relationship—the mantle array—which is thought to result from mixing between material from the depleted mantle and an enriched recycled component. Here, we model the Hf–Nd isotopic composition of oceanic basalts as a mixture of recycled oceanic crust and depleted mantle and find that recycling of basalt alone is not sufficient to reproduce the mantle array. We conclude that oceanic sediments, which have a relatively high 176Hf/177Hf ratio, must also be recycled. Combining oceanic sediments with recycled oceanic basalts and subsequent mixing with depleted mantle peridotite produces Hf and Nd isotopic compositions that coincide with the mantle array. The composition of bulk continental crust requires the existence of a complementary low 176Hf/177Hf reservoir, which we suggest is zircon-rich sediment.

Re–Os fractionation by sulfide melt–silicate melt partitioning: A new spin

Experiments have been done to assess the role of residual sulfide (melt and crystalline monosulfide solid solution, or MSS) in controlling the behavior of rhenium and osmium during basalt petrogenesis. In order to facilitate efficient separation of the sulfide and silicate phases, sulfide melt–silicate melt partitioning experiments were done at 1200 °C, 10 5 Pa and high gravitational acceleration using a furnace mounted in a centrifuge. Additional (static) high pressure experiments (1.5 GPa; 1200, 1250 °C) were performed to measure both sulfide–silicate and MSS–sulfide partitioning. Results from high pressure experiments show that MSS–sulfide melt partitioning does not significantly fractionate Re from Os, so the behavior of these elements during mantle melting will not be sensitive to the identity of the residual sulfide phase. In contrast, most experiments produced minimum values of D sulfide/silicate, Os/ D sulfide/silicate, Re > 1, with some values > 150, which is the requisite minimum to produce the observed Re/Os fractionation in mantle-derived magmas. D sulfide/silicate for Re varies over a wide range, from > 20,000 to ∼ 20, depending on the fO 2 − fS 2 conditions imposed on an experiment, and defines two coherent groupings, based on fS 2, described by the expressions: log D sulfide/silicate = − 6.95(± 0.20) + 2.39(± 0.04) {1/2 log fS 2 − 1/2 log fO 2} (ΔFMQP > 1.5) log D sulfide/silicate = − 7.79(± 0.44) + 2.90(± 0.11) {1/2 log fS 2 − 1/2 log fO 2} (ΔFMQP < 1) in which ΔFMQP is the difference between the log fS 2 of the sample and that of the fayalite–magnetite–quartz–pyrrhotite buffer at the same temperature. Values of D sulfide/silicate for Re which apply to a specific igneous system can be predicted using the results of this study, combined with estimates of the prevailing fO 2 and fS 2. By this approach, D sulfide/silicate determined from the rhenium content of coexisting sulfide globules and silicate glass for MORB (FAMOUS locality) is similar to predicted values, whereas D sulfide/silicate measured for Loihi is somewhat lower than expected. Generally speaking, values of D sulfide/silicate for Re of ∼ 400–800 are expected for melting of oceanic basalt sources, with deviations in bulk partitioning of Re corresponding to the effect of fO 2 on mineral–melt partition coefficients, or by changes in modal sulfide content. Using the experimental partitioning data, along with reasonable estimates of source sulfur content and degree of melting, the Re–Yb systematics for MORB can be reproduced from a depleted mantle composition provided magmas undergo extensive fractional crystallization and/or magma mixing.

WATER, MELTING, AND THE DEEP EARTH H2O CYCLE

Hydrous melting driven by changes in H2O storage capacity may occur in a variety of settings in the mantle, including in oceanic basalt sources and in deeper regions above and below the transition zone. The 50–200 ppm H2O in the upper mantle likely derives from a blend of sources that may include residues of hydrous partial melting, either in the deep mantle and/or beneath arcs or oceanic islands. Relative to the large storage capacity in the transition zone, low storage capacities above and below may lead to hydrous melting for material upwelling through 410 km or downwelling through 670 km. The apparently very low storage capacity of the lower mantle (&lt;20 ppm H2O) may force melting even if downwelling rocks have normal upper mantle H2O (50–200 ppm) concentrations. Very low storage capacity in the lower mantle, if verified experimentally, presents a challenge to the view that the H2O-rich sources of oceanic island basalts reside in the lower mantle.

Re-Os and Lu-Hf Isotope Constraints on the Origin and Age of Pyroxenites from the Beni Bousera Peridotite Massif: Implications for Mixed Peridotite-Pyroxenite Mantle Sources

A suite of pyroxenites from the Beni Bousera peridotite massif, northern Morocco, have been analysed for Re–Os and Lu–Hf isotopic compositions. Measured sections of the massif indicate that pyroxenite layers make up between 1 and 9% by volume of the total outcrop. Clinopyroxenes from two Cr-diopside pyroxenites have unradiogenic Hf isotope compositions (Hfi -7·7 to -8·5) whereas those of the Al-augite suite are more radiogenic (Hfi 9·4 to 25·6). In general, the Nd–Hf isotope compositions of the pyroxenites lie close to the mantle array. One garnet pyroxenite lies significantly below the mantle Hf–Nd isotope array such that it requires an ancient history characterized by high Lu/Hf and Sm/Nd but low Lu/Hf relative to Sm/Nd. As with the Sm–Nd and Rb–Sr systems, parent–daughter and isotopic ratios for the Lu–Hf system have been recently decoupled by a partial melting event associated with transfer of the massif from mantle to crust. This created highly fractionated Sm/Nd and Lu/Hf ratios in many rocks and the pyroxenites can be referred to as ‘residual’. The near-solidus extraction of a siliceous melt from the pyroxenites is also a possible explanation for the orthopyroxene-rich margins to numerous pyroxenite layers, via reaction with peridotite. Pyroxenite Os isotope compositions are much more radiogenic than their host peridotites. In contrast to the non-systematic Nd and Hf model ages, a large portion of the pyroxenite Re–Os model ages cluster between 1·2 and 1·4 Ga, within error of the model ages defined by many Ronda pyroxenites and close to the precise 1·43 ± 0·07 Ga Lu–Hf isochron defined by clinopyroxenes from the peridotites. The Re–Os system thus seems to have been more robust to late-stage melting events that decoupled Sm/Nd and Lu/Hf isotope systematics in the pyroxenites. In contrast to pyroxenites measured from Ronda, some Beni Bousera pyroxenites have relatively radiogenic Os isotope compositions at high Os concentrations (0·18 to >2 ppb), comparable with values reported for some cratonic pyroxene-rich xenoliths. In contrast to cratonic eclogites, most pyroxenites analysed here and those reported in the literature lie close to the mantle Nd–Hf isotope array. The Nd–Sr–Pb–Hf isotopic compositions and stable isotope characteristics of these pyroxenites reflect signatures from recycled oceanic crust and sediment. Hence, mixing of such material, if present within the convecting mantle, with peridotite, could account for some of the heterogeneity seen in oceanic basalts. Small amounts of pyroxenite incorporated into peridotite can also produce the radiogenic Os isotope signatures evident in the source of oceanic basalts. However, these observations alone do not require pyroxenite to be an integral part of the convecting upper-mantle magma source region. The spectrum of Nd, Hf and Os isotope compositions also makes them a suitable component to explain some of the isotopic characteristics of the source regions of ultrapotassic magmas.

The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system

We report analyses of the 176Hf/ 177Hf ratio for 25 chondrites from different classes of meteorites (C, O, and E) and the 176Lu/ 177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of 176Hf/ 177Hf= 0.282772 ± 29 and 176Lu/ 177Hf= 0.0332 ± 2. The 176Hf/ 177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 ± 29. Because the mantle array lies above the Bulk Silicate Earth in a 143Nd/ 144Nd versus 176Hf/ 177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high 3He/ 4He ratios. Comparison of observedHf/Nd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.

Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs

OCEANIC basalts are produced by melting of the Earth's mantle and are widely used to probe its composition. The observation of systematic, coupled variations in the neodymium, strontium and lead isotopic compositions of these basalts1 has been widely interpreted as reflecting the mixing of identifiable mantle components. White2 and Zindler and Hart3 have suggested that the isotopic compositions of mid-ocean-ridge and ocean island basalts (MORB and OIB) can be considered as mixtures of a depleted MORB-type mantle component with a few other components reflecting long-term enrichment of Rb/Sr, Nd/Sm and/or U/Pb ratios. It remains unclear whether these components should be considered as hypothetical mixing endmembers, or whether they exist physically as mantle reservoirs. This question may be addressed if the mixing endmembers of individual mantle plumes can be identified. To this end, we report linear Pb–Pb and curvilinear Pb–Nd and Pb–Sr isotopic covariations in Recent lavas of Heard Island, southern Indian Ocean, indicating binary mixing. The hyperbolic relationships are unique among oceanic basalts and tightly constrain the isotopic compositions of the plume source components, which do not coincide with the mantle endmember components of ref. 3. If our results are generally applicable to other plumes, they call into question the existence of large mantle reservoirs corresponding to these components, and indicate that actual oceanic basalt source reservoirs have intermediate isotopic compositions.

Variations in 143Nd/ 144Nd and 87Sr/ 86Sr ratios in oceanic basalts

143Nd/ 144Nd ratios have been determined on 37 samples of oceanic basalt, with a typical precision of±2–3 × 10 −5 (2 σ). Ocean island and dredged and cored submarine basalts are included for which reliable measurements of 87Sr/ 86Sr ratios exist in the literature or have been measured as part of this study. A strong negative correlation exists between 143Nd/ 144Nd and 87Sr/ 86Sr ratios in basalts from Iceland and the Reykjanes Ridge, but such a clear correlation does not exist for samples from the Hawaiian Islands. However, when other ocean island basalts from the Atlantic are included there is an overall correlation between these two parameters. Increases and decreases in Rb/Sr in oceanic basalt source regions have in general been accompanied by decreases and increases respectively in Sm/Nd ratios. The compatibility of the data with single-stage models is assessed and it is concluded that enrichment and depletion events, which are consistent with transfer of silicate melts, are responsible for the observed variation.

Tectonic Evolution of the Southern Gulf of Mexico

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth. Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm 3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer. In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba. The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatan and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.