Showing papers in "Geochemistry Geophysics Geosystems in 2000"

Dehydration and melting experiments constrain the fate of subducted sediments

Abstract: [1] Geochemical tracers demonstrate that elements are cycled from subducted sediments into the arc melting regime at subduction zones, although the transfer mechanism is poorly understood. Are key elements (Th, Be, Rb) lost during sediment dehydration or is sediment melting required? To investigate this question, we conducted phase equilibria and trace element partitioning experiments on a pelagic red clay for conditions appropriate to the slab beneath arc volcanoes (2–4 GPa, 600°–1000°C). Using both piston cylinders and multianvils, we determined the solidus, phase stabilities, and major element compositions of coexisting phases. The solidus (H2O + Cl fluid-saturated) was located at 775 ± 25°C at 2 GPa, 810 ± 15°C at 3 GPa, and 1025 ± 25°C at 4 GPa with noevidence for complete miscibility between melt and fluid. This sediment composition produces a profusion of phases both above and below the solidus: garnet, jadeitic pyroxene, alkali-rich amphibole, phengite, biotite, magnetite, coesite, kyanite, apatite, zircon, Cl-rich fluids, and peraluminous to peralkaline granitic melts. At 2 GPa the phengite dehydration solidus is at 800°–825°C, while biotite breaks down between 850° and 900°C. To explore trace element partitioning across the solidus at 2 GPa, we used diamonds to trap fluids and melts. Both the bulk sediment residues and diamond traps were analyzed postexperiment by inductively coupled plasma–mass spectrometry (ICP-MS) and inductively coupled plasma–atomic emission spectrometry (ICP-AES) for 40 elements for which we calculated bulk partition coefficients (D = Csolid/Cfluid). Below the solidus, Rb, Sr, Ba, and Pb showed the greatest mobility (D ∼ 0.5–1.0), while at the solidus, Th and Be became notably partitioned into the melt (D values changing from >2.0 to <1.0). K and Rb D values fall below 1.0 when the micas breakdown. Only at the solidus do Th and Rb attain similar partition coefficients, a condition required by arc data. Taken together, the experimental results indicate that critical elements (Th and Be) require sediment melting to be efficiently transferred to the arc. This conclusion is at odds with most thermal models for subduction zones, which predict slab temperatures more than 100°C lower than sediment solidi. Thus the condition of sediment melting (with oceanic crust dehydration) may provide new constraints on the next generation of thermal/geodynamical models of subduction zones.

Mantle solidus: Experimental constraints and the effects of peridotite composition

Abstract: [1] A review of experiments on natural peridotites allows improved constraints on the location of the mantle solidus. Available constraints on the location of the nominally dry peridotite solidus show considerable scatter, owing to interlaboratory uncertainties and the effects of bulk composition. When experiments on enriched peridotite are filtered from the database, the best fit to the solidus between 0 and 10 GPa is given by T(°C) = aP2 + bP + c where a = −5.104, b = 132.899, and c = 1120.661 and P is in gigapascals. Compared to previous models, the solidus in this parameterization is at lower temperature between 2 and 6 GPa, with the largest differences near 4 GPa, where it is 30°–60°C cooler. Consideration of experimental constraints on the peridotite solidus and of a theoretical model of melting in a simple analogue system suggests that a key variable affecting peridotite solidus temperature is the near-solidus liquid alkali concentration. The effect of alkalis on the solidus increases with bulk concentration in the peridotite but decreases with bulk partition coefficient. Thus small bulk concentrations of K can have a significant influence on the peridotite solidus, and the effect of Na diminishes with increasing pressure, as it becomes more compatible in the solidus residua. Mg # [=100 × MgO/(MgO + FeO)] variations are subordinate to alkali variations in controlling solidus temperature at lower pressures but may increase in relative importance as alkalis become more compatible in peridotite residua with increasing pressure. Increased clinopyroxene mode has the effect of making Na more compatible in residual solids and so diminishes the solidus-lowering tendencies of alkalis. As a consequence, experiments performed on a range of peridotite compositions may not reflect the likely effect of variable mantle composition on solidus temperature if they do not match the appropriate correlation between alkali content and clinopyroxene mode.

Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc

Abstract: [1] Plate processing at convergent margins plays a central role in the distribution of elements among major earth reservoirs. The mechanisms by which this distribution occurs, however, have remained poorly constrained. This paper provides new constraints through a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc. The data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid-ocean ridge basalt (MORB)-like isotopic compositions but high Pb/Nd; (2) a hydrous fluid from subducted sediment, with sediment-like isotopic compositions and an enrichment in fluid-mobile elements; (3) a sediment partial melt, with sediment-like isotopic compositions and high Th/Nd and Th/Nb compared to both regional and global sediments. The sediment melt is depleted in fluid-mobile elements, indicating loss of fluid prior to melting. The high Th/Nb of the sediment melt indicates presence of a Ti-rich residual phase such as rutile during partial melting. The observation that sediment fluid and sediment melt can be distinguished in different volcanic rocks suggests that they arrive separately at the sites of arc magma formation. This indicates release of multiple discrete fluid and melt phases from sediment to the overlying mantle wedge, which can be viewed as a natural consequence of progressive metamorphism of the subducting slab.

Self-consistent generation of tectonic plates in time-dependent, three-dimensional mantle convection simulations

Abstract: [1] Presented here are self-consistent, three-dimensional simulations of mantle convection, some of which display an approximation of plate tectonic behavior that is continuous in space and time. Plate behavior arises through a reasonable material description of silicate deformation, with a simple yield stress being sufficient to give first-order plate-like behavior; however, the required yield strength or fault frictional coefficient is much less than experimentally determined values. Toroidal:poloidal ratios are within geologically observed limits. The sensitivity of the system to yield strength and the form of strength envelope is systematically investigated. Optimum plate character is obtained in a narrow range of yield strength, below which diffuse boundaries, and above which episodic behavior, and eventually a rigid lid, are observed. Models with mobile lids develop very long wavelength horizontal structure, the longest wavelength possible in the domain. Two-dimensional models display much greater time dependence than three-dimensional models.

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

Abstract: We explore the hypothesis that southern Africa is actively being uplifted by a large-scale, positively buoyant structure within the mid-lower mantle. Using a new formulation in which dynamic topography and uplift rate are jointly used, we place constraints on mantle density and viscosity. The solution of the momentum equation is coupled with the advection of the density field to solve for the surface uplift rate in both an axisymmetric and fully spherical geometry. We demonstrate how dynamic topography and its rate of change depend on density and lateral and radial variations in viscosity. In the full spherical models the geometry of mantle density is derived by scaling a tomographic shear velocity model. Using a variety of geologic observations, we estimate residual topography (i.e., the topography remaining after shallow sources of density are removed) and an average Cenozoic uplift rate to be 300–600 m and 5–30 m/Myr, respectively, for southern Africa. We are able to satisfy these constraints with a mantle model in which the mid-lower mantle beneath southern Africa is 0.2% less dense and has a viscosity of ∼ 10^22 Pa s. In addition, if the continental lithosphere is thick beneath southern Africa, as suspected from seismic inversions, and has a high effective viscosity, then we find that southern Africa can be further elevated owing to increased coupling between the deep mantle and surface. We show that recent estimates of mantle density, suggesting that the lowest parts of the African anomaly may be anomalously dense are compatible with geologic constraints. We conclude that uplift rate, when combined with estimates of present-day dynamic topography, provides a powerful tool to constrain the properties of the deep mantle.

A global geochemical database structure for rocks

Abstract: [1] This technical brief describes a geochemical and petrological database structure based on the relational model that has broad applicability to chemical analyses of geological materials. Notable features of the database structure are its comprehensiveness and flexibility. The structure consists of 34 interrelated tables, which can accommodate any type of analytical values for all different materials of rock samples (volcanic glasses, minerals, inclusions, etc.) and for samples from any tectonic setting. A broad spectrum of supplementary information (metadata) is included that describes the quality of the analytical data and sample characteristics, such as petrography, geographical location, and sampling process, and that can be used to evaluate, filter, and sort the chemical data. All data in the database are linked to their original reference. The database structure can be implemented in any relational database management system (RDBMS). It is currently applied in two different rock database projects (RidgePetDB and GEOROC).

Partitioning of rare earth elements, Y, Th, U, and Pb between pargasite, kaersutite, and basanite to trachyte melts: Implications for percolated and veined mantle

Abstract: [1] A new set of partitioning data for rare earth elements (REE: La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb), Y, Th, U, and Pb has been obtained for 25 calcic amphiboles (pargasites and kaersutites) crystallized from alkali-basaltic and basanitic bulk rock compositions at pressure P = 1.4 GPa, and temperature T between 950° and 1075°C. The variations of amphibole/liquid partition coefficients and of their ratios relevant to petrogenetic studies are discussed with reference to the major element composition of the amphiboles and of the coexisting melt, and to the crystal chemical mechanisms for trace element incorporation. Our results support the conclusions that REE and actinides are incorporated into the M4 cavity in calcic amphiboles and distributed between the two available sites within that cavity and that Pb is incorporated into the A site. In our sample population, REE patterns are systematically enriched in heavy REE (HREE), as expected from the presence of significant cummingtonite component. No significant fractionation is observed between Th and U. The major factor controlling the amount of trace element incorporation is the SiO2 content of the melt. The major implication of this study is that HREE can become compatible in amphibole in systems with SiO2 content greater than ∼50 wt %, whereas LREE always remain incompatible. We use the new DREEamph/l values to calculate the effects of amphibole crystallization during melt migration in the upper mantle by reactive porous flow as well as fractional crystallization of amphibole during melt migration in veined systems. We show that both processes will lead to residual liquids and solids with extremely variable LaN/YbN ratios.

Abundance of zinc isotopes as a marine biogeochemical tracer

Abstract: [1] The Zn isotopic compositions of ferromanganese nodules, sediment trap samples, sediments, and organic reference samples have been analyzed by multiple collector inductively coupled plasma–mass spectrometry (ICP-MS). The range of isotopic variations (1‰ for 66Zn/64Zn) is significant with respect to the analytical precision (0.04‰ at the 95% confidence level). Marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from Reunion Island. We infer that the depletion of the light Zn isotopes in marine particles and in ferromanganese nodules (presumably equilibrated with seawater) results from biological activity in the upper water column. The δ66Zn values of sediment trap samples collected near the upwelling off the coast of Mauritania (central Atlantic) show a seasonal isotopic fluctuation consistent with biological pumping during the high-productivity period. Higher values of δ66Zn in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity.

Melting of a complete section of recycled oceanic crust: Trace element and Pb isotopic evidence from Iceland

Abstract: [1] We present new trace element and isotopic data on a selection of least evolved lavas in Iceland. Pb isotopic compositions are variable: 206Pb/204Pb ranges from 18.05 in picrites to 19.15 in alkali basalts. Large excesses of Sr and Pb and a deficiency of Hf and Zr relative to rare earth elements are characteristic of the picritic lavas (Ce/Pb ≈ 10, Sr/Nd ≈ 35, and Sm/Hf ≈ 1.75), whereas more normal ratios are observed in the alkali basalts (Ce/Pb ≈ 40, Sr/Nd ≈ 10, and Sm/Hf ≈ 1.4). These variations are correlated to systematic changes in Sr, Nd, and Pb isotopic compositions. Using these new data, the relationship between the Iceland plume and the Mid-Atlantic Ridge can be reexamined. The 87Sr/86Sr versus 206Pb/204Pb relationship demonstrates clearly that all isotopic variations in Iceland have their origin in the plume source. In addition, the composition of both Reykjanes and Kolbeinsey Ridges results from contamination of the depleted mantle by the Iceland plume. Trace element and isotopic heterogeneities within Iceland are related to rock type and not to location in the island. This is contrary to what would be expected if a uniform plume mixed with surrounding mid-ocean ridge basalt (MORB), in which case the geochemical values would be more MORB-like at the perimeter. Instead, it suggests that the Iceland plume itself was highly heterogeneous. We propose that the source of the Iceland plume was old recycled oceanic crust that was stored in the mantle for a long time (potentially since the end of the Archean). Melting of the basaltic portion of this crust led to the formation of the alkali basalts, and melting of the clinopyroxene-plagioclase gabbroic portion of the same oceanic crust yielded the picritic basalts. In both cases, melting of the harzburgitic portion of the recycled lithosphere provided the necessary Mg and Ni. The tholeiites represent mixtures from the two sources. Iceland is therefore, together with Hawaii, a clear example of melting of a complete section of recycled oceanic lithosphere.

Age and geochemistry of basaltic complexes in western Costa Rica: Contributions to the geotectonic evolution of Central America

Abstract: The age and origin of magmatic complexes along the Pacific Coast of Central America have important implications for the origin and tectonic evolution of this convergent plate margin Here we present new 40Ar/39Ar laser age dates, major and trace element data, and initial Sr-Nd-Pb isotope ratios The 124– 109 Ma tholeiitic portions of the Santa Elena complex formed in a primitive island arc setting, believed to be part of the Chortis subduction zone The geochemical similarities between the Santa Elena and Tortugal alkaline volcanic rocks suggest that Chortis block may extend south of the Hess Escarpment The Nicoya, Herradura, Golfito, and Burica complexes and the tholeiitic Tortugal unit formed between 95 and 75 Ma and appear to be part of the Caribbean Large Igneous Province, thought to mark the initiation of the Gala´pagos hotspot The Quepos and Osa complexes (65–59 Ma) represent accreted sections of an ocean island and an aseismic ridge, respectively, interpreted to reflect part of the Gala´pagos paleo-hotspot track An Oligocene unconformity throughout Central America may be related to the mid-Eocene accretion of the Quepos and Osa complexes

Self-consistent generation of tectonic plates in time-dependent, three-dimensional mantle convection simulations 2. Strain weakening and asthenosphere

Abstract: [1] In a previous paper [Tackley, 2000b] it was shown that a combination of temperature-dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic-like behavior in three-dimensional models of mantle convection. Here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries: strain weakening and the presence of a low-viscosity asthenosphere. Introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus (1) dramatically improves plate quality, even if the asthenosphere is restricted to regions around spreading centers, (2) gives good, smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude, and (3) gives bimodal stable solutions over a range of yield stresses; either immobile-lid or plate behavior may be obtained, depending on initial condition. By contrast, introducing an asthenosphere by using depth-dependent viscosity has a much smaller effect on system behavior. Increasing strain weakening (1) improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates, (2) weakens convergent zones and can make downwellings highly episodic, and (3) does not lead to pure transform boundaries in these calculations. Time-dependent “damage” evolution and instantaneous strain-rate weakening give very similar results. All cases that display platelike behavior have very long wavelength mantle heterogeneity, consistent with Earth's.

Changes in dissolved oxygen in the Southern Ocean with climate change

Abstract: [1] Climate models predict that increasing greenhouse gases in the atmosphere will change our climate. However, the range of observations available to assess these model predictions is limited. We have utilized an ocean biogeochemical model to project the impact of climate change on the oxygen cycle in the ocean and the air-sea exchange of oxygen. With climate change, our model predicts an outgassing of oxygen from the ocean. In the model, the Southern Ocean is the region that displays the largest change in the air-sea exchange of oxygen with climate change. The reduced oxygen uptake in the Southern Ocean is dominated by a reduction in convective mixing and in the subduction rate of bottom water which decreases the amount of dissolved oxygen in the Southern Ocean. Our model results suggest that dissolved oxygen is sensitive to changes in Southern Ocean circulation revealing the potential to use dissolved oxygen to test model predictions of climate change. From Southern Ocean observations collected 28 years a part we observed a decrease in dissolved oxygen that was consistent with model climate change simulation. This provides credibility to the climate model predictions. Additional analysis and observations of dissolved oxygen are needed to substantiate our climate model simulations.

Numerical models of the geodynamo and observational constraints

Abstract: [1] The past few years have seen the emergence of a large number of numerical simulations of the geodynamo. In parallel, both new and old geomagnetic, archeomagnetic, and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models. Naturally, model predictions should be tested against actual characteristics of the geomagnetic field. Despite huge differences (sometimes in excess of a billion) between the values of parameters used in the simulations and those estimated for the Earth, it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures, in agreement with the first-order characteristics of the geomagnetic field. Yet, when considering finer characteristics, there are significant differences, and failures to actually produce a number of fundamental characteristic features. In this presentation, we first review numerical results obtained to date, then we attempt to summarize which field characteristics derived from observational data sets can be considered robust. On the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum, we sort presumably characteristic geomagnetic features into three categories (robust, controversial, and unlikely). We conclude that numerical models should be illustrated with a number of key “predictions,” averaged over at least 10 dipole diffusion times. These predictions should be tested against the subset of robust observations only. Controversial observations should await additional confirmation.

New experimental observations on the anhydrous solidus for peridotite KLB‐1

Abstract: [1] New multianvil experimental data are reported for the anhydrous solidus for peridotite KLB-1 at 5 to 9.7 GPa. The solidus is unaffected by variable experimental run durations, but it is lowered in temperature when rhenium capsules are replaced with graphite. Using data reported previously, the anhydrous solidus for KLB-1 can be described by T = 1086 − 5.7P + 390 ln P (±68°C 2σ) at pressures that range from 2.7 to 22.5 GPa. At lower pressures the effect of composition on the temperature of the spinel peridotite solidus is small and dominated by alkali abundances; variations in Mg number have almost no effect on solidus temperature.

The 40Ar/39Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic)

Abstract: The 40Ar/39Ar ages for 35 volcanic rocks and 14C ages for two charcoal samples from the Madeira Archipelago and Ampere Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (>4.6–0.7 Ma) and a subsequent low-volume posterosional stage ( 70 Myr old hotspot which formed Porto Santo Island (11.1–14.3 Ma), Seine, Ampere (31 Ma), Corral Patch and Ormond (65–67 Ma [Feraud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70–72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at 43°36′N/ 24°33′W.

Comparison of continental and oceanic mantle electrical conductivity: Is the Archean lithosphere dry?

Abstract: [1] Electrical conductivity profiles derived from magnetotelluric and geomagnetic sounding methods provide a means of constraining upper mantle properties that is complementary to seismic studies. We analyze conductivity profiles for an Archean craton and an oceanic setting, in conjunction with independent constraints on mantle geotherms, to estimate the water content of the upper mantle in these very different geologic environments. Results from this analysis indicate that the Archean lithosphere contains less water than the oceanic mantle in the depth range between ∼150 and ∼250 km. Below ∼250 km these oceanic and continental environments show similar conductivities, suggesting that the water content of the mantle does not vary significantly between ∼250 km and the 410-km discontinuity. These observations indicate that the Archean lithosphere may be stabilized against convective instabilities partly because it has a high viscosity associated with a dry composition.

The δ44Ca-temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures

Abstract: We report direct δ44Ca-temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a 43Ca-48Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species-dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ44Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ44Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ44Ca difference between marine isotope stage 1 (MIS-1) and MIS-2 is 0.71 ± 0.24. According to the current δ44Ca-temperature calibration this value corresponds to a temperature difference between MIS-1 and MIS-2 of ∼3.0 ± 1.0°C.

Coccolith Sr/Ca as a new indicator of coccolithophorid calcification and growth rate

Abstract: [1] Polyspecific coccolith separates from core top sediments in the eastern equatorial Pacific show variations of ∼15% in Sr/Ca ratios across the equatorial upwelling zone, with Sr/Ca highest at the equator and decreasing off-axis. These variations cannot be due to changes in the Sr/Ca of seawater, which varies by less than 2% in the surface ocean. Variations in Sr/Ca of coccolith sediments are similar to variations in primary productivity and alkenone-estimated coccolithophorid growth rates in overlying surface waters and to CaCO3 rain rates measured in sediment traps. Because of these relationships and because calcification rate exerts a strong control on Sr/Ca in abiogenic calcites, we suggest that the observed Sr/Ca variations in coccoliths may be strongly controlled by coccolithophorid growth and calcification rates, although temperature may also influence coccolith Sr/Ca to a lesser degree. Changes in dissolution intensity and coccolith assemblages appear to exert a minor influence, if any, on coccolith Sr/Ca in these sediment core tops. If further work confirms relationships between coccolith Sr/Ca and coccolithophorid productivity, Sr/Ca records of past changes in coccolithophorid productivity may be useful in reconstructing past variations in the rain ratio of organic to carbonate carbon, an important control on deep ocean pH and partitioning of CO2 between the atmosphere and ocean. In addition, coccolith Sr/Ca might provide an independent record of past changes in coccolithophorid growth rates, which in combination with data on the carbon isotopic fractionation in coccolithophorid organic matter may permit more reliable calculations of past dissolved CO2 in the surface ocean.

Assessing the presence of garnet-pyroxenite in the mantle sources of basalts through combined hafnium-neodymium-thorium isotope systematics

Abstract: [1] The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel-garnet transition are different if garnet-pyroxenite is present in the peridotitic mantle Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate Melts derived from garnet-peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts

Continental crust formation by crustal delamination in subduction zones and complementary accumulation of the enriched mantle I component in the mantle

Abstract: [1] A possible mechanism for continental crust formation, which includes slab-dehydration-induced mantle melting, basalt magma generation, remelting of the initial basaltic crust, followed by delamination of the residue, is examined by geochemical modeling of dehydration, partial melting, and fluid-solid reactions. The results suggest that such processes can explain the major and trace element compositions of the andesitic bulk continental crust. Isotopic modeling suggests that a pyroxenitic delaminated component formed at 3–4 Ga possesses Sr-Nd-Pb isotopic compositions typical of the EMI component, one of the enriched geochemical endmembers in the mantle. Continental crust formation and complementary accumulation of the EMI reservoir in the deep mantle may have taken place simultaneously in Archean subduction zones.

Primitive CaO‐rich, silica‐undersaturated melts in island arcs: Evidence for the involvement of clinopyroxene‐rich lithologies in the petrogenesis of arc magmas

Abstract: On the basis of the study of olivine-hosted melt inclusions in a calc-alkaline basalt from Batan Island (Philippines) we define a distinctive type of primitive, nepheline-normative island arc magma characterized by unusually high CaO contents (up to 19.0 wt %) that cannot be simply explained by melting of the metasomatized peridotitic mantle wedge above subducting oceanic lithosphere. CaO-rich melt inclusions with these characteristics are preserved in Fo85-90 olivine, and compositional variations among the inclusions are interpreted to reflect mixing between melts such as those found in the most CaO-rich inclusions (present in Fo90 olivine) and melts similar to primitive "normal" island arc magmas (trapped in Fo85 olivine). Compilation of primitive island arc magmas from the literature shows that whole rocks and olivine-hosted melt inclusions with CaO contents >13 wt % are found in many arc volcanoes from all over the world in addition to Batan. These inclusions occur in lavas ranging from CaO-rich ankaramites to basaltic andesites with low-CaO contents (i.e., <13 wt %). The globally occurring CaO-rich inclusions and whole rocks comprise a group that although defined on the basis of their CaO contents is compositionally distinctive when compared to island arc lavas that have lower CaO contents; for example, they have lower FeO at a given SiO2 content than most arc lavas, and they are all nepheline normative, with normative nepheline contents positively correlated with CaO contents. Variations in CaO content and normative compositions of experimental partial melts of lherzolite related to changes of pressure, temperature, and source composition suggest that there are no conditions under which partial melting of peridotite can generate melts having CaO contents and other properties comparable to those observed for the primitive, CaO-rich arc-derived melts identified here. Although melting of peridotite at high pressure in the presence of CO2 can produce CaO-rich, silica-poor liquids, we consider it unlikely that this is responsible for producing the CaO-rich, silica-undersaturated melts considered in this study because there are significant differences in nearly all other compositional characteristics between the CaO-rich arc magmas and melts known or thought to be produced by melting of carbonated peridotite. Model major element compositions of partial melts of clinopyroxene-rich lithologies (mantle pyroxenites, lower crustal pyroxenites, and eclogites) calculated using the MELTS algorithm suggest that the most CaO-rich, nepheline-normative melt inclusions and whole rocks identified here could represent intermediate to high degree (~10-40 wt %) partial melts of pyroxenites at lower crustal to upper mantle pressures. Such a hypothesis is supported by the comparison between the trace element compositions of model pyroxenite sources of the Batan CaO-rich melt inclusions and naturally occurring pyroxenites. The most likely source of the primitive CaO-rich, silica-undersaturated arc melts identified here is lower crustal and shallow upper mantle pyroxene-rich cumulates from arc environments because these cumulates have CaO concentrations at the upper end of the range observed for mantle pyroxenites. They are therefore more likely to yield partial melts with the restricted range of remarkably high CaO contents of the most CaO-rich inclusions and whole rocks identified here. Moreover, these cumulates often contain amphibole, which would lower their solidus temperatures relative to the anhydrous pyroxenite equivalents to values more consistent with those expected in deep crustal or shallow subarc environments.

Variation of foraminiferal Sr/Ca over Quaternary glacial‐interglacial cycles: Evidence for changes in mean ocean Sr/Ca?

Abstract: [1] Records of Sr/Ca changes in planktonic and benthic foraminifera from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr. There is a general increase in foraminiferal Sr/Ca over the penultimate glaciation declining to minimum values during stage 5 and an increase in Sr/Ca from stage 5 through stage 2. Coincident changes in benthic foraminifera records from the Atlantic and Pacific basins imply that Sr/Ca variations are not dominated by dissolution. Planktonic culturing data provide evidence that the downcore Sr/Ca variations are not controlled by temperature changes and suggest only a small influence of salinity and pH. Variation common to the records is most readily explained by changes in mean ocean Sr/Ca. If fossil foraminifera reliably record higher glacial seawater Sr/Ca, coral Sr paleothermometry would underestimate sea surface temperature during glacialepisodes.

Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing?

Abstract: [1] The geochemical variations in basalts from Iceland and the Reykjanes Ridge have for a long time been attributed to mixing between the depleted mid-ocean ridge basalt (MORB) asthenosphere source and the enriched Iceland mantle plume (Schilling, 1973; Hart et al., 1973; Sun et al., 1975). In contrast, the occurrence of some Iceland basalts with relatively depleted incompatible trace element ratios and low Pb and Sr isotope ratios compared to normal MORB (NMORB) have been interpreted to indicate that the Iceland mantle plume transports both geochemically enriched and depleted material from the lower mantle into the upper mantle and that the depleted MORB asthenosphere source does not contribute significantly to the generation of the Iceland basalts (Thirlwall et al., 1994; Thirlwall, 1995; Hards et al., 1995; Kerr, 1995; Kerr et al., 1995; Fitton et al., 1997; Hardarson and Fitton, 1997; Hardarson et al., 1997; Kempton et al., 1998, 1999; Nowell et al., 1998). As a follow-up to this controversy, we show on the basis of new Nb, Zr, Y, La, and Sm elemental abundances and Hf isotope ratios reported here, and published trace element and isotope data for Iceland and the North Atlantic MOR (50°–79°N), that the apparent distinction between Iceland basalts and MORB can readily be accounted for by a three-component mixing model involving two incompatible trace element enriched components, one with relatively high, the other with low 206Pb/204Pb ratios, and the usual surrounding depleted MORB mantle source as the third component. The radiogenic 206Pb-rich component represents the hot Iceland mantle plume source, while the enriched but low-206Pb/204Pb EM1-like component most likely represents entrained subcontinental lithospheric material embedded in the North Atlantic depleted MORB source. The incompatible element and isotope ratio variations do not require a depleted Iceland plume (DIP) component, nor do they exclude the presence of a MORB depleted asthenosphere source component for some of the Iceland basalts, particularly in recent time.

Geologic evidence for a mantle superplume event at 1.9 Ga

Abstract: [1] Both preserved and restored areal distributions of Proterozoic marine intracratonic, passive margin, and platform sediments show a prominent peak at ∼1.9 Ga, indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time. The chemical index of alteration in shales deposited at this time was high, suggesting warm climates, possibly due to enhanced CO2 levels in the atmosphere. High sea level and warms climate may also explain an abundance of black shale, banded iron formations, and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1.9 Ga. All of these observations are consistent with a 1.9-Ga superplume event. The occurrence of only a minor positive carbon isotope shift in marine carbonates at 1.9 Ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present. Slightly low 87Sr/86Sr isotopic ratios in seawater at 1.9 Ga reflect increased mantle input of Sr from the proposed superplume event, whereas higher ratios at 1.85–1.75 Ga may reflect increased input of continental Sr from a growing supercontinent. The first massive sulfate evaporites in the geologic record at 1.8–1.6 Ga follow the possible 1.9-Ga superplume event. This may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume-related volcanism wanes.

Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites

Abstract: [1] Ophiolites are on-land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a ∼100 km wide region of partial melting into an ∼5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m ≈0.02 width−3 over a size interval from ∼0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.

Late Pleistocene sea level variations derived from the Argentine Shelf

Abstract: [1] Neritic-littoral marine sedimentary deposits from the Argentine Shelf provide a record of late Pleistocene sea level variation. Sediments interpreted to reflect the last glacial maximum low-stand are ∼150 m below present. Using a simple Airy isostatic model for hydroisostatic compensation and correcting for a minor tectonic component yields a local eustatic sea level lowering of ∼105 m at the last glacial maximum. The deglacial sea level curve records two rapid sea level rises consistent with MWP1-A and 1-B as documented by the Barbados coral-based sea level curve. Comparison with relative sea level variations predicted by the ICE4G VM2 viscoelastic model have highlighted a deficiency in the model's predicted sea level history for this region. Detailed data-model comparisons of late Pleistocene sea level variations are necessary in the face of climate change induced sea level perturbations to determine regional and or systematic biases in the treatment of lithosphere viscosity and accurate predictions of future sea level.

H2O and CO2 in magmas from the Mariana arc and back arc systems

Abstract: We examined the H2O and CO2 contents of glasses from lavas and xenoliths from the Mariana arc system, an intraoceanic convergent margin in the western Pacific, which contains an active volcanic arc, an actively spreading back arc basin, and active behind-the-arc cross-chain volcanoes. Samples include (1) glass rims from Mariana arc, Mariana trough, and cross-chain submarine lavas; (2) glass inclusions in arc and trough phenocrysts; and (3) glass inclusions from a gabbro + anorthosite xenolith from Agrigan (Mariana arc). Glass rims of submarine arc lavas contain 0.3-1.9 wt % H2O, and CO2 is below detection limits. Where they could be compared, glass inclusions in arc phenocrysts contain more H2O than their host glasses; most arc glasses and phenocryst inclusions contain no detectable CO2, with the exception of those from a North Hiyoshi shoshonite, which contains 400-600 ppm. The glass inclusions from the Agrigan xenolith contain 4-6% H2O, and CO2 is below the detection limit. Glasses from the cross-chain lavas are similar to those from the arc: H2O contents are 1.4-1.7 wt %, and CO2 is below detection limits. Volatile contents in Mariana trough lava glass rims are variable: 0.2-2.8 wt % H2O and 0-300 ppm CO2. Glass inclusions from through phenocrysts have water contents similar to the host glass, but they can contain up to 875 ppm CO2. Volatile contents of melt inclusions from trough and arc lavas and from the xenolith imply minimum depths of crystallization of ~1-8 km. H2O and CO2 contents of Mariana trough glasses are negatively correlated, indicating saturation of the erupting magma with a CO2-H2O vapor at the pressure of eruption (~400 bars for these samples), with the vapor ranging from nearly pure CO2 at the CO2-rich end of the glass array to nearly pure H2O at the H2O-rich end. Degassing of these magmas on ascent and eruption leads to significant loss of CO2 (thereby masking preeruptive CO2 contents) but minimal disturbance of preeruptive H2O contents. For submarine Mariana arc magmas, depths were low enough that degassing on ascent and eruption led to loss of both H2O and CO2; as a result, H2O contents are positively correlated with water depth for these samples. The H2O contents of primitive Mariana trough magmas richest in the slab-derived component (i.e., the most "arc-like" magmas) are ~2 wt %. Although evolved glasses with up to 4-6 wt % H2O are present among Mariana arc samples, we interpret the glass inclusion data as indicating that primitive Mariana arc liquids contain 1-3 wt % H2O. The preeruptive H2O contents of primitive cross-chain seamount liquids are >1-2 wt %.

Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge

Abstract: [1] The evolution of a gabbroic crystal mush beneath the Mid-Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153) Lithological variations occur on a vertical scale of meters and correlate with mineral compositions This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid-solution component compositions correlate with each other The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements) These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt Instead, melt migration within a crystal mush is suggested as the most likely process to explain them The meter-scale mineral compositional variations, which correlate between phases (eg, olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation

Young upper crustal chemical composition of the orogenic Japan Arc

Abstract: [1] A new geochemical estimate of the young (mainly Paleozoic age to present) upper crust of the Japan Arc shows a dacitic composition in contrast to the idea that andesite is predominant in active orogenic arcs. Temporal changes in composition are not significant from the Paleozoic age to the present for the Japan Arc. The major element composition is similar to previous models of old cratonic upper crusts. The coincidence in the major elements between young and old crusts indicates that essential mechanisms during crust formation have not changed from the Archean era to the present. In trace element compositions the average young upper crust of the Japan Arc has higher Sb and As concentrations and lower concentrations of alkaline, light rare earth, and high field strength elements with respect to previous models of continental upper crusts. The large degree of constancy of trace element composition in marine sedimentary rocks is in contrast to the large variety in igneous rocks. However, the averages for both accretionary and nonaccretionary sedimentary rocks are almost identical to the average for the igneous rocks of the Japan Arc, with the exceptions of high Sb and As concentrations in unmetamorphosed sedimentary rocks. The compositional homogeneity among different types of rocks on an arc scale implies that recycling processes mechanically mix the arc-derived igneous materials to homogenize the chemical composition during erosion, transportation, sedimentation, accretion, and uplifting. Since the contribution of oceanic crust to the composition of arc crust is small, the recycling processes have not changed the bulk upper crustal composition of the active continental margin except increase the Sb and As from sediments. Instead, the influx of differentiated acidic rocks from depth is essential to characterize the orogenic crust formation of the young Japan Arc. The characteristically low incompatible element content of the Japanese upper arc crust appears inherited from parental magmas derived from a mantle source depleted during a long-term evolution.

Chemical discrimination of Columbia River basalt flows

Abstract: [1] The homogeneity and distinctive chemical composition of individual Columbia River basalt flows are illustrated in a series of chemical plots, and the procedure for flow identification is outlined. The unique characterization of many individual eruptions permits units to be mapped from feeder dike to flow margins across the Columbia Plateau for hundreds of kilometers, and so provides critical constraints on the physical and chemical evolution of this continental flood basalt province.