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Sample records for deep upper mantle

  1. Iron-titanium oxyhydroxides which transport water into the deep upper mantle and mantle transition zone

    NASA Astrophysics Data System (ADS)

    Matsukage, K. N.; Nishihara, Y.

    2015-12-01

    We experimentally discovered a new hydrous phase in the system FeOOH-TiO2 at pressures of 10-16 GPa and temperatures of 1000-1600°C which corresponds to conditions of the deep upper mantle and the Earth's mantle transition zone. Seven different compositions in the FeOOH-TiO2 system having molar ratios of x = Ti/(Fe + Ti) = 0, 0.125, 0.25, 0.375, 0.5, 0.75 that were prepared by mixing reagent grade a-FeOOH (goethite) and TiO2 (anatase) powders were used as starting materials. High-pressure and high-temperature experiments were carried out using Kawai-type multi-anvil apparatus (Orange-1000 at Ehime University and SPI-1000 at Tokyo Institute of Technology). In this system, we identified two stable iron-titanium oxyhydroxide phases whose estimated composition is expressed by (FeH)1 - xTixO2 . One is the Fe-rich solid solution (x < 0.23) with e-FeOOH type crystal structure (e-phase, orthorhombic, P21nm) that was described by the previous studies (e.g., Suzuki 2010), and the other is the more Ti-rich solid solution (x > 0.35) with a-PbO2 type structure (a-phase, orthorhombic, Pbcn). The a-phase is stable up to 1500ºC for a composition of x = 0.5 and at least to 1600ºC for x = 0.75. Our result means that this phase is stable at average mantle temperature in the Earth's mantle transition zone. The Iron-titanium-rich hydrous phases was possible to stable in basalt + H2O system (e.g., Hashimoto and Matsukage 2013). Therefore our findings suggest that water transport in the Earth's deep interior is probably much more efficient than had been previously thought.

  2. Melting in the Earth's deep upper mantle caused by carbon dioxide

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep; Hirschmann, Marc M.

    2006-03-01

    The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10GPa and demonstrate that melting beneath ridges may occur at depths up to 330km, producing 0.03-0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of ~200-330km in the upper mantle.

  3. Melting in the Earth's deep upper mantle caused by carbon dioxide.

    PubMed

    Dasgupta, Rajdeep; Hirschmann, Marc M

    2006-03-30

    The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03-0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of approximately 200-330 km in the upper mantle. PMID:16572168

  4. Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

    USGS Publications Warehouse

    Soares, J.E.; Berrocal, J.; Fuck, R.A.; Mooney, W.D.; Ventura, D.B.R.

    2006-01-01

    A two-dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E-W transect approximately 530 km long across the Tocantins Province and western Sa??o Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the Sa??o Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goia??s ocean basin. Average crustal VP and VP/VS ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first-order tectonic structures. The 8.05 and 8.23 km s-1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic-ultramafic root beneath the Goia??s magmatic arc, and (3) block tectonics in the foreland fold-and-thrust belt of the northern Brasi??lia Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goia??s-Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain. Copyright 2006 by the American Geophysical Union.

  5. Rheology of fine-grained forsterite aggregate at deep upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Nishihara, Yu; Ohuchi, Tomohiro; Kawazoe, Takaaki; Spengler, Dirk; Tasaka, Miki; Kikegawa, Takumi; Suzuki, Akio; Ohtani, Eiji

    2014-01-01

    High-pressure and high-temperature deformation experiments on fine-grained synthetic dunite (forsterite aggregate) were conducted to determine the dominant deformation mechanism in the deep upper mantle. The sintered starting material has 90% forsterite, 10% enstatite, and an average grain size of ~1 µm. Deformation experiments were performed using a deformation-DIA apparatus at pressures of 3.03-5.36 GPa, temperatures of 1473-1573 K, and uniaxial strain rates of 0.91 × 10-5 to 18.6 × 10-5 s-1 at dry circumstances <50 H/106Si. The steady state flow stress was determined at each deformation condition. Derived stress-strain rate data is analyzed together with that reported from similar but low-pressure deformation experiments using flow law equations for diffusion creep (stress exponent of n = 1, grain-size exponent of p = 2) and for dislocation-accommodated grain-boundary sliding (GBS-disl, n = 3, p = 1). The activation volume for diffusion creep (V*dif) and for GBS-disl (V*GBS) of dunite is determined to be 8.2 ± 0.9 and 7.5 ± 1.0 cm3/mol, respectively. Calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the Earth's asthenospheric upper mantle whereas the contribution of GBS-disl is very limited, and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith. Though these conclusions are not definitive, these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure, temperature, stress, and grain-size space.

  6. Density of hydrous silicate melt at the conditions of Earth's deep upper mantle

    NASA Astrophysics Data System (ADS)

    Matsukage, Kyoko N.; Jing, Zhicheng; Karato, Shun-Ichiro

    2005-11-01

    The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions. However, melts formed under deep mantle conditions are also likely to contain some water, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of ~410km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.

  7. Rheology of the Deep Upper Mantle and its Implications for the Preservation of the Continental Roots: A Review

    SciTech Connect

    Karato, S.

    2010-01-01

    The longevity of deep continental roots depends critically on the rheological properties of upper mantle minerals under deep upper mantle conditions. Geodynamic studies suggest that the rheological contrast between the deep continental and oceanic upper mantle is a key factor that controls the longevity of the continental roots. Current understanding of rheological properties of deep upper mantle is reviewed to examine how a large enough rheological contrast between the continental and oceanic upper mantle develops that leads to the longevity of the deep continental roots. Based on the microstructures of naturally deformed deep continental rocks as well as on the observations of seismic anisotropy, it is concluded that power-law dislocation creep dominates in most of the deep upper mantle. Deformation by power-law creep is sensitive to water content and therefore the removal of water by partial melting to form depleted continental roots is a likely mechanism to establish a large rheological contrast. The results of experimental studies on the influence of temperature, pressure and water content on plastic flow by power-law dislocation creep are reviewed. The degree of rheological contrast depends critically on the dependence of effective viscosity on water content under 'wet' (water-rich) conditions but it is also sensitive to the effective viscosity under 'dry' (water-free) conditions that depends critically on the influence of pressure on deformation. Based on the analysis of thermodynamics of defects and high-temperature creep, it is shown that a robust estimate of the influence of water and pressure can be made only by the combination of low-pressure (< 0.5 GPa) and high-pressure (> 5 GPa) studies. A wide range of flow laws has been reported, leading to nearly 10 orders of magnitude differences in estimated viscosities under the deep upper mantle conditions. However, based on the examination of several criteria, it is concluded that relatively robust

  8. Density of Hydrous Ultramafic Silicate Melt under the Earth's Deep Upper Mantle Conditions

    NASA Astrophysics Data System (ADS)

    Jing, Z.; Matsukage, K. N.; Karato, S.

    2005-12-01

    Density of silicate melts is a critical material property in our understanding of geochemical evolution of the Earth. Previous studies (e.g., Agee & Walker 1993; Suzuki et al., 1995) showed that the density of dry silicate melts can be higher than that of surrounding solids under deep upper mantle conditions. However, melts formed under such conditions likely contain some water (Bercovici & Karato, 2003), which will reduce the melt density. In this study, we performed sink/float experiments between 10 and 14GPa and at 2173K to determine the density of hydrous ultramafic silicate melts, using a Kawai-type multianvil apparatus. We choose a target melt composition based on the experimental study by Litasov & Ohtani (2002). With this chemical composition, olivine reacts with the melt above the liquidus, so we used diamond as the density marker. However, diamond is much denser than a melt with a typical mantle like Fe/Mg ratio. Therefore in this study we determined the density of melts with high Fe contents, and from the relation between Fe content and melt density, we inferred the melt density with Earth-like Fe/Mg. Four Fe-rich compositions with 5wt% water and different iron content were chosen as starting materials. Density crossovers between melts and diamond were observed for all compositions. The densities of four melts at 14GPa and 2173K were calculated using the Birch-Murnaghan equation of state. The pressure derivative of isothermal bulk modulus (Kt') of the melts was estimated to be around 4. The density of mantle melt with mantle value of content and 5wt% water at 14GPa, 2173K was extrapolated to be ~3.42±0.4g/cm3. We compared our density results for hydrous melts with previous results on dry melts and found that water is more compressible than other components in melt. The estimated partial molar volume of water at 14GPa and 2173K is ~8±2cm3/mol, which is significantly lower than the value at low pressures. The conditions under which the density crossover

  9. Rheology of the upper mantle: a synthesis.

    PubMed

    Karato, S; Wu, P

    1993-05-01

    Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earth's upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths. PMID:17746109

  10. Stress relaxation experiments of olivine under conditions of subducted slab in Earth's deep upper mantle

    NASA Astrophysics Data System (ADS)

    Nishihara, Yu; Funakoshi, Ken-ichi; Higo, Yuji; Tsujino, Noriyoshi; Kawazoe, Takaaki; Kubo, Tomoaki; Shimojuku, Akira; Terasaki, Hidenori; Nishiyama, Norimasa; Suetsugu, Daisuke; Bina, Craig; Inoue, Toru; Wiens, Douglas; Jellinek, Mark

    2010-11-01

    Stress relaxation experiments of olivine were conducted under high-pressure and high-temperature conditions up to 10 GPa and 1273 K using a Kawai-type multi-anvil apparatus. A pre-sintered San Carlos olivine sample rod was inserted between two dense Al2O3 pistons to yield high stress at high-pressure within an octahedral pressure medium. Stress was determined from the two-dimensional diffraction pattern taken using monochromatic X-rays and an imaging-plate, and sample length was determined from an X-ray radiograph. In these experiments, pressure was first increased at room temperature, and then the temperature was increased and kept at 673, 873, 1073, and 1273 K. Four relaxation cycles, in total, were carried out in two experimental runs. The magnitude of deviatoric stress was calculated from five diffraction peaks with the following hkls: 0 2 1, 1 0 1, 1 3 0, 1 3 1, and 1 1 2. The calculated deviatoric stress was significantly different depending on which diffraction peak was used (up to a factor of ∼2) due to plastic deformation within the polycrystalline sample. The deviatoric stress decreased with increasing temperature in all of relaxation cycles. At given temperatures, the final-state value of deviatoric stress increased with increasing pressure. The upper bound for the plastic strain rate in the final-state was determined to be 10-7 s-1 based on a comparison between the total sample length determined from the radiograph and the d-spacings along the piston direction determined from X-ray diffraction. Present results suggest a positive activation volume for the low-temperature rheology of olivine.

  11. The statistical upper mantle assemblage

    NASA Astrophysics Data System (ADS)

    Meibom, Anders; Anderson, Don L.

    2004-01-01

    A fundamental challenge in modern mantle geochemistry is to link geochemical data with geological and geophysical observations. Most of the early geochemical models involved a layered mantle and the concept of geochemical reservoirs. Indeed, the two layer mantle model has been implicit in almost all geochemical literature and the provenance of oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB) [van Keken et al., Annu. Rev. Earth Planet. Sci. 30 (2002) 493-525]. Large-scale regions in the mantle, such as the 'convective' (i.e. well-stirred, homogeneous) upper mantle, sub-continental lithosphere, and the lower mantle were treated as distinct and accessible geochemical reservoirs. Here we discuss evidence for a ubiquitous distribution of small- to moderate-scale (i.e. 10 2-10 5 m) heterogeneity in the upper mantle, which we refer to as the statistical upper mantle assemblage (SUMA). This heterogeneity forms as the result of long-term plate tectonic recycling of sedimentary and crustal components. The SUMA model does not require a convectively homogenized MORB mantle reservoir, which has become a frequently used concept in geochemistry. Recently, Kellogg et al. [Earth Planet. Sci. Lett. 204 (2002) 183-202] modeled MORB and OIB Sr and Nd isotopic compositions as local mantle averages of random distributions of depleted residues and recycled continental crustal material. In this model, homogenization of the MORB source region is achieved by convective stirring and mixing. In contrast, in the SUMA model, the isotopic compositions of MORB and OIB are the outcome of homogenization during sampling, by partial melting and magma mixing (e.g. [Helffrich and Wood, Nature 412 (2001) 501-507]), of a distribution of small- to moderate-scale upper mantle heterogeneity, as predicted by the central limit theorem. Thus, the 'SUMA' acronym also captures what we consider the primary homogenization process: sampling upon melting and averaging. SUMA does not require the

  12. Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

    USGS Publications Warehouse

    Fuis, G.S.; Murphy, J.M.; Lutter, W.J.; Moore, T.E.; Bird, K.J.; Christensen, N.I.

    1997-01-01

    Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector. Copyright 1997 by the American Geophysical Union.

  13. GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

    USGS Publications Warehouse

    Behrendt, John C.; Hutchinson, D.R.; Lee, M.; Thornber, C.R.; Trehu, A.; Cannon, W.; Green, A.

    1990-01-01

    Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s-1 depth. ?? 1990.

  14. Carbon storage in the deep reducing mantle

    NASA Astrophysics Data System (ADS)

    Rohrbach, A.; Ghosh, S.; Schmidt, M. W.; Wijbrans, C. H.; Klemme, S.

    2014-12-01

    To understand the storage and cycling of carbon in/through Earth's deep mantle it is vital to examine carbon speciations at relevant pressure, temperature, and oxygen fugacity (fO2). In particular redox conditions of the mantle critically influence the mobility of carbon bearing phases in the silicate matrix; oxidized species are generally more mobile (carbonatites, carbonated silicate melts) or have a larger impact on silicate solidi (carbonated peridotite/eclogite) than reduced species (diamond, carbides, metals). Within garnet bearing mantle lithologies, fO2 can be expected to decrease with depth [1], eventually reaching values similar to the iron-wüstite equilibrium which implies the precipitation of a Fe-Ni metal phase at pressures corresponding to the base of the upper mantle [2]. Because Ni is more noble than Fe, Ni partitions strongly into the reduced phases such that at low metal fractions the metal phase reaches XNi > 0.5. Thermodynamic calculations suggest that the mantle contains ~0.1 wt.% Fe,Ni metal at ~300 km depth [3], increasing to ~1 wt% in the lower mantle [4]. To understand the nature of carbon bearing reduced phases in the Earth mantle, we examine experimentally phase relations and melting behavior in the system Fe-Ni-C at 10 and 23 GPa. Dependent on Fe-Ni ratio and related fO2, C content, P and T we observe a variety of phases, namely (Fe,Ni)3C and (Fe,Ni)7C3 carbides, carbon bearing Fe-Ni metal, diamond and carbon rich metal-melt. In the subsolidus, mantle bulk C contents of 50 to 500 ppm [5] would result in the phase association (Fe,Ni)3C + metal + diamond at 10 GPa. In the uppermost lower mantle, about 1 wt.% metal would dissolve ca. 100 ppm C, any further C would lead to (Fe,Ni)3C carbide saturation. The solidus temperatures of theses phase assemblages however are considerably lower than the geotherm at upper and lower mantle pressures. We therefore suggest that reduced carbon bearing phases in the deep mantle are largely molten [6]. [1

  15. Transition region of the earth's upper mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Bass, J. D.

    1986-01-01

    The chemistry of the earth's mantle is discussed using data from cosmochemistry, geochemistry, petrology, seismology, and mineral physics. The chondritic earth, the upper mantle and the 400-km discontinuity, the transition region, lower mantle mineralogy, and surface wave tomography are examined. Three main issues are addressed: (1) whether the mantle is homogeneous in composition or chemically stratified, (2) whether the major element chemistry of the mantle is more similar to upper mantle peridotites or to chondrites, and (3) the nature of the composition of the source region of basalts erupted at midocean ridges.

  16. Carbon Storage in the Mid- to Deep- Upper Mantle Constrained by Phase Relations in the Fe-Ni-Cu-C-S system

    NASA Astrophysics Data System (ADS)

    Tsuno, K.; Dasgupta, R.

    2014-12-01

    Carbon is a key element for evolution of terrestrial planets as it has influence on the chemistry and habitability of surficial environment as well as impact on mantle processes such as partial melting and element mobility. Because mantle is arguably the largest reservoir of extractable carbon, the stable form of carbon in various mantle domains needs to be constrained. In the reduced, mid- to deep- upper mantle, the host of deep carbon is graphite/diamond and/or Fe-Ni-bearing alloy melt [1]. However, high solubility of carbon in Fe-Ni alloy melt [2] suggests that diamond saturation may be restricted only to C-rich mantle domains. But such suggestions do not take into account the role of sulfides, which must interact with alloy-carbon mantle subsystems. In order to constrain the stable forms of carbon in the reduced mantle where Ni-rich alloy is likely present [3], we explore the phase relations and C solubility in Ni-rich portion of the Fe-Ni-±Cu-C-S systems. Experiments were performed in a MgO capsule using a multi-anvil with six starting mixes (Ni/(Fe+Ni) wt. ratio of 0.50-0.61, 8-16 % wt.% S, 2.0-2.5 wt.% C, and 0-0.7 wt.% Cu) at 6-8 GPa and 800-1400 °C. Low-temperature runs for all starting mixes contain C-bearing, solid Fe-Ni alloy + alloy melt + graphite, and solid alloy-out boundary is constrained, for example, at 1000-1050 °C at 6 GPa and 900-1000 °C at 8 GPa for the S-rich starting mix. The carbon solubility in the alloy melt (0.8~2.1 wt.% at 8 GPa and 1400 °C) decreases with increasing S content from 8 to 24 wt.%, increasing pressure for S-rich (18-24 wt.%) melt, and decreasing Ni/(Fe+Ni) from 0.65 to 0.53. For a mantle with ~0.1 wt.% alloy (~250 km depth) [3], diamond is likely stable coexisting with an S-rich alloy melt for ≥10 ppm bulk C. This is owing to the influence of S, which suppresses the incorporation of C in the alloy melt to stabilize diamond. Our results thus imply that diamond is a stable form of carbon even in depleted mantle

  17. Metal saturation in the upper mantle.

    PubMed

    Rohrbach, Arno; Ballhaus, Chris; Golla-Schindler, Ute; Ulmer, Peter; Kamenetsky, Vadim S; Kuzmin, Dmitry V

    2007-09-27

    The oxygen fugacity f(O2)of the Earth's mantle is one of the fundamental variables in mantle petrology. Through ferric-ferrous iron and carbon-hydrogen-oxygen equilibria, f(O2) influences the pressure-temperature positions of mantle solidi and compositions of small-degree mantle melts. Among other parameters, f(O2) affects the water storage capacity and rheology of the mantle. The uppermost mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts, but it is not known whether the shallow mantle is representative of the entire upper mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the upper mantle can no longer be regarded as being representative for the Earth's upper mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an upper veneer only about 250 km in thickness. PMID:17898766

  18. Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

    USGS Publications Warehouse

    Artemieva, I.M.; Thybo, H.; Kaban, M.K.

    2006-01-01

    We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petrological studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body-and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe. ?? The Geological Society of London 2006.

  19. Dihedral angle of carbonatite melts in mantle residue near the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Ghosh, S. K.; Rohrbach, A.; Schmidt, M. W.

    2015-12-01

    Carbonate melts are thought to be ideal metasomatic agents in the deep upper mantle (Green & Wallace, 1988) and these melts are low in viscosities (10-1-10-3 Pa·s) compared to primitive basalt (101-102 Pa·s), furthermore the ability to form an interconnected grain-edge melt network at low melt fractions (< 1%) make carbonate melts extremely mobile. They are molten at relatively low temperatures and have solidus temperatures hundreds of degrees lower than silicate melts at >3 GPa (Dasgupta et al. 2006, Ghosh et al., 2009), dissolve a number of geochemically incompatible elements much better than silicate melts (Blundy and Dalton, 2000). Previous studies of carbonate melt dihedral angles in olivine-dominated matrices yielded 25-30oat 1-3 GPa, relatively independent of melt composition (Watson et al., 1990) and temperature (Hunter and McKenzie, 1989). Dihedral angles of carbonate melts in contact with deep mantle silicate phases (e.g. garnet, wadsleyite, and ringwoodite) which constitute more than 70 % of the deep upper mantle and transition zone have not been studied yet. We have performed multi-anvil experiments on carbonate-bearing peridotites with 5.0 wt% CO2 from 13.5 to 20 GPa 1550 oC to investigate the dihedral angle of magnesio-carbonatite melts in equilibrium with garnet, olivine (and its high-pressure polymorphs), and clinoenstatite. The dihedral angle of carbonate melts in the deep upper mantle and transition zone is ~30° for majorite garnet and olivine (and its polymorphs) dominated matrices. It does not change with increasing pressure in the range 13.5-20 GPa. Our results suggest that very low melt fractions of carbonatite melt forming in the deep upper mantle and transition zone are interconnected at melt fractions less than 0.01. Consistent with geophysical observations, this could possibly explain low velocity regions in the deep mantle and transition zone.

  20. The upper mantle transition region - Eclogite

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post-spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al2O3-poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 microcalorie/sq cm-sec, and drives upper mantle convection.

  1. Extraction of magma from deep in the upper mantle: a model for the formation of Barberton komatiites

    NASA Astrophysics Data System (ADS)

    Robin-Popieul, C. C. M.; Arndt, N. T.; Chauvel, C.; Byerly, G.; Sobolev, A. V.; Wilson, A.

    2012-04-01

    Komatiites are highly magnesian volcanic rocks characteristic of the Archean. Al-depleted komatiites have low Al/Ti, relatively high concentrations of incompatible elements and depleted heavy rare earth elements (HREE); Al-undepleted komatiites have chondritic Al/Ti and flat HREE patterns; and Al-enriched komatiites have high Al/Ti, low concentrations of incompatible elements, enriched HREE and extremely depleted light rare earth elements. The oldest well-preserved examples are found in the Barberton Greenstone Belt in South Africa (3.5-3.3 Ga). All three komatiite types are found in the belt, commonly within the same stratigraphic unit. Based on a comprehensive petrological and geochemical study, we propose a new melting model for their formation. The basis of the model is the observation, from published experimental studies, that at great depths (~13GPa), the density of komatiitic liquid is similar to that of solid peridotite. At such depths the neutrally buoyant komatiite melt does not escape from residual peridotite. As the source ascends through the mantle, however, the pressure decreases and the density difference increases, eventually making the escape possible. Al-depleted komatiites form first at about 13GPa by equilibrium melting under conditions in which a large proportion of liquid (30-40%) was retained in the source and the residue contained a high proportion of garnet (15%). Al-undepleted and Al-enriched komatiites form by fractional melting at intermediate to shallow depths after the escape of a large proportion of melt and after exhaustion of residual garnet. This model reproduces the chemical characteristics of all komatiite types in the Barberton belt and can probably be applied to komatiites in other parts of the world.

  2. Interaction of the Upper-Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Sharapov, V.; Perepechko, Y.

    2006-12-01

    This work deals with the problem of interaction and combined evolution of closely spaced plumes. One of activities, which initiated this problem statement, was an attempt to explain via this mechanism the formation of large igneous provinces in the form of surface manifestations of a hot spot system. Convection in the upper mantle was simulated using the expanded Boussinesq model with non-linear state equations for mantle substance and lithosphere rocks, which considered the main solid-state phase transitions and melting processes. This system consisted of the upper mantle and mantle lithosphere, including non-uniform continental crust of a given thickness. The asthenosphere and regions of partial melting in lithosphere were formed during convection of the mantle substance. Interaction between several plumes, generated by hot spots, was considered. These plumes were located at distances, characteristic for the upper mantle (of about the lithosphere or upper mantle thickness). The initial distribution of plume sources and their physical parameters were assigned. According to numerical simulation, combined evolution of two plumes provides formation of a united igneous province in the upper geospheres. An increase in the number of closely spaced hot spots leads to division of a single igneous province into several areas of a smaller scale. A horizontal size of these areas is determined by a typical size of large dissipative structures in the upper mantle and lithosphere thickness. This research was supported by the Russian Foundation for Basic Research grant 04-05-64107, by the President's grants NSh-1573.2003.5, and by the Russian Ministry Science and Education grant RNP.2.1.1.702.

  3. Water Recycling, Lower Mantle Slab Subduction, and Viscous Layering of the Deep Mantle

    NASA Astrophysics Data System (ADS)

    Williams, Q.; McNamara, A.; Garnero, E.

    2005-12-01

    We explore the causes and possible consequences of a water/hydrogen-depleted layer in the lowermost ~1000 km of Earth`s mantle. At least three distinct, non-exclusive mechanisms exist that could generate such a layer: (1) descending melts could extract water from the deep mantle, and possibly sequester it within D``; (2) hydrogen could be stripped from deep mantle material during core formation, through formation of iron hydrides; and (3) the accreting planet could have nearly completely degassed, with the terrestrial water budget being accreted in a late hydrous veneer. In the latter two instances, the water budget of the mantle, and particularly the deep mantle, must entirely be generated by injection of water into the interior from the near surface. Our hypothesis is thus that the lower portion of Earth`s mantle might be (or have been) essentially dry, in contrast to the possible presence of 10's to 100's of ppm water in the overlying material. The principal geophysical effect of a water-depleted zone likely involves a marked increase in viscosity: for reference, such a decrease in water content produces about a 2-order of magnitude increase in the viscosity of upper mantle material. Fluid dynamic simulations show that a layer with a 2-order of magnitude viscosity increase in the bottom 1000 km of Earth`s mantle produces a substantial impediment to subduction, with subducted material laterally spreading out above this viscous layer. This behavior is compatible with tomographic images showing a lack of slab continuity into the deepest mantle, and the viscosity contrast thus produces a barrier to water ingress into the deep viscous layer, allowing it to remain anhydrous for extended time periods. Notably, the boundary between the viscous layer and overlying mantle and slab material undergoes substantial deflections, and because of the chemical similarity of the layers, should be seismically undetectable. Our results provide a straightforward mechanism through

  4. Upper mantle anisotropy - Evidence from free oscillations

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Dziewonski, A. M.

    1982-01-01

    There is a growing body of evidence that much of the upper mantle may be anisotropic to the propagation of seismic waves. The present investigation shows that a small amount of anisotropy completely changes the nature of the surface wave and normal mode problem. In particular, the apparent lack of sensitivity of many of the spheroidal modes to the compressional velocity structure is due to the degeneracy in the isotropic case. The normal mode data set appears to be adequate to resolve the five elastic constants of a transversely isotropic upper mantle. Dziewonski and Anderson (1981) have shown that these data can be fitted with anisotropy restricted to the upper 200 km of the mantle.

  5. Fe-Ni-Cu-C-S phase relations at high pressures and temperatures - The role of sulfur in carbon storage and diamond stability at mid- to deep-upper mantle

    NASA Astrophysics Data System (ADS)

    Tsuno, Kyusei; Dasgupta, Rajdeep

    2015-02-01

    Constraining the stable form of carbon in the deep mantle is important because carbon has key influence on mantle processes such as partial melting and element mobility, thereby affecting the efficiency of carbon exchange between the endogenic and exogenic reservoirs. In the reduced, mid- to deep-upper mantle, the chief host of deep carbon is expected to be graphite/diamond but in the presence of Fe-Ni alloy melt in the reduced mantle and owing to high solubility of carbon in such alloy phase, diamond may become unstable. To investigate the nature of stable, C-bearing phases in the reduced, mid- to deep-upper mantle, here we have performed experiments to examine the effect of sulfur on the phase relations of the Ni-rich portion of Fe-Ni ± Cu-C-S system, and carbon solubility in the Fe-Ni solid and Fe-Ni-S liquid alloys at 6-8 GPa and 800-1400 °C using a multianvil press. Low-temperature experiments for six starting mixes (Ni/(Fe + Ni) ∼ 0.61, 8-16 wt.% S) contain C-bearing, solid Fe-Ni alloy + Fe-Ni-C-S alloy melt + metastable graphite, and the solid alloy-out boundary is constrained, at 1150-1200 °C at 6 GPa and 900-1000 °C at 8 GPa for S-poor starting mix, and at 1000-1050 °C at 6 GPa and 900-1000 °C at 8 GPa for the S-rich starting mix. The carbon solubility in the liquid alloy significantly diminishes from 2.1 to 0.8 wt.% with sulfur in the melt increasing from 8 to 24 wt.%, irrespective of temperature. We also observed a slight decrease of carbon solubility in the liquid alloy with increasing pressure when alloy liquid contains >∼18 wt.% S, and with decreasing Ni/(Fe + Ni) ratio from 0.65 to ∼0.53. Based on our results, diamond, coexisting with Ni-rich sulfide liquid alloy is expected to be stable in the reduced, alloy-bearing oceanic mantle with C content as low as 20 to 5 ppm for mantle S varying between 100 and 200 ppm. Deep, reduced root of cratonic mantle, on the other hand, is expected to have C distributed among solid alloy, liquid alloy

  6. Isotopic Evidence For Chaotic Imprint In The Upper Mantle Heterogeneity

    NASA Astrophysics Data System (ADS)

    Armienti, P.; Gasperini, D.

    2006-12-01

    Heterogeneities of the asthenospheric mantle along mid-ocean ridges have been documented as the ultimate effect of complex processes dominated by temperature, pressure and composition of the shallow mantle, in a convective regime that involves mass transfer from the deep mantle, occasionally disturbed by the occurrence of hot spots (e.g. Graham et al., 2001; Agranier et al., 2005; Debaille et al., 2006). Alternatively, upper mantle heterogeneity is seen as the natural result of basically athermal processes that are intrinsic to plate tectonics, such as delamination and recycling of continental crust and of subducted aseismic ridges (Meibom and Anderson, 2003; Anderson, 2006). Here we discuss whether the theory of chaotic dynamical systems applied to isotopic space series along the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR) can delimit the length-scale of upper mantle heterogeneities, then if the model of marble-cake mantle (Allègre and Turcotte, 1986) is consistent with a fractal distribution of such heterogeneity. The correlations between the isotopic (Sr, Nd, Hf, Pb) composition of MORB were parameterized as a function of the ridge length. We found that the distribution of isotopic heterogenity along both the MAR and EPR is self- similar in the range of 7000-9000 km. Self-similarity is the imprint of chaotic mantle processes. The existence of strange attractors in the distribution of isotopic composition of the asthenosphere sampled at ridge crests reveals recursion of the same mantle process(es), endured over long periods of time, up to a stationary state. The occurrence of the same fractal dimension for both the MAR and EPR implies independency of contingent events, suggesting common mantle processes, on a planetary scale. We envisage the cyclic route of "melting, melt extraction and recycling" as the main mantle process which could be able to induce scale invariance. It should have happened for a significant number of times over the Earth

  7. Seismological constraints on deep mantle structure: recent results

    NASA Astrophysics Data System (ADS)

    Ritsema, J. E.

    2002-12-01

    Thanks to the development of global seismic networks, especially in the past two decades when digital broadband ground motion sensors were employed, full waveform analysis has led to the development of three-dimensional models of the structure of Earth's interior. Yet, many of the new models confirm a number of critical conclusions that F. Birch drew in his seminal 1952 JGR paper. For example, he envisioned that the upper mantle transition zone plays a prominent role in mantle circulation and he emphasized the significant effect of pressure on thermodynamic parameters. Indeed, a large number of seismic studies of the transition zone indicate that the descent of slabs of former oceanic lithosphere is, with few exceptions, impeded by the 660--km discontinuity. Furthermore, the observation of predominantly broad seismic velocity structures in the lower mantle (> 1500 km depth) may reflect sluggish convection, due to reduced thermal expansivity. I will review several recent seismological studies of the deep mantle and place them in the context of Birch's paper. In particular, I will discuss surprising new findings in the deep mantle beneath Africa, which is especially well studied with data from recent African deployments. Furthermore, I will show results of the application of a new 3D waveform modeling technique that may prove invaluable in future seismological studies of the deep mantle.

  8. Methane-derived hydrocarbons produced under upper-mantle conditions

    SciTech Connect

    Kolesnikov, Anton; Kutcherov, Vladimir G.; Goncharov, Alexander F.

    2009-08-13

    There is widespread evidence that petroleum originates from biological processes. Whether hydrocarbons can also be produced from abiogenic precursor molecules under the high-pressure, high-temperature conditions characteristic of the upper mantle remains an open question. It has been proposed that hydrocarbons generated in the upper mantle could be transported through deep faults to shallower regions in the Earth's crust, and contribute to petroleum reserves. Here we use in situ Raman spectroscopy in laser-heated diamond anvil cells to monitor the chemical reactivity of methane and ethane under upper-mantle conditions. We show that when methane is exposed to pressures higher than 2 GPa, and to temperatures in the range of 1,000-1,500 K, it partially reacts to form saturated hydrocarbons containing 2-4 carbons (ethane, propane and butane) and molecular hydrogen and graphite. Conversely, exposure of ethane to similar conditions results in the production of methane, suggesting that the synthesis of saturated hydrocarbons is reversible. Our results support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle.

  9. Preface: Deep Slab and Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Suetsugu, Daisuke; Bina, Craig R.; Inoue, Toru; Wiens, Douglas A.

    2010-11-01

    We are pleased to publish this special issue of the journal Physics of the Earth and Planetary Interiors entitled "Deep Slab and Mantle Dynamics". This issue is an outgrowth of the international symposium "Deep Slab and Mantle Dynamics", which was held on February 25-27, 2009, in Kyoto, Japan. This symposium was organized by the "Stagnant Slab Project" (SSP) research group to present the results of the 5-year project and to facilitate intensive discussion with well-known international researchers in related fields. The SSP and the symposium were supported by a Grant-in-Aid for Scientific Research (16075101) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. In the symposium, key issues discussed by participants included: transportation of water into the deep mantle and its role in slab-related dynamics; observational and experimental constraints on deep slab properties and the slab environment; modeling of slab stagnation to constrain its mechanisms in comparison with observational and experimental data; observational, experimental and modeling constraints on the fate of stagnant slabs; eventual accumulation of stagnant slabs on the core-mantle boundary and its geodynamic implications. This special issue is a collection of papers presented in the symposium and other papers related to the subject of the symposium. The collected papers provide an overview of the wide range of multidisciplinary studies of mantle dynamics, particularly in the context of subduction, stagnation, and the fate of deep slabs.

  10. Deep Mantle Fluids Bottled Up in Diamonds

    NASA Astrophysics Data System (ADS)

    Weiss, Y.; Pearson, D. G.

    2015-12-01

    Many mantle xenoliths and mineral inclusions in diamonds reflect refertilisation and enrichment by mantle metasomatism, a key mechanism for controlling abrupt changes in the chemical and physical properties of the continental lithospheric mantle (CLM) globally. However, the nature of the fluids involved can normally only be constrained indirectly from geochemical proxies or calculated using mineral/melt partition coefficients. Direct samples of mantle metasomatic fluids, shielded from any late stage alteration, are encased as microinclusions in fast-growing diamonds - "fibrous diamonds". These trapped high-density fluids (HDFs) provide a unique chemical and physical record for tracing the sources of deep mantle fluids and constraining the processes that shape their nature.Diamond HDFs vary between four major compositional types: saline, silicic and high-Mg plus low-Mg carbonatitic. A strong connection has been established between high-Mg carbonatitic HDFs and a carbonated peridotite source. In addition, the silicic and low-Mg carbonatitic HDFs have been related to hydrous eclogite (±carbonate). However, the compositionally extreme saline fluid endmember remained enigmatic and its source in the deep lithosphere has remained ambiguous. Our new data on fluid-rich diamonds show the geochemical fingerprints of a subducting slab as the source of deep mantle fluids of saline composition. In addition, for the first time, we show that these deep saline fluids are parental, via fluid rock interaction, to in-situ forming carbonatitic and silicic melts in the lithosphere. This model provides a strong platform for resolving the effects of the compositional spectrum of mantle fluids, which alter the deep lithosphere globally and play key roles in diamond formation.

  11. How the Upper Mantle Became Oxidized

    NASA Astrophysics Data System (ADS)

    Kasting, J. F.

    2004-12-01

    Today, Earth's upper mantle has an average oxygen fugacity near the quartz-fayalite-magnetite (QFM) redox buffer (1), although significant departures from this redox state occur in different localities and at different depths (2). However, early in Earth history, following the Moon-forming impact, the upper mantle was almost certainly uniformly more reduced. The impactor that formed the Moon was probably Mars-sized or larger (3) and had already differentiated an iron core. Successful models of lunar formation must account for the fact that the Moon has only 25 percent of Earth's iron abundance (4). This can be accomplished if the iron core of the impactor was accreted by the Earth, while the Moon was formed from the mantles of the impactor and the Earth. Other large impactors would also have brought in metallic iron, and all such large impacts would have melted large portions of Earth's mantle. It is therefore inevitable that the Earth's upper mantle began its existence with an oxygen fugacity at or below iron-wüstite (IW). How the upper mantle became oxidized from IW up to QFM is an interesting question. Much of the oxidation could have taken place during brief steam atmosphere stages following impacts (5,6) when hydrogen escape to space was extremely rapid (7). Continued oxidation could have been caused by cycling of volatiles through the mantle, accompanied by outgassing of reduced gases (8) and by subduction of ferric iron that had been oxidized at the surface (9). Oxidation of the uppermost 700 km of the mantle from QFM to IW would have required the equivalent of about half an ocean of water, assuming that the hydrogen was lost to space. This could have been accomplished in less than 2 b.y. if the average H2 outgassing rate was a few times the present value, 5x1012 mol/yr (10). The timing of mantle oxidation has important consequences for the composition of Earth's atmosphere at the time when life originated because it controls the oxidation state of volcanic

  12. Seismic Q of the lunar upper mantle

    NASA Astrophysics Data System (ADS)

    Nakamura, Y.; Koyama, J.

    1982-06-01

    Shallow moonquake data are used to determine the frequency dependence of Q values for both compressional and shear waves in the upper mantle of the moon at frequencies between 3 and 8 Hz. The seismic P wave Q is estimated to be at least 4000 and is nearly independent of frequency or decreases slightly with increasing frequency, while the S wave Q increases from at least 4000 at 3 Hz to at least 7000 at 8 Hz. The rate of increase of Q(S) is approximately proportional to the 0.7 + or - 0.1 power of the frequency above 5 Hz. With the absence of other dissipation mechanisms, compressional heat loss may be a dominant factor in the lunar interior. Uncertainty remains, however, in the absolute values of Q's owing to the largely unknown detailed structure of the lunar upper mantle.

  13. Seismic Q of the lunar upper mantle

    NASA Technical Reports Server (NTRS)

    Nakamura, Y.; Koyama, J.

    1982-01-01

    Shallow moonquake data are used to determine the frequency dependence of Q values for both compressional and shear waves in the upper mantle of the moon at frequencies between 3 and 8 Hz. The seismic P wave Q is estimated to be at least 4000 and is nearly independent of frequency or decreases slightly with increasing frequency, while the S wave Q increases from at least 4000 at 3 Hz to at least 7000 at 8 Hz. The rate of increase of Q(S) is approximately proportional to the 0.7 + or - 0.1 power of the frequency above 5 Hz. With the absence of other dissipation mechanisms, compressional heat loss may be a dominant factor in the lunar interior. Uncertainty remains, however, in the absolute values of Q's owing to the largely unknown detailed structure of the lunar upper mantle.

  14. Physical state of the western U.S. upper mantle

    NASA Technical Reports Server (NTRS)

    Humphreys, Eugene D.; Dueker, Kenneth G.

    1994-01-01

    Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low-velocity mantle is hot and partially molten to depths of 100-200 km, and that the high-velocity upper mantle is subsolidus. Most the high-velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high-velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high- and the low-velocity upper mantle. The only resonable way to produce the high-velocity low-density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within approximately 250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of

  15. Surface wave tomography applied to the North American upper mantle

    NASA Astrophysics Data System (ADS)

    van der Lee, Suzan; Frederiksen, Andrew

    Tomographic techniques that invert seismic surface waves for 3-D Earth structure differ in their definitions of data and the forward problem as well as in the parameterization of the tomographic model. However, all such techniques have in common that the tomographic inverse problem involves solving a large and mixed-determined set of linear equations. Consequently these inverse problems have multiple solutions and inherently undefinable accuracy. Smoother and rougher tomographic models are found with rougher (confined to great circle path) and smoother (finite-width) sensitivity kernels, respectively. A powerful, well-tested method of surface wave tomography (Partitioned Waveform Inversion) is based on inverting the waveforms of wave trains comprising regional S and surface waves from at least hundreds of seismograms for 3-D variations in S wave velocity. We apply this method to nearly 1400 seismograms recorded by digital broadband seismic stations in North America. The new 3-D S-velocity model, NA04, is consistent with previous findings that are based on separate, overlapping data sets. The merging of US and Canadian data sets, adding Canadian recordings of Mexican earthquakes, and combining fundamental-mode with higher-mode waveforms provides superior resolution, in particular in the US-Canada border region and the deep upper mantle. NA04 shows that 1) the Atlantic upper mantle is seismically faster than the Pacific upper mantle, 2) the uppermost mantle beneath Precambrian North America could be one and a half times as rigid as the upper mantle beneath Meso- and Cenozoic North America, with the upper mantle beneath Paleozoic North America being intermediate in seismic rigidity, 3) upper-mantle structure varies laterally within these geologic-age domains, and 4) the distribution of high-velocity anomalies in the deep upper mantle aligns with lower mantle images of the subducted Farallon and Kula plates and indicate that trailing fragments of these subducted

  16. Upper mantle material in the Brazilian shield

    NASA Astrophysics Data System (ADS)

    Berbert, C. O.; Svisero, D. P.; Sial, A. N.; Meyer, H. O. A.

    1981-04-01

    Information on the nature of the upper mantle can be obtained from nodules in kimberlites and basalt and from mantle-derived magmas, mineral inclusions in diamonds, as well as from the fields of geodesy, seismology, geothermy, geomagnetism and petrological models for the upper mantle. In Brazil studies of these kinds are still in the stage of data gathering. This article intends to present some of this data related to the alpine peridotites, nodules in basalts, mineral inclusions in diamonds, and kimberlites, without any pretension of deeper-going interpretation. Alpine peridotites are found all over Brazil and are grouped in three main classes: the serpentinized dunites-peridotites of small and medium size; the gabbro-pyroxenite-peridotite association in large complexes, the latter described only in the central part of Brazil; and the pyroxenite-gabbroic gneisses of the Goianira-Trindade type. Kimberlites have been described in Minas Gerais and Piaui states, but they also exist in Mato Grosso and possibly in Rondonia, Goiás, Roraima and Bahia. Inclusions in diamonds studied from Minas Gerais, Piauí, Mato Grosso, Paraná, Sa˜o Paulo and Goiás include olivine, pyroxene, garnet, chromite, sulphides, ilmenite, zircon and rutile. Ultramafic nodules in basalts and basanites from Rio de Janeiro, Rio Grande do Norte, Paraíba states and Fernando de Noronha Island are essentially Iherzolites, like the ones described from Paraguay.

  17. Diamond Formation in association with Deep Mantle Dehydration Zones

    NASA Astrophysics Data System (ADS)

    Harte, B.

    2009-12-01

    INTRODUCTION. During the last 25 years a series of publications have documented the occurrence of inclusions in diamonds that show mineral compositions and mineral associations predicted for mantle rocks at deeper levels than the mantle lithosphere (e.g Harte et al., 1999; Harte & Cayzer, 2007). Although the diamonds bearing deep mantle inclusions are not abundant in absolute terms they are widespread and have been reported from cratonic blocks on all major continents. DEPTH ZONES OF INCLUSION SUITES. The silicate inclusions and their mineral associations form a series showing good correlation with mineral assemblages expected in basic and ultrabasic rock compositions at depth. However, there is a strong bias towards assemblages from two principal zones: (1)uppermost Transition zone and Asthenosphere. (2)lowermost Transition Zone and uppermost Lower Mantle. In the case of (1) the assemblages are predominantly of majorite garnet, and majorite garnet + cpx with an affinity to eclogitic bulk compositions. In many of these inclusions the cpx appears to have exsolved from the majoritic garnet and the depth of origin of the initial inclusions may be near the top of the Transition Zone. The assemblages from (2) are predominantly of peridotic affinity and involve fPer as well as silicates. They indicate material from three depth zones near the upper mantle to lower mantle boundary (UM/LM boundary). 2a) Upper/Lower Mantle Boundary association - inclusions of: Mg2SiO4, fPer, majorite/TAPP, MgSi-perovskite(mpv), CaSi-perovskite (cpv). 2b) uppermost Lower Mantle association with: Mpv(Al-poor), fPer and cpv, majorite/tapp 2c)lower Mantle association with: Mpv(Al-rich) with fPer, and corundum. DISCUSSION. The above features show that the formation of deep mantle diamonds is concentrated in a zone around the top of the Transition Zone (ca 400-500 kms deep), and a zone at 600-800 kms embracing the bottom of the Transition Zone and the top of Lower Mantle. Associations including Mg

  18. Upper mantle structure of the Saharan Metacraton

    NASA Astrophysics Data System (ADS)

    Abdelsalam, Mohamed G.; Gao, Stephen S.; Liégeois, Jean-Paul

    2011-07-01

    The ˜500,000 km 2 Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean-Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian-Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175-250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies.

  19. Processes of deep terrestrial mantles and cores

    NASA Technical Reports Server (NTRS)

    Jeanloz, Raymond

    1991-01-01

    Ultrahigh pressure experiments are currently focused on revealing processes occurring deep inside planets. This is in addition to the traditional emphasis on the constitution of planetary interiors, such as the identification of the high pressure perovskite phase of (Mg,Fe)SiO3 as the predominant mineral inside the Earth, and probably Venus. For example, experiments show that the mechanism of geochemical differentiation, separation of partial melts, differs fundamentally in the lower mantles of Earth and Venus than at near surface conditions. In addition to structural transformations, changes in chemical bonding caused by pressure can also be significant for planetary interiors. Measurements of AC and DC electrical conductivity can be obtained at ultrahigh pressures and temperatures, to greater than 80 GPa and 3000 K simultaneously, using the laser heated diamond cell. Anhydrous lower mantle assemblages (perovskite + or - oxide phases) exhibit an electrical conductivity that depends strongly on Fe content. Contrary to traditional assumptions, temperature affects the conductivity of lower mantle assemblages relatively little. The Earth's deep focus seismicity can be explained by the recycling of water into the mantle.

  20. Attenuation Tomography of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Adenis, A.; Debayle, E.; Ricard, Y. R.

    2014-12-01

    We present a 3-D model of surface wave attenuation in the upper mantle. The model is constrained by a large data set of fundamental and higher Rayleigh mode observations. This data set consists of about 1,800,000 attenuation curves measured in the period range 50-300s by Debayle and Ricard (2012). A careful selection allows us to reject data for which measurements are likely biased by the poor knowledge of the scalar seismic moment or by a ray propagation too close to a node of the source radiation pattern. For each epicenter-station path, elastic focusing effects due to seismic heterogeneities are corrected using DR2012 and the data are turned into log(1/Q). The selected data are then combined in a tomographic inversion using the non-linear least square formalism of Tarantola and Valette (1982). The obtained attenuation maps are in agreement with the surface tectonic for periods and modes sensitive to the top 200km of the upper mantle. Low attenuation regions correlate with continental shields while high attenuation regions are located beneath young oceanic regions. The attenuation pattern becomes more homogeneous at depths greater than 200 km and the maps are dominated by a high quality factor signature beneath slabs. We will discuss the similarities and differences between the tomographies of seismic velocities and of attenuations.

  1. Small-scale upper mantle extension beneath a destroyed craton

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Zheng, T.; Chen, L.; Ai, Y.; He, Y.; Xu, X.

    2014-12-01

    The North China Craton (NCC), as an unusual craton with part of its thick lithosphere destructed, records the geodynamic processes associated with the convergence of Eurasia and the Pacific and Philippine plates lasting from the Mesozoic to the Cenozoic. How the cratonic lithosphere deformed in response to the extensional tectonics caused by the oceanic plate subduction, however, remains debated. In order to investigate the mantle deformation of the NCC, we present new shear wave splitting measurements and updated tomographic models beneath a 900-km long profile across the north NCC. Compared to our other observations in the NCC, this profile is shorter but also crosses a region that experienced strong lithospheric destruction, therefore provides a good opportunity to improve our understanding of upper mantle deformation during the craton destruction. The upper mantle deformation is studied using SKS data from 60 broadband stations with average spacing of 15 km. For the data from events occurring at distances of 85º-115º, fast polarization directions and delay times (fδt) are retrieved by a routine method, while for the events at distances < 85º, waveform modeling are applied to obtain (fδt) after separating the effects of S and SKS. The measured splitting parameters show small-scale variations from east to west: the major fast directions, trending NE-SW or NW-SE in contrast, distribute intermittently along the profile. We plot the splitting parameters overlapping on the geological map and the tomography image for a depth range of 120-300 km. Comparison shows good consistency of the splitting pattern and structural features both at shallow and deep depths: NW-SE trending fast directions are observed at stations located within the basins or extensional zones like metamorphic core complexes, with the fast direction parallel to the extensional or stretching directions; the fast directions and the shear-wave velocity anomalies within the upper mantle

  2. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

    PubMed

    Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

    2011-10-01

    A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle. PMID:21921159

  3. Towards a Global Upper Mantle Attenuation Model

    NASA Astrophysics Data System (ADS)

    Karaoglu, Haydar; Romanowicz, Barbara

    2015-04-01

    Global anelastic tomography is crucial for addressing the nature of heterogeneity in the Earth's interior. The intrinsic attenuation manifests itself through dispersion and amplitude decay. These are contaminated by elastic effects such as (de)focusing and scattering. Therefore, mapping anelasticity accurately requires separation of elastic effects from the anelastic ones. To achieve this, a possible approach is to try and first predict elastic effects through the computation of seismic waveforms in a high resolution 3D elastic model, which can now be achieved accurately using numerical wavefield computations. Building upon the recent construction of such a whole mantle elastic and radially anisotropic shear velocity model (SEMUCB_WM1, French and Romanowicz, 2014), which will be used as starting model, our goal is to develop a higher resolution 3D attenuation model of the upper mantle based on full waveform inversion. As in the development of SEMUCB_WM1, forward modeling will be performed using the spectral element method, while the inverse problem will be treated approximately, using normal mode asymptotics. Both fundamental and overtone time domain long period waveforms (T>60s) will be used from a dataset of over 200 events observed at several hundred stations globally. Here we present preliminary results of synthetic tests, exploring different iterative inversion strategies.

  4. Chemical equilibration of the Earth's core and upper mantle

    USGS Publications Warehouse

    Brett, R.

    1984-01-01

    The oxygen fugacity (fO2) of the Earth's upper mantle appears to lie somewhat above that of the iron-wu??stite buffer, its fO2 is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO2 at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely. As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of P, T, and fO2. The latter possibility seems unlikely over such a P-T range. ?? 1984.

  5. Primordial metallic melt in the deep mantle

    NASA Astrophysics Data System (ADS)

    Zhang, Zhou; Dorfman, Susannah M.; Labidi, Jabrane; Zhang, Shuai; Li, Mingming; Manga, Michael; Stixrude, Lars; McDonough, William F.; Williams, Quentin

    2016-04-01

    Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3-3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.

  6. Seismic images of the upper mantle velocities and structure of European mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Plomerova, Jaroslava; Munzarova, Helena; Vecsey, Ludek; Babuska, Vladislav

    2014-05-01

    Tomography images of seismic velocities in the Earth mantle represent significant tool for recovering first order structural features. Regional studies, based on dense networks of temporary stations allow us to focus on structure of the continental upper mantle and to study variations of body-wave velocities in greater detail. However, the standard tomography exhibits only isotropic view of the Earth, whose structure is anisotropic in general, as shown by results of various studies exploiting a broad range of methods, types of waves and scales. We present results of our studies of seismic anisotropy in tectonically different provinces that clearly demonstrate the continental mantle lithosphere consists of domains with different fossil fabrics. We detect anisotropic signal both in teleseismic P-wave travel-time deviations and shear-wave splitting and show changes of the anisotropic parameters across seismic arrays, in which stations with similar characteristics form groups. The geographical variations of seismic-wave anisotropy delimit individual, often sharply bounded domains of the mantle lithosphere, each of them having a consistent fabric. The domains can be modelled in 3D by peridotite aggregates with dipping lineation a or foliation (a,c). These findings allow us to interpret the domains as micro-plate fragments retaining fossil fabrics in the mantle lithosphere, reflecting thus an olivine LPO created before the micro-plates assembled. Modelling anisotropic structure of individual domains of the continental mantle lithosphere helps to decipher boundaries of individual blocks building the continental lithosphere and hypothesize on processes of its formation (Plomerova and Babuska, Lithos 2010). Exploiting the long memory of the deep continental lithosphere fabric, we present the lithosphere-asthenosphere boundary (LAB) as a transition between a fossil anisotropy in the mantle lithosphere and an underlying seismic anisotropy related to the present-day flow in

  7. Upper mantle anisotropy structure beneath eastern Tibet and its exploration

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Wen, L.

    2014-12-01

    Continental collision between the Indian and the Eurasian plates resulted in uplift of the Tibetan plateau and the thickening of the crust. A lot of work has been done on the crust structures beneath Tibet, and several tectonic models are proposed to explain the mechanism of the uplift and thickening. But due to the absence of the upper mantle structures, those models are still under debate. Fine upper mantle velocity and anisotropy structures can help us understand the dynamic process of the Tibetan plateau. Waveform modeling of upper mantle triplication phases can provide a good vertical resolution of upper mantle velocity structures, but present methods for calculating synthetic seismograms cannot process anisotropic media. We develop a method based on the generalized reflection and transmission method (GRTM) to calculate synthetic seismograms for wave propagating in stratified VTI media, so we can waveform model upper mantle triplications propagating in anisotropic media. In this study, we waveform model the tangential and radial seismic triplication data recorded in Chinese digital seismic stations at a epicentral distance of 10-30 degree for one events occurring in middle Tibet to constrain fine upper mantle velocity and anisotropy structures beneath eastern Tibet. The result shows that horizontal S wave velocity is larger than vertical S wave velocity in the upper mantle beneath eastern Tibet. We also build a mineral physics modeling method, which can calculate upper mantle anisotropy structures based on mantle temperatures, compositions and directions of mantle flow, and use this method to explore compositional and dynamic models that would explain the inferred seismic structures. The results suggest that in our sampling region, the water content is lower than 0.4 wt%, and there is vertical mantle flow beneath the lithospheric lid.

  8. Anisotropy of thermal diffusivity in the upper mantle.

    PubMed

    Tommasi, A; Gibert, B; Seipold, U; Mainprice, D

    2001-06-14

    Heat transfer in the mantle is a key process controlling the Earth's dynamics. Upper-mantle mineral phases, especially olivine, have been shown to display highly anisotropic thermal diffusivity at ambient conditions, and seismic anisotropy data show that preferred orientations of olivine induced by deformation are coherent at large scales (>50 km) in the upper mantle. Thus heat transport in the upper mantle should be anisotropic. But the thermal anisotropy of mantle minerals at high temperature and its relationship with deformation have not been well constrained. Here we present petrophysical modelling and laboratory measurements of thermal diffusivity in deformed mantle rocks between temperatures of 290 and 1,250 K that demonstrate that deformation may induce a significant anisotropy of thermal diffusivity in the uppermost mantle. We found that heat transport parallel to the flow direction is up to 30 per cent faster than that normal to the flow plane. Such a strain-induced thermal anisotropy implies that the upper-mantle temperature distribution, rheology and, consequently, its dynamics, will depend on deformation history. In oceans, resistive drag flow would result in lower vertical diffusivities in both the lithosphere and asthenosphere and hence in less effective heat transfer from the convective mantle. In continents, olivine orientations frozen in the lithosphere may induce anisotropic heating above mantle plumes, favouring the reactivation of pre-existing structures. PMID:11459053

  9. Global Upper Mantle Azimuthal Anisotropy From Probabilistic Tomography

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Yuan, K.

    2014-12-01

    The new model of Yuan and Beghein (2013), hereafter YBaniSV13, is the first global model to constrain 3-D azimuthal anisotropy in the deep upper mantle. It is compatible with previous models in the uppermost 200km of the mantle, but also displays 1% anisotropy above, inside, and below the Mantle Transition Zone (MTZ). Another interesting characteristic of this model is the change in fast seismic direction detected, on average, at ~250km depth and at the MTZ boundaries. These results have important consequences for our understanding of mantle deformation and convection patterns in the mantle. It is therefore important to assess the robustness if these features. We already tested that the model does not strongly depend on the reference 1-D mantle model, on the presence of discontinuities in this reference model, or on the crustal model and Moho depth used to calculate the laterally varying partial derivatives. In this work, we apply a model space approach, the Neighborhood Algorithm (NA) of Sambridge (1999), to determine quantitative model uncertainties and parameter trade-offs. First, the NA generates an ensemble of models with a sampling density that increases toward the best fitting regions of the model space, and then performs a Bayesian appraisal of the models obtained that allows us to determine the likelihood of azimuthal anisotropy in different region of Earth's interior. Such approaches have the advantage of sampling the model null-space, and therefore provide more reliable model uncertainties than traditional inverse techniques. We use YBaniSV13 as initial model, and search the model space around it, allowing for large enough deviations to test the robustness of the anisotropy amplitude. We compare results from a model space search based on the chi-square misfit and from a model space search based on the variance reduction, which is another useful measure of data fit that is independent of data uncertainties. Preliminary results for the chi-square driven

  10. Upper mantle anisotropy in the New Zealand region

    SciTech Connect

    Klosko, E.K.

    1999-02-01

    Shear-wave splitting parameters of fast polarization direction (F) and delay time (dt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that F are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific-Australian plate boundary. The dt values range from 0.6-2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear-wave splitting is an anisotropic region between 40-400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured F, and the large dt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

  11. The distribution of H2O between silicate melt and nominally anhydrous peridotite and the onset of hydrous melting in the deep upper mantle

    NASA Astrophysics Data System (ADS)

    Novella, Davide; Frost, Daniel J.; Hauri, Erik H.; Bureau, Helene; Raepsaet, Caroline; Roberge, Mathilde

    2014-08-01

    The partitioning of H2O between a mantle peridotite assemblage and low degree hydrous melt has been investigated at 6 GPa (corresponding to ∼180 km depth) at a temperature of 1400 °C. Peridotite mineral phases were analysed from 6 melting experiments performed in a natural chemical system. The experiments contained ∼80 wt% of a low degree hydrous melt that was obtained through a series of experiments where the melt composition was iteratively adjusted until saturation with the appropriate peridotite assemblage was achieved. The melt is fluid-undersaturated at the conditions of the experiment. Ion microprobe measurements of the mineral phases indicate olivine H2O concentrations of 434±61 ppm wt and average clinopyroxene (cpx) concentrations of 1268±173 ppm wt H2O. Orthopyroxene (opx) and garnet contain 700±46 ppm wt and 347±83 ppm wt H2O, respectively. The H2O content of the hydrous melts was determined by mass balance to be 11±0.5 wt% H2O. H2O partition coefficients between minerals and melt (DH2Omin/melt=XH2Omin/XH2Omelt) are 0.0040±0.0006 for olivine, 0.0064±0.0004 for opx, 0.0115±0.0016 for cpx and 0.0032±0.0008 for garnet. Using the determined H2O partition coefficients the onset and extent of melting at conditions equivalent to 180 km below mid-ocean ridges was determined as a function of mantle H2O content. Current estimates for the H2O content of the depleted mantle (50-200 ppm wt H2O) are insufficient to induce mantle melting at this depth, which requires ∼700 ppm wt H2O to produce 0.1% melting and 1600 ppm wt H2O for 1% melting, along an adiabat with a potential temperature of 1327 °C. Melting can occur at these conditions within the mantle source of ocean island basalts, which are estimated to contain up to 900 ppm wt H2O. If adiabatic temperatures are 200 °C higher within such plume related sources, then melt fractions of over 1% can be reached at 180 km depth. In addition, a model for the distribution of H2O between peridotite mineral

  12. Redox state of earth's upper mantle from kimberlitic ilmenites

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Tompkins, L. A.

    1983-01-01

    Temperatures and oxygen fugacities are reported on discrete ilmenite nodules in kimberlites from West Africa which demonstrate that the source region in the upper mantle is moderately oxidized, consistent with other nodule suites in kimberlites from southern Africa and the United States. A model is presented for a variety of tectonic settings, proposing that the upper mantle is profiled in redox potential, oxidized in the fertile asthenosphere but reduced in the depleted lithosphere.

  13. Formation of hydrocarbons under upper mantle conditions: experimental view

    NASA Astrophysics Data System (ADS)

    Kolesnikov, Anton; Kutcherov, Vladimir G.

    2010-05-01

    Main postulates of the theory of abiogenic abyssal origin of petroleum have been developed in the last 50 years in Russia and Ukraine. According to this theory, hydrocarbon compounds were generated in the mantle and migrated through the deep faults into the Earth's crust. There they formed oil and gas deposits in any kinds of rocks and in any kind of their structural positions. Until recently the main obstacle to accept the theory was the lack of reliable and reproducible experimental data confirming the possibility of the synthesis of complex hydrocarbon systems under the mantle conditions. The results received in the last decade by different groups of researchers from Russia, U.S.A. and China have confirmed the possibility of generation of hydrocarbons from inorganic materials, highly distributed in the Earth's mantle, under thermobaric conditions of 70-250 km: 2 - 5 GPa and 1000-1500 K. Experiments made in the CONAC chamber at pressures of 3-5 GPa and temperatures of 1000-1500 K by Kutcherov et al. [1, 2] have demonstrated that the mixtures of hydrocarbons with composition similar to natural hydrocarbon systems have been received as a result of chemical reactions between CaCO3, FeO and H2O. Methane formation from the same compounds was registered after heating up to 600-1500 K at pressures of 4-11 GPa in diamond anvil cells [4, 5, 6]. Influence of oxidation state of carbon donor and cooling rate of the fluid synthesized at high pressure were studied using different types of high pressure equipments. It was shown that composition of the final hydrocarbon mixture depends on these parameters. Experimental investigations of transformation of methane and ethane at 2-5 GPa and 1000-1500 K [3] confirmed thermodynamic stability of heavy hydrocarbons in the upper mantle and showed the possibility of hydrocarbon chain growth even at oxidative environment. For development of the theory of abiogenic abyssal origin of petroleum it is necessary to arrange a set of new

  14. An in situ experimental study of Zr4+ transport capacity of water-rich fluids in the temperature and pressure range of the deep crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Mysen, Bjorn

    2015-12-01

    Throughout the Earth's history, mass transport involved fluids. In order to address the circumstances under which Zr4+ may have been transported in this manner, its solubility behavior in aqueous fluid with and without NaOH and SiO2 in equilibrium with crystalline ZrO2 was determined from 550 to 950 °C and 60 to 1200 MPa. The measurements were carried out in situ while the samples were at the temperatures and pressures of interest. In ZrO2-H2O and ZrO2-SiO2-H2O fluids, the Zr4+ concentration ranges from ≤10 to ~70 ppm with increasing temperature and pressure. Addition of SiO2 to the ZrO2-H2O system does not affect these values appreciably. In these two environments, Zr4+ forms simple oxide complexes in the H2O fluid with ∆H ~ 40 kJ/mol for the solution equilibrium, ZrO2(solid) = ZrO2(fluid). The Zr4+ concentration in aqueous fluid increases about an order of magnitude upon addition of 1 M NaOH, which reflects the formation of zirconate complexes. The principal solution mechanism is ZrO2 + 4NaOH = Na4ZrO4 + 2H2O with ∆H ~ 200 kJ/mol. Addition of both SiO2 and NaOH to ZrO2-H2O enhances the Zr4+ by an additional factor of about 5 with the formation of partially protonated alkali zircon silicate complexes in the fluid. The principal solution mechanism is 2ZrO2 + 2NaOH + 2SiO2 = Na2Zr2Si2O9 + H2O with ∆H ~ 40 kJ/mol. These results, in combination with other published experimental data, imply that fluid released during high-temperature/high-pressure dehydration of hydrous mineral assemblages in the Earth's interior under some circumstances may carry significant concentrations of Zr and probably other high field strength elements (HFSEs). This suggestion is consistent with the occurrence of Zr-rich veins in high-grade metamorphic eclogite and granulite terranes. Moreover, aqueous fluids transported from dehydrating oceanic crust into overlying mantle source rocks of partial melting also may carry high-abundance HFSE of fluids released from dehydrating slabs and

  15. Upper Mantle Seismic Velocity Structure Beneath Eastern Africa and the Origin of Cenozoic Extensional Tectonism (Invited)

    NASA Astrophysics Data System (ADS)

    Nyblade, A.; Julia, J.; Adams, A. N.; Mulibo, G. D.; Tugume, F. A.

    2009-12-01

    The seismic structure of the upper mantle beneath eastern Africa will be reviewed using results from body wave tomography, surface wave tomography, and images of the 410 and 660 km discontinuities. Most of the data used for obtaining these results come from temporary deployments of broadband seismic stations in Ethiopia, Kenya, Uganda and Tanzania over the past decade. The ensemble of seismic results point to a deep-seated low velocity zone beneath the East African rift system that extends from the uppermost mantle, through the upper mantle, and into the mantle transition zone. The low velocity anomaly may also extend through the mantle transition zone and link with the low velocity zone in the lower mantle under southern Africa, commonly referred to as the African Superplume. This is in contrast to southern Africa, were there is little evidence for a pronounced low velocity anomaly in the upper mantle. The existence of a seismic low velocity zone beneath eastern African that extends to depths of more than 500 km supports the possibility that there is a geodynamic connection between the African Superplume and the origin of Cenozoic extensional tectonism in eastern Africa.

  16. Upper-mantle velocity structure beneath the Siberian platform

    NASA Astrophysics Data System (ADS)

    Priestley, Keith; Cipar, John; Egorkin, Anatoli; Pavlenkova, Nina

    1994-08-01

    We present a new velocity model for the continental upper mantle beneath central Siberia based on observations of the 1982'RIFT'Deep Seismic Sounding (DSS) profile. Three Peaceful Nuclear Explosions (PNE) were detonated to provide energy for the 2600 km long profile that extends from the Yamal Peninsula to the Mongolian border SE of Lake Baikal. In this paper, we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle-transition-zone discontinuities. The analysis combines forward-traveltime modelling and waveform matching using reflectivity synthetic seismograms. Our model for the lithosphere has velocities of 8.25-8.20 km s-1 from the Moho to 117 km depth. Between 117 and 123 km depth, a strong velocity gradient (8.30-8.53 km s-1) is required while a moderate gradient (8.53-8.55 km s-1) exists between 123 and 136 km depth. A low-velocity zone from 136 to 210 km depth terminates this phase arrival branch. The gradient again rises between 210 and 233 km and depth, culminating in a high-gradient zone (8.63-8.80 km s-1) between 233 and 235 km depth. Below the high-gradient zone, more moderate gradient (8.80-8.85 km s-1) is required from 235 to 253 km depth, terminating in a zone of lower velocity (8.62-8.64 km s-1) from 253 to 400 km depth. The upper-mantle transition zone consists of two high-gradient zones separated by a more moderate gradient. The upper zone is best modelled as a 35 km thick velocity gradient (8.64-9.45 km s-1) from 400 to 435 km depth. The existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance. A model with a first-order discontinuity predicts significant arrivals at ranges closer than 1580 km. Our observations contain no compelling evidence for a 520 km discontinuity, although a small discontinuity cannot be ruled out. The lower

  17. The North American upper mantle: density, composition, and evolution

    USGS Publications Warehouse

    Mooney, Walter D.; Kaban, Mikhail K.

    2010-01-01

    The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data

  18. The North American upper mantle: Density, composition, and evolution

    NASA Astrophysics Data System (ADS)

    Mooney, Walter D.; Kaban, Mikhail K.

    2010-12-01

    The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (-50 to -400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The

  19. Seismic structure and heterogeneity in the upper mantle

    NASA Astrophysics Data System (ADS)

    Kenntt, B. L. N.

    The earliest models of the seismic velocity structure of the upper mantle were smooth. But, since the introduction of strong gradients near 400 km depth by Jeffreys to explain the '20° discontinuity" in observed travel times, there has been a steady accumulation of detail in mantle structure. For a particular region, a smoothed and averaged representation of the seismic structure in the upper mantle can be derived from long-period body wave and higher mode surface wave observations. The vertical resolving power of such techniques is limited by the relatively long wavelengths. In contrast short-period observations offer potential resolution, but are susceptible to the influence of lateral heterogeneity. Fortunately the major features of the upper mantle can be discerned but important questions for structural processes such as the detailed nature ofthe transitions near 410 and 660 km are generally inaccessible. There is a natural tendency to overweight those observations on which particularly clear features are seen (as compared with the statistical anonymity of less spectacular data) which can lead to unwarranted generalizationsof specific results. To reconcile different views of mantle structure requires us to address the purpose for which the mantle structures are to be used. For example, fine detail in a velocity model which is insignificant for travel time studies can have a profound effect on amplitudes and short-period seismic waveforms. The variability in the patterns of body wave observations, especially atshort periods, provides strong evidence for 1-2 per cent heterogeneity on scales around 200 km in the upper mantle. Such features are superimposed on larger scale and larger amplitude lateral variations which can be mapped using surface wave studies. Much of the pattern of lateral variability in the upper mantle is likely to be due to thermal processes both directly by the influence of temperature and indirectly by compositional effects induced by flow

  20. Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling.

    PubMed

    Rohrbach, Arno; Schmidt, Max W

    2011-04-14

    Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle. PMID:21441908

  1. Structure of North Atlantic upper mantle based on gravity modelling, regional geochemistry and tectonic history

    NASA Astrophysics Data System (ADS)

    Barantseva, Olga; Artemieva, Irina; Thybo, Hans

    2016-04-01

    We study the link between deep geodynamic processes and their surface expression in the North Atlantic region which has an anomalous, complex structure compared to other oceans. We calculate a model of residual mantle gravity between the Charlie Gibbs Fracture Zone and Svalbard. The calculations are based on GOCE satellite data the regional crustal model EUNAseis (Artemieva and Thybo, 2013) ; for the crustal and topography effects, and the global totpgraphy and bathymetry model ETOPO1 from NOAA (Amante and Eakis, 2009). Results are complemented by sensitivity analysis of the various parameters' effects on the models. Our results identify strong heterogeneity in the upper mantle residual gravity, expressed as a sharp contrasts at the continent-ocean transition, positive mantle gravity below the continental blocks and negative - below oceanic blocks; the MOR has low-gravity anomaly. By introducing regional geochemical data and analysis of the tectonical history, we identify a strong correlation between residual mantle gravity anomalies and geochemical anomalies in ɛNd and Mg#. This analysis identifies three zones of North Atlantic mantle based on the correlation between upper mantle gravity and ocean floor age. In the area around Iceland, the residual mantle gravity is systematically lower than predicted from the half-space cooling model, and we estimate the thermal anomaly that could cause this shift.

  2. Helium isotopic textures in Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Graham, David W.; Hanan, Barry B.; Hémond, Christophe; Blichert-Toft, Janne; Albarède, Francis

    2014-05-01

    report 3He/4He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°E and 101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range occurs over ˜10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ˜9 RA likely indicates melting of pyroxenite-free or eclogite-free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including periodogram, redfit, multitaper method, and continuous wavelet transform. Long-wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot-type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ˜100 and ˜30-40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in-phase with axial depth along the SEIR at ˜1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (˜1000 km) and mesoscale (˜100 km) mantle flow, and by sampling during the partial melting process (˜30 km).

  3. Seismic Anisotropy in the Deep Mantle, Boundary Layers and the Geometry of Mantle Convection

    NASA Astrophysics Data System (ADS)

    Karato, S.

    An attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics. The mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation, the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime. Many of these issues are still poorly constrained, but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion, based on the solid state processes without invoking partial melting. The key notions are (i) the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep (or superplasticity), associated with a change in the stress level and/or grain-size in the convecting mantle with a high Rayleigh number, and (ii) the change in elastic anisotropy with pressure in major mantle minerals, particularly in (Mg, Fe)O. The results provide the following constraints on the style of mantle convection (i) the SH > SV anisotropy in the bottom transition zone and the SV > SH anisotropy in the top lower mantle can be attributed to anisotropy structures (lattice preferred orientation and/or laminated structures) caused by the horizontal flow in this depth range, suggesting the presence of a mid-mantle boundary layer due to (partially) layered convection, (ii) the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep (or superplasticity) and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces, and (iii) the SH > SV anisotropy in the cold thick portions of the D" layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiow

  4. Rayleigh Wave Phase Velocity in the Indian Ocean Upper Mantle

    NASA Astrophysics Data System (ADS)

    Godfrey, K. E.; Dalton, C. A.

    2015-12-01

    Current understanding of the seismic properties of the oceanic upper mantle is heavily weighted toward studies of the Pacific upper mantle. However, global seismic models indicate differences in upper-mantle properties beneath the Pacific, Atlantic, and Indian oceans. Furthermore, factors such as spreading rate, absolute plate motion, and the presence of intraplate volcanism vary between these regions. It is thus important to consider the broad range in parameters when forming ideas about mantle dynamics and lithosphere evolution within ocean basins. We are developing a high-resolution basin-wide seismic model of the Indian Ocean upper mantle. The Indian Ocean contains 16,000 km of mid-ocean ridge, with spreading rates ranging from approximately 14 mm/yr along the Southwest Indian Ridge to 55-75 mm/yr along the Southeast Indian Ridge. It also contains 12 volcanic hotspots, overlies a portion of a large low-shear-velocity province in the lower mantle, and is home to the Australian-Antarctic Discordance and a negative geoid anomaly just south of India, among other features. We measure phase velocity in the period range 30-130 seconds for fundamental-mode Rayleigh waves traversing the Indian Ocean; the data set includes 831 events that occurred between 1992 and 2014 and 769 stations. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. Variations in phase velocity in the Indian Ocean upper mantle are explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Our preliminary results indicate a strong dependence of phase velocity on seafloor age, with higher velocity associated with older seafloor, and perturbations to the age-dependent trend in the vicinity of the Australian-Antarctic Discordance and the Marion and

  5. Variations in Upper Mantle Seismic Structure Across the Ethiopian Plateau

    NASA Astrophysics Data System (ADS)

    Weeraratne, D. S.; Solomon, S. C.; Nyblade, A. A.

    2006-05-01

    The Ethiopian plateau, disrupted by the Main Ethiopian Rift (MER) and adjacent to the nearby Red Sea spreading center, resides within a complex tectonic environment. We use Rayleigh wave phase velocity data from the Ethiopian Broadband Seismic Experiment to study the upper mantle seismic structure of the plateau. Dispersion curves indicate phase velocities that are significantly lower than other continental lithosphere by as much as 8% for periods between 18 and 91 s. Lateral velocity variations show a narrow low-velocity anomaly elongated in the NE-SW direction located within the rift boundaries. Phase velocities within the western and eastern plateau are ~6% higher than within the MER. Shear wave velocity inversions indicate a high-velocity lid that extends to 100 km ± 20 km depth and marks the base of the lithosphere. Azimuthal anisotropy within the western plateau is resolvable for periods up to 60 s; a 1.6% peak-to-peak amplitude gradually rotates in azimuth from NNE at short periods to NE at 60 s. This azimuthal rotation is consistent with lateral variations in the fast direction of SKS splitting studies between the rift and western plateau. The change in Rayleigh wave anisotropy at long periods indicates that the NE splitting direction observed within the western plateau may be due to a component of deep sublithospheric flow. We suggest that a transition in anisotropic fabric is controlled by the combined effects of aligned melt pockets within the MER and pre-existing Mozambique sutures at shallow lithospheric depths and asthenospheric flow parallel to plate spreading at greater depths. Our results may also be consistent with sublithospheric flow of mantle plume material that rises beneath the Ethiopian plateau and flows northeastward to the Red Sea spreading center.

  6. The composition of mantle plumes and the deep Earth

    NASA Astrophysics Data System (ADS)

    Hastie, Alan R.; Fitton, J. Godfrey; Kerr, Andrew C.; McDonald, Iain; Schwindrofska, Antje; Hoernle, Kaj

    2016-06-01

    Determining the composition and geochemical diversity of Earth's deep mantle and subsequent ascending mantle plumes is vital so that we can better understand how the Earth's primitive mantle reservoirs initially formed and how they have evolved over the last 4.6 billion years. Further data on the composition of mantle plumes, which generate voluminous eruptions on the planet's surface, are also essential to fully understand the evolution of the Earth's hydrosphere and atmosphere with links to surface environmental changes that may have led to mass extinction events. Here we present new major and trace element and Sr-Nd-Pb-Hf isotope data on basalts from Curacao, part of the Caribbean large igneous province. From these and literature data, we calculate combined major and trace element compositions for the mantle plumes that generated the Caribbean and Ontong Java large igneous provinces and use mass balance to determine the composition of the Earth's lower mantle. Incompatible element and isotope results indicate that mantle plumes have broadly distinctive depleted and enriched compositions that, in addition to the numerous mantle reservoirs already proposed in the literature, represent large planetary-scale geochemical heterogeneity in the Earth's deep mantle that are similar to non-chondritic Bulk Silicate Earth compositions.

  7. Upper mantle fluids involved in diamond formation and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Sverjensky, D. A.

    2014-12-01

    Diamond formation coupled with metasomatic reactions involving the interaction of fluids with silicate host rocks provides important clues about the deep carbon cycle. However, quantitative modeling of diamond formation with silicate rock metasomatism has not been possible. Here the Deep Water (DEW) model [1] was used to generate equilibrium constants for irreversible chemical mass transfer calculations monitoring evolving fluid chemistry during diamond formation and coupled silicate reactions. Conceptual models for diamond formation in two environments were constructed for the purpose of illustrating the role of pH in diamond-forming systems. For cratonic diamonds, fluid at 900°C and 5.0 GPa was derived in equilibrium with a carbonated mafic part of a subducting slab consisting of pure diopside, enstatite, pyrope, phlogopite, magnesite, diamond and pyrite. The fluid was assumed to infiltrate and react at constant T and P with a model metasedimentary eclogite (jadeite, pyrope, kyanite and coesite). Abundant diamond was predicted to precipitate as reactant silicate minerals were destroyed and secondary pyrope, jadeite and kyanite were precipitated, which could represent the solid inclusions in natural diamonds. The final fluid chemistry was extremely enriched in Si and depleted in Ca relative to the initial fluid, consistent with the worldwide fluid inclusion trend from carbonatitic fluid to silicic fluid. The logfO2 changed by only 0.2, whereas pH continuously decreased as reaction with jadeite and kyanite and precipitation of secondary pyrope removed Mg2+ and added H+ to the fluid. Most of the carbon precipitated as diamond was derived from decreasing concentrations of formate and propionate. In the UHPM scenario, fluid at 600°C and 5.0 GPa in carbonated peridotite (forsterite, antigorite, clinochlore, magnesite, and pyrrhotite) in a subducting slab was assumed to infiltrate and react at constant T and P with a different model metasedimentary eclogite (jadeite

  8. Velocity-density models of the Earth's crust and upper mantle from the quartz, Craton, and Kimberlite superlong seismic profiles

    NASA Astrophysics Data System (ADS)

    Yegorova, T. P.; Pavlenkova, G. A.

    2015-03-01

    The unique deep seismic studies carried out in Russia with the use of nuclear explosions provided the possibility to identify the detailed structure of the Earth's crust, upper mantle, and transition zone to the lower mantle to a depth of 700 km in a huge territory of North Eurasia. It is shown that seismic velocities in the upper mantle mainly reflect its temperature regime. The gravity modeling along these profiles showed the absence of a direct relationship between seismic velocity and density. The Siberian Craton, which is marked with a low heat flow and high-velocity mantle, has lower density. The upper mantle of the East European Platform, with almost the same heat flow, is characterized by the highest densities and seismic velocities. Within the West Siberian Plate, high heat flow, lower seismic velocities, and increased density in the upper mantle are revealed. This combination of seismic velocities and densities suggests different composition of the upper mantle beneath the studied structures with the depleted upper mantle beneath the Siberian Craton.

  9. Postglacial rebound with a non-Newtonian upper mantle and a Newtonian lower mantle rheology

    NASA Technical Reports Server (NTRS)

    Gasperini, Paolo; Yuen, David A.; Sabadini, Roberto

    1992-01-01

    A composite rheology is employed consisting of both linear and nonlinear creep mechanisms which are connected by a 'transition' stress. Background stress due to geodynamical processes is included. For models with a non-Newtonian upper-mantle overlying a Newtonian lower-mantle, the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice-load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.

  10. Upper Mantle Structure of Eastern Africa from Body Wave Tomography

    NASA Astrophysics Data System (ADS)

    Mulibo, G.; Nyblade, A.; Fredinand, R.

    2010-12-01

    This study presents preliminary results of the upper mantle structure beneath the east Africa from body wave tomography. This work is part of an on-going study aimed at investigating the origin and structure of the African Superplume. The available global tomographic studies suggest that the African Superplume is a low velocity-anomaly extending from the core-mantle boundary upward into the mid mantle beneath southern Africa and may reach the upper mantle beneath eastern Africa. However, the limited vertical resolution of global tomographic models makes it difficult to confirm a connection from the lower to the upper mantle. Previous regional studies of upper mantle structure in east Africa have found evidence of a low velocity anomaly beneath the region that has been suggested as the upper mantle expression of the Superplume. Models from previous tomographic studies in east Africa have limited resolution below ~400 km beneath the eastern rift and are less well resolved beneath the western part of the rift due to less data coverage. This study uses teleseismic data from a wider region in east Africa than previously used. Data for this study are from a 3-year (2007-2010) deployment of 40 broadband seismic stations in Uganda and Tanzania. The dataset is supplemented by data from the 1994-1995 Tanzania broadband seismic experiment, the 2001-2002 Kenya broadband seismic experiment, the permanent AfricaArray seismic stations and IRIS/GSN stations. The data have been used for body wave tomography by computing relative travel time delays using a multi-channel cross-correlation technique and then inverting them for a 3D wave speed model. Preliminary results from the inversion of the relative delay times show that there is a broad low wave speed anomaly beneath east Africa extending from shallow upper mantle depths to at least 500 km into the mantle transition zone. The appearance and size of the low wave speed anomaly in the region indicates the presence of broad thermal

  11. Dikes, joints, and faults in the upper mantle

    USGS Publications Warehouse

    Wilshire, H.G.; Kirby, S.H.

    1989-01-01

    Three different types of macroscopic fractures are recognized in upper-mantle and lower-crustal xenoliths in volcanic rocks from around the world: 1. (1) joints that are tensile fractures not occupied by crystallized magma products 2. (2) dikes that are tensile fractures occupied by mafic magmas crystallized to pyroxenites, gabbros or hydrous-mineral-rich rocks, 3. (3) faults that are unfilled shear fractures with surface markings indicative of shear displacement. In addition to intra-xenolith fractures, xenoliths commonly have polygonal or faceted shapes that represent fractures exploited during incorporation of the xenoliths into the host magma that brought them to the surface. The various types of fractures are considered to have formed in response to the pressures associated with magmatic fluids and to the ambient tectonic stress field. The presence of fracture sets and crosscutting relations indicate that both magma-filled and unfilled fractures can be contemporaneous and that the local stress field can change with time, leading to repeated episodes of fracture. These observations give insight into the nature of deep fracture processes and the importance of fluid-peridotite interactions in the mantle. We suggest that unfilled fractures were opened by volatile fluids exsolved from ascending magmas to the tops of growing dikes. These volatile fluids are important because they are of low viscosity and can rapidly transmit fluid pressure to dike and fault tips and because they lower the energy and tectonic stresses required to extend macroscopic cracks and to allow sliding on pre-existing fractures. Mantle seismicity at depths of 20-65 km beneath active volcanic centers in Hawaii corresponds to the depth interval where CO2-rich fluids are expected to be liberated from ascending basaltic magmas, suggesting that such fluids play an important role in facilitating earthquake instabilities in the presence of tectonic stresses. Other phenomena related to the fractures

  12. Carbon-dioxide-rich silicate melt in the Earth's upper mantle.

    PubMed

    Dasgupta, Rajdeep; Mallik, Ananya; Tsuno, Kyusei; Withers, Anthony C; Hirth, Greg; Hirschmann, Marc M

    2013-01-10

    The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former. PMID:23302861

  13. Water Distribution in the Continental and Oceanic Upper Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2015-01-01

    Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

  14. Isotopic evidence for chaotic imprint in upper mantle heterogeneity

    NASA Astrophysics Data System (ADS)

    Armienti, Pietro; Gasperini, Daniela

    2010-05-01

    The intrinsic structure of the isotope data set of samples from the Mid-Atlantic Ridge and East Pacific Rise, believed to represent the isotopic composition of their mantle source, reveals a close relationship between sample spatial distribution and their geochemical features. The spatial distribution of their isotopic heterogeneity is self-similar on a scale between 5000 and 6000 km (about 1/6 of Earth's circumference), suggesting a self-organized structure for the underlying mantle. This implies the imprint of chaotic mantle processes, induced by mantle flow and likely related to an early phase of highly dynamic behavior of the Earth's mantle. The size of the identified self-organized region reflects the large length scale of upper mantle chemical variability, and it is likely frozen since the Proterozoic. The geochemical heterogeneity of the asthenosphere along the ridges is believed to record a transition in the thermal conditions of the Earth's mantle and to be reflected in the l = 6 peak expansion of several geophysical observables.

  15. Water percolation in the upper lower mantle: A way to fill the Earth's mantle transition zone?

    NASA Astrophysics Data System (ADS)

    Richard, G. C.; Monnereau, M.; Ricard, Y.; Ingrin, J.

    2003-04-01

    Numerous studies have been devoted to the determination of water solubility in mantle material [1]. They all show strong solubility variations from one mineral phase to another. Principally, water partitioning has made the transition zone a probable trap for water from the Earth's mantle [2]. Nevertheless, previous numerical study [3] suggests the presence of 'free' water in the lower mantle. We have studied the behaviour of this 'free' phase and its importance in the water distribution just below the transition zone (660 km depth) in a 2D cartesian geometry. The model takes into account water partitioning between the mantle's transition zone and the upper-mantle of 10:1 and between the lower-mantle and the transition zone of 1:100 (i.e. respectively between olivine-spinel and spinel-postspinel). We have modelled two possible transport processes for the 'free' water: Diffusion and percolation. Our numerical experiments show that the presence of a ‘free phase’ modifies the water distribution in this area. Percolation process is more efficient to hydrate the transition zone than diffusion one, even in a very high diffusivity case. [1] J. Ingrin, H. Skogby, Hydrogen in nominally anhydrous upper mantle minerals: Concentration levels and implications, Eur. J. Mineral. 12 (2000) 543-570. [2] N. Bolfan-Casanova, H. Keppler, D. C. Rubie, Water partitioning between nominally anhydrous minerals in the MgO-SiO2-H2O system up to 24 GPa: implications for the distribution of water in Earth's mantle, Earth Planet. Sci. Lett. 182 (2000) 209-221. [3] G. C. M. Richard, M. Monnereau, J. Ingrin, Is the transition zone an empty water reservoir? Inferences from numerical model of mantle dynamics, Earth. Planet. Sci. Lett. 205 (2002) 37-51.

  16. Modeling Three-Dimensional Upper Mantle Seismic Anisotropy with Higher Mode Surface Waves

    NASA Astrophysics Data System (ADS)

    Yuan, Kaiqing

    This dissertation presents a new 3-D global upper mantle model of elastic anisotropy obtained from surface wave seismic tomography. This research contributes to our understanding of deep Earth structure. The two main results are the following: (1) Our work unravels the presence of azimuthal seismic anisotropy in the mantle transition zone, to greater depths than previously found, thereby challenging common views of mantle deformation mechanisms. It also reveals a striking correlation between changes in seismic anisotropy where upper mantle phase transitions occur, which provides new constraints on the style of mantle convection; (2) We confirm the dominantly thermal nature of the oceanic lithosphere-asthenosphere boundary (LAB), and show that the Gutenberg discontinuity associated with vertical changes in azimuthal anisotropy inside the lithosphere, implying that this interface is not equivalent to the LAB, contrary to what is commonly assumed. The origin of the Gutenberg discontinuity is a result of frozen-in lithospheric structures, regional compositional variations of the mantle, or dynamically perturbed LAB.

  17. Do mantle plumes preserve the heterogeneous structure of their deep-mantle source?

    NASA Astrophysics Data System (ADS)

    Jones, T. D.; Davies, D. R.; Campbell, I. H.; Wilson, C. R.; Kramer, S. C.

    2016-01-01

    It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

  18. Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle.

    PubMed

    Simpson, F

    2001-08-01

    Seismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or 'shear-wave splitting', which has been investigated in numerous studies. Although olivine is also anisotropic with respect to electrical conductivity (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion. PMID:11493919

  19. Modeling gravity and magnetic fields for crustal and upper mantle structures

    NASA Technical Reports Server (NTRS)

    Denoyer, J. M.

    1985-01-01

    Research was conducted to: (1) make a direct comparison between the gravity and magnetic fields near the ellipsoid and at the height expected for the Geopotential Research Mission (GRM) for the same geologic model, (2) obtain realistic estimates of the gradients that can be expected at the orbit height of the GRM, and (3) demonstrate the value of data that the GRM could provide for investigating upper mantle and deep crustal anomalies.

  20. A solid-state framework for terrestrial upper mantles

    NASA Technical Reports Server (NTRS)

    Estey, L.; Douglas, B.; Spetzler, H.

    1985-01-01

    A framework is proposed for understanding the upper mantle structure of the terrestrial planets which is based on solid state dislocation processes. It is proposed that the base of the lithosphere on any planet with a Mg-Fe silicate rich upper mantle is defined by the threshold temperatures of low energy dislocation glide systems in olivine and the pyroxenes. This threshold temperature is approximately 1100 to 1200 K and is directly tied to the mobility of olivine and pyroxene dislocations. Using this definition, all terrestrial planets of the Inner Solar System are expected to have mantle lithospheres. Second, the anomalous properties of the asthenosphere in the Earth are related to the rheological properties of an olivine rich differentiate approximately 220 km thick. All of these properties can be the result of the abundant low energy glide systems in olivine which are mobile at temperatures above approximately 1200 K. It is proposed that a true planetary asthenosphere must be both olivine rich and at temperatures above approximately 1200 K. This mode for the asthenosphere does not require partial melting and is therefore less sensitive to the constraints of volatile content in the upper mantle. The thickness of planetary lithospheres is estimated as is maximum thickness of the olivine rich differentiate on each planet. The cooling of the olivine rich layer on Mars (and hence disappearance of the asthenosphere) may coincide with the cessation of global tectonic activity approximately 1 Gyr ago.

  1. Upper Mantle Discontinuity Structure Beneath the Western Atlantic Ocean and Eastern North America from SS Precursors

    NASA Astrophysics Data System (ADS)

    Schmerr, N. C.; Beghein, C.; Kostic, D.; Baldridge, A. M.; West, J. D.; Nittler, L. R.; Bull, A. L.; Montesi, L.; Byrne, P. K.; Hummer, D. R.; Plescia, J. B.; Elkins-Tanton, L. T.; Lekic, V.; Schmidt, B. E.; Elkins, L. J.; Cooper, C. M.; ten Kate, I. L.; Van Hinsbergen, D. J. J.; Parai, R.; Glass, J. B.; Ni, J.; Fuji, N.; McCubbin, F. M.; Michalski, J. R.; Zhao, C.; Arevalo, R. D., Jr.; Koelemeijer, P.; Courtier, A. M.; Dalton, H.; Waszek, L.; Bahamonde, J.; Schmerr, B.; Gilpin, N.; Rosenshein, E.; Mach, K.; Ostrach, L. R.; Caracas, R.; Craddock, R. A.; Moore-Driskell, M. M.; Du Frane, W. L.; Kellogg, L. H.

    2015-12-01

    Seismic discontinuities within the mantle arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. In particular, the depth and sharpness of upper mantle discontinuities at 410 and 660 km depth are attributed to solid-state phase changes sensitive to both mantle temperature and composition, where regions of thermal heterogeneity produce topography and chemical heterogeneity changes the impedance contrast across the discontinuity. Seismic mapping of this topography and sharpness thus provides constraint on the thermal and compositional state of the mantle. The EarthScope USArray is providing unprecedented access to a wide variety of new regions previously undersampled by the SS precursors. This includes the boundary between the oceanic plate in the western Atlantic Ocean and continental margin of eastern North America. Here we use a seismic array approach to image the depth, sharpness, and topography of the upper mantle discontinuities, as well as other possible upper mantle reflectors beneath this region. This array approach utilizes seismic waves that reflect off the underside of a mantle discontinuity and arrive several hundred seconds prior to the SS seismic phase as precursory energy. In this study, we collected high-quality broadband data SS precursors data from shallow focus (< 30 km deep), mid-Atlantic ridge earthquakes recorded by USArray seismometers in Alaska. We generated 4th root vespagrams to enhance the SS precursors and determine how they sample the mantle. Our data show detection of localized structure on the discontinuity boundaries as well as additional horizons, such as the X-discontinuity and a potential reflection from a discontinuity near the depth of the lithosphere-asthenosphere boundary. These structures are related to the transition from predominantly old ocean lithosphere to underlying continental lithosphere, as while

  2. Metasomatic oxidation of upper mantle periodotite

    USGS Publications Warehouse

    McGuire, A.V.; Dyar, M.D.; Nielson, J.E.

    1991-01-01

    metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C-O-H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT. ?? 1991 Springer-Verlag.

  3. Teleseismic array analysis of upper mantle compressional velocity structure. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1984-01-01

    Relative array analysis of upper mantle lateral velocity variations in southern California, analysis techniques for dense data profiles, the P-wave upper mantle structure beneath an active spreading center: the Gulf of California, and the upper mantle under the Cascade ranges: a comparison with the Gulf of California are presented.

  4. Constraining Upper Mantle Azimuthal Anisotropy With Free Oscillation Data (Invited)

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Resovsky, J. S.; van der Hilst, R. D.

    2009-12-01

    We investigate the potential of Earth's free oscillations coupled modes as a tool to constrain large-scale seismic anisotropy in the transition zone and in the bulk of the lower mantle. While the presence of seismic anisotropy is widely documented in the uppermost and the lowermost mantle, its observation at intermediate depths remains a formidable challenge. We show that several coupled modes of oscillations are sensitive to radial and azimuthal anisotropy throughout the mantle. In particular, modes of the type 0Sl-0T(l+1) have high sensitivity to shear-wave radial anisotropy and to six elastic parameters describing azimuthal anisotropy in the 200 km-1000 km depth range. The use of such data enables us thus to extend the sensitivity of traditionally used fundamental mode surface waves to depths corresponding to the transition zone and the top of the lower mantle. In addition, these modes have the potential to provide new and unique constraints on several elastic parameters to which surface waves are not sensitive. We attempted to fit degree two splitting measurements of 0Sl-0T(l+1) coupled modes using previously published isotropic and transversely isotropic mantle models, but we could not explain the entire signal. We then explored the model space with a forward modeling approach and determined that, after correction for the effect of the crust and mantle radial anisotropy, the remaining signal can be explained by the presence of azimuthal anisotropy in the upper mantle. When we allow the azimuthal anisotropy to go below 400 km depth, the data fit is slightly better and the model space search leads to better-resolved model than when we force the anisotropy to lie in the top 400 km of the mantle. Its depth extent and distribution are, however, still not well constrained by the data due to parameter tradeoffs and a limited coupled mode data set. It is thus clear that mode coupling measurements have the potential to constrain upper-mantle azimuthal anisotropy

  5. Magnesium stable isotope composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Handler, Monica R.; Baker, Joel A.; Schiller, Martin; Bennett, Vickie C.; Yaxley, Gregory M.

    2009-05-01

    The mantle is Earth's largest reservoir of Mg containing > 99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to > 99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite ± amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ25Mg DSM-3 = - 0.22 to - 0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤ ± 0.07‰ [2 sd]). We find no systematic δ25Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ25Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Δ25Mg pyr-ol = +0.06 ± 0.10‰ (2 sd; n = 5). Our average mantle olivine δ25Mg DSM-3 = - 0.14 ± 0.07‰ and δ26Mg DSM-3 = - 0.27 ± 0.14‰ (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have δ25Mg DSM-3 = - 0.16 to - 0.11‰ that are identical to terrestrial olivine and indistinguishable from the average δ25Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope

  6. Search for deep slabs in the Northwest Pacific mantle.

    PubMed

    Zhou, H W; Anderson, D L

    1989-11-01

    A residual sphere is formed by projecting seismic ray travel-time anomalies, relative to a reference Earth model, onto an imaginary sphere around an earthquake. Any dominant slab-like fast band can be determined with spherical harmonic expansion. The technique is useful in detecting trends associated with high-velocity slabs beneath deep earthquakes after deep-mantle and near-receiver effects are removed. Two types of corrections are used. The first uses a tomographic global mantle model; the second uses teleseismic station averages of residuals from many events over a large area centered on the events of interest. Under the Mariana, Izu-Bonin, and Japan trenches, the dominant fast bands are generally consistent with seismicity trends. The results are unstable and differ from the seismicity trend for Kurile events. The predominant fast band for most deep earthquakes under Japan is subhorizontal rather than near vertical. We find little support for the deep slab penetration hypothesis. PMID:16594080

  7. Thermophysical Properties and Phase Changes in the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Arafin, Sayyadul

    2015-11-01

    The correlation between phase changes within the upper mantle and the thermophysical properties of the minerals therein has been investigated by using the thermoelastic and thermodynamic equations. The depth dependence data of seismic velocities of Jeffreys-Bullen and density within the upper mantle are used as inputs in the analysis. The material characteristic properties like Debye temperature,Θ _D, adiabatic compressibility, κ S, Grüneisen parameter, ξ and the specific heat capacity, C_{{P}} computed as a function of depth show clearly two discontinuities at average depths of 414 km and 645 km which are in fair agreement with the presently accepted depths 410 km and 670 km from the preliminary reference earth model data.

  8. Changes in the crust and upper mantle near the Japan-Bonin Trench

    NASA Astrophysics Data System (ADS)

    Houtz, R.; Windisch, C.; Murauchi, S.

    1980-01-01

    Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multi-channel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.05 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n = 47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11-11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reduced T2/X2 analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well-observed mantle reflection and its strong 8.2-km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3-km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor. The peculiar double reflector near mantle and the marked change in velocity structure and upper mantle reflectivity at the edge of the outer slopes seem to confirm the changes in upper mantle refraction velocity reported by Talwani et al. (1977) in the Curacao trench, Caribbean Sea, but our evidence is not conclusive. In spite of the ambiguity it is clear that velocity

  9. Rheologic Controls on the Dynamic Evolution of Slabs in the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Billen, M.; Hirth, G.

    2004-12-01

    Subduction of tectonic plates is characterized by long-lived subduction zones, asymmetric subduction and slab dip angles of 25--80o in the upper mantle. Several mechanisms proposed to explain the variation in observed dip include large-scale mantle flow, trench roll-back, and interaction of the slab with the transition zone. Previous dynamic models of subduction that include only Newtonian viscosity and moderately strong slabs generally fail to predict subduction angles less than 60--90o at shallow depths (100--300 km). We find that the observed characteristics of subduction are reproduced by viscous flow models, in which the rheologic structure is consistent with experimentally determined flow laws for Newtonian and non-Newtonian visco-plastic deformation of olivine. The properties of the models required to match the observed characteristics of slabs are: non-Newtonian viscosity in the mantle producing a weak mantle wedge (1018--1019~Pa s), a stiff slab interior (1025~Pa s) limited by a plastic yield criterion and a weak plate boundary shear zone (1020--1021~Pa s). The shallow slab dip reaches a minimum of 25--30o for high convergence rates and a stiff slab, without trench roll-back or relative motion of the entire lithosphere with respect to the mantle, suggesting these other mechanisms are not the primary controls on slab geometry. The deep slab dip (350--650 km) decreases as the slab penetrates the stiffer (x10), Newtonian viscosity lower mantle, eventually stabilizing the upper mantle slab geometry.

  10. Seismic evidence for widespread serpentinized forearc upper mantle along the Cascadia margin

    NASA Astrophysics Data System (ADS)

    Brocher, Thomas M.; Parsons, Tom; Tréhu, Anne M.; Snelson, Catherine M.; Fisher, Michael A.

    2003-03-01

    Petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle. To test these models, we use the results of controlled-source seismic surveys and earthquake tomography to map the upper mantle along the Cascadia margin forearc. We find anomalously low upper-mantle velocities and/or weak wide-angle reflections from the top of the upper mantle in a narrow region along the margin, compatible with recent teleseismic studies and indicative of a serpentinized upper mantle. The existence of a hydrated forearc upper-mantle wedge in Cascadia has important geological and geophysical implications. For example, shearing within the upper mantle, inferred from seismic reflectivity and consistent with its serpentinite rheology, may occur during aseismic slow slip events on the megathrust. In addition, progressive dehydration of the hydrated mantle wedge south of the Mendocino triple junction may enhance the effects of a slab gap during the evolution of the California margin.

  11. Upper-Mantle Flow Driven Dynamic Topography in Eastern Anatolia

    NASA Astrophysics Data System (ADS)

    Sengul Uluocak, Ebru; Pysklywec, Russell; Eken, Tuna; Hakan Gogus, Oguz

    2016-04-01

    Eastern Anatolia is characterized by 2 km plateau uplift -in the last 10 Myrs-, high surface heat flow distribution, shallow Curie-point depth, anomalous gravity field. Seismological observations indicate relatively high Pn and Sn attenuation and significant low seismic velocity anomalies in the region. Moreover, the surface geology is associated predominantly with volcanic rocks in which melt production through mantle upwelling (following lithospheric delamination) has been suggested. It has been long known that the topographic loading in the region cannot be supported by crustal thickness (~45 km) based on the principle of Airy isostasy. Recent global geodynamic studies carried out for evaluating the post-collisional processes imply that there is an explicit dynamic uplift in Eastern Anatolia and its adjacent regions. In this study we investigate the instantaneous dynamic topography driven by 3-D upper-mantle flow in Eastern Anatolia. For this purpose we conducted numerous thermo-mechanical models using a 2-D Arbitrary Lagrangian Eulerian (ALE) finite element method. The available P-wave tomography data extracted along 10 profiles were used to obtain depth-dependent density anomalies in the region. We present resulting dynamic topography maps and estimated 3D mantle flow velocity vectors along these 2-D cross sections for each profile. The residual topography based on crustal thickness and observed topography was calculated and compared with other independent datasets concerning geological deformation and dynamic topography predictions. The results indicate an upper mantle driven dynamic uplift correlated with the under-compensated characteristic in Eastern Anatolia. We discuss our results combined with 3D mantle flow by considering seismic anisotropy studies in the region. Initial results indicate that high dynamic uplift and the localized low Pn velocities in concurrence with Pn anisotropy structures show nearly spatial coherence in Eastern Anatolia.

  12. Three-dimensional mapping of Seismic reflections from the crust and upper mantle, northwest of Scotland

    NASA Astrophysics Data System (ADS)

    Flack, Catherine; Warner, Mike

    1990-02-01

    A survey area 190 km by 110 km to the northwest of Scotland on the UK continental shelf, has been the site of intense exploration using the deep Seismic reflection technique. A closely spaced grid of deep reflection data totalling 1600 line km has been acquired by the BIRPS group between 1981 and 1987. The majority of the data were recorded to 15 s two-way travel time (50 km depth); profiles with recording times up to 60 s (230 km depth) were also acquired. The survey includes a two-ship synthetic aperture profile, the SLAVE line, which synthesises a 0-16 km offset CDP profile. The main features revealed and consistently imaged across the survey area by this combined dataset are: an upper crust containing half-graben style basins, bounded to the west by planar faults; a reflective lower crust; and dipping and sub-horizontal, high-amplitude reflections within the mantle. Three-dimensional mapping of the dipping reflectors shows a complex package of reflections which updomes into the lower crust in the central area of the survey, disrupting the Moho and continuing into the uppermost mantle, where it thins to a couplet of reflections. In the north and south of the survey area, this structure is observed only in the mantle, to a depth of 80 km. This dataset is unique in deep Seismic reflection profiling, in terms of the quality of the image returned, the great depth to which reflections are imaged and the intensity of surveying of the lower crust and upper mantle. The continuity and strength of the reflectors seen on these records show that even at the great depths at which these structures exist the Seismic reflection technique is able to image them as high-amplitude, coherent and highly continuous reflections. The reflections are interpreted as shear zones or faults within the lower crust and upper mantle. This indicates that in this part of the world at least, both the lower crust and upper mantle are able to sustain discrete zones of deformation.

  13. Thermo-chemical plumes rooted in the deep mantle beneath major hotspots: implications for mantle dynamics

    NASA Astrophysics Data System (ADS)

    Romanowicz, B. A.; French, S.

    2015-12-01

    The existence of mantle plumes as a possible origin for hotspots has been the subject of debate for the last 30 years. Many seismic tomographic studies have hinted at the presence of plume-like features in the lower mantle, but resolution of narrow low velocity features is difficult, and ambiguity remains as to the vertical continuity of these features and how distinct they are from other low velocity blobs. We present robust evidence for significant, vertically continuous, low velocity columns in the lower mantle beneath prominent hotspots located within the footprint of the large low shear velocity provinces (LLSVPs), from a recent global, radially anisotropic whole mantle shear-wave velocity (Vs) model, SEMUCB-WM1 (French and Romanowicz, 2014, 2015). This model was constructed by inversion of a large dataset of long period three-component seismograms down to 32s period. Because it includes surface-wave overtones, S-diffracted waves and multiply reflected waves between the surface and the CMB, this dataset provides considerably better illumination of the whole mantle volume than can be obtained with a standard set of travel times alone. In addition, accurate numerical computation of the forward wavefield using the spectral element method at each iteration of the model construction, allows us to better resolve regions of lower than average Vs. The imaged plumes have several common characteristics: they are rooted in patches of very low Vs near the core mantle boundary, some of which contain documented ULVZs, and extend vertically through the lower mantle up to ~1000 km depth, where some are deflected horizontally, or give rise to somewhat thinner conduits that meander through the upper mantle in the vicinity of the target hotpots. Combined with evidence for slab stagnation at ~1000 km depth, this suggests a change in rheology between 660 and 1000 km depth, very high viscosity throughout the bulk of the lower mantle, and lower viscosity plumes, only mildly

  14. Constraints on the heterogeneity spectrum of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Mancinelli, Nicholas; Shearer, Peter; Liu, Qinya

    2016-05-01

    We constrain the heterogeneity spectrum of Earth's upper mantle at scales from a few kilometers to tens of thousands of kilometers using observations from high-frequency scattering, long-period scattering, and tomography. Tomography and high-frequency scattering constraints are drawn from previous studies, but constraints on mantle heterogeneity at intermediate scales (5-500 km) are lacking. To address this, we stack ˜15,000 long-period P coda envelopes to characterize the globally averaged scattered wavefield at periods from 5 to 60 s and at ranges from 50 to 98°. To fit these observations, we consider models of random mantle heterogeneity and compute the corresponding global wavefield using both a ray theoretical "seismic particle" approach and full spectral element simulations. Von Kármán random media distributed throughout the uppermost 600 km of the mantle with a = 2000 km, ɛ = 10%, and κ = 0.05 provide a good fit to the time, range, and frequency dependence of the stacks, although there is a trade-off between ɛ and the thickness of the assumed scattering layer. This random media model also fits previously published 1 Hz stacks of P coda and agrees with constraints on long-wavelength structure from tomography. Finally, we explore geodynamically plausible scenarios that might be responsible for the RMS and falloff rate of the proposed spectrum, including a self-similar mixture of basalt and harzburgite.

  15. Olivine crystals align during diffusion creep of Earth's upper mantle.

    PubMed

    Miyazaki, Tomonori; Sueyoshi, Kenta; Hiraga, Takehiko

    2013-10-17

    The crystallographic preferred orientation (CPO) of olivine produced during dislocation creep is considered to be the primary cause of elastic anisotropy in Earth's upper mantle and is often used to determine the direction of mantle flow. A fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during diffusion creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose diffusion creep to be the primary means of mantle flow. PMID:24132289

  16. Upper mantle flow and lithospheric dynamics beneath the Eurasian region

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Jiang, G.; Jia, Z.; Gao, R.; Fu, R.

    2010-12-01

    Evidence from seismic tomography, geothermal and short wavelength geoid anomalies reveals the existence of small-scale convective systems in the upper mantle, with scales ranging from 500 km to 700 km. It is reasonable to suggest that these small-scale convective systems probably control the regional tectonic structure and the dynamical processes of the lithosphere. Here we have calculated the patterns of small-scale convection in the upper mantle for the Eurasian region (20°E~170°E,15°N~75°N), using the anomaly of isostatic gravity. The results show that the regional lithospheric tectonics is strongly correlated with the upper mantle flow in the Eurasian region. Two intensive convective belts against the weak background convection can be recognized from convection patterns in this region: Alpine-Himalayan collision belt and West Pacific island arc-underthrust belt. Alpine-Himalayan belt is caused by the collision between the northern plate (Eurasian plate) and the southern plates (African plate and Indian plate). West Pacific island arc-underthrust belt is caused by the subduction of the Pacific plate beneath the Eurasian plate. Both of them are also seismotectonic belts. The collision and the subduction are two important geological events occurred since Mesozoic era and Cenozoic era in the Eurasian region. Therefore, the mantle flows may be one of the main driving forces of two events. In addition, most plate boundaries in this region can be recognized and the characteristics of upper mantle convection are different completely between the Eurasian plate and the plates around it (African plate, Arabian plate, Indian plate, Philippine Sea plate and Pacific plate). Main structures and geodynamic characteristics of the Eurasian can also be explained by our model results. The Tibet plateau is located in the intensive convective belt. Around the belt, the upwelling materials push the lithosphere to lift unitarily and form the plateau. Towards the north of the Tibet

  17. Anisotropy and shear-velocity heterogeneities in the upper mantle

    NASA Technical Reports Server (NTRS)

    Nataf, H.-C.; Nakanishi, I.; Anderson, D. L.

    1984-01-01

    Long-period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle. The dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure. The data are corrected for the effect of surface layers and both Love and Rayleigh waves are used. Shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km. The shear wave velocity distribution to 200 km depth correlates with surface tectonics, except in a few anomalous regions. Below that depth the correlation vanishes. Cold subducted material shows up weakly at 350 km as fast S-wave anomalies. In the transition region a large scale pattern appears with fast mantle in the South-Atlantic. S-anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones. The Pacific shows a NW-SE fabric.

  18. Upper Mantle Seismic Anisotropy around the Plate Edge beneath Northern Taiwan

    NASA Astrophysics Data System (ADS)

    Liang, W.; Hsu, Y.; Huang, B.

    2006-12-01

    We analyze the shear wave splitting to investigate the seismic anisotropy in the upper mantle around a plate edge beneath northern Taiwan. Both local shear waves generated from deep earthquakes and distant SKS phases from the Feb. 22, 2006 Mozambique earthquake (Mw=7) were used to derive the anisotropic parameters. The measurements show significant spatial variation in seismic anisotropy across the slab edge. Result obtained from local S waves shows that a trench-parallel polarization of the leading shear-wave (fast direction) exists in the mantle wedge beneath northern Taiwan, which is consistent with previous observations along the Ryukyu arc to the east. The fast directions of SKS confirm this feature for stations (WFSB, YNGF, and IGKF) above the mantle wedge. However, it is perpendicular to the one derived from local shear waves at ANPB and is comparable with those stations to the south. The ANPB is located right above the edge of the subducted Philippine Sea plate, where the slab is colliding with the continental lithosphere and a minimum slab rollback is expected. In this case, we interpret this observation as an evidence of B-type olivine fabric in the mantle wedge, rather than the slab edge flow driven by slab rollback. On the other hand, the fast directions in the upper mantle are subparallel to the orientation of the mountain strike. In addition, the split time is roughly proportional to the mountain height. This may be caused by the mountain parallel mantle flow induced by lithospheric collision between the Philippine Sea plate and the Eurasian plate. This work has provided an important contraint on the geodynamic modeling at the plate edge associated with collision and subduction processes in the Taiwan region.

  19. Deep-focus earthquakes and recycling of water into the earth's mantle

    NASA Technical Reports Server (NTRS)

    Meade, Charles; Jeanloz, Raymond

    1991-01-01

    For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of the planet.

  20. Experimentally determined water storage capacity in the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Ferot, A.; Bolfan-Casanova, N.

    2010-12-01

    Trace amounts of hydrogen dissolved as defects in nominally anhydrous minerals (NAMs) in the mantle are believed to play a key role in physical and chemical processes in the Earth’s upper mantle. Hence, the estimation of water storage in mantle phases and solubility mechanisms are important in order to better understand the effect of water. Experimental data on water solubility in NAMs are available for upper mantle minerals such as olivine, pyroxenes and garnet. However, the majority of studies are based on the study of single phases, and at temperatures or pressures that are too low for the Earth’s upper mantle. The aim of this study is to constrain the combined effects of pressure, temperature and composition on water solubility in olivine and orthopyroxene under upper mantle conditions. The solubility of water in coexisting orthopyroxene and olivine was investigated by simultaneously synthesizing the two phases at high pressure and high temperature in a multi-anvil press. Experiments were performed under water-saturated conditions in the MSH systems with Fe and Al at 2.5, 5, 7.5 and 9 GPa and temperatures between 1175 and 1400°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on doubly polished thin sections of randomly oriented crystals. Water solubility in olivine increases with pressure and decreases with temperature as has been described previously (Bali et al., 2008). The aluminum content strongly decreases in olivine with pressure from 0.09 wt% at 2.5 GPa and 1250°C to 0.04 wt% at 9 GPa and 1175°C. The incorporation of this trivalent cation in the system enhances water solubility in olivine even if present in trace amounts, however this behavior appears to reverse at high pressure. The effect of temperature on water solubility follows a bell-shaped curve with a maximum solubility in olivine and orthopyroxene at 1250°C. Aluminum is incorporated in orthopyroxene following the Tschermak substitution and strongly

  1. Plume's buoyancy and heat fluxes from the deep mantle estimated by an instantaneous mantle flow simulation based on the S40RTS global seismic tomography model

    NASA Astrophysics Data System (ADS)

    Yoshida, Masaki

    2012-11-01

    It is still an open question as to how much heat is transported from the deep mantle to the upper mantle by mantle upwelling plumes, which would impose a strong constraint on models of the thermal evolution of the earth. Here I perform numerical computations of instantaneous mantle flow based on a recent highly resolved global seismic tomography model (S40RTS), apply new simple fluid dynamics theories to the plume's radius and velocity, considering a Poiseuille flow assumption and a power-law relationship between the boundary layer thickness and Rayleigh number, and estimate the plume's buoyancy and heat fluxes from the deep lower mantle under varying plume viscosity. The results show that for some major mantle upwelling plumes with localized strong ascent velocity under the South Pacific and Africa, the buoyancy fluxes of each plume beneath the ringwoodite to perovskite + magnesiowüstite ("660-km") phase decomposition boundary are comparable to those inferred from observed hotspot swell volumes on the earth, i.e., on the order of 1 Mg s-1, when the plume viscosity is 1019-1020 Pa s. This result, together with previous numerical simulations of mantle convection and the gentle Clausius-Clapeyron slope for the 660-km phase decomposition derived from recent high-pressure measurements under dehydrated/hydrated conditions in the mantle transition zone, implies that mantle upwelling plumes in the lower mantle penetrate the 660-km phase decomposition boundary without significant loss in thermal buoyancy because of the weak thermal barrier at the 660-km boundary. The total plume heat flux under the South Pacific is estimated to be about 1 TW beneath the 660-km boundary, which is significantly smaller than the core-mantle boundary heat flux. Previously published scaling laws for the plume's radius and velocity based on a plume spacing theory, which explains well plume dynamics in three-dimensional time-dependent mantle convection, suggest that these plume fluxes depend

  2. Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle

    USGS Publications Warehouse

    Foulger, G.R.; Pritchard, M.J.; Julian, B.R.; Evans, J.R.; Allen, R.M.; Nolet, G.; Morgan, W.J.; Bergsson, B.H.; Erlendsson, P.; Jakobsdottir, S.; Ragnarsson, S.; Stefansson, R.; Vogfjord, K.

    2001-01-01

    range ??? 100-300 km beneath east-central Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north-south and generally underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convective upwellings driven by basal heating, passive upwelling in response to plate separation and lateral temperature gradients. Although we cannot resolve structure deeper than ??? 450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle transition zone. Such models thus suggest a shallow origin for the Iceland hotspot rather than a deep mantle plume, and imply that the hotspot has been located on the spreading ridge in the centre of the north Atlantic for its entire history, and is not fixed relative to other Atlantic hotspots. The results are consistent with recent, regional full-thickness mantle tomography and whole-mantle tomography images that show a strong, low-wave-speed anomaly beneath the Iceland region that is confined to the upper mantle and thus do not require a plume in the lower mantle. Seismic and geochemical observations that are interpreted as indicating a lower mantle, or core-mantle boundary origin for the North Atlantic Igneous Province and the Iceland hotspot should be re-examined to consider whether they are consistent with upper mantle processes.

  3. Ultradeep (greater than 300 kilometers), ultramafic upper mantle xenoliths.

    PubMed

    Haggerty, S E; Sautter, V

    1990-05-25

    Geophysical discontinuities in Earth's upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid-state dissolution of pyroxene into garnet with increasing depth. These predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet-bearing xenoliths from a diamondiferous kimberlite. Conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars, placing the origin of these xenoliths at depths of 300 to 400 kilometers. These ultradeep xenoliths support the theory that the 400-km seismic discontinuity is marked by a transition from peridotite to eclogite. PMID:17745405

  4. Seismic velocity, attenuation and rheology of the upper mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.; Minster, J. B.

    1980-01-01

    Seismic and rheological properties of the upper mantle in the vicinity of the low-velocity zone are expressed in terms of relaxation by dislocation glide. Dislocation bowing in the glide plane explains seismic velocities and attenuation. Climbing at higher stresses for longer periods of time give the observed viscosity, and explain the low velocity and high temperature attenuation found at seismic frequencies. Due to differing parameters, separate terms for thermal, seismic and rheological lithospheres are proposed. All three lithospheres, however, are related and are functions of temperature, and must be specified by parameters such as period, stress, and stress duration.

  5. Upper Mantle Discontinuities Underneath Central and Southern Mexico

    NASA Astrophysics Data System (ADS)

    Perez-Campos, X.; Clayton, R. W.

    2011-12-01

    Central and southern Mexico are affected by the subduction of Cocos plate beneath North American plate. The MesoAmerican Subduction Experiment (MASE) and the Veracruz-Oaxaca (VEOX) project have mapped the geometry of the Cocos slab. It is characterized in central Mexico by a shallow horizontal geometry up to ~300 km from the trench, then it dives steeply (70°) into the mantle, to its apparent end at 500 km depth. In contrast, some 400 km to the south, the slab subducts smoothly, with a dip angle of ~26° to a depth of 150 km. We use receiver functions from teleseismic events, recorded at stations from MASE, VEOX, and the Servicio Sismológico Nacional (SSN, Mexican National Seismological Service) to map the upper mantle discontinuities and properties of the transition zone in central and southern Mexico. We also use data from the Mapping the Rivera Subduction Zone (MARS) Experiment to get a complete picture of the subduction regime in central Mexico and compare the mantle transition zone in a slab tear regime. The 410 discontinuity shows significant variation in topography in central Mexico, particularly where the slab is expected to reach such depth. The 660 discontinuity shows a smoother topography, indicating that the slab does not penetrate this far down. The results will be compared with a ridge regime in the Gulf of California.

  6. Komatiites reveal a hydrous Archaean deep-mantle reservoir.

    PubMed

    Sobolev, Alexander V; Asafov, Evgeny V; Gurenko, Andrey A; Arndt, Nicholas T; Batanova, Valentina G; Portnyagin, Maxim V; Garbe-Schönberg, Dieter; Krasheninnikov, Stepan P

    2016-03-31

    Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth's mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth's history. PMID:27029278

  7. Komatiites reveal a hydrous Archaean deep-mantle reservoir

    NASA Astrophysics Data System (ADS)

    Sobolev, Alexander V.; Asafov, Evgeny V.; Gurenko, Andrey A.; Arndt, Nicholas T.; Batanova, Valentina G.; Portnyagin, Maxim V.; Garbe-Schönberg, Dieter; Krasheninnikov, Stepan P.

    2016-03-01

    Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.

  8. Upper extremity deep vein thrombosis

    PubMed Central

    Saseedharan, Sanjith; Bhargava, Sunil

    2012-01-01

    A 56-year-old female, recently (3 months) diagnosed with chronic kidney disease (CKD), on maintenance dialysis through jugular hemodialysis lines with a preexisting nonfunctional mature AV fistula made at diagnosis of CKD, presented to the hospital for a peritoneal dialysis line. The recently inserted indwelling dialysis catheter in left internal jugular vein had no flow on hemodialysis as was the right-sided catheter which was removed a day before insertion of the left-sided line. The left-sided line was removed and a femoral hemodialysis line was cannulated for maintenance hemodialysis, and the next day, a peritoneal catheter was inserted in the operation theater. However, 3 days later, there was progressive painful swelling of the left hand and redness with minimal numbness. The radial artery pulsations were felt. There was also massive edema of forearm, arm and shoulder region on the left side. Doppler indicated a steal phenomena due to a hyperfunctioning AV fistula for which a fistula closure was done. Absence of relief of edema prompted a further computed tomography (CT) angiogram (since it was not possible to evaluate the more proximal venous segments due to edema and presence of clavicle). Ct angiogram revealed central vein thrombosis for which catheter-directed thrombolysis and venoplasty was done resulting in complete resolution of signs and symptoms. Upper extremity DVT (UEDVT) is a very less studied topic as compared to lower extremity DVT and the diagnostic and therapeutic modalities still have substantial areas that need to be studied. We present a review of the present literature including incidences, diagnostic and therapeutic modalities for this entity. Data Sources: MEDLINE, MICROMEDEX, The Cochrane database of Systematic Reviews from 1950 through March 2011. PMID:22624098

  9. Seismic tomography of the Colorado Rocky Mountains upper mantle from CREST: Lithosphere-asthenosphere interactions and mantle support of topography

    NASA Astrophysics Data System (ADS)

    MacCarthy, J. K.; Aster, R. C.; Dueker, K.; Hansen, S.; Schmandt, B.; Karlstrom, K.

    2014-09-01

    The CREST experiment (Colorado Rocky Mountains Experiment and Seismic Transects) integrated the EarthScope USArray Transportable Array with portable and permanent stations to provide detailed seismic imaging of crust and mantle properties beneath the highest topography region of the Rocky Mountains. Inverting approximately 14,600 P- and 3600 S-wave arrival times recorded at 160 stations for upper mantle Vp and Vs structure, we find that large Vp perturbations relative to AK135 of 7% and Vs variations of 8% take place over very short (approaching tens of kilometers) lateral distances. Highest heterogeneity is observed in the upper 300 km of the mantle, but well resolved low velocity features extend to the top of the transition zone in portions of these images. The previously noted low velocity upper mantle Aspen Anomaly is resolved into multiple features. The lowest Vp and Vs velocities in the region are found beneath the San Juan Mountains, which is clearly distinguished from other low velocity features of the northern Rio Grande Rift, Taos/Latir region, Aspen region, and below the Never Summer Mountains. We suggest that the San Juan anomaly, and a similar feature below the Taos/Latir region of northern New Mexico, are related to delamination and remnant heat (and melt) beneath these sites of extraordinarily voluminous middle-Cenozoic volcanism. We interpret a northeast-southwest grain in velocity structure that parallels the Colorado Mineral belt to depths near 150 km as being reflective of control by uppermost mantle Proterozoic accretionary lithospheric architecture. Further to the north and west, the Wyoming province and northern Colorado Plateau show high velocity features indicative of thick (∼150 km) preserved Archean and Proterozoic lithosphere, respectively. Overall, we interpret the highly heterogeneous uppermost mantle velocity structure beneath the southern Rocky Mountains as reflecting interfingered chemical Proterozoic lithosphere that has been, is

  10. Upper mantle conductivity in Fennoscandia as imaged by the BEAR array

    NASA Astrophysics Data System (ADS)

    Korja, T.; Bear Working Group

    2003-04-01

    The Baltic Electromagnetic Array Research (BEAR) project realizes a deep electromagnetic sounding experiment with the use of a shield-wide magnetotelluric and magnetometer array of simultaneous recordings. The BEAR project focuses on determining the electrical conductivity of the upper mantle beneath Fennoscandia and thereby aims to gain deeper insights into the structure, evolution and current dynamics of the continental lithosphere beneath cratons. We shall first briefly review the results of specific research related to e.g. data processing (incl. efforts to determine long period responses and eliminate source effects), studies on the nature and effects of the non-uniform source, studies on crustal distortions (decompositions), and 3D modelling and multi-dimensional inversion (incl. resolution studies) studies. In the latter, the recently compiled crustal 3D conductivity model over the entire Fennoscandia had a major role. Thereafter we concentrate on possible anisotropy of continental lithosphere and on the upper mantle conductivity structure beneath Fennoscandia as imaged by the BEAR array. The major findings, to be discussed together with other geophysical and geological models, are: - upper mantle contains an excess of up to 5000 S of conductive material with respect to an average 1D reference model of Fennoscandia; - mantle lithospheric conductors are observed at the depth interval of 80-120 km in several parts of the Shield; - a significant increase of conductivity takes place at the depths ranging from 170 km to 250 km; - strong anisotropic signatures in data (e.g. impedance phase split) can be explained primarily by isotropic crustal 3D structures (a small “remaining “ part may be due to anisotropic lithosphere and/or 3D isotropic lithosphere); - conductivity at depths below c. 400 km is compatible with global averages (Olsen, 1998; Semenov, 1999).

  11. Study on 3-D velocity structure of crust and upper mantle in Sichuan-yunnan region, China

    USGS Publications Warehouse

    Wang, C.; Mooney, W.D.; Wang, X.; Wu, J.; Lou, H.; Wang, F.

    2002-01-01

    Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others appear the characteristic of tectonic boundary, indicating that the faults litely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the Sichuan-Yunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the Indian and the Asian plates. The crustal velocity in the Sichuan-Yunnan rhombic block generally shows normal.value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

  12. Plumes do not Exist: Plate Circulation is Confined to Upper Mantle

    NASA Astrophysics Data System (ADS)

    Hamilton, W. B.

    2002-12-01

    Plumes from deep mantle are widely conjectured to define an absolute reference frame, inaugurate rifting, drive plates, and profoundly modify oceans and continents. Mantle properties and composition are assumed to be whatever enables plumes. Nevertheless, purported critical evidence for plume speculation is false, and all data are better interpreted without plumes. Plume fantasies are made ever more complex and ad hoc to evade contradictory data, and have no predictive value because plumes do not exist. All plume conjecture derives from Hawaii and the guess that the Emperor-Hawaii inflection records a 60-degree change in Pacific plate direction at 45 Ma. Paleomagnetic latitudes and smooth Pacific spreading patterns disprove any such change. Rationales for other fixed plumes collapse when tested, and hypotheses of jumping, splitting, and gyrating plumes are specious. Thermal and physical properties of Hawaiian lithosphere falsify plume predictions. Purported tomographic support elsewhere represents artifacts and misleading presentations. Asthenosphere is everywhere near solidus temperature, so melt needs a tensional setting for egress but not local heat. Gradational and inconsistent contrasts between MORB and OIB are as required by depth-varying melt generation and behavior in contrasted settings and do not indicate systematically unlike sources. MORB melts rise, with minimal reaction, through hot asthenosphere, whereas OIB melts react with cool lithosphere, and lose mass, by crystallizing refractories and retaining and assimilating fusibles. The unfractionated lower mantle of plume conjecture is contrary to cosmologic and thermodynamic data, for mantle below 660 km is more refractory than that above. Subduction, due to density inversion by top-down cooling that forms oceanic lithosphere, drives plate tectonics and upper-mantle circulation. It organizes plate motions and lithosphere stress, which controls plate boundaries and volcanic chains. Hinge rollback is the

  13. Double layering of a thermochemical plume in the upper mantle beneath Hawaii

    NASA Astrophysics Data System (ADS)

    Ballmer, M. D.; Ito, G.; Wolfe, C. J.; Cadio, C.; Solomon, S. C.

    2012-04-01

    Volcanism far from plate boundaries has traditionally been explained by "classical" plume theory. Classical plumes are typically described as narrow thermal upwellings that rise through the entire mantle to be deflected into a thin (<100 km) "pancake" beneath the overriding lithosphere. The pancake is thought to be deflected by the drag of the overriding plate and hence to support a hotspot swell that is parabolic in map view and symmetric about the direction of plate motion. Many hotspots and their swells, such as Cape Verde and Iceland, are indeed well explained by near-classical thermal plumes. High-resolution seismic velocity images obtained from the PLUME project support the concept of a deep-rooted mantle plume beneath the Hawaiian hotspot. However, in detail these images challenge traditional concepts inasmuch as they indicate a low-velocity body in the upper mantle that is too thick (~400 km) and asymmetric to be interpreted as a classical pancake. Classical plume theory is, moreover, inconsistent with several geochemical characteristics of Hawaiian magmas, which point to a heterogeneous mantle source involving mafic lithologies such as eclogite and not an exclusively thermal (i.e., isochemical) origin¹. To explore the dynamical and melting behavior of plumes containing a substantial fraction (~15%) of eclogite, we performed three-dimensional numerical simulations of thermochemical convection. Relative to ambient-mantle peridotite, eclogite is intrinsically dense. This density contrast is sensitive to phase changes in the upper mantle; the contrast peaks at 410-300 km and lessens at about 250-190 km depth, where eclogite is subsequently removed by melting. For a plume core with an eclogite content >12%, these effects locally increase the density beyond that of the ambient mantle. Therefore, the upwelling column forms a broad and thick pool at depths of 450-300 km (which we term the deep eclogite pool, or DEP). As the DEP is well supported by the deeper

  14. Reservoirs of Undegassed Material in the Deep Mantle and the Origin of Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Deschamps, Frédéric; Tackley, Paul; Cobden, Laura; Kaminski, Edouard

    2013-04-01

    The large scattering in the isotopic Helium ratio (4He/3He) observed in Ocean Island Basalts (OIB) suggests that the plumes at the origin of OIB sample several reservoirs. The low values (< 30000) of the Helium ratio indicates that OIB sample an undegassed reservoir. Its lowest value, around 15000, imposes a constraint on the entrainment of primitive material by plumes, which should not exceed 10%. Numerical experiments of thermo-chemical convection in 3D-Cartesian and spherical geometries showed that reservoirs of primordial material can be maintained at the bottom of the system, the shape and stability of these reservoirs depending on the chemical density contrast and on the thermal viscosity contrast. In addition, plumes are generated at the top of these reservoirs, entraining small fraction of primordial material up to the surface. Numerical experiments showed that this entrainment quantitatively agrees with OIB data, with values around 9%. The location of the undegassed reservoirs is still a matter of debate. Images of slabs penetrating in the deep mantle indicate that the lower mantle itself is not isolated. The undegassed reservoirs may instead consist of pools of chemically distinct material located in the lowermost mantle. Possible candidates for these pools are the low shear-wave velocity provinces (LLSVP) observed by seismic tomography. Additional observations, including the anti-correlation between shear- and bulk-sound velocity anomalies, show that these structures are caused by large scale thermo-chemical anomalies. The exact nature of the chemical component of these anomalies is still unclear, two end-members hypotheses (namely the recycling of MORB by subduction, and the survival of primordial deep reservoirs) being usually advocated. The combination of mineral physics data and global tomographic models shows that LLSVP are better explained by material enriched in iron and silicates than by high pressure MORB, unless these LLVSP are hotter than the

  15. Upper Mantle Structure beneath the Chinese Capital Region from Teleseismic Finite-Frequency Seismic Tomography

    NASA Astrophysics Data System (ADS)

    Yang, F.; Huang, J.

    2009-12-01

    In this study, we applied the finite-frequency seismic tomography(FFST) to teleseismic waveform data to determine 3-D P-wave velocity structure of the upper mantle under the Chinese capital region. The seismic waveform data from more than 300 teleseismic events recorded by the Chinese digital Capital Seismic Network during the period from September 2003 to December 2005 was used in this study. We obtained 18499 high accuracy P-wave relative travel-times by filtering these waveform data on the vertical component into high-, intermediate-, low-frequency bands (1.0-2.0, 0.1-1.0 and 0.05-0.1 hz, respectively) and the multi-channel waveform cross correlation measurement. The 3-D Fréchet sensitivity kernels were calculated by paraxial approximation for each frequency band. We established observation equations with these measured relative travel-times and 3-D Fréchet sensitivity kernels and then determined the 3-D velocity structure by inverting the observation equations. Our results show there are distinct differences of deep velocity structure down to 150 km depth under the four tectonic units of present study region. The Yanshan uplift exhibited the high velocity(high-V) feature. Under the Taihangshan uplift, broad low velocity(low-V) are visible, but it also shows up as small high-V anomalies. A large scale prominent low-V anomaly was revealed in the shallow upper mantle under the North China basin and Bohai bay. In the North China basin the low-V anomaly generally extend from 50 km to 150 km depth, but in the Bohai bay, this low-V anomaly gradually extend down to 200 km depth. The depth of this low-V anomaly is 50-70 km under the North China basin and Bohai bay, which is consistent with the depth of high conductivity layer in the upper mantle determined by the measurement of magnetotelluric sounding and heat flow. This result shows lithosphere thinning in the North China basin and Bohai bay. Most of large earthquakes occurred in the Zhangjiakou-Penglai fault zone

  16. Finite Frequency Upper Mantle Tomography Using the Spectral Element Method

    NASA Astrophysics Data System (ADS)

    Lekic, V.; Romanowicz, B.

    2007-12-01

    In the past quarter century, global tomography based on ray theory and first-order perturbation methods has imaged long-wavelength velocity heterogeneities of the Earth's mantle. While these models have contributed significantly to our understanding of mantle circulation, the development of higher resolution images of the Earth's interior holds tremendous promise for understanding the nature of the observed heterogeneities. This endeavor confronts us with two challenges. First, it requires extracting a far greater amount of information from the available seismograms than is generally used. Second, the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with short-wavelength heterogeneity. We have developed a novel hybrid approach to long-period waveform tomography in which forward-modeling is performed using the Coupled Spectral Element Method (CSEM: Capdeville et al., 2003), which can accurately model seismic wave propagation in a 3D earth with both short and long wavelength structure, while in the inversion step, the sensitivity kernels are calculated using an approximate, non-linear normal mode summation approach (NACT: Li and Romanowicz, 1995). Our dataset consists of complete 3-component time domain seismograms filtered at periods greater than 80 s for 100 earthquakes observed at well over 100 stations of the IRIS/GSN, GEOSCOPE, GEOFON and various regional broadband networks. Modeling is performed in an iterative fashion, and convergence is achieved as long as the sign of the sensitivity kernels is correct. A further advantage of this hybrid approach is that it allows us - for the first time in global tomography - to accurately account for the effects of crustal structure on the observed seismograms. We illustrate these effects and the consequences of common assumptions such as linear crustal corrections. We present a preliminary model of velocity and radial anisotropy variations in the upper 800 km of

  17. Wavefront Healing and Tomographic Resolution of Deep Mantle Superplumes

    NASA Astrophysics Data System (ADS)

    Xue, Jing; Zhou, Ying

    2014-05-01

    Seismic tomography revealed two large low shear velocity province (LLSVP) at the bottom of the mantle, one under Africa and the other under the southern Pacific. Traditional tomographic results show strong anti-correlation between bulk sound speed and S wave speed perturbations, supporting at least partially chemical origin of deep mantle superplumes. The core-mantle boundary regions are best sampled by core diffracted waves while wave front healing effects of diffracted waves have been ignored in traditional tomographic studies. To investigate the resolution of deep mantle superplumes as well as the robustness of the anti correlation between bulk sound speed and S wave speed, we use Spectral Element Method (SEM) to simulate global seismic wave propagation in 3-D plume models at periods down to 10s. We measure frequency-dependent P-wave and S-wave travel time anomalies caused by 3-D plume structures using a multi-taper technique, and calculate bulk sound speed perturbations based on measured P-wave and S-wave traveltimes. The comparison between measured delay times and ray-theory predictions shows that different healing rates between P waves and S waves in thermal plume models can lead to significant artifact as anti-correlation between bulk sound speed and S-wave speed perturbations. The strength of this artifact depends on epicenter distance and wave frequency. The artifact in anti-correlation is also confirmed in tomographic inversions based on ray theory using Pdiff and Sdiff traveltimes measured from SEM seismograms. We show that resolutions of superplumes as well as artifacts in the anti-correlation are dependent upon the length scale of the anomalies, the frequency of the wave as well as source-receiver distribution. Finally we compare calculations based on finite-frequency theory and ray theory and show that different healing rates between P waves and S waves can be properly accounted for in finite-frequency tomography.

  18. Hunting for the Tristan plume - An upper mantle tomography around the volcanic island Tristan da Cunha

    NASA Astrophysics Data System (ADS)

    Schlömer, Antje; Geissler, Wolfram H.; Jokat, Wilfried; Jegen, Marion

    2016-04-01

    Tristan da Cunha is a volcanic island in the South Atlantic close to the Mid-Atlantic Ridge. It is part of an area consisting of widely scattered seamounts and small islands at the western and youngest end of the aseismic Walvis Ridge. Tristan da Cunha together with the Walvis Ridge represents the classical example of a mantle plume track, because of the connection to the Cretaceous Etendeka flood basalt province in NW Namibia. The genesis of the island has so far remained enigmatic. It is hotly debated, if Tristan da Cunha sits actually above a deep mantle plume or if it is only originated by upwelling material from weak (leaky) fracture zones. It also has to be clarified if there are any indications for a plume-ridge interaction. Geochemical investigations have shown complex compositions of magmatic samples from Tristan da Cunha, which could be interpreted as a mixing of plume-derived melts and depleted upper mantle sources. To improve our understanding about the origin of Tristan and to test the mantle plume hypothesis, we deployed 24 broadband ocean-bottom seismometers and 2 seismological land stations around and on the island during an expedition in January 2012 with the German research vessel Maria S. Merian. After acquiring continuous seismological data for almost one year, the seismometers were recovered in early January 2013. We cross-correlated the arrival times of teleseismic P and PKP phases to perform a finite-frequency tomography of the upper mantle beneath the study area. Here we show the 3D mantle structure in terms of velocity variations: We do not image a "classical" plume-like structure directly beneath Tristan da Cunha, but we observe regions of low velocities at the edges of our array that we relate to local mantle upwelling from potentially deeper sources. Additionally we discuss local seismicity within the Tristan da Cunha region, which show processes along the nearby mid-ocean ridge and transform faults. Furthermore, the local seismicity

  19. Insights on the upper mantle beneath the Eastern Alps

    PubMed Central

    Bianchi, Irene; Miller, Meghan S.; Bokelmann, Götz

    2014-01-01

    Analyses of Ps and Sp receiver functions from datasets collected by permanent and temporary seismic stations, image a seismic discontinuity, due to a negative velocity contrast across the entire Eastern Alps. The receiver functions show the presence of the discontinuity within the upper mantle with a resolution of tens of kilometers laterally. It is deeper (100–130 km) below the central portion of the Eastern Alps, and shallower (70–80 km) towards the Pannonian Basin and in the Central Alps. Comparison with previous studies renders it likely that the observed discontinuity coincides with the lithosphere–asthenosphere boundary (LAB) east of 15°E longitude, while it could be associated with a low velocity zone west of 15°E. PMID:25843967

  20. Identification of crustal and upper mantle heterogeneity by modelling of controlled-source seismic data

    NASA Astrophysics Data System (ADS)

    Nielsen, L.; Thybo, H.

    2006-04-01

    High-frequency controlled-source seismic sections with dense spatial sampling show the existence of heterogeneity at different depth levels of the continental crust and upper mantle. Our sources of information are the Peaceful Nuclear Explosion (PNE) seismic data sets recorded to large offsets in the former Soviet Union supplemented by recordings from the North American Early Rise deep seismic experiment and normal-incidence reflection seismic sections collected in northwest Europe. Heterogeneity in the crust and upper mantle can be uniquely identified in reversed high-frequency (2-10 Hz) PNE seismic sections collected with dense spatial sampling (nominal receiver spacing of 10-15 km) out to 4000 km offset. We document pronounced seismic scattering from three heterogeneous zones: The lower crust from ˜20 km to ˜40 km depth, an ˜80 km thick low-velocity zone below ˜100 km depth, and the ˜320-460 km depth interval around the top of the mantle transition zone. We calculate the full seismic wavefield in heterogeneous crust-mantle models with a two-dimensional finite-difference algorithm. We represent the heterogeneous layers by random fluctuations of the elastic parameters and Q-values. The spatial (horizontal and vertical) correlation lengths and the standard deviation of the scattering media are constrained by comparison of observed and calculated seismic sections. The lower crustal heterogeneity causes a coda to the upper mantle arrivals at all recorded frequencies. This coda is a prominent feature for whispering-gallery phases (teleseismic Pn), which travel as multiply reflected refractions below the Moho to more than 3000 km offset from the PNE sources. The heterogeneous mantle low-velocity zone causes a scattered coda trailing the first arrivals in the ˜800-1400 km offset range. The best fit to the observations along profile Kraton in Siberia is obtained by an 80 km thick heterogeneous low-velocity zone below 100 km depth, represented by fluctuations

  1. Crust and Upper Mantle Structure of Antarctica from Rayleigh Wave Tomography

    NASA Astrophysics Data System (ADS)

    Wiens, D. A.; Heeszel, D. S.; Sun, X.; Chaput, J. A.; Aster, R. C.; Nyblade, A.; Anandakrishnan, S.; Wilson, T. J.; Huerta, A. D.

    2012-12-01

    We combine data from three temporary arrays of seismometers (AGAP/GAMSEIS 2007-2010, ANET/POLENET 2007-2012, TAMSEIS 2001-2003) deployed across Antarctica, along with permanent stations in the region, to produce a large scale shear velocity model of the continent extending from the Gamburtsev Subglacial Mountains (GSM) in East Antarctica, across the Transantarctic Mountains (TAM) and West Antarctic Rift System (WARS) to Marie Byrd Land (MBL) in West Antarctica. Our combined dataset consists of Rayleigh wave phase and amplitude measurements from 112 stations across the study region. We first invert for 2-D Rayleigh wave phase velocities using the two-plane wave method. These results are then inverted for shear velocity structure using crustal thicknesses derived from ambient noise tomography and teleseismic receiver functions. We refine our shear velocity model by performing a Monte Carlo simulation that explores the tradeoff between crustal thickness and upper mantle seismic velocities. The resulting model is higher resolution than previous studies (~150 km resolution length) and highlights significant differences in crustal and uppermost mantle structure between East and West Antarctica in greater detail than previously possible. East Antarctica is underlain by thick crust (reaching ~55 km beneath the GSM) and fast, cratonic lithosphere. West Antarctica is defined by thinner crust and slow upper mantle velocities indicative of its more recent tectonic activity. The observed boundary in crustal thickness closely follows the TAM front. MBL is underlain by a thicker lithosphere than that observed beneath the WARS, but slow mantle velocities persist to depths greater than 200 km, indicating a 'deep seated' (i.e. deeper than the deepest resolvable features of our model) thermal source for volcanism in the region. The slowest seismic velocities at shallow depths are observed in the Terror Rift region of the Ross Sea along an arc following the TAM front, where the most

  2. Seismic Anisotropy And Upper Mantle Structure In Se Brazil

    NASA Astrophysics Data System (ADS)

    Heintz, M.; Vauchez, A.; Assumpcao, M.; Egydio-Silva, M.

    We present preliminary shear wave splitting measurements performed in south-east Brazil in a quite complex region, from a geological point of view. Seismic anisotropy is the result of a preferred orientation of anisotropic minerals (olivine) in the upper mantle, due to deformation. Splitting parameters Ø (direction of the fastest S wave) are compared to large-scale tectonic structures of the area, in order to infer to which extent the deformations in the upper mantle and in the crust are mechanically coupled. The field of study is a region of 1000 by 1000 km, along the Atlantic coast from São Paulo to 500 km north of Rio de Janeiro. This region is made up of large scale geological units as the southern termination of the São Francisco craton, from archean age, surrounded by two neoproterozoic belts (the Ribeira belt to the east and the Brasilia belt to the west), and the Parana basin, which is a vast flood basalt region. Teleseisms used were acquired by 39 seismological stations well distributed in the region of interest. The results highlight the fact that the orientations of the polarization plane of the fast split shear wave vary a lot in this region, and measurements could be splitted into 5 groups : directions are parallel to the NE-SW trending of the Ribeira belt, some are parallel to the NW-SE trending of the Brasilia belt, in the NE-SW direction of the Transbrasiliano lineament, parallel to the absolute plate maotion (APM) that is EW in this region, or turning around a cylindrical low velocity anomaly imaged in the Parana basin and supposed to be the fossil plume head conduit of the Tristan da Cunha plume head.

  3. Conditions of the Asthenosphere Layer Appearance during Upper-Mantle Convection

    NASA Astrophysics Data System (ADS)

    Sharapov, V. N.; Perepechko, Y. V.; Sorokin, K.

    2012-12-01

    The upper mantle parameters responsible to the asthenosphere appearance during convection have been studied. Many geophysical models of the earth mantle have a continuous layer of a partially melted upper-mantle material under the lithosphere plate. From the period of the early earth this structure is possible only if there is the upper-mantle convection due to 660 km depth phase transition. The previous research considered hydrodynamic conditions of the general mantle convective systems taking into account phase boundaries within the upper mantle. In this work, based on numerical modeling, we discuss thermodynamic conditions of the asthenosphere layer evolution under a homogeneous lithospheric plate. Convection in the upper mantle is modeled using the Boussinesq approximation provided spatially distributed phase transitions within the upper mantle. The problem was solved using the control volume method, which provides integral conservation laws. The results of modeling show that the convective instability is possible when the temperature at the mantle boundary is Tb~1410÷1450°C. Decompression melting of the upper mantle rocks take place if Tb>1650°C. The starting temperatures at the lower boundary of the upper mantle are 1700÷1950°C result in the appearance of the asthenosphere layer existing for 30÷100 Ma. In this case the development of complex, separated asthenosphere zones with additional convective cells is typical. Further evolution results in decay of the melting elements and their final disappearance. The initial phase of the evolution of the asthenosphere zones lasts for about 30÷100 Ma. The second, longer phase of decompression melting contains periodical melting elements with the sizes coextensive to lava sheets of intraplate volcanoes. If Tb>1950°C, the evolution of the asthenosphere is different: after the decay of large asthenosphere zones we observe a reconstruction of convective cells; in this case spatial 'wandering' of varying melting

  4. Single-crystal elasticity of the deep-mantle magnesite at high pressure and temperature

    NASA Astrophysics Data System (ADS)

    Yang, Jing; Mao, Zhu; Lin, Jung-Fu; Prakapenka, Vitali B.

    2014-04-01

    Magnesite (MgCO3) is considered to be a major candidate carbon host in the Earth's mantle, and has been found to exist as an accessory mineral in carbonated peridotite and eclogite. Studying the thermal elastic properties of magnesite under relevant pressure-temperature conditions of the upper mantle is thus important for our understanding of the deep-carbon storage in the Earth's interior. Here we have measured the single-crystal elasticity of a natural magnesite using in situ Brillouin spectroscopy and X-ray diffraction in a diamond anvil cell up to 14 GPa at room temperature and up to 750 K at ambient pressure, respectively. Using the third-order Eulerian finite-strain equations to model the elasticity data, we have derived the aggregate adiabatic bulk, KS0, and shear moduli, G0, at ambient conditions: KS0=114.7 (±1.3) GPa and G0=69.9 (±0.6) GPa. The pressure derivatives of the bulk and shear moduli at 300 K are (∂KS/∂P)T=4.82 (±0.10) and (∂G/∂P)T=1.75 (±0.10), respectively, while their temperature derivatives at ambient pressure are (∂Ks/∂T)P=-24.0 (±0.2) MPa/K and (∂G/∂T)P=-14.8 (±0.7) MPa/K. Based on the thermal elastic modeling of the measured elastic constants along an expected normal upper-mantle geotherm and a cold subducting slab, magnesite exhibits compressional wave (VP) anisotropy of approximately 46-49% and shear wave (VS) splitting of 37-41% that are much larger than those of major constituent minerals in the Earth's upper mantle including olivine, pyroxene, and garnet. The modeled aggregate VP and VS velocity in moderately carbonated peridotite and eclogite containing approximately 10 wt.% magnesite (approximately 5 wt.% CO2) show minimal effects of magnesite on the seismic profiles of these rock assemblages at upper mantle conditions, suggesting that the presence of magnesite is likely difficult to be detected seismically. However, due to its unusually high VP and VS anisotropies, magnesite with strong preferred orientations

  5. Seismic character of the crust and upper mantle beneath the Sierra Nevada

    NASA Astrophysics Data System (ADS)

    Frassetto, A.; Gilbert, H.; Zandt, G.; Owens, T. J.; Jones, C.

    2008-12-01

    Recent geophysical studies of the Southern Sierra Nevada suggest that the removal of a gravitationally unstable, eclogitic residue links to recent volcanism and uplift in the Eastern Sierra. The Sierra Nevada EarthScope Project (SNEP) investigates the extent of this process beneath Central and Northern Sierra Nevada. We present receiver functions, which provide estimates of crustal thickness and Vp/Vs and image the response of the crust and upper mantle to lithospheric removal. For completeness this study combines data from the 2005-2007 SNEP broadband experiment, EarthScope's BigFoot Array, regional backbone stations, and earlier PASSCAL deployments. We analyze transects of teleseismic receiver functions generated using a common-conversion-point stacking algorithm. These identify a narrow, "bright" conversion from the Moho at depths of ~25-35 km along the crest of the Eastern Sierra and adjacent Basin and Range northward to the Cascade Arc. Trade-off analysis using the primary conversion and reverberations shows a high Vp/Vs (~1.9) throughout the Eastern Sierra, which may relate to partial melt present in the lower crust. To the west the crust-mantle boundary vanishes beneath the western foothills. However, low frequency receiver functions do image the crust-mantle boundary exceeding 50 km depth along the foothills to the west and south of Yosemite National Park. Unusually deep, intraplate earthquakes (Ryan et al., this session) occur in the center of this region. The frequency dependence of the Moho conversion implies a gradational increase from crust to mantle wavespeeds over a significant depth interval. The transition from a sharp to gradational Moho probably relates to the change from a delaminated granitic crust to crust with an intact, dense, eclogitic residue. The spatial correlation and focal mechanisms of the deep earthquakes suggest that a segment of this still intact residue is currently delaminating.

  6. How to interpret upper mantle structure under the Eastern Alps?

    NASA Astrophysics Data System (ADS)

    Brückl, Ewald; Keller, G. Randy; Mitterbauer, Ulrike

    2013-04-01

    Recent controlled source seismic investigations, supplemented by potential field studies, have substantially improved our knowledge about the lithospheric structure of the Eastern Alps. Crustal structures due to collision and escape tectonics were imaged and an improved Moho map revealed the fragmentation of the mantle lithosphere into three blocks, the European plate (EU), the Adriatic micro-plate (AD), and a newly interpreted Pannonian domain (PA) comprising the mantle lithosphere below ALCAPA, Tisza, and the Dinarides. The EU, AD, and PA blocks compose a triple junction near the southeastern border of the Tauern window. Images of the upper mantle supplied by seismic tomography provide a better understanding of plate tectonic processes. These studies identified a slab below the EU-AD plate boundary, with its eastern termination near the triple junction. We interpret the European lithospheric mantle to be connected to this slab (East Alpine slab, EAS), and thus, identify it as former lower European lithosphere. Another interpretation has been proposed based on an apparent NE directed dip of the EAS resolved by teleseismic tomography carried out as part of the TRANSALP project. In this interpretation, the EAS is connected to the Adriatic mantle lithosphere thus inferring a subduction polarity flip near the Brenner normal fault terminating the Tauern window in the west. However, we conclude that arguments based only on the slab geometry are not sufficient to determine the nature of the EAS. We suggest a plate tectonic model of the East Alpine collision and extrusion processes based on the structure of the lithospheric mantle and the slab geometry. We reconstruct the passive EU margin of the Penninic Ocean (Alpine Tethys) by restoration of the EAS to the EU plate. The Adriatic domain, including ALCAPA, represents the active margin. Collision of the Adriatic domain with EU starts at the southern end of the restored EAS. After subduction of the entire Penninic Ocean

  7. Noble gas isotope signals of mid-ocean ridge basalts and their implication for upper mantle structure

    NASA Astrophysics Data System (ADS)

    Stroncik, Nicole A.; Niedermann, Samuel

    2016-04-01

    The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic-Ridge (MAR) between 4 and 12° S, an area well known for its high degree of lithophile isotope heterogeneity and exhibiting anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopies along this segment of the MAR range from ultra-depleted (more than NMORB) to highly enriched, and different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as e.g. 3He/22NeM or 4He/40Ar*. 3He/4He, 21Ne/22Neextra and 40Ar/36Ar range from 7.3 to 9.3 RA, from 0.05 to 0.08, and from 346 to 37,400, respectively. Nevertheless, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne-three-isotope diagram, represented by the equation 20Ne/22Ne=70.5 x 21Ne/22Ne + 7.782, with a slope distinctly different from that of the MORB line, indicating that the ultra-depleted material is characterised by a significantly more nucleogenic 21Ne/22Ne isotopy than the normal depleted mantle. We show, based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and 178Hf/177Hf, that the ultra-depleted material erupted at this MAR segment was most likely produced by an ancient, deep melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may also be due to differences in the depth and degree of melting of upper mantle material within

  8. Numerical Modeling of Deep Mantle Flow: Thermochemical Convection and Entrainment

    NASA Astrophysics Data System (ADS)

    Mulyukova, Elvira; Steinberger, Bernhard; Dabrowski, Marcin; Sobolev, Stephan

    2013-04-01

    One of the most robust results from tomographic studies is the existence of two antipodally located Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle, which appear to be chemically denser than the ambient mantle. Results from reconstruction studies (Torsvik et al., 2006) infer that the LLSVPs are stable, long-lived, and are sampled by deep mantle plumes that rise predominantly from their margins. The origin of the dense material is debated, but generally falls within three categories: (i) a primitive layer that formed during magma ocean crystallization, (ii) accumulation of a dense eclogitic component from the recycled oceanic crust, and (iii) outer core material leaking into the lower mantle. A dense layer underlying a less dense ambient mantle is gravitationally stable. However, the flow due to thermal density variations, i.e. hot rising plumes and cold downwelling slabs, may deform the layer into piles with higher topography. Further deformation may lead to entrainment of the dense layer, its mixing with the ambient material, and even complete homogenisation with the rest of the mantle. The amount of the anomalous LLSVP-material that gets entrained into the rising plumes poses a constraint on the survival time of the LLSVPs, as well as on the plume buoyancy, on the lithospheric uplift associated with plume interaction and geochemical signature of the erupted lavas observed at the Earth's surface. Recent estimates for the plume responsible for the formation of the Siberian Flood Basalts give about 15% of entrained dense recycled oceanic crust, which made the hot mantle plume almost neutrally buoyant (Sobolev et al., 2011). In this numerical study we investigate the mechanics of entrainment of a dense basal layer by convective mantle flow. We observe that the types of flow that promote entrainment of the dense layer are (i) upwelling of the dense layer when it gets heated enough to overcome its stabilizing chemical density anomaly, (ii

  9. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal)

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep

    2013-04-01

    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (<< 1 wt.%), its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present down to ~200 km [4-6] but large melt fractions is thought to be restricted to shallower depths. In this Kuno Award lecture, I will discuss the recent advancements on our understanding of deeper silicate melt generation induced by CO2-H2O volatiles and the relative stability of silicate versus carbonatitic melt in various tectonic settings. I will present recent experiments on carbonated peridotites that constrain the location and the slope of the onset of silicate melting in the mantle [7]. The new finding is that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and as a consequence the silicate melting of dry peridotite+CO2 beneath ridges commences at ~180 km. Accounting for the effect of 50-200 ppm of mantle H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ~100 ppm CO2 becomes as deep as ~220-300 km [7]. This melting generates a kimberlitic magma with ~25 wt.% dissolved CO2 and 1-5 wt.% dissolved H2O. Based on the recent constraints of oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated

  10. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal)

    NASA Astrophysics Data System (ADS)

    Dasgupta, Rajdeep

    2013-04-01

    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (<< 1 wt.%), its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present down to ~200 km [4-6] but large melt fractions is thought to be restricted to shallower depths. In this Kuno Award lecture, I will discuss the recent advancements on our understanding of deeper silicate melt generation induced by CO2-H2O volatiles and the relative stability of silicate versus carbonatitic melt in various tectonic settings. I will present recent experiments on carbonated peridotites that constrain the location and the slope of the onset of silicate melting in the mantle [7]. The new finding is that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and as a consequence the silicate melting of dry peridotite+CO2 beneath ridges commences at ~180 km. Accounting for the effect of 50-200 ppm of mantle H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ~100 ppm CO2 becomes as deep as ~220-300 km [7]. This melting generates a kimberlitic magma with ~25 wt.% dissolved CO2 and 1-5 wt.% dissolved H2O. Based on the recent constraints of oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated

  11. Hafnium isotope evidence from Archean granitic rocks for deep-mantle origin of continental crust

    NASA Astrophysics Data System (ADS)

    Guitreau, Martin; Blichert-Toft, Janne; Martin, Hervé; Mojzsis, Stephen J.; Albarède, Francis

    2012-07-01

    Combined whole-rock and zircon MC-ICP-MS Lu-Hf isotope data are reported for a large collection of Archean granitoids belonging to typical tonalite-trondhjemite-granodiorite (TTG) suites. Our data demonstrate that the time-integrated Lu/Hf of the mantle source of TTGs has not significantly changed over the last 4 Gy. Continents therefore most likely grew from nearly primordial unfractionated material extracted from the deep mantle via rising plumes that left a depleted melt residue in the upper mantle. The deep mantle could retain its primitive relative element abundances over time because sinking plates are largely stripped barren of their oceanic and continental crust components at subduction zones; this process results in only small proportions (<15-25%) of present-day continental mass getting recycled to great depths. Zircon populations extracted from the analyzed TTGs have Hf isotopic compositions broadly consistent with those of their host whole-rocks, whereas the U-Pb system in the same grains is often disturbed, causing a discrepancy that creates spurious initial ɛHf values. This problem is endemic to the Archean detrital zircon record and consistent with experimental results bearing on the relative retentivity of Hf vs. U and Pb in zircon. We argue that this behavior biases the Archean zircon record toward negative ɛHf values, which are at odds with the present TTG data set. If Hadean Jack Hills zircons are considered in light of these results, the mantle source of continents has remained unchanged for the last 4.3 Gy.

  12. Supercontinents, Plate Tectonics, Large Igneous Provinces and Deep Mantle Heterogeneities

    NASA Astrophysics Data System (ADS)

    Torsvik, T. H.; Steinberger, B.; Burke, K.; Smethurst, M. A.

    2008-12-01

    The formation and break-up of supercontinents is a spectacular demonstration of the Earth's dynamic nature. Pangea, the best-documented supercontinent, formed at the end of the Palaeozoic era (320 Ma) and its dispersal, starting in the Early Jurassic (190 Ma), was preceded by and associated with widespread volcanic activity, much of which produced Large Igneous Provinces (LIPs), but whether any of the heat or material involved in the generation of LIP rocks comes from greater depths has remained controversial. Two antipodal Large Low Shear wave Velocity Provinces with centre of mass somewhat south of the equator (African and Pacific LLSVPs), isolated within the faster parts of the deep mantle dominate all global shear- wave tomography models. We have tested eight global models and two D" models: They all show that deep- plume sourced hotspots and most reconstructed LIPs for the last 300 million years project radially downwards to the core-mantle-boundary near the edges of the LLSVPs showing that the plumes that made those hotspots and LIPS came only from those plume generation zones. This is a robust result because it is observed in multiple reference frames, i.e. fixed/moving hotspot and palaeomagnetic frames, and in the latter case whether the effect of True Polar Wander (TPW) is considered or not. Our observations show that the LLSVPs must have remained essentially stable in their present position for the last 300 million years. LIPs have erupted since the Archean and may all have been derived from the margins of LLSVPs but whether the African and Pacific LLSVPs have remained the same throughout Earth's history is less certain although analogous structures on Mars do indicate long-term stability on that planet. Deep mantle heterogeneities and the geoid have remained very stable for the last 300 million years, and the possibility is therefore open for speculating on links to Pangea assembly. In a numerical model, Zhong et al. (2007, EPSL) argued that Pangea

  13. Mantle seismic structure beneath USArray: slab segmentation and deep roots of the Yellowstone hotspot (Invited)

    NASA Astrophysics Data System (ADS)

    Schmandt, B.

    2013-12-01

    The Yellowstone hotspot track is a dramatic example of how EarthScope data are helping to reveal systematic connectivity between deep Earth convection, plate tectonics, and surface geologic activity in the plate interior. Two types of seismic constraints provide evidence for buoyant ascent of a lower mantle thermal plume beneath Yellowstone. First, teleseismic body-wave tomography images of smooth 3-D velocity structure show a vertically elongated and irregular low-velocity volume extending to about 1000 km depth. Second, upward deflection of the endothermic postspinel phase transition that represents the boundary between the lower and upper mantle specifically indicates local upwelling of hot lower mantle. Beyond establishing the present day existence of the Yellowstone plume, recent seismic images afford further insight into hotspot initiation in a region where subducted slabs might be expected to block ascent of a lower mantle plume. Several body-wave tomography using EarthScope data reveal a dissected distribution of high-velocity slabs beneath the western U.S., thus providing potential pathways for plume ascent and origins for the plume's vertical irregularity. The possibility of plume ascent through such a slab gap has recently been substantiated by other workers' numerical forward modeling of Farallon subduction constrained by a global plate tectonic model, which predicts a trench-normal slab segmentation event that is spatially consistent with present day tomography and temporally consistent with Miocene initiation of the Yellowstone hotspot track near the edge of Precambrian North America lithosphere.

  14. Crustal and upper mantle velocity structure of the Salton Trough, southeast California

    USGS Publications Warehouse

    Parsons, T.; McCarthy, J.

    1996-01-01

    This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (???21-22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (???27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (???24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (???5-6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (???2-3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ???15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6-7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part

  15. Multiple-frequency tomography of the upper mantle beneath the African/Iberian collision zone

    NASA Astrophysics Data System (ADS)

    Bonnin, Mickaël; Nolet, Guust; Villaseñor, Antonio; Gallart, Josep; Thomas, Christine

    2014-09-01

    During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study, we take advantage of the dense broad-band station networks now available in the Alborán Sea region, to develop a high-resolution 3-D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will better constraint the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centred between 0.03 and 1.0 Hz, and for the first time interpreted using multiple frequency tomography. Our model shows, beneath the Alborán Sea, a strong (4 per cent) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly, and its extent at depth, are coherent with a lithospheric slab, thus favouring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper mantle, high intensity slow anomalies are widespread in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at the surface with the position of the Rif and Atlas orogens and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot?) upper mantle, but without clear indication for a lateral spreading of the Canary plume to the east.

  16. Tomography of the upper mantle beneath the African/Iberian collision zone

    NASA Astrophysics Data System (ADS)

    Mickael, B.; Nolet, G.; Villasenor, A.; Josep, G.; Thomas, C.

    2013-12-01

    During Cenozoic, geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study we take advantage of the dense broadband-station networks now available in Alborán Sea region, to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will bring new constraints on the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centered between 0.03 and 1.0 Hz, and interpreted using multiple frequency tomography. Our model shows, beneath Alborán Sea, a strong (~ 4%) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly and its extent at depth are coherent with a lithospheric slab, thus favoring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper-mantle, several high intensity slow anomalies are widely observed in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at surface with the position of the orogens (Rif and Atlas) and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot) upper mantle, with piece of evidence for a lateral connection with the Canary volcanic islands, likely indicating a lateral spreading of the Canary plume to the east.

  17. Upper mantle shear and compressional velocity structures beneath southern Africa

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Wen, L.; Weidner, D.

    2005-12-01

    The velocity structures in the upper mantle play an important role in understanding mantle composition and temperature. In this study, we constrain the fine seismic shear and compressional velocity structures in the upper mantle beneath southern Africa by waveform modeling the seismic data recorded in the Kaapvaal array at the distance range of 9°-28° for an event occurring near Lake Tanganyika in east Africa. We then explore mineralogical models that would explain the inferred seismic structures. The seismic data recorded at this distance range provide excellent sampling of both the SH and P velocity structures in the top 800 km of the mantle. The first direct arrivals in both the P and SH data become weak at an epicentral distance of about 20°, indicating presence of a low velocity zone beneath southern Africa at a depth of about 150 km. In the SH data, the observed travel times of the reflected and triplicated phases off the 410-km discontinuity require a large shear velocity reduction in the low velocity zone and a small shear velocity jump across the 410-km discontinuity; the observed triplications at the 660-km discontinuity require a large shear velocity jump across the 660-km discontinuity. In the P wave data, the observed travel times of the triplication at the 410-km discontinuity suggest a small P wave velocity reduction in the low velocity zone, a large velocity jump across the 410-km discontinuity and a high Vp/Vs ratio in the transition zone; the triplication at the 660-km discontinuity is indiscernible, suggesting a small P wave velocity jump across the 660-km discontinuity. Overall, the seismic data can be explained by a 150-km thick high-velocity lid overlying a low velocity zone between 150 km and 405 km depths with a P wave velocity reduction of -1.5% and an SH wave velocity reduction of -9%, followed by a small shear velocity jump of 3% and a large P velocity jump of 10% across the 410-km discontinuity, a transition zone with a high Vp

  18. Anisotropic Peridotite Rheology and Regional Upper Mantle Flow Patterns

    NASA Astrophysics Data System (ADS)

    Blackman, D. K.; Boyce, D.; Dawson, P.; Castelnau, O.

    2014-12-01

    We investigate the rheologic impact of strong lattice preferred orientation (LPO), such as develops due to plate-driven shear, on the pattern of upper mantle flow near plate boundaries. We use finite element models to simulate a regional system of mantle flow, that includes LPO evolution in olivine polycrystal aggregates tracked along flow paths and anisotropic viscosity tensors based on the LPO. Our first, loosely coupled approach begins with a flow field based on a scalar viscosity. The results are postprocessed to compute LPO by integration along streamlines, and an anisotropic viscosity tensor field is derived from LPO. A new flow field is then computed based on the viscosity tensor field. For this case, the predicted flow field differed in a modest but geologically relevant way from the isotropic case. In preparation for incorporating the LPO and effective viscosity calculation directly into the flow code, we have been testing this step separately to assess the sensitivity of the computed tensor to specified deformation parameters. New work explores a power law stress:strain rate relation for the LPO development, upon which the aggregate's effective viscosity tensor depends. The pattern and amplitude of predicted deviation from isotropic viscosity are stronger than for the previously assumed linear stress:strain rate case, as expected. Initial runs that employ the power law viscosity tensor in updated flow calculations are underway at the time of this writing. In addition to the stress exponent for LPO and the resulting viscosity tensor, flow model parameters that notably impact the predictions include the specified stiffening as asthenosphere cools to lithospheric temperatures and mesh resolution within the axial and the base of lithosphere regions. We will present results for subaxial oceanic spreading center flow and report the outcomes of model parameter testing.

  19. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Stein, S.; Okal, E.A.; Rubie, David C.

    1996-01-01

    Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher-pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine ??? spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer-grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes

  20. Seismic structure of the European upper mantle based on adjoint tomography

    NASA Astrophysics Data System (ADS)

    Zhu, Hejun; Bozdağ, Ebru; Tromp, Jeroen

    2015-04-01

    We use adjoint tomography to iteratively determine seismic models of the crust and upper mantle beneath the European continent and the North Atlantic Ocean. Three-component seismograms from 190 earthquakes recorded by 745 seismographic stations are employed in the inversion. Crustal model EPcrust combined with mantle model S362ANI comprise the 3-D starting model, EU00. Before the structural inversion, earthquake source parameters, for example, centroid moment tensors and locations, are reinverted based on global 3-D Green's functions and Fréchet derivatives. This study consists of three stages. In stage one, frequency-dependent phase differences between observed and simulated seismograms are used to constrain radially anisotropic wave speed variations. In stage two, frequency-dependent phase and amplitude measurements are combined to simultaneously constrain elastic wave speeds and anelastic attenuation. In these two stages, long-period surface waves and short-period body waves are combined to simultaneously constrain shallow and deep structures. In stage three, frequency-dependent phase and amplitude anomalies of three-component surface waves are used to simultaneously constrain radial and azimuthal anisotropy. After this three-stage inversion, we obtain a new seismic model of the European curst and upper mantle, named EU60. Improvements in misfits and histograms in both phase and amplitude help us to validate this three-stage inversion strategy. Long-wavelength elastic wave speed variations in model EU60 compare favourably with previous body- and surface wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from the smooth starting model. Subducting slabs, slab detachments, ancient suture zones, continental rifts and backarc basins are well resolved in model EU60. We find an anticorrelation between shear wave speed and anelastic attenuation at depths < 100 km. At greater depths, this anticorrelation becomes

  1. Combined teleseismic surface wave and receiver function analysis of the crust and upper mantle of Madagascar

    NASA Astrophysics Data System (ADS)

    Pratt, M. J.; Aleqabi, G. I.; Wysession, M. E.; Wiens, D. A.; Nyblade, A.; Shore, P.; Rambolamanana, G.; Tsiriandrimanana, R.; Andriampenomanana Ny Ony, F. S. T.

    2014-12-01

    The continental crust and upper mantle velocity structure beneath Madagascar remained poorly constrained until recent deployments of broadband seismic instrumentation across the island. The MACOMO (MAdagascar, COmoros and MOzambique), RHUM-RUM (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) and the Madagascar Seismic Profile experiments have opened up this region to be studied in detail for the first time. The island is an amalgamation of an Archean craton, associated with the Western Dhawar craton of southern India, and a series of Proterozoic terranes that comprise the backbone of the island (Tucker et al., 2010). A receiver-function analysis has provided both the first Moho depth measurements and spatially discrete 1-D shear velocity results that matched well with known tectonic regions. To provide a more continuous 2-D and 3-D velocity structure map, teleseismic surface wave analysis is employed. Using Helmholtz tomography as implemented by the ASWMS package (Ge, Gaherty and Hutko; 2014), we are able to map phase velocities from the cross-correlation of station pairs at periods 20-100 s. At periods 20-40 s our results compare well with ambient noise analysis results (see poster by Wysession et al. (this meeting)). The prominent features of these results are a distinct low phase-velocity sector beneath the central Itasy region, with a secondary low phase-velocity region to the north of the island. Both the central part of the island and the northern region have experienced geothermal activity in recent times as well as volcanic activity within the last 10,000 years. This may suggest that the crust and underlying mantle in these regions remains at relatively higher temperatures than the surrounding rock. Combining this information with receiver-function analysis, we jointly invert our data for the shear velocity structure. These analyses will constrain the upper mantle seismic velocities in the region, allowing further analysis from body waves to

  2. Melt migration modeling in partially molten upper mantle

    NASA Astrophysics Data System (ADS)

    Ghods, Abdolreza

    The objective of this thesis is to investigate the importance of melt migration in shaping major characteristics of geological features associated with the partial melting of the upper mantle, such as sea-floor spreading, continental flood basalts and rifting. The partial melting produces permeable partially molten rocks and a buoyant low viscosity melt. Melt migrates through the partially molten rocks, and transfers mass and heat. Due to its much faster velocity and appreciable buoyancy, melt migration has the potential to modify dynamics of the upwelling partially molten plumes. I develop a 2-D, two-phase flow model and apply it to investigate effects of melt migration on the dynamics and melt generation of upwelling mantle plumes and focusing of melt migration beneath mid-ocean ridges. Melt migration changes distribution of the melt-retention buoyancy force and therefore affects the dynamics of the upwelling plume. This is investigated by modeling a plume with a constant initial melt of 10% where no further melting is considered. Melt migration polarizes melt-retention buoyancy force into high and low melt fraction regions at the top and bottom portions of the plume and therefore results in formation of a more slender and faster upwelling plume. Allowing the plume to melt as it ascends through the upper mantle also produces a slender and faster plume. It is shown that melt produced by decompressional melting of the plume migrates to the upper horizons of the plume, increases the upwelling velocity and thus, the volume of melt generated by the plume. Melt migration produces a plume which lacks the mushroom shape observed for the plume models without melt migration. Melt migration forms a high melt fraction layer beneath the sloping base of the impermeable oceanic lithosphere. Using realistic conditions of melting, freezing and melt extraction, I examine whether the high melt fraction layer is able to focus melt from a wide partial melting zone to a narrow region

  3. Modeling the Crust and Upper Mantle in Northern Beata Ridge (CARIBE NORTE Project)

    NASA Astrophysics Data System (ADS)

    Núñez, Diana; Córdoba, Diego; Cotilla, Mario Octavio; Pazos, Antonio

    2016-05-01

    The complex tectonic region of NE Caribbean, where Hispaniola and Puerto Rico are located, is bordered by subduction zone with oblique convergence in the north and by incipient subduction zone associated to Muertos Trough in the south. Central Caribbean basin is characterized by the presence of a prominent topographic structure known as Beata Ridge, whose oceanic crustal thickness is unusual. The northern part of Beata Ridge is colliding with the central part of Hispaniola along a transverse NE alignment, which constitutes a morphostructural limit, thus producing the interruption of the Cibao Valley and the divergence of the rivers and basins in opposite directions. The direction of this alignment coincides with the discontinuity that could explain the extreme difference between west and east seismicity of the island. Different studies have provided information about Beata Ridge, mainly about the shallow structure from MCS data. In this work, CARIBE NORTE (2009) wide-angle seismic data are analyzed along a WNW-ESE trending line in the northern flank of Beata Ridge, providing a complete tectonic view about shallow, middle and deep structures. The results show clear tectonic differences between west and east separated by Beata Island. In the Haiti Basin area, sedimentary cover is strongly influenced by the bathymetry and its thickness decreases toward to the island. In this area, the Upper Mantle reaches 20 km deep increasing up to 24 km below the island where the sedimentary cover disappears. To the east, the three seamounts of Beata Ridge provoke the appearance of a structure completely different where sedimentary cover reaches thicknesses of 4 km between seamounts and Moho rises up to 13 km deep. This study has allowed to determine the Moho topography and to characterize seismically the first upper mantle layers along the northern Beata Ridge, which had not been possible with previous MCS data.

  4. Modeling the Crust and Upper Mantle in Northern Beata Ridge (CARIBE NORTE Project)

    NASA Astrophysics Data System (ADS)

    Núñez, Diana; Córdoba, Diego; Cotilla, Mario Octavio; Pazos, Antonio

    2015-10-01

    The complex tectonic region of NE Caribbean, where Hispaniola and Puerto Rico are located, is bordered by subduction zone with oblique convergence in the north and by incipient subduction zone associated to Muertos Trough in the south. Central Caribbean basin is characterized by the presence of a prominent topographic structure known as Beata Ridge, whose oceanic crustal thickness is unusual. The northern part of Beata Ridge is colliding with the central part of Hispaniola along a transverse NE alignment, which constitutes a morphostructural limit, thus producing the interruption of the Cibao Valley and the divergence of the rivers and basins in opposite directions. The direction of this alignment coincides with the discontinuity that could explain the extreme difference between west and east seismicity of the island. Different studies have provided information about Beata Ridge, mainly about the shallow structure from MCS data. In this work, CARIBE NORTE (2009) wide-angle seismic data are analyzed along a WNW-ESE trending line in the northern flank of Beata Ridge, providing a complete tectonic view about shallow, middle and deep structures. The results show clear tectonic differences between west and east separated by Beata Island. In the Haiti Basin area, sedimentary cover is strongly influenced by the bathymetry and its thickness decreases toward to the island. In this area, the Upper Mantle reaches 20 km deep increasing up to 24 km below the island where the sedimentary cover disappears. To the east, the three seamounts of Beata Ridge provoke the appearance of a structure completely different where sedimentary cover reaches thicknesses of 4 km between seamounts and Moho rises up to 13 km deep. This study has allowed to determine the Moho topography and to characterize seismically the first upper mantle layers along the northern Beata Ridge, which had not been possible with previous MCS data.

  5. Upper Mantle Structure Around the Trans-European Suture Zone

    NASA Astrophysics Data System (ADS)

    Janutyte, Ilma; Majdanski, Mariusz; Voss, Peter H.; Kozlovskaya, Elena

    2014-05-01

    The Trans-European Suture Zone (TESZ) is the transition between old Proterozoic lithosphere in Northern and Eastern Europe and the younger Phanerozoic lithosphere in Central and Western Europe. The presented study is a part of the PASSEQ 2006-2008 project which is linked to the TOR project realized during 1996-1997. The PASSEQ and the TOR projects aimed to study the lithosphere and asthenosphere structure around the TESZ, but the latter was focused on the northwestern part of the TESZ between Sweden and Denmark - Germany, while the PASSEQ project was focused on the TESZ mainly beneath Poland. During the PASSEQ project 139 short-period and 49 broadband temporary seismic stations were deployed along the transect stretching from Germany throughout Czech Republic and Poland to Lithuania. The array recorded continuous seismic data from May, 2006 to June, 2008. In our study we used data of all available PASSEQ seismic stations and seismic stations of the national seismological networks of the participating countries and compiled a data set of teleseismic P-wave arrivals. The full data set consists of 8308 manually picked arrivals. Due to limited computational power we used the data of the highest quality only, i.e. 6008 picks. The non-linear teleseismic tomography algorithm TELINV was used to obtain the model of P-wave velocity perturbations in the upper mantle around the TESZ. We recovered the upper mantle structure from 70 km down to 350 km in the study area. The results show ±6.5 % P-wave velocity variations compared to the IASP91 velocity model. We found higher velocities beneath the old East European Craton (EEC) east of the TESZ and lower ones beneath the younger Western Europe west of the TESZ. The thickest litosphere was found beneath the EEC (Lithuania) where the higher velocities continue to about 300 km or even more. To the west of the TESZ under the Variscides the average depth of the lithosphere-asthenosphere boundary (LAB) is about 100 km. The TESZ appears

  6. A kinematic model for the late Cenozoic development of southern California crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Humphreys, Eugene D.; Hager, Bradford H.

    1990-01-01

    A model is developed for the young and ongoing kinematic deformation of the southern California crust and upper mantle. The kinematic model qualitatively explains both the overall seismic structure of the upper mantle and much of the known geological history of the late Cenozoic as consequences of ongoing convection beneath southern California. In this model, the high-velocity upper-mantle anomaly of the Transverse ranges is created through the convergence and sinking of the entire thickness of subcrustal lihtosphere, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4 percent partial melt related to adiabatic decompression during mantle upwelling.

  7. Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

    USGS Publications Warehouse

    Kelemen, P.B.; Dick, H.J.B.; Quick, J.E.

    1992-01-01

    Many mantle peridotite samples are too rich in SiO2 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

  8. Density Structure of the Upper Mantle in the Middle East and Surroundings: Interaction of Diverse Tectonic Processes

    NASA Astrophysics Data System (ADS)

    Kaban, M. K.; El Khrepy, S.; Al-Arifi, N. S.

    2015-12-01

    The Middle East is a very complex region combining several tectonic regimes, which are linked together. Density heterogeneity of the upper mantle, which is related to temperature and compositional variations, is one of the principal factors governing tectonic processes. Therefore, a comprehensive density model of the upper mantle is a key for understanding of these processes. Here we use seismic, gravity and tomography data to construct a 3D density model of the lithosphere and upper mantle and to identify main factors responsible for density variations. At the first stage we use a recent crustal model (Stolk et al., 2013) to estimate gravity effect of the crust and to remove it from the observed fields. As a result, the residual mantle gravity anomalies and residual topography are calculated. In addition we remove the impact of deep density variations below 325 km as estimated by a recent instantaneous dynamic model of the mantle (Kaban et al., 2014). We invert the residual fields jointly with seismic tomography data to image density distribution within the crust and upper mantle. The inversion technique accounts for the fact that the residual gravity and residual topography are controlled by the same factors but in a different way, e.g. depending on depth and wavelength. This provides a possibility for remarkably better vertical resolution of the resulting density model. As the initial approximation, we employ the seismic tomography model of Schaeffer and Lebedev (2013). Velocity variations are converted to density by applying mineral physics constrains. This model is adjusted in the inversion to fit both residual mantle gravity and topography. The obtained density variations are very significant; their amplitude somewhere exceeds 60 kg/m3 relative to a reference model. The most pronounced decrease of the mantle density corresponds to the Gulf of Aden spreading axis, the Red sea and the Afar zone. The maximum density of the upper mantle is associated with the

  9. Mantle Temperature, Mantle Composition, Mantle Heterogeneity, and the Composition of the Upper Mantle: The View from a Global Synthesis of MORB

    NASA Astrophysics Data System (ADS)

    Langmuir, C. H.; Gale, A.; Dalton, C. A.

    2012-12-01

    A new comprehensive review of global MORB can address outstanding issues such mantle temperature vs. mantle composition in controlling MORB compositions, the mean composition of ocean ridge basalts, the K/U ratio of the MORB reservoir, and the implications for silicate Earth mass balance of the composition of the upper mantle. We created a global catalogue of ridge segments to assign every sample to a segment. We carried out interlaboratory corrections for major elements, and examined data from each segment to ensure appropriate fractionation correction. We included large unpublished data sets from the Langmuir and Schilling laboratories, assembling the most comprehensive data set for MORB. Data averaged by segment permit calculation of averages that include weighting by segment length and spreading rate. The segment-based approach, comprehensive data set, individualized fractionation correction and interlaboratory corrections distinguish these results from earlier efforts. We also carried out bootstrapping statistical tests for meaningful errors on average compositions. The mean composition of the ocean crust is best determined by a segment length and spreading rate weighted arithmetic mean. As with other recent efforts, notably Su (2002) and also Arevalo and McDonough (2009), the mean composition is substantially more enriched than previous MORB estimates. Average MORB implies a MORB mantle Sm/Nd and Nd isotopic composition similar to the 'non-chondritic primitive mantle' composition based on 142Nd. Then continental crust/MORB mantle mass balance is not possible using a non-chondritic (depleted) bulk silicate earth composition, unless there is a large unsampled depleted reservoir. In contrast to Arevalo and McDonough, who suggested a K/U ratio for MORB of 19,000, we find K/U of 12,340±810, in line with earlier estimates. The discrepancy can be understood from contrasts in methodology, as we determine average K/ average U, while they determine average K/U. To

  10. Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track

    NASA Astrophysics Data System (ADS)

    Kelbert, A.; Egbert, G. D.

    2012-12-01

    We have used high-quality electromagnetic data obtained through the EarthScope USArray project to obtain detailed three-dimensional images of electrical resistivity / conductivity in the crust and upper mantle beneath the Snake River Plain/Yellowstone (SRP/Y) volcanic province (Idaho and Wyoming, United States). The lowest resistivities in the area can only plausibly be explained by partial melt and/or fluids, providing valuable new information about the distribution of these phases deep within the Earth beneath the volcanic system. Unexpectedly, in light of the mantle plume models often used to explain Yellowstone volcanism, the electromagnetic data imply that there is no interconnected melt in the lower crust and uppermost mantle directly beneath the modern Yellowstone caldera. Instead, low resistivities consistent with 1-3% melt in the uppermost mantle (depths of 40-80 km) extend at least 200 km southwest of Yellowstone. Shallower areas of reduced resistivity extend upward into the mid-crust around the edges of the seemingly impermeable Snake River Plain province, including beneath Yellowstone. We suggest that the elevated temperatures beneath the active volcanic center have resulted in greater permeability, allowing magma to ascend to shallower depths and pool in the crust. Little melt is entering the system from below at present, perhaps due to intermittency of supply. We describe these results in the context of larger scale electrical resistivity and seismic tomography models of the western US and employ joint interpretation to formulate hypotheses that would explain this unexpected melt distribution beneath the SRP/Y. Our 3-D model is available at http://www.iris.edu/dms/products/emc/models/SRPY-MT.htm

  11. Crustal and upper mantle velocity structure of the Hoggar swell (Central Sahara, Algeria)

    NASA Astrophysics Data System (ADS)

    Ayadi, A.; Dorbath, C.; Lesquer, A.; Bezzeghoud, M.

    2000-02-01

    The Hoggar region is known as one of the most important swells in the African continent. Its altitude culminates at 2908 m in the Tahat hill (Atakor). The Hoggar and other massifs of central Africa (Aı̈r, Eghei, Tibesti, Darfur, Cameroon mount, …) form a system of domal uplifts with similar scale, morphology and volcanic activity. The knowledge of the structure beneath the Hoggar swell will help us to understand the origin of continental swells. In order to get an image of the lithosphere in this region, we have performed a teleseismic field experiment. The 33 short-period seismic stations have been maintained for 2 1/2 month along a 700-km long NNW-SSW profile. This experiment crossed the Central Hoggar and extended northward into the In-Salah Sahara basin which is characterized by high heat flow values of deep origin. The high quality of the data recorded during this experiment allows us to perform a velocity inversion. The Hoggar appears to be characterized by lower mantle velocities. The anomalous zone extends from the upper lithosphere to the mantle. The weak velocity contrast is interpreted in agreement with gravity, geothermal and petrological data as due to extensive mantle modifications inherited from Cenozoic volcanic activity. It confirms that the Hoggar swell is not due to a large-scale uplift of hot asthenospheric materials but corresponds to a now cooled-off modified mantle. On the contrary, local low-velocity zones associated with the Atakor and Tahalra volcanic districts show that hot materials still exist at depths in relation with recent basaltic volcanism.

  12. Density heterogeneity of the North American upper mantle from satellite gravity and a regional crustal model

    NASA Astrophysics Data System (ADS)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans

    2014-05-01

    We present a regional model for the density structure of the North American upper mantle. The residual mantle gravity anomalies are based on gravity data derived from the GOCE geopotential models with crustal correction to the gravity field being calculated from a regional crustal model. We analyze how uncertainties and errors in the crustal model propagate from crustal densities to mantle residual gravity anomalies and the density model of the upper mantle. Uncertainties in the residual upper (lithospheric) mantle gravity anomalies result from several sources: (i) uncertainties in the velocity-density conversion and (ii) uncertainties in knowledge of the crustal structure (thickness and average Vp velocities of individual crustal layers, including the sedimentary cover). In this study, we address both sources of possible uncertainties by applying different conversions from velocity to density and by introducing variations into the crustal structure which corresponds to the uncertainty of its resolution by high-quality and low-quality seismic models. We examine the propagation of these uncertainties into determinations of lithospheric mantle density. Given a relatively small range of expected density variations in the lithospheric mantle, knowledge on the uncertainties associated with incomplete knowledge of density structure of the crust is of utmost importance for further progress in such studies. The new regional density model for the North American upper mantle complements an on-going study of the regional upper mantle velocity and density structure by other methods. Our new regional density model is compared to regional and world-wide petrological data on upper mantle densities constrained by mantle-derived xenoliths.

  13. Evidence for primordial water in Earth's deep mantle.

    PubMed

    Hallis, Lydia J; Huss, Gary R; Nagashima, Kazuhide; Taylor, G Jeffrey; Halldórsson, Sæmundur A; Hilton, David R; Mottl, Michael J; Meech, Karen J

    2015-11-13

    The hydrogen-isotope [deuterium/hydrogen (D/H)] ratio of Earth can be used to constrain the origin of its water. However, the most accessible reservoir, Earth's oceans, may no longer represent the original (primordial) D/H ratio, owing to changes caused by water cycling between the surface and the interior. Thus, a reservoir completely isolated from surface processes is required to define Earth's original D/H signature. Here we present data for Baffin Island and Icelandic lavas, which suggest that the deep mantle has a low D/H ratio (δD more negative than -218 per mil). Such strongly negative values indicate the existence of a component within Earth's interior that inherited its D/H ratio directly from the protosolar nebula. PMID:26564850

  14. Evidence for primordial water in Earth’s deep mantle

    NASA Astrophysics Data System (ADS)

    Hallis, Lydia J.; Huss, Gary R.; Nagashima, Kazuhide; Taylor, G. Jeffrey; Halldórsson, Sæmundur A.; Hilton, David R.; Mottl, Michael J.; Meech, Karen J.

    2015-11-01

    The hydrogen-isotope [deuterium/hydrogen (D/H)] ratio of Earth can be used to constrain the origin of its water. However, the most accessible reservoir, Earth’s oceans, may no longer represent the original (primordial) D/H ratio, owing to changes caused by water cycling between the surface and the interior. Thus, a reservoir completely isolated from surface processes is required to define Earth’s original D/H signature. Here we present data for Baffin Island and Icelandic lavas, which suggest that the deep mantle has a low D/H ratio (δD more negative than -218 per mil). Such strongly negative values indicate the existence of a component within Earth’s interior that inherited its D/H ratio directly from the protosolar nebula.

  15. Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

    PubMed Central

    Ohuchi, Tomohiro; Kawazoe, Takaaki; Higo, Yuji; Funakoshi, Ken-ichi; Suzuki, Akio; Kikegawa, Takumi; Irifune, Tetsuo

    2015-01-01

    Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6 to 1020.7 Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle. PMID:26601281

  16. Constraints on melt migration in the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Garapic, Gordana

    Melting and melt segregation are key processes in the geochemical evolution of the Earth. However, mechanism and time scale of melt transport from the source to the surface are still not well understood and are dependent on the grain-scale distribution of melt. A related question is the retention of melt in partially molten regions of the Earth's upper mantle. Seismic observations from mid-ocean ridges (MOR) and subduction zones are interpreted to show in-situ melt contents up to 3%, while geochemical observations from MOR basalts are inferred to indicate very efficient extraction of melt (porosities of order of 0.1%). Earlier theoretical models of the melt distribution were based on the balance of surface tension between melt and uniform crystalline grains, predicting a simple network of melt along three-grain edges. Analyses of experimentally produced samples of olivine and basaltic melt show that the melt geometry is much more complex, and includes wetted two-grain boundaries. I reconstructed the 3-D model of melt geometry of two experimentally produced samples by serial sectioning and rendering of the pore space which demonstrates for the first time that melt exists in thin layers on two-grain boundaries. This confirms the inferences from previous 2-D observations and has significant implications for physical properties of partially molten regions, for example seismic velocities and attenuation. The wetted two-grain boundaries are inferred to be a consequence of continuous grain growth. Due to the complexity of the 3-D melt geometry the permeability of partially molten rocks can not be predicted from simple models. I therefore investigated the permeability as a function of porosity for both synthetic and experimentally determined pore geometries using a lattice-Boltzmann method. The calculated permeability is not a simple function of porosity, but increases rapidly at a critical fraction of wetted two-grain boundaries. To extrapolate the experimentally based

  17. a Method for Determining Upper Mantle P Velocities Using Apparent Velocity Measurements and Earthquake Sources

    NASA Astrophysics Data System (ADS)

    Ebel, J.; Hertzog, J.; Cipar, J. J.

    2013-12-01

    An important challenge in the study of earth structure is to determine with high vertical and horizontal resolutions the distribution of seismic velocities in the uppermost mantle from the Moho to 100 km depth. Such determinations are vital to understanding the composition, physical state and history of the lower half of the lithosphere. Receiver-function studies can provide good constraints on depths of the Moho and upper mantle discontinuities, but less constraint on upper mantle velocities. Surface-wave analyses can provide accurate average velocities in the upper mantle, but with only broad vertical and lateral resolution. Seismic refraction methods can provide strong constraints on upper mantle seismic velocities and interface depths, but they must rely on strategically placed sources and receivers. In this study we demonstrate a inversion method that uses an array of receivers and a set of earthquake and explosion sources to image the lateral and vertical P velocity variations of the upper mantle. Preliminary work in northeastern North America and in the midcontinent demonstrate the utility of this method and the vertical and horizontal resolutions on that P velocity structure that can be achieved. In particular, variations in the upper mantle P velocity structure between the North American craton in Quebec and the accreted terranes in New England have been found with this method. As the EarthScope Transportable Array sweeps across the eastern US, it will provide a unique data set that can be used for mapping the edge of the North American craton in the upper mantle along with the seismic properties of the upper mantle of the terranes that have been accreted onto the craton. These structural images should provide new information on the history of the assembly of the North American continent.

  18. Calculations of upper-mantle velocity from published Soviet earthquake data

    USGS Publications Warehouse

    Rodriquez, Robert G.

    1965-01-01

    The lack of information on mantle velocities and crustal structure of the U.S.S.R. has led to a preliminary examination of published Soviet earthquake bulletins in the hope of deriving useful velocity and structure information from the data they contain. Mantle velocities deduced from earthquake data on several Russian earthquakes are in excellent agreement with results of Soviet deep seismic sounding.

  19. The upper-mantle transition zone beneath the Chile-Argentina flat subduction zone

    NASA Astrophysics Data System (ADS)

    Bagdo, Paula; Bonatto, Luciana; Badi, Gabriela; Piromallo, Claudia

    2016-04-01

    The main objective of the present work is the study of the upper mantle structure of the western margin of South America (between 26°S and 36°S) within an area known as the Chile-Argentina flat subduction zone. For this purpose, we use teleseismic records from temporary broad band seismic stations that resulted from different seismic experiments carried out in South America. This area is characterized by on-going orogenic processes and complex subduction history that have profoundly affected the underlying mantle structure. The detection and characterization of the upper mantle seismic discontinuities are useful to understand subduction processes and the dynamics of mantle convection; this is due to the fact that they mark changes in mantle composition or phase changes in mantle minerals that respond differently to the disturbances caused by mantle convection. The discontinuities at a depth of 410 km and 660 km, generally associated to phase changes in olivine, vary in width and depth as a result of compositional and temperature anomalies. As a consequence, these discontinuities are an essential tool to study the thermal and compositional structure of the mantle. Here, we analyze the upper-mantle transition zone discontinuities at a depth of 410 km and 660 km as seen from Pds seismic phases beneath the Argentina-Chile flat subduction.

  20. Global mantle convection: Evidence from carbon and nitrogen isotopes in super-deep diamonds (Invited)

    NASA Astrophysics Data System (ADS)

    Palot, M.; Cartigny, P.; Harris, J.; Kaminsky, F. V.; Stachel, T.

    2009-12-01

    Constraining the convective regime of the Earth’s mantle has profound implications for our understanding of the Earth’s cooling and the geodynamics of plate tectonics. Although subducting plates seem to be occasionally deflected at 660 km, evidence from seismic tomography and fluid dynamics suggest that substantial amounts of material reach the core-mantle boundary. Most geochemists, on the other hand, based on evidence from noble gases, would argue for the presence of separate upper and lower mantle reservoirs. Diamond provides a unique opportunity to sample those parts of the mantle that remains inaccessible by any other means. Some mineral associations in diamond, such as majoritic garnet, calcic and magnesian perovskite and manganoan ilmenite with ferropericlase have been recognised as originated from the transition zone down to the lower mantle (Stachel et al., 1999; Kaminsky et al., 2001). In addition, nitrogen in these diamonds is potentially a good tracer for mantle geodynamics. Exchanges between an inner reservoir (characterised by negative δ15N) via degassing at oceanic ridges with an outer reservoir (characterised by positive δ15N) via recycling at a subduction zones can lead to isotopic contrast in a stratified mantle. Because of common super-deep mineral inclusion assemblages in diamonds from Juina (Brazil) and Kankan (Guinea), we carried out a detailed study of nitrogen and carbon isotopes. The Juina diamonds show broadly similar ranges of δ15N from +3.8‰ down to -8.8‰ for both upper (UM) and lower (LM) mantle diamonds. This important feature is also found for UM and LM diamonds from Kankan, although the range of δ15N differs with values from +9.6‰ down to -39.4‰. Both sets of results suggest extensive material-isotopic exchange through the 660km discontinuity, contrary to the idea of an isolated reservoir. Transition zone (TZ) diamonds are enriched in 13C with δ13C from -3.1‰ up to +3.8‰ at Kankan but those of Juina are depleted

  1. A Preliminary Look at the Crust and Upper Mantle of North Africa Using Libyan Seismic Data

    SciTech Connect

    Pasyanos, M

    2005-08-05

    In recent years, LLNL has been developing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, etc. We proposed applying this method to some recently available data from several Libyan stations, as we believe the technique has not been applied to any stations in Libya. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and North Africa. We recently requested seismic data from stations GHAR (Gharyan) and MARJ (Al Marj) in Libya for about 20 events. The events were large events at regional distances suitable for making dispersion measurements. An example of waveforms recorded at the two stations from an earthquake in Italy is shown in Figure 1. The paths traverse the Ionian Sea. Notice the slow short period group velocities of the surface waves across the Mediterranean, particularly to the easternmost station MARJ. However, because of data availability, signal-to-noise ratio, etc. we were unable to make measurements for every one of these events at both stations. Figure 2 shows a map of paths for 20 sec Rayleigh waves in the eastern Mediterranean region. Paths measured at the two Libyan stations are shown in green. Rayleigh wave dispersion measurements at 20 sec period are sensitive to velocities in the upper 20 km or so, and reveal sediment thickness, crustal velocity, and crustal thickness. Tomographic inversions reveal the sharp group velocity contrast between regions with deep sedimentary basins and those without. Figure 3, the result of an inversion made before adding the new dispersion measurements, shows slow group velocities in the Black Sea, Adriatic Sea, and Eastern Mediterranean. In general, these features correspond well with the sediment thickness model from Laske, shown in Figure

  2. Peeling linear inversion of upper mantle velocity structure with receiver functions

    NASA Astrophysics Data System (ADS)

    Shen, Xuzhang; Zhou, Huilan

    2012-02-01

    A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3 h M ( h M denotes the depth of Moho) and 5 h M due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3 h M and 5 h M is sacrificed.

  3. Tectonic geomorphometrics of the western United States: Speculations on the surface expression of upper mantle processes

    NASA Astrophysics Data System (ADS)

    Coblentz, D.; Karlstrom, K. E.

    2011-11-01

    The topography of the western United States provides a classic field laboratory for investigations of the relationship between surface features and sub-crustal dynamic processes. The interpretation of recently collected, high-resolution seismic images of the upper mantle beneath the central Colorado Rocky Mountains substantiates the notion that much of the high elevation coincides with thin or attenuated continental crust (with respect to predicted Airy crustal thicknesses), necessitating topographic support by anomalously buoyant mantle. This is highly suggestive that broad-scale topographic features may be correlated with buoyancy variations in the upper mantle. In an attempt to sharpen our understanding of the underlying geodynamics, we evaluate the correlation between the surface topographic character and data sets that provide information about density variations indicative of buoyancy in the upper mantle, including the lithospheric geoid, upper mantle seismic velocity anomalies, and crustal (Lg) Q. Our general conclusion is that mantle buoyancy is driving differential surface uplift throughout the western United States and this driver of topography is manifested by measureable anomalies in the topographic roughness at short wavelengths (tens of kilometer) and elevated spectral power in the topography at longer (several hundred kilometers) wavelengths. A provocative conclusion is that the long-recognized physiographic provinces of the Colorado Plateau, Rocky Mountains, and Rio Grande rift are also neotectonic provinces that are related to convective processes and related buoyancy in the upper mantle.

  4. Global Transition Zone Anisotropy and Consequences for Mantle Flow and Earth's Deep Water Cycle

    NASA Astrophysics Data System (ADS)

    Beghein, C.; Yuan, K.

    2011-12-01

    The transition zone has long been at the center of the debate between multi- and single-layered convection models that directly relate to heat transport and chemical mixing throughout the mantle. It has also been suggested that the transition zone is a reservoir that collects water transported by subduction of the lithosphere into the mantle. Since water lowers mantle minerals density and viscosity, thereby modifying their rheology and melting behavior, it likely affects global mantle dynamics and the history of plate tectonics. Constraining mantle flow is therefore important for our understanding of Earth's thermochemical evolution and deep water cycle. Because it can result from deformation by dislocation creep during convection, seismic anisotropy can help us model mantle flow. It is relatively well constrained in the uppermost mantle, but its presence in the transition zone is still debated. Its detection below 250 km depth has been challenging to date because of the poor vertical resolution of commonly used datasets. In this study, we used global Love wave overtone phase velocity maps, which are sensitive to structure down to much larger depths than fundamental modes alone, and have greater depth resolution than shear wave-splitting data. This enabled us to obtain a first 3-D model of azimuthal anisotropy for the upper 800km of the mantle. We inverted the 2Ψ terms of anisotropic phase velocity maps [Visser, et al., 2008] for the first five Love wave overtones between 35s and 174s period. The resulting model shows that the average anisotropy amplitude for vertically polarized shear waves displays two main stable peaks: one in the uppermost mantle and, most remarkably, one in the lower transition zone. F-tests showed that the presence of 2Ψ anisotropy in the transition zone is required to improve the third, fourth, and fifth overtones fit. Because of parameter trade-offs, however, we cannot exclude that the anisotropy is located in the upper transition zone as

  5. Towards the Petrophysics and Petrology of Earth's Deep Mantle and the Core Mantle Boundary

    NASA Astrophysics Data System (ADS)

    Mueller, H. J.

    2010-12-01

    valid under deep mantle conditions. Unfortunately we miss these rocks, because there is no natural or technological way to bring them to the surface, especially in original state. So, we should make them for experiments! Because of the complexity of rocks and rock crystallization conditions including the factor time this is not possible so simple. But there seems to be a promising innovative complex approach. It consists among other things of: 1. Measurement of elastic and thermophysical properties of natural rocks under experimentally simulated in situ conditions of the uppermost mantle to find the quantitative relations between structural and physical properties under extreme conditions 2. Systematic rising of the maximum pressure in multi-anvil devices by multi-staging, i.e. higher pressures in bigger volumes with less temperature gradients 3. Measurement of elastic and thermophysical properties of artificial polymineral rock samples in conjunction with synchrotron radiation (third generation, highly brilliant) and neutrons under lower mantle in situ conditions using the multi-staging technique 4. Development of theoretical models and numerical simulation tools for extension and upgrading of the experimental results from 3 to unknown rocks of natural complexity

  6. Inferring Chemical, Thermal and Mechanical Heterogeneities in the Upper Mantle From Seismological Observations

    NASA Astrophysics Data System (ADS)

    Karato, S.; Shito, A.

    2003-12-01

    Inferring heterogeneity in the mantle is critical for our understanding of evolution and dynamics of this planet. Most previous efforts in this direction have been concerned with mapping anomalies in temperature, partial melting and/or major element chemistry. We show that in addition to these anomalies, anomalies in trace elements such as hydrogen (water) and the stress level can now be mapped using seismological observations when combined with the latest results of mineral physics. The effects of hydrogen on seismic wave propagation are mostly through its effects on attenuation (Q) and anisotropy. A theoretical analysis shows that the effects of water on attenuation and seismic wave velocities can be parameterized using the rheologically effective temperature (Karato, 2003). This formulation predicts that the velocity heterogeneity caused by anomalies in temperature or water content must have correlation with anomalies in Q. Furthermore the slopes of correlation between Q and velocity anomalies are different between thermal and water origin. Consequently a comparison of anomalies in average seismic wave velocities and Q provides a useful tool to identify the cause of these anomalies. Such an analysis on wedge mantle in the western Pacific suggests that significant heterogeneity in major element chemistry is present in the shallow (<200km) upper mantle whereas anomalies in the deep upper mantle are most likely attributed to the heterogeneity in water content (Shito and Shibutani, 2003). Recent laboratory studies also show that the nature of seismic anisotropy is sensitive to various parameters including water content, temperature and stress magnitude (Jung and Karato, 2001; Katayama et al., 2003). A commonly observed trend of fast shear wave polarization (trench parallel near trench to trench normal anisotropy away from trench) can be attributed to the regional variation in stress level (and water content) in the subduction zone: high stress (plus high water

  7. Potassium:rubidium ratio in ultramafic rocks: differentiation history of the upper mantle.

    PubMed

    Stueber, A M; Murthy, V R

    1966-08-12

    The increase in K:Rb ratio with decrease in potassium content found in basaltic rocks does not seem to apply to ultramafic rocks. The ratios in a series of alpine ultramafic rocks and ultramafic inclusions in basals and kimberlite pipes are about 200 to 500-significantly lower than those in oceanic tholeiites. This characteristic of ultramafic rocks appears to be consistent with a simplified model in which early differentiation of the primitive mantle led to formation of an upper mantle region enriched in alkali elements and having a low K:Rb ratio. Alpine ultramafic rocks may be residuals from such an upper mantle region. PMID:17791130

  8. Evidence for deep mantle circulation from global tomography

    USGS Publications Warehouse

    Van Der Hilst, R. D.; Widiyantoro, S.; Engdahl, E.R.

    1997-01-01

    Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate oonvergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength hetorogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

  9. Crust and upper mantle of Kamchatka from teleseismic receiver functions

    NASA Astrophysics Data System (ADS)

    Levin, Vadim; Park, Jeffrey; Brandon, Mark; Lees, Jonathan; Peyton, Valerie; Gordeev, Evgenii; Ozerov, Alexei

    2002-11-01

    Teleseismic receiver functions (RFs) from a yearlong broadband seismological experiment in Kamchatka reveal regional variations in the Moho, anisotropy in the supra-slab mantle wedge, and, along the eastern coast, Ps converted phases from the steeply dipping slab. We analyze both radial- and transverse-component RFs in bin-averaged epicentral and backazimuthal sweeps, in order to detect Ps moveout and polarity variations diagnostic of interface depth, interface dip, and anisotropic fabric within the shallow mantle and crust. At some stations, the radial RF is overprinted by near-surface resonances, but anisotropic structure can be inferred from the transverse RF. Using forward modeling to match the observed RFs, we find Moho depth to range between 30 and 40 km across the peninsula, with a gradational crust-mantle transition beneath some stations along the eastern coast. Anisotropy beneath the Moho is required to fit the transverse RFs at most stations. Anisotropy in the lower crust is required at a minority of stations. Modeling the amplitude and backazimuthal variation of the Ps waveform suggests that an inclined axis of symmetry and 5-10% anisotropy are typical for the crust and the shallow mantle. The apparent symmetry axes of the anisotropic layers are typically trench-normal, but trench-parallel symmetry axes are found for stations APA and ESS, both at the fringes of the central Kamchatka depression. Transverse RFs from east-coast stations KRO, TUM, ZUP and PET are fit well by two anisotropic mantle layers with trench-normal symmetry axes and opposing tilts. Strong anisotropy in the supra-slab mantle wedge suggests that the mantle "lithosphere" beneath the Kamchatka volcanic arc is actively deforming, strained either by wedge corner flow at depth or by trenchward suction of crust as the Pacific slab retreats.

  10. Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle.

    PubMed

    Mainprice, David; Tommasi, Andréa; Couvy, Hélène; Cordier, Patrick; Frost, Daniel J

    2005-02-17

    The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below. PMID:15716950

  11. Structural change in molten basalt at deep mantle conditions.

    PubMed

    Sanloup, Chrystèle; Drewitt, James W E; Konôpková, Zuzana; Dalladay-Simpson, Philip; Morton, Donna M; Rai, Nachiketa; van Westrenen, Wim; Morgenroth, Wolfgang

    2013-11-01

    Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth's interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning. PMID:24201283

  12. Upper Mantle Structure of the Eastern Africa from Body Wave Tomography

    NASA Astrophysics Data System (ADS)

    Mulibo, G. D.; Nyblade, A.; Ferdinand, R. W.; Reusch, A. M.; Adams, A. N.; Tugume, F. A.

    2009-12-01

    This study presents preliminary results of the upper mantle structure beneath the east Africa from body wave tomography. This work is part of an on-going study aimed at investigating the origin and structure of the African Superplume. The available global tomographic studies suggest that the African Superplume is a low velocity-anomaly extending from the core-mantle boundary upward into the mid mantle beneath southern Africa and may reach the upper mantle beneath eastern Africa. However, the limited vertical resolution of global tomographic models makes it difficult to confirm a connection from the lower to the upper mantle. Previous regional studies of upper mantle structure in east Africa have found evidence of a low velocity anomaly beneath the region that has been suggested as the upper mantle expression of the Superplume. Models from previous tomographic studies in east Africa have limited resolution below ~400 km beneath the eastern rift and are less well resolved beneath the western part of the rift due to less data coverage. This study uses teleseismic data from a wider region in east Africa than previously used. Data for this study are from a 3-year (2007-2010) deployment of 40 broadband seismic stations in Uganda and Tanzania. The dataset is supplemented by data from the 1994-1995 Tanzania broadband seismic experiment, the 2001-2002 Kenya broadband seismic experiment, the permanent AfricaArray seismic stations and IRIS/GSN stations. The data have been used for body wave tomography by computing relative travel time delays using a multi-channel cross-correlation technique and then inverting them for a 3D wave speed model. Preliminary results from the inversion of the relative delay times show that there is a low wave speed anomaly beneath east Africa extending from shallow upper mantle depths to at least 500 km.

  13. A shear-wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms

    NASA Astrophysics Data System (ADS)

    Legendre, C.; Meier, T.; Lebedev, S.; Friederich, W.; Viereck-Götte, L.

    2012-04-01

    Broadband waveforms recorded at stations in Europe and surrounding regions were inverted for shear-wave velocity of the European upper mantle. For events between 1995 and 2007 seismograms were collected from all permanent stations for which data are available via the data centers ORFEUS, GEOFON, ReNaSs and IRIS. In addition, we incorporated data from temporary experiments, including SVEKALAPKO, TOR, Eifel Plume, EGELADOS and other projects. Automated Multimode Inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. Successful waveform fits for about 70,000 seismograms yielded over 300,000 independent linear equations that were solved together for a three-dimensional tomographic model. Resolution of the imaging is particularly high in the mantle lithosphere and asthenosphere. The highest velocities in the mantle lithosphere of the East European Craton are found at about 150 km depth. There are no indications for a large scale deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved by our model as well. The locations of diamond bearing kimberlites correlate with reduced S-wave velocities in the cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the western margin of the East European Craton and hint to erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath Western Europe and between the Tornquist-Teyissere Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with

  14. Relationship between observed upper mantle structures and recent tectonic activity across the Southeastern United States

    NASA Astrophysics Data System (ADS)

    Biryol, C. Berk; Wagner, Lara S.; Fischer, Karen M.; Hawman, Robert B.

    2016-05-01

    The lithospheric structure of the Southeastern United States is a product of earlier episodes of continental collision and breakup. The region is located in the interior of the North American Plate, away from active plate margins. However, there is ongoing tectonism in the region with multiple zones of seismicity, uplifting arches, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the state of stress remain enigmatic. Two important factors are plate strength and preexisting, inherited structures. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Examples include the relatively thick lithospheric mantle of stable North America that abruptly thins beneath the Paleozoic Appalachian orogeny, and the slow upper mantle of the Proterozoic Reelfoot rift. Our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. This provides a viable explanation for seismicity, uplifting, and young intraplate volcanism. We postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

  15. The Asthenosphere Melting Regimes Alteration due to Changing Conditions of Upper Mantle

    NASA Astrophysics Data System (ADS)

    Perepechko, Y. V.; Sharapov, V. N.; Sorokin, K., Jr.

    2014-12-01

    Analyzed in the article are different asthenosphere magma generation regimes above the upper mantle hot spots as thermodynamic and geometric parameters of the upper mantle and the conditions on its boundaries vary. The two-layer mantle model is applied to consider the formation of decompression melting areas. The thickness of metasomatically altered lithospheric mantle is determined by the mantle substance rheology and the location of the upper boundary of asthenosphere. We also take into consideration the principal solid state phase transitions by using the mantle substance state equation. The sizes and distribution of hot spots as well as their maximal temperature were defined by the thermodynamic conditions of the perovskite transition existence. The numerical analysis results demonstrate the manifestation of three main mantle dynamics modes; the conditions necessary to form the partial melting zones are not reached; some melting areas with the 30 to 65 Ma existence time do occur; the melting areas that are formed exist permanently. The permanently existing asthenosphere zones are marked by quasiperiodical variation in thickness and the degree of melting. The typical temperatures of a hot spot sharing these modes are the 1740°С and 2020°С correspondingly. The originally presupposed heating degree and the temperature ratio of the upper mantle do influence the decompression melting degree substantially and - to a lesser extent - they influence the size of melting zones. The primary evolution of the second mode is described by the development of a complex system of asthenosphere zones that lead to the occurrence of additional convectional cells dividing the partial melting zone. The variation in the rheological properties of the mantle substance also contributes to the manifestation of the complex structure of asthenosphere zone. The work was made with support of the Russian Foundation for Basic Research grant #12-05-00625.

  16. Duration of the hydrocarbon fluid formation under thermobaric conditions of the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Mukhina, Elena; Kolesnikov, Anton; Kutcherov, Vladimir

    2016-04-01

    Deep abiogenic formation of hydrocarbons is an inherent part of the Earth's global carbon cycle. It was experimentally confirmed that natural gas could be formed from inorganic carbon and hydrogen containing minerals at pressure and temperature corresponding to the Earth's upper mantle conditions. Reaction between calcite, wustite and water in the large volume device was studied in several works. It was previously proposed that reaction is possible only after 40 minutes of exposure at high pressure and temperature. In this work similar experiment at P = 60 kbar and T = 1200 K were carried out in "Toroid" type chamber with the 5 seconds duration of thermobaric exposure. Gas chromatographic analysis of the reaction products has shown the presence of hydrocarbon mixture comparable to 5 minutes and 6 hours exposure experiments. Based on this fact it is possible to conclude that the reaction of natural gas formation is instant at least at given thermobaric conditions. This experiment will help to better understand the process of deep hydrocarbon generation, particularly its kinetics.

  17. Small-scale convective instability and upper mantle viscosity under california.

    PubMed

    Zandt, G; Carrigan, C R

    1993-07-23

    Thermal calculations and convection analysis, constrained by seismic tomography results, suggest that a small-scale convective instability developed in the upper 200 kilometers of the mantle under California after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the San Andreas transform boundary. The upper bound for the upper mantle viscosity in the slab window, 5 x 10(19) pascal seconds, is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions. These model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time-dependent small-scale convection that can generate localized areas of uplift and subsidence. PMID:17770025

  18. Stress-driven relaxation of heterogeneous upper mantle and time-dependent afterslip following the 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Hu, Yan; Bürgmann, Roland; Uchida, Naoki; Banerjee, Paramesh; Freymueller, Jeffrey T.

    2016-01-01

    Understanding of postseismic deformation following great subduction zone earthquakes is complicated by the combined effects of viscoelastic relaxation of earthquake-induced stresses in the upper mantle and time-dependent afterslip on the megathrust. We integrate geodetic observations and constraints on afterslip from small repeating earthquakes on the megathrust to better distinguish contributions from these two postseismic processes. We have developed a three-dimensional, spherical viscoelastic finite element model to study the postseismic deformation of the 2011 Mw9.0 Tohoku earthquake that has been recorded at unprecedented high resolution in space and time. We model stress-driven afterslip in a 2 km thick weak shear zone away from historic rupture zones on the megathrust. We model both the viscoelastic relaxation of the upper mantle and shear zone deformation with a transient Burgers body rheology. The transient Kelvin viscosity is assumed to be one order of magnitude lower than that of the Maxwell viscosity. Viscoelastic relaxation of the mantle wedge alone causes postseismic uplift and seaward motion in the upper plate, opposite to the pattern from relaxation of just the oceanic upper mantle. Afterslip on the fault produces uplift updip of the afterslip zone and subsidence over its downdip edge and mostly seaward motion above the afterslip zone. The best fit Maxwell viscosity of the shear zone at depths ≤50 km is 1017 Pa s, constrained by afterslip estimates from repeating earthquakes. The optimal viscosities of the deep weak shear zone, continental mantle wedge, and oceanic upper mantle are determined to be 5 × 1017 Pa s, 3 × 1019 Pa s, and 5 × 1019 Pa s, respectively. The stress-driven afterslip in the shear zone is up to ~3.5 m in the first 2 years after the earthquake, equivalent to an Mw8.4. Our model reproduces the first-order pattern of the GPS observations both in horizontal and in vertical directions. Seafloor geodetic observations of subsidence

  19. Incipient fluid migration through the deep mantle by dissolution-precipitation: crystal growth constraints

    NASA Astrophysics Data System (ADS)

    Shatskiy, Anton; Litasov, Konstantin; Borzdov, Yury; Katsura, Tomoo; Ohtani, Eiji

    2010-05-01

    The mechanism and driving forces for transport of incipient C-O-H-bearing fluid/melt species through the Earth's mantle is a key issue in geosciences. Several mechanisms of the volatile transport, mainly solid state diffusion and fluid/melt porous flow have been considered. The grain boundary diffusion of hydrogen and carbon is in the range of 10-10-10-11 m2/s. These values suggest failure of the solid state diffusion hypothesis to explain melt migration. Since the extensive partial melting of the Earth's mantle seems improbable, the porous flow model of fluid or melt migration, accepted for the shallow upper mantle, can not be applied for the volatile transport through the deep mantle. At the mantle conditions water and carbonates are the excellent silicate solvents. Hence the migration of insulated portions of fluid through the solid matrix should proceed by means of the dissolution-precipitation mechanism. The major driving force for this process would be pressure or temperature gradient, differences in stable and metastable phase solubilities, and stress. In order to estimate the reliability of proposed mechanism we measured migration rate of carbonate, water-carbonate, or water-rich liquid layer through the solid silicate matrix at the upper and lower mantle PT conditions. The thermal gradient was employed as a driving force. The kinetic constant of the migration rates were estimated to be 8×10-8 m/s/K for H2O, 5×10-9 m/s/K for K2Mg(CO3)2+2H2O, and 3×10-10 m/s/K for K2Mg(CO3)2 solvents. In order to extrapolate obtained data to the Earth we assumed that (a) mass transfer of silicate components through the melt layer is limited by diffusion and (b) the thickness of the melt layer is not enough to establish convection. The large lateral thermal gradient, 1-4 oC/km, proposed for mantle plumes reveals lateral fluid migration rate relevant to the plume lifetime (25-50 Ma). Nevertheless, the radial (vertical) migration rate is quite slow, about 1 km in 12.5 Ga

  20. Crust and Upper Mantle Velocity Structure beneath NE China from Joint Inversion of Local and Teleseismic Data

    NASA Astrophysics Data System (ADS)

    Huang, J.; Liu, Z.; Yang, F.

    2014-12-01

    Northeast China, which is located in the composite part of Paleo Asia ocean and Pacific ocean Domain, has undergone multi-stage tectonism. The geological structure is complicated and volcanic activity is strong. In this region, two major geologic and geophysical boundaries are distinct, the NNE-trending North South Gravity Lineament (NSGL) and Tanlu fault. With respect to North China Craton (NCC), Northeast China is more closely adjacent to the subduction zone of Pacific slab, along the eastern boundary of Northeast China the subducting Pacific plate approaches depths of 600 km, many deep earthquakes often occurred here, thus, this region become an ideal place to investigate deep structure related to deep subduction, deep earthquakes as well as intraplate volcanism. In this study, we determined high-resolution three dimensional P- and S-wave velocity models of the crust and upper mantle to 800 km depth by jointly inverting arrival times from local events and relative residuals from teleseismic events recorded by more than 400 seismic stations which belong to the six regional seismic Networks and a PASSCALL temporary array (NECESSArray). Our results show that main velocity anomalies exhibited block feature and are generally oriented in NE to NNE direction, which is consistent with regional tectonic direction. The NSGL is characterized by a high-velocity (high-V) anomaly belt with a width of approximately 100 km, and the high-V anomaly extents to the bottom of upper mantle or mantle transition zone. The songliao basin, which is located between NSGL and Tanlu fault tectonic boundaries, obvious low-velocity anomaly extends to about depth of 200 km. Under the Great Xing'an Range on the west side of NSGL, the velocity is slightly low in the lithosphere. Our results also revealed that most of deep earthquakes all occurred in deep subduction zone with high-velocity anomaly,but in the subduction zone with low-velocity, almost deep earthquakes have not occurred. Further

  1. The effect of water and iron content on electrical conductivity of upper mantle rocks.

    NASA Astrophysics Data System (ADS)

    Wang, D.; Yi, L.

    2008-12-01

    Geophysical observations (MT and GDS) show the conductivity anomaly which may be related to the presence of water and melting. Recently, several researchers have estimated the water content in the transition zone (Huang et al. 2005; Yoshino et al. 2008) and the upper mantle (Wang et al.2006; Yoshino et al. 2006) by electrical conductivity methods. They may underestimate the water content, especially, Yoshino et al did too much underestimate. However, other coexisting phases such as pyroxene and its high-pressure polymorphs may also contribute to the bulk conductivity of the mantle. To test this hypothesis, we measured the electrical conductivity of upper mantle rocks- dunite, pyroxenite and lherzolite at ~ 2-3 GPa and ~1273-1573 K using impedance spectra within a frequency range of 0.1~1000000 Hz. The oxygen fugacity was controlled by a Mo-MoO2 solid buffer. The results show that the electrical conductivity of lherzolite and pyroxenite are ~ half and one order of magnitude higher than that of dunite. These differences were interpreted through a preliminary model involving water and iron content effects on the electrical conductivity. We extrapolated our results and compared the results with some of geophysical observations of the upper mantle. Our results indicate the maximum water content in oceanic upper mantle is as high as ~ 0.09wt % and suggest that pyroxenes dominate the bulk conductivity of upper mantle in hydrous conditions. These results indicated that our model with various water contents could explain the conductivity anomaly in the oceanic upper mantle without involving the presence of partial melt at these depths. This work was supported by national natural science foundation of china (40774036); the special grant from the president of Chinese Academy of Sciences and Graduate University of Chinese Academy Sciences.

  2. Numerical Simulation Analysis of Deformation Effect of The Upper Mantle Flow to Ordos and Its Surroundings

    NASA Astrophysics Data System (ADS)

    Yun, S.; Ping, L. C.; Qi, D.

    2014-12-01

    Ordos block is a typical representative of cratonic lithosphere in North China. It is stable in the block ,but around the block there are a series of faulted basins and folded mountains, the new tectonic movement around the block is intense. Some scholars propose that the upper mantle flow is an important factor to the extension activity of the fault zone around the block. But it has never been discussed in detail that how the upper mantle flow affects the movement and deformation around Ordos block? A 3D viscoelastic modeling is realized for studying the deformation effect of the upper mantle flow to Ordos and its surroundings, based on the comprehensive geological and geophysical data ,such as 3d rheological structure, the active blocks of China, thermal structure, shear wave splitting, et al. The modeling results indicate that in the vertical direction, compared with the local uplift and depression caused by the compression among the plates, the uplifting of Ordos block as a whole is mainly effected by mantle upwelling. In general the upper mantle surrounding of Ordos block is upwelling, Linfen basin goes up more faster. In the horizontal direction, The general flow direction of upper mantle in the study area is NE, basically the same as Qingzang block movement direction. But there is a bifurcation flow along the southwestern margin of Liupanshan. Generally speaking, the regional deformation is drive mainly by the movement of Qingzang block and adjacent blocks pushing into each other,the deformation effect of the upper mantle flow to Ordos and its surroundings is a superposition and partial adjustment.

  3. Crustal and upper mantle structure of central Qiangtang terrane (Tibet Plateau) imaged with magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Zeng, S.; Hu, X.; Li, J.

    2013-12-01

    Since the Tethys Ocean closed, the ongoing collision between India and Aisa continents has created the Tibet Plateau, which is the most spectacular topographic feature on the surface of the earth. In the last decades, a large number of geological and geophysical studies have been undertaken in the Tibet Plateau, but most of these studies were focused on southern Tibet, where the collision of the Indian tectonic plate with Eurasia was occurred, and southeast Tibet, where lateral extrusion of crustal material may be occurred, absent in the central Tibet. As research continues, it has become clear that a complete understanding of the formation and deformation of the Tibet Plateau requires a study of the entire plateau. The Qiangtang terrane is located in the central Tibet Plateau. In 1993-1994, three profiles of broadband MT data (320 Hz to 2000 s) along N-S trending ranges from 86°E to 91°E were collected by China University of Geoscience in central Qiangtang terrane for the purpose of oil and gas exploration, the previous interpretation was focused on the shallow structures. In this study, we reanalyze the three MT profiles to produce more detailed images of the deep electrical structure of the Qiangtang terrane. Dimensionality analysis and geoelectric strike analysis of these data show that they appear to be two dimensional. 2-D inversion model show that there is a pervasive conductivity layer in the mid- to lower crustal and upper mantle, especially in the north Qiangtang terrane, which was considered to be the result of partial melt. The partial melt fraction is sufficient for crustal flow to occur. The similarity of the inversion models of the three profiles show that there is west-east crustal flow along the Jinsha River suture in central Qiangtang terrane, which seems to be western extension of the crustal flow observed in southeast Tibet by Bai et al. (2010). The inversion results also show difference of the electrical structure between the west and east

  4. Osmium isotopes suggest fast and efficient mixing in the oceanic upper mantle.

    NASA Astrophysics Data System (ADS)

    Bizimis, Michael; Salters, Vincent

    2010-05-01

    The depleted upper mantle (DUM; the source of MORB) is thought to represent the complementary reservoir of continental crust extraction. Previous studies have calculated the "average" DUM composition based on the geochemistry of MORB. However the Nd isotope compositions of abyssal peridotites have been shown to extend to more depleted compositions than associated MORB. While this argues for the presence of both relatively depleted and enriched material within the upper mantle, the extent of compositional variability, length scales of heterogeneity and timescales of mixing in the upper mantle are not well constrained. Model calculations show that 2Ga is a reasonable mean age of depletion for DUM while Hf - Nd isotopes show the persistence of a depleted terrestrial reservoir by the early Archean (3.5-3.8Ga). U/Pb zircon ages of crustal rocks show three distinct peaks at 1.2, 1.9, and 2.7Ga and these are thought to represent the ages of three major crustal growth events. A fundamental question therefore is whether the present day upper mantle retains a memory of multiple ancient depletion events, or has been effectively homogenized. This has important implications for the nature of convection and time scales of survival of heterogeneities in the upper mantle. Here we compare published Os isotope data from abyssal peridotites and ophiolitic Os-Ir alloys with new data from Hawaiian spinel peridotite xenoliths. The Re-Os isotope system has been shown to yield useful depletion age information in peridotites, so we use it here to investigate the distribution of Re-depletion ages (TRD) in these mantle samples as a proxy for the variability of DUM. The probability density functions (PDF) of TRD from osmiridiums, abyssal and Hawaiian peridotites are all remarkably similar and show a distinct peak at 1.2-1.3 Ga (errors for TRD are set at 0.2Ga to suppress statistically spurious age peaks). The Hawaiian peridotites further show a distinct peak at 1.9-2Ga, but no oceanic mantle

  5. Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

    NASA Astrophysics Data System (ADS)

    Priestley, Keith; Tilmann, Frederik

    2009-04-01

    The lithosphere-asthenosphere boundary corresponds to the base of the "rigid" plates - the depth at which heat transport changes from advection in the convecting deeper upper mantle to conduction in the shallow upper mantle. Although this boundary is a fundamental feature of the Earth, mapping it has been difficult because it does not correspond to a sharp change in temperature or composition. Various definitions of the lithosphere and asthenosphere are based on the analysis of different types of geophysical and geological observations. The depth to the lithosphere-asthenosphere boundary determined from these different observations often shows little agreement when they are applied to the same region because the geophysical and geological observations (i.e., seismic velocity, strain rate, electrical resistivity, chemical depletion, etc.) are proxies for the change in rheological properties rather than a direct measure of the rheological properties. In this paper, we focus on the seismic mapping of the upper mantle high velocity lid and low velocity zone and its relationship to the lithosphere and asthenosphere. We have two goals: (a) to examine the differences in how teleseismic body-wave travel-time tomography and surface-wave tomography image upper mantle seismic structure; and (b) to summarise how upper mantle seismic velocity structure can be related to the structure of the lithosphere and asthenosphere. Surface-wave tomography provides reasonably good depth resolution, especially when higher modes are included in the analysis, but lateral resolution is limited by the horizontal wavelength of the long-period surface waves used to constrain upper mantle velocity structure. Teleseismic body-wave tomography has poor depth resolution in the upper mantle, particularly when no strong lateral contrasts are present. If station terms are used, features with large lateral extent and gradual boundaries are attenuated in the tomographic image. Body-wave models are not

  6. Multi-scale seismic heterogeneity and convection in the western U.S. upper mantle (Invited)

    NASA Astrophysics Data System (ADS)

    Schmandt, B.; Humphreys, E.

    2010-12-01

    New P- and S-wave tomography of the U.S. upper mantle from the Pacific Coast to the Great Plains reveals strong multi-scale heterogeneity closely correlated with tectonic and magmatic activity. We invert teleseismic travel-time residuals from the USArray and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1200 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel-time residuals, and frequency-dependent 3-D sensitivity kernels to map travel-time residuals, measured in multiple frequency bands, into velocity structure. In addition to separate Vp and Vs models, we jointly invert the two datasets for Vp/Vs perturbations by imposing a smoothness constraint on the dlnVs/dlnVp field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of Vp, Vs, and Vp/Vs variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small-scale convection. Unreasonably large mantle temperature variations, up to ~900 C at 100 km depth, are required if the entire magnitude of velocity structure is attributed to temperature. Partially molten mantle is inferred beneath Yellowstone and the eastern Snake River Plain (SRP), the Salton Trough, and the Clear Lake volcanic field. The inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern SRP and Yellowstone. A northeast trending swath of relatively high-velocity mantle extends from the Colorado Plateau to northeastern Wyoming suggesting that considerable compositional heterogeneity in the lithosphere is necessary to reconcile the high mean elevation and negligible geoid anomaly of the region. This swath of high-velocity mantle is juxtaposed against generally low-velocity mantle beneath the Basin and

  7. Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies

    NASA Technical Reports Server (NTRS)

    Anderson, Don L.

    1987-01-01

    Factors which influence the lateral heterogeneity in density and seismic velocity with depth in the upper earth mantle are discussed. It is emphasized that most of the increases in density and seismic velocity with depth are caused by pressure-induced solid-solid phase changes in the high-density high-velocity phases of mineral assemblage, due to variations in temperature. In particular, the ilmenite form of MgSiO3 and the gamma-spinel form of Mg2SiO4 have broad stability fields in cold mantle and are not stable in hotter mantle. It is emphasized that the density and velocity anomalies associated with temperature-induced phase changes in mineral assemblage must be taken into account in the thermal models of the slabs; when these effects are accounted for, the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and with layered models of mantle convection.

  8. A new density model of the upper mantle of North America

    NASA Astrophysics Data System (ADS)

    Kaban, Mikhail K.; Mooney, Walter D.

    2010-05-01

    We investigate the density structure of the North America upper mantle based on the integrative analysis of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust from the observed field to determine the mantle gravity anomalies. We use a new crustal model, which is based on nearly all existing seismic determinations including the most recent. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (-50 to -400 mgal) beneath western NA and the adjacent oceanic region, and positive anomalies (+50 to +350 mgal) east of the NA Cordillera. This pattern reflects the well-known division of NA into the stable eastern region and the tectonically active western region. In the same way we estimate the residual topography, which represents the part of the surface topography not- (or over-) compensated by the crustal structure. We invert these fields jointly with seismic tomography data to image density distribution within the crust and upper mantle. The inversion technique accounts for the fact that the residual gravity and residual topography are controlled by the same factors but in a different way, e.g. depending on depth. In the final stage we separate the effect of mantle temperature variations, which is estimated from seismic tomography models constrained by geothermal modelling. Some features of the composition density distribution, which are invisible in the seismic tomography data, are for the first time detected in the upper mantle. The strongest positive anomaly is co-incident with the Gulf of Mexico, and indicates possibly a high-density eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen: one with a NE-SW trend in the eastern USA roughly coincident with the Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as due either to: (1) the presence of remnants of

  9. A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico

    USGS Publications Warehouse

    Spence, W.; Gross, R.S.

    1990-01-01

    To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, the lateral variations of P wave velocity in the upper mantle beneath these features has been investigated. Teleseismic P wave delays recorded at a 22-station network were used to perform a damped least squares, three-dimensional inversion for these lateral variations. Results infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle. -from Authors

  10. Deep mantle mineralogy and novel materials synthesis using multianvil high-pressure technology (Robert Wilhelm Bunsen Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Tetsuo, Irifune

    2016-04-01

    Phase relations in mantle and slab materials have been studied using Kawai-type multianvil apparatus (KMA) under pressure and temperature conditions of the mantle transition region and the uppermost lower mantle of the Earth. The associated density and sound velocity changes of these materials have also been determined using the KMA technology combined with synchrotron in situ X-ray and ultrasonic interferometry measurements. The results show that the mantle transition region is made of a pyrolitic composition, while the presence of a harzburgite-rich layer is suggested in the lower parts of this region. Use of sintered diamond anvils for KMA has allowed expansion of these measurements toward deeper region of the lower mantle. Our preliminary results of such measurements indicate that at least upper part of the lower mantle is made of the pyrolitic composition contrary to a recent study based on Brillouin scattering measurements in diamond anvil cell, which concluded a more Si-rich lower mantle. On the other hand, we have been applying KMA technology to synthesis of novel functional materials utilizing its capability of producing very high static pressures and homogeneous temperatures in relatively large sample volumes. These include ultrahard nano-polycrystalline diamond (NPD) directly converted from graphite, which is now being used for applications to abrasive and cutting tools as well as for some scientific applications such as anvils for some high-pressure devices. Another example of such a novel material is hard and tough nano-polycrystalline stishovite (NPS), which is also potentially important for some industrial applications. Moreover, we recently succeeded in making highly transparent nano-polycrystalline garnet (NPG), which is ideal for the measurements of sound velocities by various methods, such as Brillouin scattering and GHz ultrasonic interferometry. Thus, the KMA technology opens the door to the synthesis of transparent nano

  11. Teleseismic P wave spectra from USArray and implications for upper mantle attenuation and scattering

    NASA Astrophysics Data System (ADS)

    Cafferky, Samantha; Schmandt, Brandon

    2015-10-01

    Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt* for multiple frequency bands within 0.08-2 Hz. All frequency bands show high Δt* regions in the southwestern U.S., southern Rocky Mountains, and Appalachian margin. Low Δt* is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt* magnitudes. Even the two standard deviation Δt* magnitude for the widest band is ˜2-7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt* signal is nonthermal in origin. Nonthermal contributions are further indicated by only a moderate correlation between Δt* and P travel times. Some geographic variations, such as high Δt* in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3-D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt* are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt* stations. The large magnitude of Δt* estimates and the T/Z spectra are consistent with major contributions to Δt* from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt* magnitude and the moderate correlation of Δt* with travel times.

  12. Sharpness of upper-mantle discontinuities determined from high-frequency reflections

    USGS Publications Warehouse

    Benz, H.M.; Vidale, J.E.

    1993-01-01

    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transition zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth.

  13. Sharpness of upper-mantle discontinuities determined from high-frequency reflections

    USGS Publications Warehouse

    Benz, H.M.; Vidale, J.E.

    1993-01-01

    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth. ?? 1993 Nature Publishing Group.

  14. Deep mantle forces and the uplift of the Colorado Plateau

    SciTech Connect

    Moucha, R; Forte, A M; Rowley, D B; Mitrovica, J X; Simmons, N A; Grand, S P

    2009-06-23

    Since the advent of plate tectonics, it has been speculated that the northern extension of the East Pacific Rise, specifically its mantle source, has been over-ridden by the North American Plate in the last 30 Myrs. Consequently, it has also been postulated that the opening of the Gulf of California, the extension in the Basin and Range province, and the uplift of the Colorado Plateau are the resulting continental expressions of the over-ridden mantle source of the East Pacific Rise. However, only qualitative models based solely on surface observations and heuristic, simplified conceptions of mantle convection have been used in support or against this hypothesis. We introduce a quantitative model of mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 Myrs and its relative advance towards the interior of the southwestern USA. The onset and evolution of the crustal uplift in the central Basin and Range province and the Colorado Plateau is determined by tracking the topographic swell due to this mantle upwelling through time. We show that (1) the extension and magmatism in the central Basin and Range province between 25 and 10 Ma coincides with the reconstructed past position of this focused upwelling, and (2) the southwestern portion of the Colorado Plateau experienced significant uplift between 10 Ma and 5 Ma that progressed towards the northeastern portion of the plateau. These uplift estimates are consistent with a young, ca. 6 Ma, Grand Canyon model and the recent commencement of mafic magmatism.

  15. Activation volume for creep in the upper mantle.

    PubMed

    Ross, J V; Ave'lallemant, H G; Carter, N L

    1979-01-19

    The activation volume for creep, V*, of olivine-rich rocks has been determined in pressure-differential creep experiments on dunite at temperatures from 1100 degrees to 1350 degrees C and confining pressures from 5 to 15 kilobars. Values of V* range from 10.6 to 15.4 cubic centimeters per mole with a mean value of 13.4 cubic centimeters per mole, near that expected for oxygen ion self-diffusion. The quantity V* is incorporated into existing flow equations; in combination with observations on naturally deformed mantle xenoliths, estimates are given of the variation with depth of stress, strain rate, and viscosity. PMID:17738997

  16. Consequences of experimental transient rheology. [of earth lower crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Sabadini, Roberto; Smith, Brad K.; Yuen, David A.

    1987-01-01

    Recent analyses of transient creep data for lower crustal and upper mantle substances are used to constrain the viscosity contrast across the 670 km discontinuity, and a 4-layer earth model in which both upper and lower mantles are described by a Burgers body rheology is assumed. Results indicate that the previously inferred viscosity variations would be reduced by a factor of ten using the new transient models. This result is in agreement with a revised viscosity estimate based on long-wavelength geoid anomalies and seismic tomography. Time-dependent perturbations to the gravity field from recent ice movements are shown to be significant.

  17. Lunar thermal regime to 300 km. [in crust and upper mantle

    NASA Technical Reports Server (NTRS)

    Keihm, S. J.; Langseth, M. G.

    1977-01-01

    Coupling of the global heat flow, crustal heat source enrichment, thermal conductivity, and temperature in the crust and upper mantle of the moon is examined. A steady-state moon in which conductive heat transfer dominates is assumed. Heat-flow measurements from the Apollo 15 and 17 missions and gamma-ray mapping of thorium conducted by the Apollo 15 and 16 missions provide data for the study of the lunar thermal regime. Temperatures in the range of 1100 to 1600 K are found for the 300-km depth level. In the upper mantle, temperature gradients are in the range of 1.8 to 3.2 K/km.

  18. The osmium isotopic composition of convecting upper mantle deduced from ophiolite chromites

    NASA Astrophysics Data System (ADS)

    Walker, Richard J.; Prichard, Hazel M.; Ishiwatari, Akira; Pimentel, Márcio

    2002-01-01

    Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γ Os values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/ 188Os of a filtered suite yields a slope of -0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γ Os value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory. Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab

  19. Tracking deep mantle reservoirs with ultra-low velocity zones

    NASA Astrophysics Data System (ADS)

    McNamara, Allen K.; Garnero, Edward J.; Rost, Sebastian

    2010-10-01

    Some regions of the Earth's lowermost mantle exhibit anomalous seismic properties within a thin zone, less than tens of kilometers in thickness, that directly overlies the core-mantle boundary (CMB). These regions have been dubbed Ultra-Low Velocity Zones (ULVZs) due to their greater than 10% drop in seismic velocities. High resolution seismic array studies have found small, localized ULVZs (e.g., 10 km thick and 50-100 km wide) with a large increase in ULVZ density (~ 10%) relative to the background mantle. Many studies note that ULVZ material may be chemically distinct, though P-to-S-wave velocity reductions are sometimes consistent with partial melting. The apparent absence of ULVZs in many regions of the CMB is consistent with having a distinct chemical signature, regardless of melt content. However, it is unknown how a small volume of very dense ULVZ material can be locally elevated, particularly in the presence of large-scale compositional reservoirs predicted by seismology, geochemistry, and geodynamics. We perform ultra-high resolution, kilometer-scale, thermochemical convection calculations for an entire mantle system containing three distinct compositional components in order to investigate how a ULVZ interacts with large-scale lower mantle compositional reservoirs. We demonstrate that convection can dynamically support small scale accumulations of dense ULVZ material, consistent with the size and density inferred from seismology. Furthermore, we show that ULVZs preferentially reside at the boundaries of large compositional reservoirs, which periodically break apart and merge together in response to changes in downwelling patterns. As they do, ULVZ material migrates and recollects in a systematic fashion. ULVZ material can become entrained in mantle plumes forming from reservoir boundaries, contributing to isotopic anomalies found in hotspot volcanism. Thus ULVZ detection helps to constrain large-scale mantle convection patterns, the locations of

  20. High surface topography related to upper mantle flow beneath Eastern Anatolia

    NASA Astrophysics Data System (ADS)

    Komut, Tolga

    2015-11-01

    Eastern Anatolia region between north-south colliding Arabian and Eurasian plates has no significant crustal root and shallow (upper) mantle flow beneath seems to be vertically supporting its high topography. It has a high surface heat flow and the underlying mantle is characterized by low seismic velocity zones. Using a mantle density/temperature variation field derived from P-wave seismic velocity, current shallow mantle flow and resultant dynamic topography of Eastern Anatolia and adjacent Arabian foreland and Caucasus areas were calculated along a vertical section. The section crosses the tectonic boundaries interrelated with slab bodies (high seismic velocity/cold regions) and the low velocity zones above the slabs. According to the modelling experiments, the surface topography of Eastern Anatolia seems to be supported by shallow mantle flow dynamics. On the other hand, residual topography for the region was calculated using high resolution crustal thickness data. Positive residual topography that suggests an undercompensated state of Eastern Anatolia is in concordance with the dynamic topography anomaly. The modelled local shallow mantle flow support due to the density contrast between hot (low velocity) zones and underlying cold slab bodies beneath the area may be the present-day snapshot of the mantle flow uplift in Eastern Anatolia presence of which was previously suggested.

  1. Shear zones in the upper mantle - relation between geochemical enrichment and deformation in mantle peridotites

    SciTech Connect

    Downes, H. )

    1990-04-01

    Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometers to tens of kilometers). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimeters to meters) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. These is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rate earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low {epsilon}Sr and high {epsilon}Nd. In contrast, deformed peridotites, with or without amphibole, often have higher {epsilon}Sr and low {epsilon}Nd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.

  2. Geophysical constraints on partial melt in the upper mantle

    SciTech Connect

    Shankland, T.J.; O'Connell, R.J.; Waff, H.S.

    1981-08-01

    This paper adresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical 'anomaly'. Typical observed anomalies are high electrical conductivity of the order of 0.1 S/m or greater, velocity decreases of 7--10%, seismic Q values less than 100, and a frequency band for seismic effects in the region mear 1 Hz. Existing theories of electrical conduction in partial melts and of frequency-dependent seismic properties together with recent measurements of melt electrical conductivity, viscosity, and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods. From electrical anomalies, mainly sensitive to melt volume and its interconnection, one can require a minimum melt fraction of several percent at temperatures close to the solidus (1150/sup 0/--1300/sup 0/C). However, seismic models demand only a small volume in very flattened shapes (aspect ratio approx. =0.001, melt fraction approx.0.1%). Further, if melt configuration permits seismic dissipation in bulk, that is, there exist flattened voids intersecting more or less equant voids, then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies. Observed equilibrium textures of partly melted peridotite together with inferred melt-solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern. Thus if partial melt causes mantle geophysical anomalies, it should exist in a variety of void shapes and probably of sizes. While the association of electrical and elastic anomalies with indications of reduced density, volcanism, and high heat flow makes the hypothesis of partial melting an attractive explanation, the minimum physical requirement is for existence of relatively high temperature.

  3. P/n/ velocity and cooling of the continental lithosphere. [upper mantle compression waves in North America

    NASA Technical Reports Server (NTRS)

    Black, P. R.; Braile, L. W.

    1982-01-01

    The average upper mantle compressional wave velocity and heat flow figures presently computed for continental physiographic provinces in North America exhibit an inverse relationship, and possess a statistically significant correlation coefficient. A correlation is also demonstrated between compressional wave velocity and material temperature by estimating crust-mantle boundary temperatures from heat flow values. The dependency of compressional wave velocity on temperature implies that the observed geographical distribution in upper mantle seismic velocity may be due to the temperature effect character of upper mantle compressional wave velocity variation.

  4. Seismic imaging of the upper mantle beneath the northern Central Andean Plateau: Implications for surface topography

    NASA Astrophysics Data System (ADS)

    Ward, K. M.; Zandt, G.; Beck, S. L.; Wagner, L. S.

    2015-12-01

    Extending over 1,800 km along the active South American Cordilleran margin, the Central Andean Plateau (CAP) as defined by the 3 km elevation contour is second only to the Tibetan Plateau in geographic extent. The uplift history of the 4 km high Plateau remains uncertain with paleoelevation studies along the CAP suggesting a complex, non-uniform uplift history. As part of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we use surface waves measured from ambient noise and two-plane wave tomography to image the S-wave velocity structure of the crust and upper mantle to investigate the upper mantle component of plateau uplift. We observe three main features in our S-wave velocity model including (1), a high velocity slab (2), a low velocity anomaly above the slab where the slab changes dip from near horizontal to a normal dip, and (3), a high-velocity feature in the mantle above the slab that extends along the length of the Altiplano from the base of the Moho to a depth of ~120 km with the highest velocities observed under Lake Titicaca. A strong spatial correlation exists between the lateral extent of this high-velocity feature beneath the Altiplano and the lower elevations of the Altiplano basin suggesting a potential relationship. Non-uniqueness in our seismic models preclude uniquely constraining this feature as an uppermost mantle feature bellow the Moho or as a connected eastward dipping feature extending up to 300 km in the mantle as seen in deeper mantle tomography studies. Determining if the high velocity feature represents a small lithospheric root or a delaminating lithospheric root extending ~300 km into the mantle requires more integration of observations, but either interpretation shows a strong geodynamic connection with the uppermost mantle and the current topography of the northern CAP.

  5. Multi-observable thermochemical tomography of the lithosphere and upper mantle beneath the Western/Central US

    NASA Astrophysics Data System (ADS)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Schutt, D.; Fullea, J.; Jones, A. G.

    2013-12-01

    We use a novel multi-observable 3D inversion method (Afonso et al., 2013a; b) to study the present-day thermal and compositional structures of the lithosphere and sublithospheric upper mantle beneath the Western and Central US (between 256-246o long and 33-43o lat). We jointly invert Rayleigh wave phase velocity maps for periods up to 150 sec (from ambient noise and earthquake data), P and S teleseismic travel time residuals (>63000 for P-waves and > 28000 for S-waves), geoid and gravity anomalies, surface heat flow, gravity gradients, and absolute elevation. These observables have different sensitivities to deep/shallow, thermal/compositional anomalies and therefore they provide complementary constraints to the inversion. The method is based on a thermodynamically-constrained, nonlinear probabilistic (Bayesian) approach and includes the effect of potential dynamic contributions from density anomalies in the sublithospheric mantle via full solutions of the Stokes-flow problem. From this joint inversion, we obtain the 3D density, compositional, electrical conductivity and thermal structure for the entire lithosphere (including a multi-layer crust) and sublithospheric upper mantle down to 400 km. The resulting models show a number of robust features that carry important implications for supporting or disapproving current evolutionary models for this region. References: - Afonso, J.C., Fullea J., Griffin, W.L., Yang, Y., Jones, A.G., Connolly, J.A.D., O'Reilly, S.Y. (2013a), 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle I: a priori information and geophysical observables. J. Geophys. Res., 118, 2586-2617, doi:10.1002/jgrb.50124. - Afonso, J.C., Fullea J., Yang, Y., Connolly, J.A.D., Jones, A.G. (2013b), 3D multi-observable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle II: General methodology and resolution analysis. J. Geophys. Res

  6. Three-dimensional radial anisotropic structure of the North American upper mantle from inversion of surface waveform data

    NASA Astrophysics Data System (ADS)

    Marone, Federica; Gung, Yuancheng; Romanowicz, Barbara

    2007-10-01

    Seismic anisotropy provides insight into palaeo and recent deformation processes and, therefore, mantle dynamics. In a first step towards a model for the North American upper mantle with anisotropy characterized by a symmetry axis of arbitrary orientation, aimed at filling the gap between global tomography and SKS splitting studies, we inverted long period waveform data simultaneously for perturbations in the isotropic S-velocity structure and the anisotropic parameter , in the framework of normal mode asymptotic coupling theory (NACT). The resulting 2-D broad-band sensitivity kernels allow us to exploit the information contained in long period seismograms for fundamental and higher mode surface waves at the same time. To ensure high quality of the retrieved regional upper-mantle structure, accurate crustal corrections are essential. Here, we follow an approach which goes beyond the linear perturbation approximation and split the correction into a linear and non-linear part. The inverted data set consists of more than 40000 high quality three component fundamental and overtone surface waveforms, recorded at broad-band seismic stations in North America from teleseismic events and provides a fairly homogeneous path and azimuthal coverage. The isotropic part of our tomographic model shares the large-scale features of previous regional studies for North America. We confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central/eastern stable shield, as well as the presence of subducted material (Juan de Fuca and Farallon Plate) at transition zone depths. The new regional 3-D radial anisotropic model indicates the presence of two distinct anisotropic layers beneath the cratonic part of the North American continent: a deep asthenospheric layer, consistent with present day mantle flow, and a shallower lithospheric layer, possibly a record of ancient tectonic events.

  7. Probing the oxidation state of iron in the deep mantle using high P,T Mössbauer spectroscopy

    NASA Astrophysics Data System (ADS)

    McCammon, C. A.; Kupenko, I.; Sinmyo, R.; Cerantola, V.; Potapkin, V.; Chumakov, A. I.; Kantor, A.; Rüffer, R.; Dubrovinsky, L. S.

    2014-12-01

    The bulk of the Earth's interior is not directly accessible, yet redox processes occurring deep within drive many of the events observed at its surface. Laboratory studies of the relevant minerals at the appropriate pressure and temperature conditions have been pivotal in advancing our understanding of the Earth's interior. Iron plays an important role because it is the only major element with multiple electronic configurations (oxidation and spin state); however most of our knowledge regarding the oxidation state of iron in deep mantle phases is based on measurements of samples quenched from high temperature and sometimes also from high pressure. Mössbauer spectroscopy has been a key player in such studies and results on quenched samples have shown that the oxidation state of iron varies considerably through the mantle, from predominantly ferrous iron in upper mantle and transition zone phases to roughly 50% ferric iron in silicate perovskite (bridgmanite) due to the strong affinity of ferric iron for that phase in the presence of trivalent aluminium. Mössbauer measurements at pressures and temperatures along the geotherm are impractical using conventional radioactive sources due to their low brightness and the extremely limited possibilities for focusing in a laboratory setting. To address these limitations, we have developed an energy domain Synchrotron Mössbauer Source (SMS) on beamline ID18 at the European Synchrotron Radiation Facility that enables rapid collection of high quality energy domain Mössbauer spectra, and coupled with a portable double-sided laser heating system, SMS spectra can be collected on iron-containing mantle phases at pressures and temperatures up to those near the Earth's core in only a few hours (or less). The presentation will showcase recent results documenting in situ determination of iron oxidation state in lower mantle phases and discuss their implications for redox sensitive processes taking place throughout the Earth

  8. Refining Upper Mantle Structure in the North American continent using Spectral Element method

    NASA Astrophysics Data System (ADS)

    Romanowicz, B. A.; Yuan, H.; Cupillard, P.

    2011-12-01

    /Appalachian orogens down to 100 km, indicating persistent influence of orogeny boundaries deep in the lithosphere. In contrast, the northwestern Atlantic plate shows large positive velocity perturbations, reflecting its age of over 150Ma. At 100 km depth, the neighboring oceans show persistent Vsh>Vsv indicating well aligned horizontal shear in the oceanic lithosphere and asthenosphere. The continent, on the other hand, is characterized at this depth by Vsvupper mantle vertical motions.

  9. Osmium Isotopic Compositions of Chondrites and Earth's Primitive Upper Mantle: Constraints on the Late Veneer

    NASA Technical Reports Server (NTRS)

    Walker, R. J.; Horan, M. F.; Morgan, J. W.; Meisel, T.

    2001-01-01

    The 187 Os/188 Os of carbonaceous chondrites averages approximately 2% lower than for enstatite and ordinary chondrites. The primitive upper mantle ratio for the Earth best matches that of ordinary and enstatite chondrites. Additional information is contained in the original extended abstract.

  10. Crust and Upper Mantle of North Africa Using Libyan Seismic Data

    NASA Astrophysics Data System (ADS)

    Pasyanos, M. E.; Eshwehdi, A.

    2005-12-01

    We investigate the crust and upper mantle structure of North Africa using Libyan seismic data. Libya sits at the transition between the relatively aseismic continental crust of the African plate and the seismically active oceanic crust under the Mediterranean Sea which is subducting under the Eurasian Plate along the Calabrian, Hellenic, and Cyprean Arcs. The country also encompasses the Sirte Basin to the north and the smaller Murzuk and Kufra basins in the south. Broadband data from several seismic stations in Libya provide an opportunity for studying the velocity structure of the region. We have made some preliminary dispersion measurements from these stations and have found notable improvements in the group velocity tomography model by incorporating the additional measurements. We will be adding to this analysis by making dispersion measurements from regional events and receiver functions for teleseismic events. Recently, we have been employing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, as well as more detail about the upper mantle (lithospheric thickness and presence of anisotropy). We propose to apply this method to data from several Libyan stations where we can and, in the absence of receiver functions, invert the dispersion data only. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and how it fits into the larger tectonic picture of North Africa.

  11. Olivine-mica pyroxenite xenoliths from northern Tanzania: metasomatic products of upper-mantle peridotite

    NASA Astrophysics Data System (ADS)

    Dawson, J. B.; Smith, J. V.

    1992-04-01

    Olivine-mica-pyroxene blocks in Neogene pyroclastics from Oldoinyo Lengai and Loluni, Tanzania, result from K, Ca, Fe, Ti, Al, REE, Cl, F and OH metasomatism of upper-mantle peridotite. Deformed olivine relicts and high Cr and Ni in bulk-rock analyses indicate a peridotite precursor.

  12. Carbon Dioxide Carbonates in the Earth;s Mantle: Implications to the Deep Carbon Cycle

    SciTech Connect

    Yoo, Choong-Shik; Sengupta, Amartya; Kim, Minseob

    2012-05-22

    An increase in the ionic character in C-O bonds at high pressures and temperatures is shown by the chemical/phase transformation diagram of CO{sub 2}. The presence of carbonate carbon dioxide (i-CO{sub 2}) near the Earth's core-mantle boundary condition provides insights into both the deep carbon cycle and the transport of atmospheric CO{sub 2} to anhydrous silicates in the mantle and iron core.

  13. Inference of upper-mantle density structure from seismic velocities

    NASA Astrophysics Data System (ADS)

    Nettles, M.; Dziewonski, A. M.

    2005-12-01

    The inverse problem for the determination of density structure from perturbations in the gravity field is highly nonunique. The combination of gravity data and other observables can, however, be used to make inferences about the Earth's density structure in three dimensions. We use the three-dimensional shear-wave velocity model of Nettles and Dziewonski (2005) to make a forward prediction of the Earth's gravity field using simple assumptions about the relationship between perturbations in shear velocity and density. A scaling factor f=0.25 relating perturbations in shear velocity and density (δ/ρρ = f · δvS/vS) is determined empirically by comparison of observed variations in shear velocity in oceanic regions with density variations predicted from a simple model of conductive cooling. This value agrees well with f=0.27 based on the laboratory results of Jackson et al. (1992). The observed gravity signal in the oceans is explained well by this simple thermal-scaling approach. Behavior in some continental regions, such as the Basin and Range and the East African rift zone, is found to be similar to that in the oceans: the high topography in these regions appears to be supported by hot, low-density mantle underneath, a result also found by Kaban and Mooney (2001) for the Basin and Range. A velocity-to-density scaling relationship based only on thermal considerations is clearly inadequate in regions of continental craton, where such scaling leads to unrealistically large perturbations in the predicted gravity field. This result suggests that non-thermal effects must counteract the high density that would occur due to thermal effects alone, consistent with the suggestion of Jordan (1975) and other workers that density increases due to cool temperatures in the continental roots must be balanced by density decreases due to compositional variations. Using the compositional derivatives for density and shear velocity with respect to Mg# determined by Lee (2003), and an

  14. Receiver function probing of the crust and the upper mantle underneath north German basin, Scandinavia and the Baltic shield: evidence of ancient collision zones

    NASA Astrophysics Data System (ADS)

    Alinaghi, A.; Kind, R.; Bock, G.

    2002-05-01

    Teleseismic earthquakes (distance: 30-90 degrees)recorded by TOR (passive-source seismic experiment, north Germany and Scandinavia, 1996-1997), GRSN (German Regional Seismic Network, 1992-1999), and SVEKALAPKO (seismic experiment, Finland, 1998-1999) have been used for calculating receiver functions. Source and epicentral distance equalized receiver functions were utilized to probe into the crust and upper mantle beneath the recording stations. Detailed images of the crust-mantle boundary across the studied regions show a pronounced increase of the crustal thickness at the junction between northern Europe Paleozoic and Precambrian provinces known as TESZ( TransEuropean Suture Zone). This is accompanied by some north dipping intracrustal structures which run deep into the lower crust and are believed to be remnants of an ancient subduction zone. Another significance of the TESZ is its effect on the two major upper mantle discontinuities and thereby on the upper mantle transition zone. The deepening of the moho is associated with early arrival of 410 km discontinuity compared to predicted time by IASP91 velocity model, while the 660 km discontinuity has been affected to a lesser degree. The crust, underneath the Baltic shield, is thicker (on average 45-50 km) than beneath the north German Basin( 30-35 km on average ), and the moho shows a complex topography in sharp contrast to rather flat surface topography. Beneath SVEKALAPKO, both in stacked time domain and migrated receiver function sections the 410 km and 660 km upper mantle discontinuities arrive earlier than predicted by IASP91 velocity model. This is an indication of higher shear velocity of upper mantle in the Baltic shield compared to that of the Paleozoic northern Europe. We found no sign of the lithosphere-asthenosphere in our study which might be explained by the masking effect of the moho multiples and/or lack of strong Ps conversions due to gradual lithosphere-asthenosphere boundary

  15. OH solubility in olivine in the peridotite-COH system under reducing conditions and implications for water storage and hydrous melting in the reducing upper mantle

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi

    2015-12-01

    at ˜100 and 210 km depth, respectively, and is even larger at greater depths. These values exceed the typical H2O abundance (˜ 100 ± 50 ppm) in the upper mantle, suggesting that pervasive hydrous melting at reducing depths of the oceanic upper mantle is not likely. Similar arguments may also be casted for the reducing deep upper mantle in the continental regions.

  16. Os Isotope Heterogeneity of the Convecting Upper Mantle: The Mayari-Baracoa Ophiolitic Belt (Eastern Cuba)

    NASA Astrophysics Data System (ADS)

    Frei, R.; Gervilla, F.; Meibom, A.; Proenza, J. A.

    2005-12-01

    Chromite separates from a set of historically important chromite deposits from the 90 Ma old Mayarí-Baracoa Ophiolitic Belt in eastern Cuba were inspected for Re-Os isotopic systematics in an attempt to quantify the extent of Os isotopic heterogeneities within a restricted upper mantle portion represented by a single ophiolite. Compositional variations of chromites indicate their crystallization from hydrous melts varying in composition from back-arc basin basalts (Al-rich chromites; Cr# = 0.43-0.55; low Pd/Ir) to boninites (Cr-rich chromites; Cr# = 0.60-0.83; high Pd/Ir) in a supra-subduction zone setting. Initial Os isotopic compositions of the studied chromites can be grouped according to their distribution in 3 regional districts. Results indicate systematically negative calculated initial γOs values varying from -1.06 ± 0.79 (Moa-Baracoa district), -1.77 ± 0.80 (Sagua de Tanamo district) and -2.79 ± 0.31 (Mayari district). These suprachondritic values are distinctly (3.5-5.2%) less radiogenic than the estimated minimum 187Os/188Os composition of the primitive upper mantle of 0.1296 ± 8 and can be explained by Re depletion during ancient partial melting and melt percolation events. Old Os isotope model ages (<2100 Ma)of some of the chromites (or platinum-group minerals included in them) show and confirm previous findings that ancient Os isotopic signatures can survive in the Earth's upper mantle. Our systematically negative initial γOs values do not improve the definition of an already statistically poorly defined present-day Os isotopic composition of the convecting upper mantle, but instead indicate a complex history for the convecting upper mantle which precludes the calculation of a uniform regional Os isotopic signature for this reservoir.

  17. Global Upper Mantle Radially Anisotropic Model Developed Using the Spectral Element Method

    NASA Astrophysics Data System (ADS)

    Lekic, V.; Romanowicz, B.

    2008-12-01

    Improving the resolution of global upper mantle tomographic models of shear wavespeed and anisotropy is crucial for understanding the nature and morphology of upper mantle heterogeneities. Traditional methods of global tomography that rely on infinite-frequency and first-order perturbation theory become increasingly inadequate as shorter-wavelength heterogeneities are investigated. The spectral element method, on the other hand, permits accurate calculation of wave propagation through highly heterogeneous structures, and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the earth such as the upper mantle (cSEM: Capdeville et al., 2003). Importantly, cSEM allows a dramatic improvement in accounting for the effects of crustal structure. We have implemented a new method for global tomography, which uses cSEM for forward modeling in conjunction with approximate 2D finite frequency kernels for the inversion step, calculated using non-linear asymptotic coupling theory (NACT: Li and Romanowicz, 1995). In order to avoid biasing our results toward existing 3D upper mantle models, we start our iterative inversion procedure with a 1D model. We verify that the use of approximate kernels does not prevent our iterative procedure from converging. With each iteration, we include additional waveforms that would be rejected based on a comparison with the 1D starting model. We obtain the first global model of upper mantle velocity and radial anisotropy developed by applying the SEM to modeling 3-component long- period (corner frequency : 80s) fundamental- and higher-mode waveforms. Our model confirms the large- scale features observed by previous researchers. In particular, we retrieve the relatively shallow, seismically slow velocities beneath volcanic arcs and mid-ocean ridges, the deeper fast roots underlying cratons, slow velocities in the central Pacific below 250km depth, and enhanced fast velocities anomalies

  18. Upper mantle can in-situ fracture: an implication from a cataclastic peridotite xenolith from Megata, Northeast Japan arc

    NASA Astrophysics Data System (ADS)

    Takeuchi, Miyuki; Arai, Shoji

    2015-04-01

    A peculiar peridotite xenolith with cataclastic texture was found from Ichinomegata crater, Megata volcano, the Northeast Japan arc. This peridotite xenolith is the same in mineral assemblage and mineral chemistry (olivine, Fo90; spinel, Cr/(Cr + Al) atomic ratio, 0.2) to some fertile mantle lherzolites, but quite different in texture from all the documented mantle peridotite xenoliths from Ichinomegata and other localities on Earth. The peridotite is a mixture of coarse and fine grains of olivine, orthopyroxene, clinopyroxene, chromian spinel. The coarse mineral grains are angular but never elongated like those in mylonite. All minerals have been fragmented, and the fragmented pyroxenes and chromian spinel form thin streaks in fine-grained olivine-rich matrix. These features indicate that brittle fracturing does occur even in the upper mantle possibly along pre-existing deep-seated faults that have been frequent in the Northeast Japan arc. Some of the fine grains (<100 μm across) of olivine and pyroxenes display strong intra-grain and inter-grain chemical variations; some are more refractory and the others are more evolved in chemistry than the coarse grains. This suggests a possibility of very small degree frictional melting of peridotite upon cataclastic fracturing.

  19. Fault-controlled hydration of the upper mantle during continental rifting

    NASA Astrophysics Data System (ADS)

    Bayrakci, G.; Minshull, T. A.; Sawyer, D. S.; Reston, T. J.; Klaeschen, D.; Papenberg, C.; Ranero, C.; Bull, J. M.; Davy, R. G.; Shillington, D. J.; Perez-Gussinye, M.; Morgan, J. K.

    2016-05-01

    Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world’s rifted margins. Serpentinite is also inferred to exist in the downgoing plate at subduction zones, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

  20. P wave radial anisotropy tomography of the upper mantle beneath the North China Craton

    NASA Astrophysics Data System (ADS)

    Wang, Jian; Wu, Huohua; Zhao, Dapeng

    2014-06-01

    present the first P wave radial anisotropy tomography of the crust and upper mantle beneath the North China Craton (NCC), determined using a large number of high-quality arrival-time data of local earthquakes and teleseismic events. Our results show a prominent high-velocity (high-V) anomaly down to ˜250 km depth beneath the Ordos block, a high-V anomaly in the mantle transition zone beneath the eastern NCC, and a low-velocity (low-V) anomaly down to ˜300 km depth beneath the Trans-North China Orogen (TNCO). The Ordos block exhibits significant negative radial anisotropy (i.e., vertical Vp > horizontal Vp), suggesting that its cratonic lithosphere has kept the frozen-in anisotropy formed by vertical growth via high-degree melting mantle plume in the early Earth. Prominent low-V anomalies with positive radial anisotropy (i.e., horizontal Vp > vertical Vp) exist beneath the Qilian and Qaidam blocks down to ˜400 km depth, suggesting that the horizontal material flow resulting from the Tibetan Plateau is blocked by the Ordos thick lithosphere. Beneath the eastern NCC, high-V anomalies with negative radial anisotropy exist in the upper mantle, possibly reflecting sinking remains of the Archean cratonic lithosphere. A high-V anomaly with positive radial anisotropy is revealed in the mantle transition zone under the eastern NCC, which reflects the stagnant Pacific slab.

  1. Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle

    SciTech Connect

    Rapp, R.P.; Irifune, T.; Shimizu, N.; Nishiyama, N.; Norman, M.D.; Inoue, T.

    2008-10-08

    Isotopic and trace element geochemical studies of ocean island basalts (OIBs) have for many years been used to infer the presence of long-lived ({approx} 1-2 Ga old) compositional heterogeneities in the deep mantle related to recycling of crustal lithologies and marine and terrigenous sediments via subduction [e.g., Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annu. Rev. Earth Planet. Sci. 14, 493-571; Weaver, B.L., 1991. The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104, 381-397; Chauvel, C., Hofmann, A.W., Vidal, P., 1992. HIMU-EM: the French Polynesian connection. Earth Planet. Sci. Lett. 110, 99-119; Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219-229; Willbold, M., Stracke, A., 2006. Trace element composition of mantle end-members: Implications for recycling of oceanic and upper and lower continental crust. Geochem. Geophys. Geosyst. Q04004. 7, doi:10.1029/2005GC001005]. In particular, models for the EM-1 type ('enriched mantle') OIB reservoir have invoked the presence of subducted, continental-derived sediment to explain high {sup 87}Sr/{sup 86}Sr ratios, low {sup 143}Nd/{sup 144}Nd and {sup 206}Pb/{sup 204}Pb ratios, and extreme enrichments in incompatible elements observed in OIB lavas from, for example, the Pitcairn Island group in the South Pacific [Woodhead, J.D., McCulloch, M.T., 1989; Woodhead, J.D., Devey, C.W., 1993. Geochemistry of the Pitcairn seamounts, I: source character and temporal trends. Earth Planet. Sci. Lett. 116, 81-99; Eisele, J., Sharma, M., Galer, S.J.G., Blichert-Toft, J., Devey, C.W., Hofmann, A.W., 2002. The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Sci. Lett. 196, 197-212]. More recently, ultrapotassic, mantle-derived lavas (lamproites) from Gaussberg, Antarctica have been interpreted as the product

  2. Upper Mantle Anisotropy Beneath the Ordos Basin in China and its geodynamic significance

    NASA Astrophysics Data System (ADS)

    Wang, Liangshu; Mi, Ning; Huang, Zhouchuan; Xu, Mingjie

    2016-04-01

    The Ordos basin is a stable block between the Eastern and Western China, and surrounded by active thrust belts and extensional graben systems. Investigations on the upper-mantle deformation and flowing pattern beneath the Ordos basin will help to illuminate how the different geodynamical processes affect the intra-continental deformation in China. From five portable seismic arrays in the southern Ordos block, SKS and SKKS phases are used to estimate the S-wave splitting parameters. The results show distinct anisotropy in the upper mantle beneath the Ordos area. To the southwest of the Ordos, the orientations of anisotropy are NNW-SSE, which are subparallel to the thrust belt and boundary faults between the Ordos and the Northeast Tibetan Plateau, mapping a clockwise mantle flow induced by the eastward extrusion of the Northeast Tibetan Plateau and deflected by the Ordos block. To the south of the Ordos, mantle flow direction is nearly E-W, parallel to the strike-slip direction of the Weihe graben, indicating an eastward mantle flow from the NE Tibetan plateau to the eastern part of China. To the east of the Ordos, the direction of fast S-wave is changing slowly from NWW-SSE to E-W, perpendicular to the main tectonic direction in Shanxi graben system, showing an extension feature similar to that of the North China. Above results illuminate much information on the mass deformation and migration in the upper mantle resulting from the interactions between the Ordos block and its surrounding dynamic systems. It can be seen that the thrust faults and extensional grabens around the Ordos block are the positions where the anisotropy shows obvious change. As the boundary area of different blocks, they are the key areas to adjust the transformation between different geodynamic systems.

  3. Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

    NASA Astrophysics Data System (ADS)

    Heeszel, David S.; Wiens, Douglas A.; Anandakrishnan, Sridhar; Aster, Richard C.; Dalziel, Ian W. D.; Huerta, Audrey D.; Nyblade, Andrew A.; Wilson, Terry J.; Winberry, J. Paul

    2016-03-01

    The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

  4. Crust and upper mantle electrical structure of Haiyuan-Liupanshan Thrust Belt and its vicinity revealed by magnetotelluric(MT) detection

    NASA Astrophysics Data System (ADS)

    Han, S.; Liu, G.; Han, J.

    2015-12-01

    Under the auspices of SinoProbe Project, an array of 91 broad-band magnetotelluric(MT) sites across the southern segment of the Haiyuan-Liupanshan Thrust Belt (HLTB) was occupied to determine the crust and upper mantle structure of the transition zone between the Ordos Block (OB) and the Qilian Orogenic Belt (QOB).An electrical structure model of the crust and the upper mantle was finally obtained after data processing, qualitative analysis and 2D inversion of the observed data.The model revealed the deep structure of the profile.The upper crust of the HLTB is modelled as resistive while the other two tectonic units are modelled as less resistive.The massive high resistive blocks in the upper crust are seen in the HLTB.On the contrast,the lower crust is revealed as conduvtive on the whole.Middle to lower crustal high conductive layers (HCL) are seen both in the QOB and the OB.A strong lower crust conductor is revealed in the HLTB.Electrical structure of the upper mantle is revealed as resistive,respectively.The wedge structure is seen in the uppermost mantle under the Liupanshan Mountain.According to the electrical structure of the profile,the study region can be divided into three tectonic units:the QOB,the Liupan Transition Zone (LTZ,expansion of the HLTB) and the OB.The tectonic deformation for the QB manifest as thrust nappe in the upper crust and shortening strain in the lower crust.The east-dipping conductor in the west of QOB may represent the accumulated weak material in the form of middle crust flow and the HCL of the QB may be the migration channel.The fluctuation of HCL may indicate interior deformation of the OB.The LTZ is quite different from the adjacent tectonic units that fragment structure exists in the upper crust and deep thrust faults cut through the upper crust.The conductor located in the lower crust is interpreted as partial melt zone as a result of the vertical decompression process.Joint interpretation of the electrical structure and the

  5. Thermal-Mechanical Behavior of Oceanic Transform Faults- Implications for Hydration of the Upper Oceanic Mantle

    NASA Astrophysics Data System (ADS)

    Roland, E. C.; Behn, M. D.; Hirth, G.

    2007-12-01

    The presence of water at oceanic transform faults influences the thermal structure, rheology, and petrology of the upper mantle. Serpentinization at ridges and transforms plays an important role for the large-scale water budget of the mantle and eventual flux melting that is responsible for arc volcanism at convergent margins. The extent to which hydrous minerals (e.g., serpentine and talc) are incorporated into the upper mantle at oceanic transform faults is highly dependent on the thermal structure and stress state. Previous numerical modeling studies have suggested that the mantle beneath oceanic transform faults is anomalously cold, with depressed isotherms relative to a half-space cooling model [1,2,3]. However, recent models, that incorporate brittle rheology, show that transform faults may represent a region of enhanced mantle upwelling and elevated temperatures [4]. To investigate the thermal-mechanical behavior of oceanic transform faults, we utilize a 3D finite element model, assuming mantle convection, conduction, and steady-state incompressible mantle flow. Our model incorporates a non-linear viscous rheology with a visco-plastic approximation to simulate lithospheric brittle failure. The introduction of water into the lithosphere causes rheological changes with additional feedbacks on the thermal and rheologic structure such as enhanced conductive cooling and changes in frictional behavior. We incorporate the effects of these feedbacks, and our derived thermal structures are integrated with the estimated zone of permeable fluid flow to approximate the stability fields of hydrous phases in the upper mantle. Through examining a rage of parameters, including spreading rate, fault length, and the efficiency of hydrothermal circulation, we constrain the potential for transform faults to act as a source for mantle hydration, and estimate the amount of water that could be bound in hydrous phases as a result of brittle cracking at oceanic faults. 1. Furlong et

  6. Thermal-Mechanical Behavior of Oceanic Transform Faults- Implications for Hydration of the Upper Oceanic Mantle

    NASA Astrophysics Data System (ADS)

    Roland, E. C.; Behn, M. D.; Hirth, G.

    2004-12-01

    The presence of water at oceanic transform faults influences the thermal structure, rheology, and petrology of the upper mantle. Serpentinization at ridges and transforms plays an important role for the large-scale water budget of the mantle and eventual flux melting that is responsible for arc volcanism at convergent margins. The extent to which hydrous minerals (e.g., serpentine and talc) are incorporated into the upper mantle at oceanic transform faults is highly dependent on the thermal structure and stress state. Previous numerical modeling studies have suggested that the mantle beneath oceanic transform faults is anomalously cold, with depressed isotherms relative to a half-space cooling model [1,2,3]. However, recent models, that incorporate brittle rheology, show that transform faults may represent a region of enhanced mantle upwelling and elevated temperatures [4]. To investigate the thermal-mechanical behavior of oceanic transform faults, we utilize a 3D finite element model, assuming mantle convection, conduction, and steady-state incompressible mantle flow. Our model incorporates a non-linear viscous rheology with a visco-plastic approximation to simulate lithospheric brittle failure. The introduction of water into the lithosphere causes rheological changes with additional feedbacks on the thermal and rheologic structure such as enhanced conductive cooling and changes in frictional behavior. We incorporate the effects of these feedbacks, and our derived thermal structures are integrated with the estimated zone of permeable fluid flow to approximate the stability fields of hydrous phases in the upper mantle. Through examining a rage of parameters, including spreading rate, fault length, and the efficiency of hydrothermal circulation, we constrain the potential for transform faults to act as a source for mantle hydration, and estimate the amount of water that could be bound in hydrous phases as a result of brittle cracking at oceanic faults. 1. Furlong et

  7. New interpretation of the deep mantle structure beneath eastern China

    NASA Astrophysics Data System (ADS)

    Ma, Pengfei; Liu, Shaofeng; Lin, Chengfa; Yao, Xiang

    2016-04-01

    Recent study of high resolution seismic tomography presents a large mass of high velocity abnormality beneath eastern China near the phase change depth, expanding more than 1600km-wide in East-west cross-section across the North China plate. This structure high is generally believed to be the subducted slab of Pacific plate beneath the Eurasia continent, while its origin and dynamic effect on the Cenozoic tectonic evolution of eastern China remain to be controversial. We developed a subduction-driven geodynamic mantle convection model that honors a set of global plate reconstruction data since 230Ma to help understand the formation and evolution of mantle structure beneath eastern China. The assimilation of plate kinematics, continuous evolving plate margin, asymmetric subduction zone, and paleo seafloor age data enables the spatial and temporal consistency between the geologic data and the mantle convection model, and guarantees the conservation of the buoyancy flux across the lithosphere and subducted slabs. Our model achieved a first order approximation between predictions and the observed data. Interestingly, the model suggests that the slab material stagnated above discontinuity didn't form until 15Ma, much later than previous expected, and the fast abnormality in the mid-mantle further west in the tomographic image is interpreted to be the remnants of the Mesozoic Izanagi subduction. Moreover, detailed analysis suggests that the accelerated subduction of Philippine Sea plate beneath Eurasia plate along the Ryukyu Trench and Nankai Trough since 15Ma may largely contribute to extending feature above 670km discontinuity. The long distance expansion of the slab material in the East-west direction may be an illusion caused by the approximate spatial perpendicularity between the cross-section and the subduction direction of the Philippine Sea plate. Our model emphasizes the necessity of the re-examination on the geophysical observation and its tectonic and

  8. the P-wave upper mantle structure beneath an active spreading center: The Gulf of California

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1983-01-01

    Detailed analysis of short period travel time, and waveform data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km. More than 1400 digital seismograms from earthquakes in Mexico and central America recorded at SCARLET yield 1753 travel times and 58 direct measurements of short period travel time as well as high quality, stable waveforms. The 29 events combine to form a continuous record section from 9 deg to 40 deg with an average station spacing of less than 5 km. First the travel times are inverted. Further constraints arise from the observed relative amplitudes of mantle phases, which are modeled by trial and error.

  9. Evidence for a heterogeneous upper mantle in the cabo ortegal complex, Spain.

    PubMed

    Girardeau, J; Ibarguchi, J I; Jamaa, N B

    1989-09-15

    A well-preserved fragment of a heterogeneous upper mantle is present in the Cabo Ortegal Complex (Spain). This section is made of harzburgite containing a large volume of pyroxenite. The pyroxenite is concentrated in a layer 300 meters thick by 3 kilometers long. In this layer, ultramafic rocks, essentially pyroxenite (massive websterite and clinopyroxenite) and minor dunite, alternate without any rhythmicity. Part of this layering is of primary magmatic origin and possibly resulted from crystallization of magmas in dikes intruded into the host peridotite under mantle conditions. PMID:17747886

  10. Origin of a 'Southern Hemisphere' geochemical signature in the Arctic upper mantle.

    PubMed

    Goldstein, Steven L; Soffer, Gad; Langmuir, Charles H; Lehnert, Kerstin A; Graham, David W; Michael, Peter J

    2008-05-01

    The Gakkel ridge, which extends under the Arctic ice cap for approximately 1,800 km, is the slowest spreading ocean ridge on Earth. Its spreading created the Eurasian basin, which is isolated from the rest of the oceanic mantle by North America, Eurasia and the Lomonosov ridge. The Gakkel ridge thus provides unique opportunities to investigate the composition of the sub-Arctic mantle and mantle heterogeneity and melting at the lower limits of seafloor spreading. The first results of the 2001 Arctic Mid-Ocean Ridge Expedition (ref. 1) divided the Gakkel ridge into three tectonic segments, composed of robust western and eastern volcanic zones separated by a 'sparsely magmatic zone'. On the basis of Sr-Nd-Pb isotope ratios and trace elements in basalts from the spreading axis, we show that the sparsely magmatic zone contains an abrupt mantle compositional boundary. Basalts to the west of the boundary display affinities to the Southern Hemisphere 'Dupal' isotopic province, whereas those to the east-closest to the Eurasian continent and where the spreading rate is slowest-display affinities to 'Northern Hemisphere' ridges. The western zone is the only known spreading ridge outside the Southern Hemisphere that samples a significant upper-mantle region with Dupal-like characteristics. Although the cause of Dupal mantle has been long debated, we show that the source of this signature beneath the western Gakkel ridge was subcontinental lithospheric mantle that delaminated and became integrated into the convecting Arctic asthenosphere. This occurred as North Atlantic mantle propagated north into the Arctic during the separation of Svalbard and Greenland. PMID:18451860

  11. Mineralogy of the lower mantle: A review of 'super-deep' mineral inclusions in diamond

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix

    2012-01-01

    Starting from the late 1980s, several groups of lower-mantle mineral inclusions in diamond have been found. Three associations were established among them: juvenile ultramafic, analogous to eclogitic, and carbonatitic. The juvenile ultramafic association strongly predominates, and it is composed of ferropericlase, MgSi-, CaSi- and CaTi-perovskites, stishovite, tetragonal almandine-pyrope phase (TAPP), and some others. The association analogous to the upper-mantle eclogitic association, formed from subducting lithosphere, comprises: majorite, CaSi-perovskite bearing compositional Eu anomalies, phase 'Egg' with a tetragonal structure, and stishovite. The carbonatitic association is represented by various carbonates, halides, and associated minerals. Some mineral associations (wüstite + periclase and native iron + iron carbides) are, possibly, related to the D″ layer at the core/mantle boundary. The mineralogical composition of the lower mantle is now understood to be more complex than had been suggested in theoretic and experimental works. The proportion of ferropericlase in the lower mantle is higher than it was suggested before, and its composition is more iron-rich ( mg = 0.36-0.90) as compared to experimental and theoretical data. Free silica (stishovite) is always present in lower-mantle associations, and a separate aluminous phase (TAPP) has been identified in several areas. These discrepancies suggest that the composition of the lower mantle differs to that of the upper-mantle, and experiments based solely on 'pyrolitic' compositions are not, therefore, applicable to the lower mantle. These data indicate a probability of an alternative to the CI-chondrite model of the Earth's formation, for example, an enstatite-chondrite model.

  12. Teleseismic wave front anomalies at a Continental Rift: no mantle anomaly below the central Upper Rhine Graben

    NASA Astrophysics Data System (ADS)

    Kirschner, Stephanie; Ritter, Joachim; Wawerzinek, Britta

    2011-08-01

    The deep structure of the Upper Rhine Graben (URG), a continental rift in SW Germany and E France, is still poorly known. This deficit impedes a full understanding of the geodynamic evolution of this prominent rift. We study the lithosphere-asthenosphere structure using teleseismic waveforms obtained from the passive broad-band TIMO project across the central URG. The recovered, crust-corrected traveltime residuals relative to the iasp91 earth model are tiny (mostly less than 0.2-0.3 s). The average measured slowness (<1 s deg-1) and backazimuth (<5°) deviations are also very small and do not show any systematic wave front anomalies. These observed perturbation values are smaller than expected ones from synthetic 3-D ray tracing modelling with anomalies exceeding 2-3 per cent seismic velocity in the mantle. Thus there is no significant hint for any deep-seated anomaly such as a mantle cushion, etc. This result means that the rifting process did not leave behind a lower lithospheric signature, which could be clearly verified with high-resolution teleseismic experiments. The only significant traveltime perturbation at the central URG is located at its western side in the upper crust around a known geothermal anomaly. The upper crustal seismic anomaly with traveltime delays of 0.2-0.3 s cannot be explained with increased temperature alone. It is possibly related to a zone of highly altered granite. In the west of our network a traveltime anomaly (0.6-0.7 s delay) related with the Eifel plume is confirmed by the TIMO data set.

  13. Clockwise Rotation of Upper-Mantle Strain and Crust-Mantle Coupling Beneath the Eastern Syntaxis Tibet

    NASA Astrophysics Data System (ADS)

    Sol, S.; Meltzer, A.; Zurek, B.; Zeitler, P.; Zhang, X.; Zhang, J.

    2005-12-01

    a potential contribution from the crust. This argues for the presence of an effective crust-mantle coupling beneath the eastern syntaxis, in contrast with the presence of a low strength (weak) decoupling lower crust relative to the upper mantle that has been suggested by data from the central plateau and some geodynamic modelling of the whole orogen. Our results indicate that although the lithosphere in the syntaxis appears to deform internally, fault block rotation via strike-slip tectonics plays an important role in the southeastward extrusion of the plateau.

  14. High-Pressure Orthorhombic Ferromagnesite as a Potential Deep-Mantle Carbon Carrier

    PubMed Central

    Liu, Jin; Lin, Jung-Fu; Prakapenka, Vitali B.

    2015-01-01

    Knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth. PMID:25560542

  15. High-pressure orthorhombic ferromagnesite as a potential deep-mantle carbon carrier

    DOE PAGESBeta

    Liu, Jin; Lin, Jung -Fu; Prakapenka, Vitali B.

    2015-01-06

    In this study, knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions ofmore » approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.« less

  16. Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

    NASA Astrophysics Data System (ADS)

    Goncharov, A. G.; Ionov, D. A.

    2012-11-01

    Oxygen fugacity ( fO2) affects melting, metasomatism, speciation of C-O-H fluids and carbon-rich phases in the upper mantle. fO2 of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe3+/ΣFe ratios in garnet and spinel obtained by Mössbauer spectroscopy, major element data and P- T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, -0.1 vs. -2.5 Δlog fO2(FMQ)). For intermediate garnet-spinel peridotites, fO2 estimates from spinel-based oxybarometers are 1.5-3.2 Δlog fO2(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel-garnet reaction and concomitant changes in mineral chemistry do not keep up with P- T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter-meter scale. The spinel-based fO2 estimates may not be correct while garnet-based fO2 values provide conditions before the heating. The T (780-1,100 °C) and fO2 ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5-2.0 Δlog fO2(FMQ)) than cratonic garnet peridotites at similar depths, and the "water maximum" conditions (>80 % H2O) in the off-craton mantle exist in a more shallow and narrow depth range (60-85 km) than in cratonic roots (100-170 km). The base of the off-craton lithospheric mantle (≥90 km) at 2.5 GPa and 1,150 °C has fO2 of -3.0 ∆log fO2(FMQ), with dominant CH4 and H2O and minor H2 in the fluid. Melting near the base of off

  17. Origin of Ultra-Deep Diamonds: Chemical Interaction of Ca-CARBONATE and the Earth's Lower Mantle Minerals

    NASA Astrophysics Data System (ADS)

    Spivak, A. V.; Dubrovinsky, L. S.; Litvin, Yu. A.

    2012-04-01

    The main goal of the work is experimental study of physicochemical conditions of origin of ultra-deep diamonds in the substance of the Earth's lower mantle (LM) based on the experimental criterium of syngenesis of diamond and primary inclusions of LM mineral. Magnesiowustite (Mg,Fe)O, Mg-Fe perovskite (Mg,Fe)(Si,Al)O3 and Ca-perovskite CaSiO3 mainly present the LM substance and are frequently disclosed as primary inclusions in ultra-deep diamonds together with Ca-, (Ca, Mg, Fe)-, Na-Ca-carbonates. For the upper mantle conditions, the mantle-carbonatite conception of diamond genesis was developed based on the effects of congruent melting of carbonates and complete liquid miscibility of carbonate-silicate melts. Melting of Ca-carbonate and CaCO3 - (Mg,Fe)O, CaCO3 - (Mg,Fe)(Si,Al)O3 systems, stability of the melts and their decomposition were studied in static high pressure experiments at pressures of 16 to 55 GPa and temperatures of 1600 to 3900 K using diamond anvil cell technique with laser heating. It was determined that melting of Ca-carbonate is congruent at the PT-conditions of the lower mantle and characterized by an expanded field of liquid Ca-carbonate phase. We observed formation of graphite (below 16 GPa) and diamond (between 16 and 43 GPa) on decomposition of the CaCO3 melt at temperatures above 3400 K. At temperatures below 3400 K congruent melting of calcium carbonate was confirmed. Also it was shown that CaCO3 - (Mg,Fe)O - (Mg,Fe)(Si,Al)O3 system is capable to form diamonds together with Ca-carbonate, magnesiowustite and perovskite as syngenesis minerals at PT-conditions of the lower mantle. We observed formation diamond (between 40 and 55 GPa) on decomposition of the CaCO3 from CaCO3 - (Mg,Fe)(Si,Al)O3 melt at temperatures above 2000 K. The experimental data on phase relations at the melting and decomposition of CaCO3 and CaCO3-(Mg,Fe)O-(Mg,Fe)(Si,Al)O3 system as well as diamond crystallization are applied to the problem of formation of natural ultra-deep

  18. Geodynamic and seismic constraints on the thermochemical structure and dynamics of convection in the deep mantle.

    PubMed

    Forte, Alessandro M; Mitrovica, Jerry X; Espesset, Aude

    2002-11-15

    We revisit a recent study by Forte & Mitrovica in which global geophysical observables associated with mantle convection were inverted and the existence of a strong increase in viscosity near a depth of 2000 km was inferred. Employing mineral-physics data and theory we also showed that, although there are chemical anomalies in the lowermost mantle, they are unable to inhibit the dominant thermal buoyancy of the deep-mantle mega-plumes below Africa and the Pacific Ocean. New Monte Carlo simulations are employed to explore the impact of uncertainties in current mineral-physics constraints on inferences of deep-mantle thermochemical structure. To explore the impact of the high-viscosity peak at a depth of 2000 km on the evolution of lower-mantle structure, we carried out time-dependent convection simulations. The latter show that the stability and longevity of the dominant long-wavelength heterogeneity in the lowermost mantle are controlled by this viscosity peak. PMID:12460479

  19. Toward mineralogical interpretation of LLVSP: High-P,T elasticity of deep mantle materials

    NASA Astrophysics Data System (ADS)

    Tsuchiya, J.; Tsuchiya, T.

    2010-12-01

    Seismological studies have clarified that although most part of the lower mantle is fairly homogeneous, substantial heterogeneities exist at the bottom a few hundred km. They, in particular low-velocity anomalies observed beneath central-Pacific and Africa often called large low shear velocity provinces (LLSVP), attract great interest, since clarification of their nature is a key to understanding of chemical and dynamical properties of the Earth's mantle. Although they would be produced associated with temperature and/or compositional heterogeneities, details are still largely unknown. Elastic property of possible mantle constituents is one of the most important properties to clarify this issue. So many studies on the high-P,T elasticity of minerals have been performed to date. However, those are still limited for some major phases in the lowermost mantle condition, such as Mg-perovskite, post-perovskite, periclase, and Ca-perovskite. We therefore performed new ab initio simulations on the high-P,T elasticity of some other phases, which are expected not to be abundant in the average silicate mantle but to be substantial when considering differentiated materials. We will discuss possible compositional heterogeneity by constructing mineralogical models of the deep mantle based on the obtained elasticity. Research supported by JSPS Grant-in-Aid for Scientific Research Grants 20001005 and 21740379 and the Ehime Univ G-COE program "Deep Earth Mineralogy".

  20. Evidence for small-scale mantle convection in the upper mantle beneath the Baikal rift zone

    NASA Astrophysics Data System (ADS)

    Gao, Stephen S.; Liu, Kelly H.; Davis, Paul M.; Slack, Phillip D.; Zorin, Yuliy A.; Mordvinova, Valentina V.; Kozhevnikov, Vladimir M.

    2003-04-01

    Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis ("central high"). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis ("flank lows"). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small-scale mantle convection system that is associated with the rifting.

  1. African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

    PubMed

    King, S D; Ritsema, J

    2000-11-10

    Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere. PMID:11073447

  2. A new gravity model of the crust and upper mantle of Asia based on seismic data

    NASA Astrophysics Data System (ADS)

    Baranov, A. A.; Tikhotsky, S. A.

    2009-04-01

    Density structure of the crust and upper mantle provides us the information about tectonic processes and evolution of the lithosphere. One of the important problems of the gravity modelling is to distinguish the crustal gravity effect and gravity effect of the upper mantle. By removing the crustal gravity effect from the observed gravity anomalies we can obtain the residual anomalies that reflect the upper mantle inhomogeneities. A digital 3D density model of the Central and Southern Asia crust is constructed based on seismic reflection, refraction and receiver functions data as well as geological data. Corresponding gravity effect is calculated. At the first step we construct a new digital model of the Asia crust, which is based on local maps showing three main crustal layers and available seismic determinations. The crustal thickness reaches 70 km beneath the Tibet and only 5 to 6 km at the oceanized parts in the central and southern portions of the Red Sea median trough. By constraining crustal thickness and structure with seismic data and density values from the velocity distribution by means of the Nafe-Drake and Birch relationships, we computed density models for the crust and upper mantle. The complex model consists of four layers: upper, middle and lower crust and sediments and specified on a 1°x1° grid within (-10-55°N, 20 W-155° E.). The intensity of the gravity field and its regional pattern correlate closely with the topographic features of the region. Intense negative anomalies characterize central Asia (area of the plates collision), and positive anomalies are observed in Southeast Asia.

  3. Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

    NASA Astrophysics Data System (ADS)

    Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

    2016-04-01

    Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

  4. Inferred rheology and upper mantle conditions of western Nevada and southern California-northwest Mexico

    NASA Astrophysics Data System (ADS)

    Dickinson, Haylee L.

    Understanding the viscous strength (rheology) of the mantle is essential for understanding the dynamics and evolution of the Earth. Rheology affects many geologic processes such as mantle convection, the earthquake cycle, and plate tectonics. This study uses tectonic (postseismic) and non-tectonic (lake unloading) events that have induced differential stress changes within the crust and mantle, which in turn, create surface deformation. The viscoelastic relaxation is constrained using geodetic methods, such as GPS, InSAR, or measurements of shoreline rebound. We can use these observed surface displacements to constrain numerical models of the relaxation processes that can be used to infer a viscosity structure. These studies allow us to infer the mechanical nature of the lithosphere and asthenosphere using 3D finite element models. When we combine our inferred viscosity structure with calculations of conductive geothermal gradients and models of mantle melting, we can infer environmental conditions of the upper mantle like water content, mineralogy, and degree of melt. In our first study, we seek to reduce non-uniqueness issues that plague in situ rheology studies by simultaneously modeling the response of the crust and mantle for a single region of western Nevada to multiple processes constrained by multiple observational data sets. Western Nevada has experienced a series of Mw >6.5 earthquakes over the last ~150 years, from the 1872 Owen's Valley earthquake to the 1954 Dixie Valley event, as well as the loading/unloading of Pleistocene-aged Lake Lahontan. Our goal was to answer whether a single Newtonian viscosity structure can explain all of the geodetic constraints. We found a strong lower crust underlain by a relatively weak upper mantle can explain all observational constraints. We also infer the decreases in viscosity we observed are due to hydration possibly from the subduction of the Farallon slab and melt content. In the next study, we investigate the

  5. Upper-mantle tomography and dynamics beneath the North China Craton

    NASA Astrophysics Data System (ADS)

    Lei, Jianshe

    2012-06-01

    A high-resolution tomographic model of the upper mantle beneath the North China Craton (NCC) is determined using a large number of precisely hand-picked teleseismic P wave arrival times. The results are generally consistent with previous results but high-quality arrivals provide new insights into the dynamics beneath the NCC. Obviously north-south trending low-velocity (low-V) zones are revealed down to ˜300-400 km depth under the Shanxi rift and Tanlu fault zone, while a north-south trending high-velocity (high-V) zone representing the remainder of detached lithosphere is visible down to ˜200 km depth under the western portion of eastern NCC. High-V anomalies representing the detached lithosphere are detected at 200-400 km depth under central and eastern NCC. Under the Ordos block high-V anomalies are visible above ˜400 km depth, indicating intact lithosphere. Broad high-V anomalies representing the stagnant Pacific slab are imaged with a low-V anomaly from Datong volcano to the edge of Bohai Sea in the mantle transition zone beneath eastern and central NCC, suggesting that the Pacific slab has subducted to central NCC but with a gap. A continuously Y-shaped low-V structure is clearly imaged under Datong volcano and Bohai Sea from the lower mantle through this gap in the mantle transition zone to the upper mantle, indicating the existence of a lower mantle plume. These results suggest that in addition to the subduction of the Pacific plate, the plume has also played an important role in lithospheric destruction by thermal erosion of the asthenosphere and detachment of the lithosphere beneath the NCC.

  6. Upper mantle seismic velocity structure beneath the Kenya Rift and the Arabian Shield

    NASA Astrophysics Data System (ADS)

    Park, Yongcheol

    Upper mantle structure beneath the Kenya Rift and Arabian Shield has been investigated to advance our understanding of the origin of the Cenozoic hotspot tectonism found there. A new seismic tomographic model of the upper mantle beneath the Kenya Rift has been obtained by inverting teleseismic P-wave travel time residuals. The model shows a 0.5--1.5% low velocity anomaly below the Kenya Rift extending to about 150 km depth. Below ˜150 km depth, the anomaly broadens to the west toward the Tanzania Craton, suggesting a westward dip to the structure. The P- and S-wave velocity structure beneath the Arabian Shield has been investigated using travel-time tomography. Models for the seismic velocity structure of the upper mantle between 150 and 400 depths reveal a low velocity region (˜1.5% in the P model and ˜3% in the S model) trending NW-SE along the western side of the Arabian Shield and broadening to the northeast beneath the MMN volcanic line. The models have limited resolution above 150 km depth everywhere under the Shield, and in the middle part of the Shield the resolution is limited at all depths. Rayleigh wave phase velocity measurements have been inverted to image regions of the upper mantle under the Arabian Shield not well resolved by the body wave tomography. The shear wave velocity model obtained shows upper mantle structure above 200 km depth. A broad low velocity region in the lithospheric mantle (depths of ≤ ˜100 km) across the Shield is observed, and below ˜150 km depth a region of low shear velocity is imaged along the Red Sea coast and MMN volcanic line. A westward dipping low velocity zone beneath the Kenya Rift is consistent with an interpretation by Nyblade et al. [2000] suggesting that a plume head is located under the eastern margin of the Tanzania Craton, or alternatively a superplume rising from the lower mantle from the west and reaching the surface under Kenya [e.g., Debayle et al., 2001; Grand et al., 1997; Ritsema et al., 1999]. For

  7. Tectonic denudation of upper mantle along passive margins: a model based on drilling (ODP Leg 103) and diving (Galinaute cruise) results, western Galicia Margin, Spain

    SciTech Connect

    Boillot, G.; Winterer, E.L.; Recq, M.; Girardeau, J.; Kornprobst, J.; Loreau, J.P.; Malod, J.; Mougenot, D.

    1987-05-01

    During ODP Leg 103 (April-June 1985) and the Galinaute cruise (June-July 1986), serpentinized peridotite (clinopyroxene-spinel harzburgite) was recovered within the basement approximately at the boundary between the North Atlantic ocean crust to the west and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle-derived peridotite on the sea floor occurred at the end of the period of rifting, roughly 110 Ma. Ductile shear zones observed in the peridotite are consistent with movements along a deep, low-angle normal fault rooted within the upper mantle and dipping eastward beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, they use a nonuniform stretching model for the lithosphere, set up from Wernicke's model.

  8. Redox state and water content in the upper mantle: Linkages to the atmosphere, hydrosphere and continents

    NASA Astrophysics Data System (ADS)

    Li, Zhengxue

    Geochemical and petrologic tools were deployed to investigate the redox state and water content of the earth's upper mantle. Study results are discussed in the context of their linkages to the atmospheric oxygen level, hydrospheric water budget and lithospheric evolution of continents. Because the partitioning of V is redox-sensitive and otherwise similar to that of Sc which is not redox sensitive, the V/Sc ratios of basalts of different ages act as a natural recorder of the redox states of the upper mantle. Through a comparison between global mid-ocean ridge basalts and Archean basalts, the fO2 of the upper mantle was inferred to have changed by no more than 0.3 log units since Archean. Combined with results from a thermodynamic model simulating the redox reactions of volcanic gases, this observation argues against the idea that the increase in oxygen in the atmosphere ˜2.3 billion years ago was caused by redox transition in the upper mantle. Through a geochemical and petrologic study at the Feather River Ophiolite (in northern California), global water recycling rates at subduction zones were estimated based on reconstructed serpentinization depths for the oceanic lithospheric mantle. Within uncertainties, the estimated water recycling rates roughly match global volcanic dewatering rates, which suggest the hydrospheric water storage is current at steady-state. Based on water contents measured in mantle xenoliths from the Colorado Plateau and vicinity, the idea that the lithospheric mantle beneath the western North America was rehydrated by the dewatering of the flat-subducting Farallon slab is confirmed. As predicted by an updated flow law for olivine aggregates, hydration might have weakened the basal lithosphere beneath the Colorado Plateau and thus induced lithospheric thinning by ˜15 km as a result of basal erosion. Extrapolation of the flow law to thick, cratonic lithosphere further suggests lithospheric thinning of much larger extents can occur if enough

  9. Early differentiation and volatile accretion recorded in deep-mantle neon and xenon.

    PubMed

    Mukhopadhyay, Sujoy

    2012-06-01

    The isotopes (129)Xe, produced from the radioactive decay of extinct (129)I, and (136)Xe, produced from extinct (244)Pu and extant (238)U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon ((129)Xe/(130)Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir. However, the low (129)Xe/(130)Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and (20)Ne/(22)Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower (129)Xe/(130)Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because (129)I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth's mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation. PMID:22678288

  10. Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan

    SciTech Connect

    Ando, M.; Ishikawa, Y.; Yamazaki, F.

    1983-07-10

    Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three-component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan. Eighty nine high-quality seismograms were selected from a period of 3 years. The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30/sup 0/ to the earth's surface in order to avoid phase shifts in the shear wave train. To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane. The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy-corrected particle motion is compared with the focal mechanism for a cross-check of the observed anisotropy. Directions of the maximum axes are plotted on azimuth-incidence angle stereograms at each station. The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate locations between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0/sup 0/ to 30/sup 0/ from the north (north anisotropy) and the other in the south with it trending 90/sup 0/ to 120/sup 0/ (south anisotropy). The maximum time delays between the two shear waves along a vertical seismic ray is about 1 s for both the anisotropic regions. The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km. perhaps prevalent in west Honshu.

  11. Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift

    NASA Astrophysics Data System (ADS)

    Pollitz, Fred F.; Mooney, Walter D.

    2014-09-01

    Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben-Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (∼50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

  12. The whole elephant: a comprehensive study of seismic anisotropy in the upper mantle beneath Kamchatka

    NASA Astrophysics Data System (ADS)

    Levin, V.; Park, J.

    2003-04-01

    Seismic waves propagating through Earth's upper mantle commonly display evidence of anisotropic (i.e. direction-dependent) wavespeed. Common techniques for identifying and quantifying these properties all suffer from inherent non-uniqueness. In combination, however, different anisotropy-aware analysis tools make possible a description of how anisotropic properties are distributed in the volume of interest. We performed a set of studies of seismic anisotropy in Kamchatka, using core-refracted phases (SKS splitting), teleseismic P-to-S converted body-waves (receiver functions), mode-converted (quasi-Love) surface waves an shear wave birefringence from seismic events in the subducting Pacific plate. These four types of observations are sensitive to different aspects of anisotropic structure, in terms of the wavelength (from 100s of km for surface waves to sub-kilometer structures for receiver functions) and the sampling (from whole upper mantle for SKS phases to 10s of km for local S splitting). Futhermore, observations of shear-wave birefringence reflect path-integrated effects, while mode-converted phases identify strong gradients in anisotropic properties. Our studies allow us to identify regions of coherent fabric in the upper mantle beneath Kamchatka with some confidence. We see evidence for sub-slab trench-parallel flow of mantle material, and for a rapid reorientation of this flow at the northern edge of the Pacific plate. We see some evidence for trench-normal fabric above the slab, consistent with subduction-driven corner flow. However trench-normal fabric is not pervasive, especially near the end of the Kamchatka subduction zone. We also find ample evidence for strong fabric at the crust-mantle boundary beneath Kamchatka, possibly indicating mobility in the continental lithosphere.

  13. Towards the Next Generation Upper-Mantle 3D Anelastic Tomography

    NASA Astrophysics Data System (ADS)

    Karaoglu, H.; Romanowicz, B. A.

    2015-12-01

    In order to distinguish the thermal and compositional heterogeneities in the mantle, it is crucial to resolve the lateral variations not only in seismic velocities but also in intrinsic attenuation. Indeed, the high sensitivity of intrinsic attenuation to temperature and water content, governed by a form of Arrhenius equation, contrasts with the quasi-linear dependence of velocities on both temperature and major element composition. The major challenge in imaging attenuation lies in separating its effects on seismic waves from the elastic ones. The latter originate from the wave propagation in media with strong lateral elastic gradients causing (de)focusing and scattering. We have previously developed a 3D upper-mantle shear attenuation model based on time domain waveform inversion of long period (T > 60s) fundamental and overtone surface wave data (Gung & Romanowicz, 2004). However, at that time, resolution was limited to very long wavelength structure, because elastic models were still rather smooth, and the effects of focusing could only be estimated approximately, using asymptotic normal mode perturbation theory.With recent progress in constraining global mantle shear velocity from waveform tomography based on the Spectral Element Method (e.g. SEMUCB_WM1, French & Romanowicz, 2014), we are now in a position to develop an improved global 3D model of shear attenuation in the upper mantle. In doing so, we use a similar time domain waveform inversion approach, but (1) start with a higher resolution elastic model with better constraints on lateral elastic gradients and (2) jointly invert, in an iterative fashion, for shear attenuation and elastic parameters. Here, we present the results of synthetic tests that confirm our inversion strategy, as well as preliminary results towards the construction of the next generation upper-mantle anelastic model.

  14. A crust and upper mantle model of Eurasia and North Africa for Pn travel time calculation

    SciTech Connect

    Myers, S; Begnaud, M; Ballard, S; Pasyanos, M; Phillips, W S; Ramirez, A; Antolik, M; Hutchenson, K; Dwyer, J; Rowe, C; Wagner, G

    2009-03-19

    We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km{sup 2} to 994 km{sup 2}, and the number of ellipses with area less than 1000 km{sup 2}, which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%.

  15. Seismic evidence of on-going sublithosphere upper mantle convection for intra-plate volcanism in Northeast China

    NASA Astrophysics Data System (ADS)

    Guo, Zhen; Chen, Y. John; Ning, Jieyuan; Yang, Yingjie; Afonso, Juan Carlos; Tang, Youcai

    2016-01-01

    A 3-D crustal and upper mantle S-wave velocity model of NE China is constructed by inversion of phase velocity dispersion curves at 6-140 s periods from ambient noise tomography and two-plane surface wave tomography. The seismic data used in this study are collected from 120 China Earthquake Administration (CEA) permanent stations and 127 portable stations of NECESSArray. We observe strong low S-wave velocity beneath the Changbaishan volcano in the upper mantle to at least 200-km depth, which is interpreted as a mantle upwelling beneath the Changbaishan volcano that is consistent with the body wave tomographic image. The Songliao Basin is dominated by a high velocity extending to at least 200-km depth. Built upon the observed velocity anomalies, we propose a sub-lithosphere mantle convection model for NE China in which the upwelling of upper mantle materials from the mantle transition zone to the Changbaishan volcano could induce a local sub-lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the Songliao Basin corresponds to the downwelling limb of this convection cell. The downwelling beneath the Songliao Basin could also induce secondary local convection in the asthenosphere to the west, leading to local asthenospheric upwelling beneath the Abaga and Halaha volcanoes in the Xing'an-Mongolia Orogenic Belt.

  16. New constraints on upper mantle creep mechanism inferred from silicon grain-boundary diffusion rates

    NASA Astrophysics Data System (ADS)

    Fei, Hongzhan; Koizumi, Sanae; Sakamoto, Naoya; Hashiguchi, Minako; Yurimoto, Hisayoshi; Marquardt, Katharina; Miyajima, Nobuyoshi; Yamazaki, Daisuke; Katsura, Tomoo

    2016-01-01

    The creep in the Earth's interior is dominated either by diffusion creep which causes Newtonian mantle flow, or by dislocation creep which results in non-Newtonian mantle flow. Although previous deformation studies on olivine claimed a transition from dislocation creep to diffusion creep with depth in the upper mantle, they might misunderstand the creep rates due to experimental difficulties. Since creep in olivine is controlled by silicon diffusion, we measured the silicon grain-boundary diffusion coefficient in well-sintered iron-free olivine aggregates as a function of temperature, pressure, and water content, showing activation energy, activation volume, and water content exponent of 220 ± 30 kJ /mol, 4.0 ± 0.7 cm3 /mol, and 0.26 ± 0.07, respectively. Our results based on Si diffusion in forsterite predict that diffusion creep dominates at low pressures and low temperatures, whereas dislocation creep dominates under high pressure and high temperature conditions. Water has negligible effects on both diffusion and dislocation creep. There is a transition from diffusion creep in the shallow upper mantle to dislocation creep in deeper regions. This explains the seismic anisotropy increases at the Gutenberg discontinuity beneath oceans and at the mid-lithosphere discontinuity beneath continents.

  17. Elasticity of Diopside to 8 GPa and 1073K and Implications for the Upper Mantle

    SciTech Connect

    B Li; D Neuville

    2011-12-31

    Simultaneous measurements of elastic wave velocities and density have been conducted on diopside by a combined ultrasonic interferometry and X-ray diffraction methods at pressure and temperature conditions relevant to the Earth's upper mantle. The current study not only yielded the first direct measurement of the shear modulus at high pressures, but also enabled a simultaneous determination of the bulk and shear properties and their pressure and temperature derivatives from the measured density and velocities. Finite strain analysis of the experimental data results in K{sub S0} = 116.4(7) GPa, K'S0 = 4.9(1), ({partial_derivative}K{sub S}/{partial_derivative}T){sub P} = -0.012(1) GPa GPa, G'{sub 0} = 1.6(1) and ({partial_derivative}G/{partial_derivative}T){sub P} = -0.011(1) GPa/K. With these results and the same finite strain equations, the P and S wave velocities of diopsidic mantle clinopyroxene were calculated along a 1600 K adiabatic geotherm. In comparison with other mantle minerals, the seismic velocities of diopsidic clinopyroxene at upper mantle depths are 1-3% higher than those of orthopyroxene, 1-2% and 6-8% lower than those of olivine and majoritic garnet, respectively.

  18. Crustal and upper mantle structure of the northern and central Sierra Nevada.

    USGS Publications Warehouse

    Mavko, B.B.; Thompson, G.A.

    1983-01-01

    Teleseismic data were recorded within the Sierra Nevada to look for lateral variations in the upper mantle. The data were collected at both temporary and permanent stations, and P wave residuals were computed. After correcting the P residual data for crustal and topographic effects, there is still a variation of as much as 0.5-0.6 s from the N end of the Sierra Nevada to Mono Lake, located E of the central part of the range. In addition, there are significant variations in travel time patterns, depending on the azimuth of wave arrivals. Two simple modeling approaches have been used to infer the upper mantle velocity structure from the observed variations in travel time. -from Authors

  19. The Indian Ocean gravity low - Evidence for an isostatically uncompensated depression in the upper mantle

    NASA Technical Reports Server (NTRS)

    Ihnen, S. M.; Whitcomb, J. H.

    1983-01-01

    The broad gravity low in the equatorial Indian Ocean south of Sri Lanka is the largest and most striking feature in the gravitational field of the earth. The most negative long-wavelength free-air gravity anomalies are found there and the sea surface (geoid) lies more than 100 meters below the best fitting ellipsoid. A model of the lithosphere and upper mantle is proposed which accurately predicts the observed free-air gravity and geoid elevation. This model is consistent with bathymetry and sediment thickness data and suggests that the crust south of India currently floats as much as 600 meters lower than would be expected if the region were isostatically compensated. This residual depression of the crust is apparently confirmed by observations of ocean depth. An uncompensated depression is consistent with the presence of a mechanical wake left in the upper mantle behind India as it traveled toward Asia.

  20. Crust and Upper Mantle Structure in the Sarfartoq Kimberlite Province, West Greenland: A Receiver Function Study

    NASA Astrophysics Data System (ADS)

    Dahl-Jensen, T.; Voss, P.; Larsen, L. M.; Steensgaard, B. M.; Pinna, L. G. B.

    2014-12-01

    A marked change in crustal thickness is seen at the deformation boundary between undisturbed Archean core in the south and reworked Archean gneiss in the foreland of the Nagssugtoqidian orogen in West Greenland. In addition, intra-crustal boundaries can be tentativly interpreted. Interpretations on upper mantle structures are less clear. This is the first information on crust and upper mantle structure in the area, which is known for kimberlite, carbonatite and ultramafic lamprophyre occurrences, and diamond exploration. The data consists of two summer seasons of passive seismological data recorded on 5 broad-band seismological stations placed on an almost 200 km long profile crossing the deformation boundary. The stations were installed in the remote area with solar panels and batteries, and recorded two summer seasons. Between 7 and 28 events on the stations were used for the Receiver Function analysis.

  1. Crust and Upper Mantle Structure in the Sarfartoq Kimberlite Province, West Greenland: A Receiver Function Study

    NASA Astrophysics Data System (ADS)

    Dahl-Jensen, Trine; Voss, Peter H.; Møller Steensgaard, Bo; Pinna, Line G.

    2015-04-01

    A marked change in crustal thickness is seen at the deformation boundary between undisturbed Archean core in the south and reworked Archean gneiss in the foreland of the Nagssugtoqidian orogen in West Greenland. In addition, intra-crustal boundaries can be tentativly interpreted. Interpretations on upper mantle structures are less clear. This is the first information on crust and upper mantle structure in the area, which is known for kimberlite, carbonatite and ultramafic lamprophyre occurrences, and diamond exploration. The data consists of two summer seasons of passive seismological data recorded on 5 broad-band seismological stations placed on an almost 200 km long profile crossing the deformation boundary. The stations were installed in the remote area with solar panels and batteries, and recorded two summer seasons. Between 7 and 28 events on the stations were used for the Receiver Function analysis.

  2. Dislocation damping and anisotropic seismic wave attenuation in Earth's upper mantle.

    PubMed

    Farla, Robert J M; Jackson, Ian; Fitz Gerald, John D; Faul, Ulrich H; Zimmerman, Mark E

    2012-04-20

    Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle. PMID:22517856

  3. Correlation Length Scales of Isotopic Variations Along Mid-Ocean Ridges and Upper Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Graham, D. W.; Spera, F. J.

    2003-12-01

    How isotopic variations in basalts erupted at the Earth's surface are linked to convective mixing in the underlying mantle is a central problem in geodynamics. The objective of this study is to quantify the length scales of upper mantle heterogeneity through spatial statistical analysis of MORB. We define a characteristic length scale, the scale of segregation L, computed from the spatial self-correlations for 3He/4He, 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb in "zero age" lavas from mid-ocean ridges. Our working hypothesis is that small scale convection in the upper mantle controls dispersion of geochemical tracers. Differences in L between ocean basins may then be quantitatively related to unsteadiness in this convection, due to thickening of the lithosphere, plume impingement, or lateral temperature/compositional differences between continental and oceanic lithosphere induced by batholith formation. The correlation coefficient R, and the separation distance r, are calculated for every i,j pair of points. Ri,j is given by the product of the deviations in isotope composition from the population mean, normalized to the population variance, and R(r) is computed as an ensemble average. The total number of point pairs (N) for n sample locations is given by N=n(n-1)/2. For the global MORB data set (n=1265 and 735 for Sr and He, respectively), N exceeds 105 (799480 and 269745, respectively). A value of R(r) close to 1 indicates that an isotope ratio above (or below) the population average is likely to be associated with an above (or below) average value at a distance r away. A value of R(r) close to zero implies a random relationship, and a value close to -1 implies an anti-correlation. R(r) approaches unity at small r by definition, as points close together are from the same "clump" of mantle. The value of r at which R first goes to zero is denoted as r*. On a diagram of R(r) vs. r (the correlogram), the integral of R(r) from r=0 to r=r* is the scale of

  4. The influence of deep mantle heterogeneity on the rhythms and scales of surface topography evolution

    NASA Astrophysics Data System (ADS)

    Arnould, Maëlis; Coltice, Nicolas; Flament, Nicolas

    2016-04-01

    Earth's surface, the interface between external processes and internal dynamics (lithosphere motions and mantle convection), is continuously reorganised. A large part of Earth's topography is generated by mantle motions and lithospheric stresses [1], which impacts for instance the global sea-level, the dynamics of sedimentary basins and the geoid. Studying how surface topography evolves in both space and time thus not only provides information on the rhythms and scales of evolution of those processes, but would also be a tool for the study of the mantle motions and properties from which it originates [2]. In this study, we propose to characterise the spatial and temporal scales of evolution of surface topography in 2D spherical annulus numerical models of mantle convection developing a plate-like behaviour. We use the geodynamical code StagYY [3] to first determine a mantle convection regime generating a surface topography with Earth-like amplitudes and realistic mantle dynamics at first order (e.g. high Rayleigh number, reasonable lithosphere thickness, pseudo-plastic lithosphere rheology generating plate tectonics). We then use this convection regime to investigate how the presence of stable deep-rooted thermochemical heterogeneities influence the rhythms of evolution of surface topography. We analyse our results to identify how the timescales of evolution are connected with the lengthscales of topography, in light of the tectonic histories produced by the models. References: [1] M. Gurnis, Long-term controls of eustatic and epeirogenic motions by mantle convection, GSA Today, 2(7):141-157, 1992. [2] B.H. Hager, R.W. Clayton, M.A. Richards, R.P. Comer, and A.M. Dziewonski, Lower mantle heterogeneity, dynamic topography and the geoid, Nature, 313:541-545, 1985. [3] J.W. Hernlund and P.J. Tackley, Modeling mantle convection in the spherical annulus, Phys. Earth Planet. Interiors, 171(1):48-54, 2008.

  5. Seismic tomography of the upper mantle beneath the Bohemian Massif (central Europe)

    NASA Astrophysics Data System (ADS)

    Karousova, H.; Plomerova, J.; Vecsey, L.; Munzarova, H.

    2012-04-01

    We present a comprehensive test for teleseismic tomography of the upper mantle beneath the southern part of the Bohemian Massif (BM) based on data of passive experiments BOHEMA III and the northern part of the ALPASS (Mitterbauer et al., Tectonophysics 2011) as well as preliminary results. A new semi-automatic picker was applied for measuring P-wave arrival times from correlated extremes of waveforms recorded at 58 temporary seismic stations and 55 permanent observatories during 2005-2006. To calculate P-velocity perturbations, we selected 173 events from epicentral distances between 25° and 90°, and with magnitude higher than 4.5. Before the travel-time inversion itself, we analysed carefully relative P-wave residuals and cleaned the dataset of the travel-times from outliers and instabilities in timing for further processing. To eliminate leakage of crustal effects into the upper mantle velocity images, we corrected the observed travel-times for crustal structure according to 3D models of the BM and Eastern Alps crust (Karousova et al., Studia Geophys. Geod. 2012; Behm et al., GJI 2007). In order to optimize model parameterization, initial velocities and damping factors we perform different synthetic tests. Checkerboard and synthetic tests with artificial heterogeneities and shifted parameterization are calculated to explore sensitivity and resolution in individual nodes. Models with indistinctive velocity perturbations in the resolved parts tend to be more sensitive to ray geometry in the upper mantle and consequently could accentuate even insignificant heterogeneities. We show series of velocity perturbation images in three parts of the BM retrieved in three successive passive seismic experiments BOHEMA I-III. No distinct 'tube-like' low velocity heterogeneity, which could be interpreted as a small plume beneath the Eger Rift is imaged in tomography in western BM from the BOHEMA I data. Relatively small velocity perturbations exist in the upper mantle beneath

  6. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming

    USGS Publications Warehouse

    McCallum, M.E.; Eggler, D.H.

    1976-01-01

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) Iherzolite at a depth of 130 to 180 kilometers.

  7. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers. PMID:17831161

  8. Compositional effects on the density of volatile-bearing magmatic liquids in the upper mantle

    NASA Astrophysics Data System (ADS)

    Seifert, R.; Malfait, W.; Sanchez-Valle, C.; Petitgirard, S.; Mezouar, N.

    2013-12-01

    The density of silicate liquids is a key control on many magmatic processes, including magma chamber dynamics and volcanic eruptions, melt extraction from residual rocks during partial melting, fractional crystallization processes and crystal settling. However, the experimental data on the density and compressibility of silicate melts relevant for magmatic processes in the crust and upper mantle remain scarce until now, especially for volatile-bearing compositions. In this contribution we review recent experimental studies to determine the density of silicate liquids with magmatic composition at upper mantle conditions (up to 3.5 GPa and 2000 K). The investigated compositions include dry and hydrous granitic (4.5 and 7.7 wt% H2O), alkaline (phonolite, 4.3 wt% H2O) and andesitic (6 wt% H2O) melts. The experiments were performed using a panoramic Paris-Edinbugh and the density was determined from the X-ray absorption contrast between the samples and a diamond capsule used as sample container. The run products were analyzed by electron microprobe, infrared spectroscopy and SIMS to verify the chemical composition and volatile content of the samples. The results, combined with literature data on silicate melt density at atmospheric pressure, provides the first experimentally derived equations of state for magmatic liquids with a broad range of silica contents at crustal and upper mantle conditions. The resulting equations of state allow constraining pressure and compositional effects on the compressibility of dissolved water in silicate melts. The implications of these results for the ascent rate of slab melts in subduction zones, the dynamics of magma chambers and crystal-liquid buoyancy relations and crystal settling velocities in the upper mantle will be discussed.

  9. Upper Mantle Seismic Structure for NE Tibet From Multiscale Tomography Method

    NASA Astrophysics Data System (ADS)

    Guo, B.; Liu, Q.; Chen, J.

    2013-12-01

    In the real seismic experiments, the spatial sampling of rays inside the studied volume is basically nonuniform because of the unequispaced distribution of the seismic stations as well as the earthquake events. The conventional seismic tomography schemes adopt fixed size of cells or grid spacing while the actual resolution varies. As a result, either the phantom velocity anomalies may be aroused in regions that are poorly illuminated by the seismic rays, or the best detailed velocity model is unable to be extracted from those with fine ray coverage. We present an adaptive wavelet parameterization solution for three-dimensional traveltime seismic tomography problem and apply it to the study of the tectonics in the Northeast Tibet region. Different from the traditional parameterization schemes, we discretize the velocity model in terms of the Haar wavelets and the parameters are adjusted adaptively based on both the density and the azimuthal coverage of rays. Therefore, the fine grids are used in regions with the good data coverage, whereas the poorly resolved areas are represented by the coarse grids. Using the traveltime data recorded by the portable seismic array and the regional seismic network in the northeastern Tibet area, we investigate the P wave velocity structure of the crust and upper mantle. Our results show that the structure of the crust and upper mantle in the northeastern Tibet region manifests a strong laterally inhomogeneity, which appears not only in the adjacent areas between the different blocks, but also within each block. The velocity of the crust and upper mantle is highly different between the northeastern Tibet and the Ordos plateau. Of these two regions, the former possesses a low-velocity feature while the latter is referred to a high-velocity pattern. Between the northeastern Tibet and the Ordos plateau, there is a transition zone of about 200km wide, which is associated with an extremely complex velocity structure in crust and upper

  10. An upper mantle model for a western rim of the East European Craton

    NASA Astrophysics Data System (ADS)

    Dec, M.; Malinowski, M.; Nita, B.; Perchuc, E.

    2012-04-01

    The upper mantle structure is a subject of many seismological analysis but existent global models are often too general to depict regional variations. Our seismic model is a trial to construct a new reference model for the regional upper mantle structure in the western rim of the East European Craton. It is based on the P-wave traveltime analysis from seismograms recorded on Suwalki (SUW) seismic station belonging to the Polish Seismological Network. SUW station is situated in NE part of Poland on the East European Craton. The data from 249 natural seismic events were divided into four groups referring to the epicenters in the Western Mediterranean Sea region, Greece and Turkey region, Caucasus region and Mid-Atlantic Ridge region. Our analysis is based on the P-wave traveltimes observed up to 3000 km distance, which is sufficient to investigate upper mantle structure down to about 500 km. For each region, we established a single model which was fitted to all sections. 1D model was calculated for all regions except Jan Mayen region, for which we had to estimate 2D model because waves propagate through both oceanic and continental structure. However, the continental part of the Jan Mayen region model is similar to 1D model established for other regions. We also include data from TOR and SVEKALAPKO experiments to check the presence of the 300-km discontinuity. Our model of the upper mantle in the western rim of the East European Craton documents low velocity zone (LVZ), 300-km discontinuity and zone with the reduction of P-wave velocity above 410-km discontinuity. We attribute the existence of the 300-km discontinuity to the paleotectonic interaction between Laurentia, Baltica and Avalonia during the closure of the Tornquist Sea.

  11. An anomalous upper mantle unit beneath southern Norway revealed by P-wave travel time residuals.

    NASA Astrophysics Data System (ADS)

    Bondo, A.; Balling, N.; Jacobsen, B. H.; England, R. W.; Kind, R.; Bödvarsson, R.; Weidle, C.; Gregersen, S.; Voss, P.

    2009-04-01

    We investigate whether high topography in southern Norway is associated with an anomalous upper mantle and we identify the western boundary of thick shield lithosphere. Several studies describe crustal structure in southern Scandinavia, whereas high-resolution information on upper mantle structures is sparse. We present relative P-wave travel time residuals (P-residuals) and preliminary tomography from southern Norway, southern Sweden and northern Denmark. We analyze distant earthquakes registered by seismological stations in projects CENMOVE, CALAS, MAGNUS and SCANLIPS together with selected TOR stations, and permanent stations in southern Sweden, southern Norway and Denmark. Station means of P-residuals corrected for topography and contributions from the crust varies by up to about 1 s across the study area. We associate early arrivals to the east of the Sorgenfrei-Tornquist Zone (STZ) and east of the Oslo Graben with thick shield lithosphere. Late arrivals observed in the Norwegian-Danish Basin southwest of the STZ are consistent with thinned lithosphere related to the basin formation. In southern Norway west of the Oslo Graben area, late arrivals indicate reduced P-wave velocity in the upper mantle and perhaps some regional isostatic buoyancy from the upper mantle. However, arrivals are early in the northern part of southern Norway, still in areas of high topography. Thus, a clear spatial correlation with areas of high topography is not observed. We identify the western boundary of thick shield lithosphere by interpretation of station means of P-residuals, together with the azimuthal dependence of single P-residuals in southern Scandinavia. We find this boundary to follow the STZ from the southeast into the northern part of Jutland. From there it proceed northwards. In southern Norway the western boundary of thick shield lithosphere is found around the Oslo Graben, proceeding to the northwest approaching the Norwegian coast.

  12. Deep mantle structure and the postperovskite phase transition.

    PubMed

    Helmberger, D; Lay, T; Ni, S; Gurnis, M

    2005-11-29

    Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid-solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals. PMID:16217029

  13. The Upper Mantle Anisotropy around the Ordos Block in China from Shear Wave Splitting

    NASA Astrophysics Data System (ADS)

    Wang, L.; Mi, N.; Huang, Z.; Xu, M.; Li, H.; Yu, D.

    2010-12-01

    Affected by the India-Eurasia collision and subsequent intrusion of the India plate into the Eurasia plate during the Cenozoic, the western China manifests mainly as intense shortening and uplifting, while the eastern China shows widespread extension caused by the subduction of the western Pacific plate. The Ordos is a stable block lies between the Eastern China and Western China, which is surrounded by active thrust belts and extensional graben systems. Investigations on the uppermantle deformation and flowing pattern beneath the Ordos will help to illuminate how those two different geodynamical processes affect the intracontinental deformation in China. Based on the seismic data from five portable broadband seismic arrays deployed in the southern Ordos from 2004 to 2010, we implemented the technology of Silver and Chan [1991] to investigate the upper mantle anisotropy in this area. Well-recorded SKS and SKKS phases are used to estimate the shear wave splitting parameters. The calculation results show distinct anisotropy in the upper mantle beneath the Ordos area. The anisotropy in different tectonic units gives different characteristics. To the southwest of the Ordos, the orientations of anisotropy are NNW-SSE, which are subparallel to the thrust belt and boundary faults between the Ordos and the Northeast Tibetan Plateau, mapping a clockwise mantle flow induced by the eastward extrusion of the Northeast Tibetan Plateau and deflected by the stable Ordos block. To the south of the Ordos, mantle flow direction is nearly E-W, parallel to the strike-slip direction of the Weihe graben system, indicating an eastward mantle flow from the NE Tibetan plateau to the eastern part of China. To the east of the Ordos, the direction of fast S-wave is changing slowly from NWW-SSE to E-W, perpendicular to the main tectonic direction in Shanxi graben system, showing an extension feature similar to that of the North China. Above results illuminate much information on the mass

  14. A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow.

    PubMed

    Hassan, Rakib; Müller, R Dietmar; Gurnis, Michael; Williams, Simon E; Flament, Nicolas

    2016-05-12

    Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth's mantle. Seismic imaging reveals that these plumes can be of deep origin--probably rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific. PMID:27172048

  15. A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow

    NASA Astrophysics Data System (ADS)

    Hassan, Rakib; Müller, R. Dietmar; Gurnis, Michael; Williams, Simon E.; Flament, Nicolas

    2016-05-01

    Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth’s mantle. Seismic imaging reveals that these plumes can be of deep origin—probably rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian–Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian–Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian–Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.

  16. P wave velocity of Proterozoic upper mantle beneath central and southern Asia

    NASA Astrophysics Data System (ADS)

    Nyblade, Andrew A.; Vogfjord, Kristin S.; Langston, Charles A.

    1996-05-01

    P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source-receiver path of one event crosses central Africa and lies outside the African superswell while the source-receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL-Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL-Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s-1, velocities between 8.3 and 8.5 km s-1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s-1, velocities between 7.9 and 8.4 km s-1 at a depth of 130 km, and velocity gradients between -0.001 and 0.001 s-1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150-200 km, whereas for southern Africa they bottom at ˜100-150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not

  17. Mantle phase changes and deep-earthquake faulting in subducting lithosphere

    USGS Publications Warehouse

    Kirby, S.H.; Durham, W.B.; Stern, L.A.

    1991-01-01

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

  18. Mantle phase changes and deep-earthquake faulting in subducting lithosphere.

    PubMed

    Kirby, S H; Durham, W B; Stern, L A

    1991-04-12

    Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events. PMID:17769266

  19. Average Potential Temperature of the Upper Mantle and Excess Temperatures Beneath Regions of Active Upwelling

    NASA Astrophysics Data System (ADS)

    Putirka, K. D.

    2006-05-01

    The question as to whether any particular oceanic island is the result of a thermal mantle plume, is a question of whether volcanism is the result of passive upwelling, as at mid-ocean ridges, or active upwelling, driven by thermally buoyant material. When upwelling is passive, mantle temperatures reflect average or ambient upper mantle values. In contrast, sites of thermally driven active upwellings will have elevated (or excess) mantle temperatures, driven by some source of excess heat. Skeptics of the plume hypothesis suggest that the maximum temperatures at ocean islands are similar to maximum temperatures at mid-ocean ridges (Anderson, 2000; Green et al., 2001). Olivine-liquid thermometry, when applied to Hawaii, Iceland, and global MORB, belie this hypothesis. Olivine-liquid equilibria provide the most accurate means of estimating mantle temperatures, which are highly sensitive to the forsterite (Fo) contents of olivines, and the FeO content of coexisting liquids. Their application shows that mantle temperatures in the MORB source region are less than temperatures at both Hawaii and Iceland. The Siqueiros Transform may provide the most precise estimate of TpMORB because high MgO glass compositions there have been affected only by olivine fractionation, so primitive FeOliq is known; olivine thermometry yields TpSiqueiros = 1430 ±59°C. A global database of 22,000 MORB show that most MORB have slightly higher FeOliq than at Siqueiros, which translates to higher calculated mantle potential temperatures. If the values for Fomax (= 91.5) and KD (Fe-Mg)ol-liq (= 0.29) at Siqueiros apply globally, then upper mantle Tp is closer to 1485 ± 59°C. Averaging this global estimate with that recovered at Siqueiros yields TpMORB = 1458 ± 78°C, which is used to calculate plume excess temperatures, Te. The estimate for TpMORB defines the convective mantle geotherm, and is consistent with estimates from sea floor bathymetry and heat flow (Stein and Stein, 1992), and

  20. Deep depleted and shallow enriched mantle sources of Karoo CFBs: geochemical evidence from Antarctica

    NASA Astrophysics Data System (ADS)

    Luttinen, A. V.; Heinonen, J. S.

    2011-12-01

    The Karoo continental flood basalts and associated intrusive rocks are typified by a great diversity of geochemical types that are most readily identified based on incompatible element and isotopic characteristics. The principal mantle source in the Karoo province has been frequently ascribed to lithospheric mantle, possibly affected by previous subduction-related fluids and/or melts; only a few rock types show unambiguous affinity to significant asthenospheric source components. Most recently, uncontaminated dyke rocks with depleted mantle geochemical characteristics (e.g. initial epsilon Nd values of up to +8) and strong garnet signatures have been discovered in Vestfjella, in the Antarctic extension of the Karoo CFB province. Our geochemical modeling implicates that a rich variety of low-Ti and high-Ti daughter magma types, notably similar to Karoo CFB types in Vestfjella, can be generated by contamination of a single depleted mantle-derived parental magma type with lithospheric material: We generalize that the deep depleted mantle source of the Vestfjella dykes may also have been the principal source of the numerous low-Ti and high-Ti magma types that are found within or adjacent to the Kaapvaal-Grunehogna craton in the Karoo province. In contrast, the low-Ti magma types that lack a garnet signature and are found outside the craton were derived from shallow, possibly subduction-contaminated mantle sources.

  1. Some recent advances in understanding the mineralogy of Earth's deep mantle

    SciTech Connect

    Duffy, T S

    2008-12-09

    Understanding planetary structure and evolution requires a detailed knowledge of the properties of geological materials under the conditions of deep planetary interiors. Experiments under the extreme pressure-temperature conditions of the deep mantle are challenging, and many fundamental properties remain poorly constrained or are inferred only through uncertain extrapolations from lower pressure-temperature states. Nevertheless, the last several years have witnessed a number of new developments in this area, and a broad overview of the current understanding of the Earth's lower mantle is presented here. Some recent experimental and theoretical advances related to the lowermost mantle are highlighted. Measurements of the equation of state and deformation behaviour of (Mg,Fe)SiO{sub 3} in the CaIrO{sub 3}-type (post-perovskite) structure yield insights into the nature of the core-mantle boundary region. Theoretical studies of the behaviour of MgSiO3 liquids under high pressure-temperature conditions provide constraints on melt volumes, diffusivities and viscosities that are relevant to understanding both the early Earth (e.g. deep magma oceans) and seismic structure observed in the present Earth (e.g. ultra-low-velocity zones).

  2. Investigating Potential Causes for An Abrupt Change of Thermal State in Earth's Upper Mantle During the Great Oxygenation Event

    NASA Astrophysics Data System (ADS)

    Li, M.; McNamara, A. K.

    2014-12-01

    The oxygenic photosynthesis might have well evolved about 3 billion years ago, but there seems no great increase of atmospheric oxygen until the great oxygenation event (GOE) at about 2.4 Ga. One possibility for the suppressing of atmospheric oxygen level before the GOE is through consumption of oxygen by reduced volcanic gasses. The amount of atmospheric oxygen that could be consumed by volcanic gases depends on the absolute amount of volcanic gases as well as the redox state of the upper mantle. Evidence from the redox sensitive V/Sc ratio have shown that the redox state of the upper mantle have remained constant for the last 3.5 billion years (e.g., Li and Lee, 2004). If so, abrupt changes in thermal state of Earth's upper mantle could explain the rapid changes of degassing rate at the time of GOE. The Earth's lowermost mantle has been shown to be compositionally heterogeneous, which could be caused by the presence of dense, primordial material resulting from early differentiation processes. An important question is how do chemical heterogeneities in the lowermost mantle influence the secular cooling of the upper mantle. Here, we performed numerical calculations to explore the effects of themochemical convection on the thermal evolution of Earth's upper mantle. A large parameter space is explored, with varying Rayleigh number, viscosity, internal heating and density of chemical heterogeneities. We start with an initially hot mantle with a layer of dense material in the lowermost mantle. We found that when the mantle is hot, the dense material remains layered and covers the entire CMB, leading to low CMB heat flux. In this stage, the upper mantle cools down rapidly. However, as the mantle cools, the dense material is swept into discrete thermochemical piles by cold downwellings, leading to increasing CMB heat flux. The cooling rate of the mantle is temporarily reduced as this transition occurs. This occurs at a time consistent with the GOE event. Li, Z. X. A. and

  3. Towards Tidal Tomography: Using Earth's Body-Tide Signal to Constrain Deep-Mantle Density Structure

    NASA Astrophysics Data System (ADS)

    Lau, Harriet; Yang, Hsin-Ying; Davis, James; Mitrovica, Jerry; Tromp, Jeroen; Latychev, Konstantin

    2015-04-01

    Luni-solar forcings drive long wavelength deformation at timescales ranging from 8 hours to 18.6 years. We propose that globally distributed GPS estimates of this deformation within the semi-diurnal band provide a new and independent constraint on long-wavelength deep mantle structure. A particular target of "tidal tomography" is the buoyancy structure of LLSVPs, which constitute a large volumetric fraction of the mantle. Constraining this structure is the key to understanding the longevity of the LLSVPs, and indeed the evolution of the entire mantle and Earth system. To this end, we begin by reporting on the development of a new normal-mode theory, based on relatively recent advances in free oscillation seismology, which is capable of predicting semi-diurnal body tides on a laterally heterogeneous, rotating and anelastic Earth. We next present the results of a suite of benchmark tests involving comparisons with predictions based on both classical tidal Love number theory for 1-D Earth models and finite-volume simulations that incorporate 3-D elastic and density structure. We find that body tide deformation is most sensitive to long wavelength, deep mantle structure, and, in particularly, to shear wave velocity and density structure. When combined with results from seismological datasets, this sensitivity provides a powerful tool to investigate the buoyancy structure of the LLSVPs. For example, adopting a variety of seismic tomography models a priori, we perform an extensive parameter search to determine misfits between model predictions based on the new theory and GPS-derived estimates of the semi-diurnal body tide displacements. Preliminary results, focusing only on density structure, have indicated that the observations are best fit when the LLSVPs have a bulk density greater than average mantle, in broad agreement with previous inferences based upon seismic normal mode inversions. In follow-up work, we have mapped out trade-offs related to the adopted seismic

  4. Developing a Crustal and Upper Mantle Velocity Model for the Brazilian Northeast

    NASA Astrophysics Data System (ADS)

    Julia, J.; Nascimento, R.

    2013-05-01

    Development of 3D models for the earth's crust and upper mantle is important for accurately predicting travel times for regional phases and to improve seismic event location. The Brazilian Northeast is a tectonically active area within stable South America and displays one of the highest levels of seismicity in Brazil, with earthquake swarms containing events up to mb 5.2. Since 2011, seismic activity is routinely monitored through the Rede Sismográfica do Nordeste (RSisNE), a permanent network supported by the national oil company PETROBRAS and consisting of 15 broadband stations with an average spacing of ~200 km. Accurate event locations are required to correctly characterize and identify seismogenic areas in the region and assess seismic hazard. Yet, no 3D model of crustal thickness and crustal and upper mantle velocity variation exists. The first step in developing such models is to refine crustal thickness and depths to major seismic velocity boundaries in the crust and improve on seismic velocity estimates for the upper mantle and crustal layers. We present recent results in crustal and uppermost mantle structure in NE Brazil that will contribute to the development of a 3D model of velocity variation. Our approach has consisted of: (i) computing receiver functions to obtain point estimates of crustal thickness and Vp/Vs ratio and (ii) jointly inverting receiver functions and surface-wave dispersion velocities from an independent tomography study to obtain S-velocity profiles at each station. This approach has been used at all the broadband stations of the monitoring network plus 15 temporary, short-period stations that reduced the inter-station spacing to ~100 km. We expect our contributions will provide the basis to produce full 3D velocity models for the Brazilian Northeast and help determine accurate locations for seismic events in the region.

  5. Upper mantle structure of the Pacific and Philippine Sea plates revealed by seafloor seismic array observations

    NASA Astrophysics Data System (ADS)

    Isse, Takehi; Shiobara, Hajime; Suetsugu, Daisuke; Sugioka, Hiroko; Ito, Aki

    2016-04-01

    Seismic tomography studies have revealed the structure and dynamics of Earth's interior since the 1980s. However, the spatial resolution of the oceanic region is not good enough caused by sparse distribution of the seismic stations. The observations with broadband ocean-bottom seismographs (BBOBSs) since the 2000s enabled us to obtain seismic tomography models with higher spatial resolution. Our Japanese BBOBS group deployed more than 100 BBOBSs in the Pacific Ocean and obtained a high-resolution (300-500 km) three-dimensional shear wave velocity structure in the upper mantle beneath northwestern and south Pacific Ocean by using surface wave tomography technique. In the northwestern Pacific Ocean, where the Pacific plate subducts beneath the Philippine Sea plate, we found that the shear wave structure in the Philippine sea plate is well correlated with the seafloor age in the upper 120 km, three separate slow anomalies in the mantle wedge at depth shallower than 100 km beneath the Izu-Bonin-Mariana arc, which have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios, and that the Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate. In the South Pacific Ocean, where midplate hotspots are concentrated, we found that the localized slow anomalies are found near hotspots in the upper mantle, estimated thickness of the lithosphere is about 90 km in average and is thinned by ~20 km in the vicinity of hotspots, which may represent thermal erosion due to mantle plumes.

  6. Inversion of Multiple Traveltime Datasets for Crust and Upper Mantle Structure in Southeast Australia

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.

    2015-12-01

    The inversion of seismic traveltimes for 2-D or 3-D velocity structure still represents the most common form of seismic tomography in use today. Studies generally focus on a particular class of data, such as teleseismic arrival time residuals, local earthquake arrival times, refraction and wide-angle reflection traveltimes or the traveltimes of regional or global phases. Group or phase traveltimes extracted from dispersion analysis of surface waves can also be inverted for velocity structure. A much smaller number of studies attempt to combine multiple data types in a single inversion; this can be of benefit when the datasets provide overlapping coverage. For instance, local earthquake and teleseismic datasets are often complementary, because while they can both sample the crust and upper mantle, there are many situations in which the teleseisms offer poor constraint on crustal structure, and local earthquakes do not constrain upper mantle structure particularly well. A joint inversion can therefore provide good recovery throughout the crust and mantle lithosphere. In this study, an updated version of the FMTOMO package will be used to jointly invert refraction, wide-angle reflection, teleseismic and local earthquake traveltime datasets for crustal, Moho and upper mantle structure beneath southeast Australia. The main target region is Tasmania, which lies at the southeastern tip of the Australian continent and represents the southern-most expression of the Tasmanides, a large Paleozoic fold belt that abuts the Pre-Cambrian shield region of central and western Australia. Broadside refraction and wide-angle reflection coverage is provided by off-shore airguns recorded by on-shore stations, and several passive seismic arrays supply teleseismic and local earthquake data. The combined dataset is able to resolve the trade-off between velocity and interface structure in the neighbourhood of the Moho, which results in a detailed picture of the Tasmanian lithosphere.

  7. Shear Wave Structure in the Crust and Upper Mantle Beneath the Wyoming Craton

    NASA Astrophysics Data System (ADS)

    Dave, R.; Li, A.

    2013-12-01

    The Wyoming craton was pervasively deformed during the Laramide and has been encroached by the Yellowstone hotspot. It is therefore an ideal place to study the survival and evolution of cratonic lithosphere. We have analyzed Rayleigh wave data recorded by the USArray and applied the two-plane-wave tomography method to determine average and 2-D phase velocities in the Wyoming craton area. These phase velocities were inverted to construct 3-D shear wave structure of the crust and upper mantle. Vertical component seismograms from 82 events at 103 stations were filtered at 18 center frequencies with a narrow bandwidth of 10 mHz. The average phase velocity varies from 3.48 km/s at 20 s to 4.36 km/s at 166 s. Compared to the average global phase velocities, these values are lower at longer periods, indicating a generally slower upper mantle. 2-D variation of phase velocity shows significantly low velocities at the hotspot and relatively high velocities in the north of the stable craton. The same pattern of velocity variation is also imaged in the 3-D shear wave model. The lowest velocity tilted to north and east to the current hotspot location with depth, reflecting shearing of the hotspot material due to the relative motion of the North American plate. In addition, a low velocity anomaly is imaged at central-eastern portion of the craton at depths greater than 150 km, correlated with a thin crust. Relative high velocity anomaly is found in between this slow anomaly and the hotspot. This slow-fast-slow velocity variation in the upper mantle could be associated with small-scale mantle convection of upwelling-downwelling-upwelling trigged by the hotspot, which could thermally erode the cratonic lithosphere.

  8. Platinum-group element systematics and petrogenetic processing of the continental upper mantle: A review

    NASA Astrophysics Data System (ADS)

    Lorand, Jean-Pierre; Luguet, Ambre; Alard, Olivier

    2013-04-01

    The platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as «mantle metasomatism») precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of

  9. Thermal and compositional constrains on the upper mantle beneath the northwestern Pacific imposed by marine magnetotellurics

    NASA Astrophysics Data System (ADS)

    Baba, Kiyoshi; Tada, Noriko; Matsuno, Tetsuo; Shimizu, Hisayoshi; Zhang, Luolei; Liang, Pengfei; Utada, Hisashi

    2016-04-01

    Oceanic upper mantle beneath the northwestern Pacific has large-scale lateral heterogeneity that is impossible to attribute to just an age-dependency of the thermal structure based on a cooling of homogeneous mantle with age. This surprising fact was revealed from seafloor magnetotelluric (MT) data collected in three areas, northwest (Area A) and southeast (Area B) of the Shatsky Rise, and off the Bonin Trench (Area C), through the Normal Oceanic Mantle Project and the Stagnant Slab Project. One-dimensional structures of electrical conductivity representing each area show significant difference in the thickness of the upper resistive layer that may be interpreted as cool lithosphere. The thickness of the layer that is more resistive than 0.01 S m‑1 is ˜90 km for Area A, ˜100 km for Area B, and ˜180 km for Area C. The conductivity below the resistive layer is similar to ˜0.03 S m‑1 for all areas. The thermal structures for the lithospheric age representing the areas (130, 140, and 147 Ma for Areas A, B, and C, respectively) predicted from a simple plate cooling model are almost identical and thus cannot reproduce such variations in electrical conductivity. Then, in this study, thermal and compositional states of the mantle beneath the three areas were investigated to discuss the cause of the variations. Combination of five model parameters, electrical conductivity of crust, mantle potential temperature, thickness of thermally conductive plate, and H2O and CO2 contents in the asthenospheric mantle were searched by forward modeling and the χ2 misfit between the MT responses observed and predicted were assessed with 95% acceptable level. The possibility of partial melting was taken into account by a self-consistent manner comparing to the solidus of peridotite that is reduced by H2O and CO2. We assumed that the mantle conductivity may be represented by the mixture of hydrous olivine and hydrous carbonated melt. This procedure enables us to discuss how water

  10. Dynamics and Upper Mantle Structure Beneath the Northwestern Andes: Subduction Segments, Moho Depth, and Possible Relationships to Mantle Flow

    NASA Astrophysics Data System (ADS)

    Monsalve, G.; Yarce, J.; Becker, T. W.; Porritt, R. W.; Cardona, A.; Poveda, E.; Posada, G. A.

    2014-12-01

    The northwestern South American plate shows a complex tectonic setting whose causes and relationship to mantle structure are still debated. We combine different techniques to elucidate some of the links between slabs and surface deformation in Colombia. Crustal structure beneath the Northern Andes was inferred from receiver functions where we find thicknesses of nearly 60 km beneath the plateau of the Eastern Cordillera and underneath the southern volcanic area of the Central Cordillera. We infer that such crustal thickening resulted from shortening, magmatic addition, and accretion-subduction. Analyses of relative teleseismic travel time delays and estimates of residual surface topography based on our new crustal model suggest that there are at least two subduction segments underneath the area. The Caribbean slab lies at a low angle beneath northernmost Colombia and steepens beneath the Eastern Cordillera. Such steepening is indicated by negative travel time relative residuals in the area of the Bucaramanga Nest, implying a cold anomaly in the upper mantle, and by positive residual topography just off the east of this area, perhaps generated by slab-associated return flow. Results for the western Andes and the Pacific coastal plains are consistent with "normal" subduction of the Nazca plate: travel time relative residuals there are predominantly positive, and the residual topography shows an W-E gradient, going from positive at the Pacific coastline to negative at the Magdalena Valley, which separates the eastern cordillera from the rest of the Colombian Andean system. Azimuthal analysis of relative travel time residuals further suggests the presence of seismically slow materials beneath the central part of the Eastern Cordillera. Azimuthal anisotropy from SKS splitting in that region indicates that seismically fast orientations do not follow plate convergence, different from what we find for the western Colombian Andes and the Caribbean and Pacific coastal plains

  11. The Effect of Upper to Lower Mantle Viscosity Jump on the Regime Diagram of Slab Deformation in the Mantle Transition Zone

    NASA Astrophysics Data System (ADS)

    Davies, J. H.; Garel, F.; Goes, S. D. B.; Davies, R.; Kramer, S. C.; Wilson, C. R.

    2014-12-01

    Slabs display a wide range of morphologies in the mantle transition zone. This slab transition zone deformation is likely caused by a barrier arising from a jump of viscosity between upper and lower mantle, and/or from the endothermic phase transitions at 660-km depth. We use 2-D thermo-mechanical models of a two-plate subduction system, modeled with the finite-element, adaptive-mesh code Fluidity, to investigate the influence of the viscosity jump on slab morphologies. We implement a temperature- and stress-dependent rheology, and variable viscosity increases from upper to lower mantle of 10, 30 or 100 (no mineral phase transitions). Various end-member subduction modes arise, ranging from vertical folding to horizontally deflected to retreating and penetrating slabs. For each viscosity contrast between upper and lower mantle, we build a regime diagram for subduction dynamics based on the initial subducting and overriding plate ages. Trench motion is facilitated by smaller upper-lower-mantle viscosity contrasts, and, for all but the oldest subducting plate cases, simulations with a 100-fold viscosity increase exhibit a stationary trench later in their evolution. Slower sinking rates also lead to weaker (lower-viscosity) slabs encountering the viscosity jump. These effects, together with the increased resistance to penetration associated with a more viscous lower mantle, produce increasingly deformed and stalling slabs at depth, as the viscosity contrast increases. Slab deformation in the transition zone leads to an alternation between phases of penetration into the lower mantle and stagnation phases, reflected in subducting plate velocity. The periodicity and amplitude of such oscillations is directly controlled by the magnitude of the viscosity jump. Hence, our dynamic models help to interpret present-day observations of slab morphologies, along with the time-evolution of plate surface velocities, in terms of Earth rheology. The range of observed slab morphologies

  12. Experimental Deformation of Olivine Crystals at Mantle P and T: Evidences for a Pressure-Induced Slip Transition and Implications for Upper-Mantle Seismic Anisotropy and Low Viscosity Zone

    NASA Astrophysics Data System (ADS)

    Raterron, P.; Chen, J.; Geenen, T.; Girard, J.

    2009-04-01

    [100](001) and [001](100) systems together. Constant applied stress < 300 MPa and specimen strain rates were monitored in situ using time-resolved X-ray diffraction and radiography, respectively. Run products were investigated by transmission electron microscopy (TEM) in order to verify the actual activation of the tested dislocation slip systems. The obtained data were compared with rheological data previously obtained at comparable T and conditions, but at room P (Darot and Gueguen, 1981, JGR, 86, 6219; Bai et al., 1991, JGR, 96, 2441), resulting in creep power laws which quantify the effect of P on olivine rheology. The new data confirm the occurrence of a P -induced [100]/[001] slip transition, and suggest that [001](010) system dominates olivine deformation in the deep upper mantle. Extrapolation of the obtained rheological laws to natural condition along upper-mantle geotherms, shows that the [100] / [001] slip transition should occur in the Earth at ~ 200 km depth, thus can explain the attenuation of seismic anisotropy in the deep upper mantle. The obtained rheological laws were also integrated into a straightforward olivine aggregate model, then extrapolated to mantle condition using a 2-D geodynamic modeling application (Van den Berg et al., 1993, Geophys. J. International, 115, 62), which is the simplest approach to investigate upper-mantle steady-state deformation. In the application, the velocity of the lower boundary (the transition-zone boundary at 410-km depth) was set to 0, while that at the Earth's surface was set to 2 cm/year. Results from this modeling suggest that the combine activity of [100] and [001] slips in olivine aggregates may significantly decrease mantle viscosity below the oceanic lithosphere, thus, may contribute to the low viscosity zone (LVZ) required in plate tectonics to decouple rigid plates from the more ductile asthenophere underneath.

  13. Measurement of activation volume for creep of anhydrous olivine at upper mantle pressures

    NASA Astrophysics Data System (ADS)

    Dixon, N. A.; Durham, W. B.; Kohlstedt, D. L.; Mei, S.; Xu, L.

    2012-12-01

    Olivine is the most abundant and likely the weakest phase in the upper mantle, and thus its rheological properties have a critical role in controlling convective flow in this region. A persistent obstacle to understanding the behavior of olivine in the mantle has been the difficulty of determining activation volume (V*), the influence of hydrostatic pressure on high-temperature creep. The bulk of previous studies examining V* were conducted at low pressure (<300 MPa) and over small pressure ranges in gas-medium deformation machines, limiting precision and raising questions about application to relevant geological conditions. For this study, we conducted numerous deformation experiments on dry polycrystalline olivine in the D-DIA apparatus to pressures 1.5 to 9 GPa at 1373 K. Stress and strain were measured in-situ with synchrotron x rays. Refinement of diffraction technique has allowed stress resolution of ±0.02 GPa. For the pressure range in this study, we have measured an average activation volume of about 11-17 cm3/mol for dry polycrystalline San Carlos olivine. This is a substantial pressure effect, representing a pressure-induced viscosity increase seven orders of magnitude from the base of the lithosphere to the bottom of the upper mantle. The diffraction technique used for stress measurement in these experiments also illuminates the relative strength of differently oriented grains in our polycrystalline sample, providing new experimental evidence for preferred dislocation slip systems in olivine at high pressure.

  14. Measurement of activation volume for creep of dry olivine at upper mantle pressure

    NASA Astrophysics Data System (ADS)

    Dixon, N. A.; Durham, W. B.; Suzuki, A. M.; Mei, S.; Kohlstedt, D. L.; Hustoft, J. W.

    2011-12-01

    Olivine is the most abundant and weakest phase in the upper mantle, and thus its rheological properties have a critical role in controlling convective flow in this region. A resilient obstacle to understanding the behavior of olivine in the mantle has been the difficulty of determining activation volume (V*), the influence of hydrostatic pressure on flow strength. The bulk of previous studies examining V* were conducted at low pressure (<300 MPa) and small pressure ranges in gas-medium deformation apparatuses, limiting precision and raising questions about application to relevant geological conditions. For this study, we conducted deformation experiments on dry polycrystalline olivine in the D-DIA apparatus. The development of a new hybrid soft-fired pyrophyllite/mullite sample assembly allowed for a broadened pressure range (2-9 GPa), while stress and strain were measured in-situ with synchrotron x rays. Refinement in diffraction technique has allowed stress resolution of ±0.01 GPa. For the pressure range in this study, we have measured an average activation volume of about 17 cm^3/mol for dry polycrystalline San Carlos olivine. This is a substantial pressure effect, representing a pressure-induced viscosity increase of nearly 7 orders of magnitude from the base of the lithosphere to the bottom of the upper mantle.

  15. High-pressure orthorhombic ferromagnesite as a potential deep-mantle carbon carrier

    SciTech Connect

    Liu, Jin; Lin, Jung -Fu; Prakapenka, Vitali B.

    2015-01-06

    In this study, knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.

  16. Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

    USGS Publications Warehouse

    Mooney, W.D.; Gettings, M.E.; Blank, H.R.; Healy, J.H.

    1985-01-01

    The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea. Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives. The Mohorovic??ic?? discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth. The crustal and upper mantle velocity structure of the Arabian Shield is

  17. Fine scale heterogeneity in the Earth's upper mantle - observation and interpretation

    NASA Astrophysics Data System (ADS)

    Thybo, Hans

    2014-05-01

    High resolution seismic data has over the last decade provided significant evidence for pronounced fine scale heterogeneity in the Earth's mantle at an unprecedented detail. Seismic tomography developed tremendously during the last 20-30 years. The results show overall structure in the mantle which can be correlated to main plate tectonic features, such as oceanic spreading centres, continental rift zones and subducting slabs. Much seismological mantle research is now concentrated on imaging fine scale heterogeneity, which may be detected and imaged with high-resolution seismic data with dense station spacing and at high frequency, e.g. from the Russian Peaceful Nuclear Explosion (PNE) data set and array recordings of waves from natural seismic sources. Mantle body waves indicate pronounced heterogeneity at three depth levels whereas other depth intervals appear transparent, at least in the frequency band of 0.5-15 Hz: (1) The Mantle Low-Velocity Zone (LVZ) is a global feature which has been detected in more than 50 long-range seismic profiles (Thybo and Perchuc, Science, 1997). Since then numerous studies based on receiver functions, surface waves, and controlled source seismology have confirmed the presence of this zone. The data demonstrates that the top of the LVZ everywhere is at a depth of 100±20 km. A pronounced coda shows that the zone is highly heterogeneous at characteristic scale lengths of 5-15 by 2-6 km. We interpret that the rocks in the LVZ have a temperature close to the solidus or even may contain small fractions of partial melt. The solidus of mantle rocks is very low below a depth of ca. 90 km if volatiles are present due to a characteristic kink in the solidus which is much lower than for dry mantle rocks. We suggest that the rocks are in a totally solid state below the LVZ and that the depth to the interface to fully solid rocks is an indicator of the thermal state of the upper mantle. (2) Significant scattering from around the top of the

  18. Fine scale heterogeneity in the Earth's upper mantle - observation and interpretation

    NASA Astrophysics Data System (ADS)

    Thybo, Hans

    2013-04-01

    High resolution seismic data has over the last decade provided significant evidence for pronounced fine scale heterogeneity in the Earth's mantle at an unprecedented detail. Seismic tomography developed tremendously during the last 20-30 years. The results show overall structure in the mantle which can be correlated to main plate tectonic features, such as oceanic spreading centres, continental rift zones and subducting slabs. Much seismological mantle research is now concentrated on imaging fine scale heterogeneity, which may be detected and imaged with high-resolution seismic data with dense station spacing and at high frequency, e.g. from the Russian Peaceful Nuclear Explosion (PNE) data set and array recordings of waves from natural seismic sources. Mantle body waves indicate pronounced heterogeneity at three depth levels whereas other depth intervals appear transparent, at least in the frequency band of 0.5-15 Hz: (1) The Mantle Low-Velocity Zone (LVZ) is a global feature which has been detected in more than 50 long-range seismic profiles (Thybo and Perchuc, Science, 1997). Since then numerous studies based on receiver functions, surface waves, and controlled source seismology have confirmed the presence of this zone. The data demonstrates that the top of the LVZ everywhere is at a depth of 100±20 km. A pronounced coda shows that the zone is highly heterogeneous at characteristic scale lengths of 5-15 by 2-6 km. We interpret that the rocks in the LVZ have a temperature close to the solidus or even may contain small fractions of partial melt. The solidus of mantle rocks is very low below a depth of ca. 90 km if volatiles are present due to a characteristic kink in the solidus which is much lower than for dry mantle rocks. We suggest that the rocks are in a totally solid state below the LVZ and that the depth to the interface to fully solid rocks is an indicator of the thermal state of the upper mantle. (2) Significant scattering from around the top of the

  19. Deep dehydration and physical and chemical nature of the mantle above the stagnant slab (Invited)

    NASA Astrophysics Data System (ADS)

    Ohtani, E.; Zhao, D.; Kuritani, T.; Tajima, F. C.

    2010-12-01

    Recent seismic tomography studies imply that the slab is stagnant in some regions such as beneath Japan and NE China [1]. Dehydration is expected from the slabs due to decomposition of hydrous and nominally anhydrous minerals in the slabs. There are two phase boundaries between the phases with a large contrast of the water contents; i.e., the olivine-wadsleyite boundary and the decomposition boundary of ringwoodite. Dehydration could occur at the boundaries in plumes or slabs crossing the boundaries. The low velocity beneath Eastern China and United State (e.g., [2]) suggests existence of gravitationally stable hydrous melts at the base of the upper mantle. Body waveforms analysis suggested existence of highly localized low velocity anomalies at the base of the transition zone [3], which are consistent with decomposition of hydrous ringwoodite in slabs. Measurement of hydrogen diffusion in wadsleyite and ringwoodite revealed that the diffusion rates of hydrogen are comparable with that of olivine suggesting heterogeneity in hydrogen contents in the transition zone [4]. Based on hydrogen diffusion coefficients together with reported electrical conductivity of mantle minerals [5] the water content in the mantle transition zone and upper mantle can be estimated combining the electrical conductivity observations and seismic tomography data. These analyses indicate that transition zone is generally more hydrous beneath Japan compared to beneath Europe [6], and the water is localized within the wet transition zone [7]. The stagnant slabs have an important effect on the overlying transition zone and upper mantle. A big mantle wedge (BMW) model has been proposed by Zhao [1], in which the stagnant slab in the transition zone could play an essential role in the intra-plate volcanic activities overlying the slab. Water released by the stagnant slab could be important for these igneous activities, such as Changbaishan in Northeast China. The recent isotopic data of basaltic

  20. Magnetotelluric constraints on the fluid content in the upper mantle beneath the southern Canadian Cordillera: Implications for rheology

    NASA Astrophysics Data System (ADS)

    Rippe, Dennis; Unsworth, Martyn J.; Currie, Claire A.

    2013-10-01

    Long-period magnetotelluric data were collected on two parallel profiles extending across the southern Canadian Cordillera and used to derive models of the electrical resistivity of the crust and mantle from the Cascadia subduction zone to the Alberta Basin. In the fore arc, the resistivity models indicate an east dipping conductor and conductive mantle wedge, caused by the release of aqueous fluids from the Juan de Fuca plate. Low resistivities are also found beneath the volcanic arc, associated with water released from the subducting slab and mantle melts. Low resistivities in the back-arc upper mantle at depths less than 60 km suggest a shallow asthenosphere compared to the adjacent North American craton where the lithosphere-asthenosphere boundary is at ~200 km. The resistivity of the back-arc upper mantle was interpreted using geotherms, laboratory studies of mineral properties and melting points to determine the type and quantity of fluids present. The low resistivities in the back-arc upper mantle require aqueous fluids, with water content increasing from 0.005 wt % at 50 km to 0.03 wt % at 150 km depth. In addition, melt fractions of up to 1.5% are required at depths less than ~135 km to explain the observed resistivities. The presence of these quantities of aqueous fluids and partial melt will lower the viscosity of the upper mantle, as required by geodynamic models that include vigorous convection in the back arc to explain the observed heat flow.

  1. Model of the Arctic evolution since the Cretaceous to present, based on upper mantle convection linked with Pacific lithosphere subduction

    NASA Astrophysics Data System (ADS)

    Lobkovsky, Leopold

    2015-04-01

    The present paper comprises a model of Arctic basin evolution since early-mid Cretaceous to present. The model is based on the mechanism of upper mantle substance circulation beneath the Arctic lithosphere linked with Pacific lithosphere subduction. Seismic tomography data obtained for the Pacific-Eurasia-Arctic joint area indicate that Pacific lithosphere slab sinking to the mantle in subduction zone transforms into the horizontal layer upon reaching the upper mantle foot, this layer extending for two or more thousands km beneath the Eurasian continent. This pattern of seismic tomography indicates the presence of a horizontal convective cell where a flow of substance moving along the upper mantle foot from a subduction zone into the continent is compensated by a return flow moving along the lithosphere foot towards the subduction zone. The return mantle flow makes continental lithosphere extension, giving rise to processes of rifting, magmatism and spreading. The convective cell being continuously supplied with new substance which is transported through the subduction zone it is sure to expand horizontally. The above cell expansion occurs first, due to ocean ward movement of subduction zone (roll back) and secondly, due to the cell front propagation into the continent. The given model allows to understand main features for the Arctic evolution since early-mid Cretaceous to present. Numerous seismic profiling data obtained for shelf and deep water sedimentary basins in the Arctic Ocean as well as on land geological investigation reveal that since Aptian up to present the Arctic region has been characterized by sublatitudinal lithosphere extension. This extension is explained by the effect the return mantle flow related to the subduction of the Northern part of the Pacific plate acts on the Arctic lithosphere foot. The model shows the phenomenon of Arctic plume to be caused by the convective cell uprising flow. In fact lower horizontal flow of convective cell moving

  2. Upper mantle P velocity structure beneath the Baikal Rift from modeling regional seismic data

    NASA Astrophysics Data System (ADS)

    Brazier, Richard A.; Nyblade, Andrew A.

    2003-02-01

    Uppermost mantle P wave velocity structure beneath the Baikal rift and southern margin of the Siberian Platform has been investigated by using a grid search method to model Pnl waveforms from two moderate earthquakes recorded by station TLY at the southwestern end of Lake Baikal. The results yielded a limited number of successful models which indicate the presence of upper mantle P wave velocities beneath the rift axis and the margin of the platform that are 2-5% lower than expected. The magnitude of the velocity anomalies and their location support the presence of a thermal anomaly that extends laterally beyond the rift proper, possibly created by small-scale convection or a plume-like, thermal upwelling.

  3. Upper mantle and transition zone structure beneath Leizhou-Hainan region: Seismic evidence for a lower-mantle origin of the Hainan plume

    NASA Astrophysics Data System (ADS)

    Le, Ba Manh; Yang, Ting; Gu, Shenyi

    2015-11-01

    The origin of the widespread volcanism at the Leizhou-Hainan (Leiqiong) region in the Southern China remains obscure. We take advantage of the highly active seismicity and dense seismic networks surrounding this region to investigate its upper mantle and Mantle Transition Zone (MTZ) structure. Over 5000 P-wave waveforms whose raypaths bottom at depths around the MTZ are collected, and traveltimes of their first arrivals are hand-picked. By matching the traveltime curve variation over the epicentral distance range from 10° to 35°, we first construct a 1-D upper mantle and MTZ velocity structure for the region. This initial model is then refined by forward modeling, in which the observed triplicated waveforms from selected earthquakes are compared with the synthetic seismograms with varying velocity structure. In our preferred model for Leiqiong, the P-wave velocities deeper than 200 km at the upper mantle are 0.8-1.2% lower than the IASP91, and 0.6% slower in the MTZ, while the top and bottom boundaries of the MTZ depresses 12 km and slightly uplifted, respectively, compared to the global averages. This model provides independent constraints on the structure beneath Leiqiong, suggesting a thermal anomaly within the MTZ and a lower mantle origin for the volcanism seen in this region.

  4. Episodic entrainment of deep primordial mantle material into ocean island basalts

    NASA Astrophysics Data System (ADS)

    Williams, Curtis D.; Li, Mingming; McNamara, Allen K.; Garnero, Edward J.; van Soest, Matthijs C.

    2015-11-01

    Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots.

  5. Episodic entrainment of deep primordial mantle material into ocean island basalts

    PubMed Central

    Williams, Curtis D.; Li, Mingming; McNamara, Allen K.; Garnero, Edward J.; van Soest, Matthijs C.

    2015-01-01

    Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots. PMID:26596781

  6. Episodic entrainment of deep primordial mantle material into ocean island basalts.

    PubMed

    Williams, Curtis D; Li, Mingming; McNamara, Allen K; Garnero, Edward J; van Soest, Matthijs C

    2015-01-01

    Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of (3)He/(4)He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of (3)He in OIBs has motivated hypotheses that high (3)He/(4)He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like (3)He/(4)He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable (3)He/(4)He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of (3)He/(4)He ratios similar to that observed in OIBs worldwide and indicate MORB-like (3)He/(4)He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots. PMID:26596781

  7. The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle

    NASA Astrophysics Data System (ADS)

    Cottrell, Elizabeth; Kelley, Katherine A.

    2011-05-01

    Micro-analytical determination of Fe3+/∑Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure (μ-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe3+/∑Fe ratios of 0.16 ± 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the quartz-fayalite-magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe3+/∑Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or < 10%) of the difference between Fe3+/∑Fe ratios determined by micro-colorimety and XANES can be attributed to the Mössbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe3+/∑Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe3+ behaving incompatibly in shallow MORB magma chambers. MORB Fe3+/∑Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na2O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe3+ may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at ~ QFM. Both explanations, in combination with the measured MORB Fe3+/∑Fe ratios, point to a fertile MORB source with greater than 0.3 wt.% Fe2O3.

  8. Seismic Tomography for the Crust and Upper Mantle behind the Japan Trench

    NASA Astrophysics Data System (ADS)

    Wang, Z.

    2014-12-01

    The Pacific plate subducts WNW under the Eurasian plates with a ~30° angle of dip and a rate of ~8 cm/yr from the Japan-Kuril Trench. The Kuril-NE Japan arc of the uppermost mantle, overlying the subducting Pacific slab, is the locus of important processes, including serpentinization of the forearc mantle wedge, repeated genesis of megathrust earthquakes, slab dehydration, arc magmatism and interplate coupling. To improve our knowledge of crustal and upper mantle structures through tomographic imaging, we determined the three-dimensional (3-D) velocity (Vp, Vs) and Vp/Vs structures under the Kuril-NE Japan subduction zone. The Vp, Vs and Vp/Vs models provide compelling evidence for a highly hydrated and serpentinized forearc mantle and the fluids related to low-velocity and high-Vp/Vs anomalies associated with the slab dehydration. Significant slow anomalous Vp and Vs with a high-Vp/Vs ratio are clearly imaged along the volcanic front with an extended depth of ~100 km under the Kuril-NE Japan arc, showing good consistency with the results of previous studies. This is caused mainly by the fluids associated with the extensive dehydration of the subducting Pacific slab. Fluid-related anomalies under the Kuril-NE Japan arc system, attributed to various processes such as slab dehydration and serpentinization of the forearc mantle wedge, are contributed mainly by arc magmatism, interplate coupling and the repeated generation of megathrust earthquakes. The characteristic distribution of high and low Vp/Vs in the forearc continental crust along the trench-parallel direction may reflects the spatial heterogeneity of the amount of the subducted water which related to the difference of the sedimentary unit and seismic activity in the oceanic crust. Our study demonstrates that the directly optimization of Vp/Vs tomographic procedure provides more stable and reliable Vp/Vs image than previous method.

  9. Refining Estimates of the Seismic Velocities of the Crust and Upper Mantle

    NASA Astrophysics Data System (ADS)

    BARMIN, M.; SHAPIRO, N. M.; Ritzwoller, M. H.; Levin, V.; Park, J.

    2001-12-01

    We discuss recent efforts to improve a global shear-velocity model of the crust and upper mantle by advancing surface wave methodology as well as by introducing new types of geophysical data in the inversion. The primary data-set used to construct the model consists of broad-band Rayleigh and Love wave group-velocity (CU-Boulder) and phase-velocity (Harvard, Utrecht) dispersion curves. The first step of the inversion is surface wave tomography in which group and phase velocity maps are constructed. We present a new method of surface wave tomography called "diffraction tomography" that is based on a physical model of the surface wave Fresnel zone rather than on ray-theory and ad hoc regularization. Diffraction tomography accounts for path-length dependent sensitivity, wave-form healing and associated diffraction effects, and provides a more accurate assessment of spatially variable resolution than traditional tomographic methods. The second step is Monte-Carlo inversion of the dispersion maps for an ensemble of acceptable shear velocity models of the crust and uppermost mantle. Because surface waves have limited vertical resolution, we apply constraints on the model derived from other types of geophysical observations. We consider two types of additional data: teleseismic receiver functions and heat flow measurements. Receivers functions are formed by P-S converted waves that arise from sharp boundaries close to the Earth's surface, and thus provide important constraints on the crustal structure. Their use in the inversion mitigates the tradeoff between the crust (where surface waves have poor sensitivity) and the deeper part of the model. Heat-flow data constrain mantle shear velocities through the conversion of heat-flow into temperature and subsequently into shear velocity at the top of the upper mantle. We present results from the joint inversion and discuss how the combination of different types of data reduces both uncertainties and systematic bias in the

  10. Crust and upper mantle structures beneath Northeast China from receiver function studies

    NASA Astrophysics Data System (ADS)

    Guo, Zhen; Cao, Yuliang; Wang, Xianguang; John Chen, Y.; Ning, Jieyuan; He, Weiguang; Tang, Youcai; Feng, Yongge

    2014-06-01

    P-wave and S-wave receiver function analyses have been performed along a profile consisted of 27 broadband seismic stations to image the crustal and upper mantle discontinuities across Northeast China. The results show that the average Moho depth varies from about 37 km beneath the Daxing'anling orogenic belt in the west to about 33 km beneath the Songliao Basin, and to about 35 km beneath the Changbai mountain region in the east. Our results reveal that the Moho is generally flat beneath the Daxing'anling region and a remarkable Moho offset (about 4 km) exists beneath the basin-mountain boundary, the Daxing'anling-Taihang Gravity Line. Beneath the Tanlu faults zone, which seperates the Songliao Basin and Changbai region, the Moho is uplift and the crustal thickness changes rapidly. We interpret this feature as that the Tanlu faults might deeply penetrate into the upper mantle, and facilitate the mantle upwelling along the faults during the Cenozoic era. The average depth of the lithosphere-asthenosphere boundary (LAB) is ~80 km along the profile which is thinner than an average thickness of a continental lithosphere. The LAB shows an arc-like shape in the basin, with the shallowest part approximately beneath the center of the basin. The uplift LAB beneath the basin might be related to the extensive lithospheric stretching in the Mesozoic. In the mantle transition zone, a structurally complicated 660 km discontinuity with a maximum 35 km depression beneath the Changbai region is observed. The 35 km depression is roughly coincident with the location of the stagnant western pacific slab on top of the 660 km discontinuity revealed by the recent P wave tomography.

  11. Effects of topography on upper mantle discontinuities for array detections of PP precursors

    NASA Astrophysics Data System (ADS)

    Thomas, C.; Lessing, S.; Rost, S.; Vanacore, E. A.; Schmerr, N. C.

    2014-12-01

    PP underside reflections off upper mantle discontinuities are frequently used to map discontinuity topography, impedance contrasts and to interpret these with respect to thermal and/or mineralogical variations. While the seismic discontinuities at 410 km and 660 km depth should be a global feature, several events show no detections or reduced amplitudes of the precursors. In this study, we investigate effects of topography on upper mantle discontinuities on array detections of PP precursors. Using the 2.5-D axisymmetric finite difference technique PSVaxi, we compute P-SV synthetic seismograms for two-dimensional model geometries with correct 3-D geometrical spreading. Retaining dominant periods of ~ 2 s, we investigate Gaussian-shaped upward or downward deflections of the 660 km discontinuity with varying lateral dimensions. Furthermore, we investigate effects of double discontinuities at ~660 km depth which are due to phase transformations in the non-olivine component of the mantle at subduction zone temperatures. Analyses of travel residuals indicate that topography of downward deflections of discontinuities is underestimated by 10 to 20 km while upward deflections are recovered within 10 km. Amplitude measurements show focussing and defocussing of PP precursor amplitudes by 60-70%. Reduced amplitudes of PP precursors are close or below the average noise level at seismic arrays and could result in non-detections. Double discontinuities at 660 km depth are recovered for lateral dimensions larger than 10° and if the two discontinuities are separated by at least 20 km. The observed effects in synthetic seismograms raise caution for interpretation of PP precursors in terms of impedance contrasts as well as thermal and mineralogical variations in the mantle.

  12. Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

    NASA Astrophysics Data System (ADS)

    Brandt, Martin B. C.; Grand, Stephen P.; Nyblade, Andrew A.; Dirks, Paul H. G. M.

    2012-04-01

    We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997-1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.

  13. Global Upper-Mantle Tomography With the Automated Multimode Inversion of Surface and S Wave Forms

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; van der Hilst, R. D.

    2006-12-01

    We apply the Automated Multimode Inversion (AMI) to a large global dataset, examine the accuracy of our techniques and assumptions, and compute an Sv-velocity model of the upper mantle (crust--660 km) using 61000 seismograms. Structure of the mantle and crust is constrained by waveform information from 306000 time-frequency windows with the fundamental-mode Rayleigh waves (periods from 20 to 400 s) and from windows with 19600 distinct higher-mode wavepackets (S and multiple S wave arrivals). We implement AMI with a 3D reference model; linear equations obtained from all the seismograms of the dataset are inverted for anomalies relative to the 3D reference, in this study composed of a 3D model of the crust and a 1D depth profile in the mantle. Waveform information is related to S- and P-velocity structure within approximate waveform sensitivity areas. Inverting for isotropic variations in S- and P-wave velocities, we also allow for S-wave azimuthal anisotropy---in order to minimize errors due to mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred km, varying with data sampling. We validate the imaging technique with a novel, "spectral-element" resolution test: inverting a global synthetic data set computed with the spectral-element method (Capdeville et al. 2003) through a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-Sv-velocity anomalies beneath mid-ocean ridges and back-arc basins extend down to ~100 km depth only; this corresponds to estimates of primary melt production depth ranges there. Seismic lithosphere beneath cratons bottoms at depths up to 200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare

  14. Partitioning of copper between olivine, orthopyroxene, clinopyroxene, spinel, garnet and silicate melts at upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Liu, Xingcheng; Xiong, Xiaolin; Audétat, Andreas; Li, Yuan; Song, Maoshuang; Li, Li; Sun, Weidong; Ding, Xing

    2014-01-01

    Previously published Cu partition coefficients (DCu) between silicate minerals and melts cover a wide range and have resulted in large uncertainties in model calculations of Cu behavior during mantle melting. In order to obtain true DCumineral/melt values, this study used Pt95Cu05 alloy capsules as the source of Cu to experimentally determine the DCu between olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (spl), garnet (grt) and hydrous silicate melts at upper mantle conditions. Three synthetic silicate compositions, a Komatiite, a MORB and a Di70An30, were used to produce these minerals and melts. The experiments were conducted in piston cylinder presses at 1.0-3.5 GPa, 1150-1300 °C and oxygen fugacities (fO2) of from ∼2 log units below to ∼5 log units above fayalite-magnetite-quartz (FMQ). The compositions of minerals and quenched melts in the run products were measured with EMP and LA-ICP-MS. Attainment of equilibrium is verified by reproducible DCu values obtained at similar experimental conditions but different durations. The results show that DCu for ol/, opx/, spl/ and possibly cpx/melt increase with increasing fO2 when fO2 > FMQ + 1.2, while DCu for cpx/ and spl/melt also increase with increasing Na2O in cpx and Fe2O3 in spinel, respectively. In the investigated P-T-fO2 conditions, the DCumineral/melt values are 0.04-0.14 for ol, 0.04-0.09 for opx, 0.02-0.23 for cpx, 0.19-0.77 for spl and 0.03-0.05 for grt. These results confirm that Cu is highly incompatible (DCu < ∼0.2) in all the silicate minerals and oxides of the upper mantle with the exception of the high-Fe spinel, in which Cu is moderately incompatible (DCu = 0.4-0.8) and thus Cu will be enriched in the derived melts during mantle partial melting and magmatic differentiation if sulfide is absent. These experimental DCu values are used to assess the controls on Cu behavior during mantle melting. The model results suggest that MORBs and most arc basalts must form by sulfide

  15. Surface Wave Phase Speed Models of the Crustal and Upper Mantle Beneath Turkey

    NASA Astrophysics Data System (ADS)

    Bakirci, Taciser; Yoshizawa, Kazunori; Fırat Özer, Mithat

    2010-05-01

    We investigated the crustal and upper mantle structure beneath Turkey using phase speed of the fundamental-mode Rayleigh waves. To determine the Rayleigh wave phase dispersion, the two-station method is employed to analyze vertical-component seismograms of 165 broad-band seismic stations in Turkey and surrounding areas. We used 289 teleseismic events with moment magnitude 5.5 and greater, and with focal depth shallower than 100 km occurred in the world between January, 2006 and December, 2008. The data is obtained from two national seismic networks operated by Boğaziçi University Kandilli Observatory and Earthquake Research Institute (KOERI), and General Directorate of Disaster Affairs Earthquake Research Department (ERI). In the first step of the study, the path-average dispersion curves along the great circle for each event are determined using the conventional two-station technique. In the second step, Rayleigh-wave phase speed maps of the entire region of Turkey are obtained as a function of frequency using the LSQR algorithm. The phase speed models are constructed in a period range from 30 to 160 sec. The fast phase speed anomalies are found in the western and southern parts of Turkey (particularly conspicuous in the north-western part; i.e., the Marmara Region) and the phase speed becomes slow in the eastern part in almost all phase speed models. In the eastern region, prominent slow anomalies are found in the short period range below 40 seconds, which corresponds to the depth of the crust and the uppermost mantle, while the faster phase speed anomaly becomes dominant in the longer periods, which sample the deeper part of the mantle. We may explain the observed fast phase speed anomaly in the northwestern part of Turkey by the cooling of the upper mantle, which can affect the immersion of oceanic lithosphere of the Black Sea beneath southward margin of the Marmara region. The slow anomalies of phase speed in the eastern Turkey, may represent the presence of

  16. Adjoint tomography of crust and upper-mantle structure beneath Continental China

    NASA Astrophysics Data System (ADS)

    Chen, M.; Niu, F.; Liu, Q.; Tromp, J.

    2013-12-01

    Four years of regional earthquake recordings from 1,869 seismic stations are used for high-resolution and high-fidelity seismic imaging of the crust and upper-mantle structure beneath Continental China. This unprecedented high-density dataset is comprised of seismograms recorded by the China Earthquake Administration Array (CEArray), NorthEast China Extended SeiSmic Array (NECESSArray), INDEPTH-IV Array, F-net and other global and regional seismic networks, and involves 1,326,384 frequency-dependent phase measurements. Adjoint tomography is applied to this unprecedented dataset, aiming to resolve detailed 3D maps of compressional and shear wavespeeds, and radial anisotropy. Contrary to traditional ray-theory based tomography, adjoint tomography takes into account full 3D wave propagation effects and off-ray-path sensitivity. In our implementation, it utilizes a spectral-element method for precise wave propagation simulations. The tomographic method starts with a 3D initial model that combines smooth radially anisotropic mantle model S362ANI and 3D crustal model Crust2.0. Traveltime and amplitude misfits are minimized iteratively based on a conjugate gradient method, harnessing 3D finite-frequency kernels computed for each updated 3D model. After 17 iterations, our inversion reveals strong correlations of 3D wavespeed heterogeneities in the crust and upper mantle with surface tectonic units, such as the Himalaya Block, the Tibetan Plateau, the Tarim Basin, the Ordos Block, and the South China Block. Narrow slab features emerge from the smooth initial model above the transition zone beneath the Japan, Ryukyu, Philippine, Izu-Bonin, Mariana and Andaman arcs. 3D wavespeed variations appear comparable to or much sharper than in high-frequency P-and S-wave models from previous studies. Moreover our results include new information, such as 3D variations of radial anisotropy and the Vp/Vs ratio, which are expected to shed new light to the composition, thermal state, flow

  17. Upper mantle structure beneath the Hangay dome, central Mongolia and implications for high topography and magmatism

    NASA Astrophysics Data System (ADS)

    Souza, Stephanie

    Origin and support of high topography in an intracontinental setting is not fully understood. The Hangay Dome in central Mongolia spans an area of ˜200,000 km2 and reaches elevations of ˜4,000 m. It has a complex accretionary history associated with the Central Asian Orogenic Belt and is bound to the north, south, and west by active strike-slip faults. The extent to which the accretionary history or present day deformation contributes to current topography remain open questions. Geodynamic models that have been proposed to account for current topography include far-field effects of Pacific Plate subduction or the India-Asia collision, rifting stemming from the Lake Baikal region, mantle-plume activity, upwelling of the asthenospheric mantle, lithospheric delamination, and/or the underplating of magmatic rocks at the base of the crust. In order to determine which whether upper mantle structure might contribute to the origin of high topography in the Hangay, two years of teleseismic P and S body wave data are inverted for 3D velocity variations in Vp and Vs in the upper mantle beneath the Hangay. Velocity perturbations range between +/-3% for the P wave model and +/-7% for the S wave model. Changes in velocity are a function of temperature, density, composition, and presence of melt or fluid. Thermal anomalies are the primary causes for velocity perturbations in the upper mantle. The Hangay is underlain by non-uniform low velocity zones that correlate well with areas of the Hangay that have experienced volcanism in the past ˜30 Ma. High velocity zones are located off the edges of the dome to the west, east, and south. One low velocity anomaly in particular, is located near the headwaters of the Orkhon River beneath a region that had experienced magmatism ˜15-20 million years ago. Interestingly, this region sits between two areas ˜50 km away on either side that have experienced magmatism in the last 3 million years. This low velocity anomaly has a DeltaVp of -4

  18. Experimental Deformation of Polyphase Aggregates at Pressures and Temperatures of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Bejina, F.; Bystricky, M.; Ingrin, J.

    2011-12-01

    Modelling the solid-state flow of the upper mantle requires a thorough understanding of its rheology and therefore necessitates to perform deformation experiments on mantle rocks (or analogues) at very high pressures and temperatures. Minerals other than olivine constitute up to 40 vol% of upper mantle rocks and may have a significant effect on the rheological behavior of these rocks. Nevertheless, most experimental studies to date have focused on the deformation properties of olivine single crystals or monomineralic olivine aggregates. In this study, and as a first step before focusing on more realistic mantle-like compositions, we have performed deformation experiments on polymineralic model aggregates of forsterite and MgO at upper mantle pressures and temperatures. Commercial powders of Mg2SiO4 and MgO were mixed and ground in WC grinders and dried in a one-atmosphere furnace at 1000°C. Powders with different volume proportions of the two phases were sintered by spark plasma sintering (SPS) at 1300-1400°C and 100 MPa for a few minutes, resulting in dense pellets 8 mm in diameter and 3-4 mm in length. Microstructural analysis by SEM reveals equilibrated microstructures with forsterite and MgO grain sizes of a few microns. Deformation experiments on samples 1.2 mm in diameter and ~1.2mm in length were performed at 3-8 GPa and 1000-1300°C in a D-DIA apparatus coupled with synchrotron X-ray radiation. The technique permits in situ measurement of macroscopic strain rates as well as stress levels sustained by different subpopulations of grains of each phase. Typically, two specimens, respectively a monomineralic and a polymineralic aggregate, were deformed concurrently in order to minimize the relative uncertainties in temperature and pressure and to facilitate the comparison of their rheological properties. The samples were deformed to total strains of 15-25%. As expected, the harder phase, forsterite, sustains much higher stress levels than MgO, in agreement

  19. "DOBREfraction'99" - Velocity models of the crust and upper mantle beneath the Donbas Foldbelt (SE Ukraine)

    NASA Astrophysics Data System (ADS)

    Stephenson, R. A.; Dobrefraction'00 Working Group,.

    2002-12-01

    The Pripyat-Dniepr-Donets basin (PDD) is a Late Devonian rift basin located on the southwestern part of the East-European Craton (EEC). This rift basin strikes in a southeasterly direction and extends from Belarus through Ukraine, where it connects with the Donbas foldbelt and its continuation as the deformed southern margin of the craton (Karpinsky Swell) in southern Russia. The Pripyat and Dniepr-Donets basins are important hydrocarbon provinces. The Donbas foldbelt (DF) is the uplifted and deformed part of the 20-km thick Dniepr-Donets basin. In 1999, an international cooperative deep seismic sounding (DSS) experiment (DOBREfraction'99) was undertaken. This effort involved 11 in-line shotpoints and deployment of some 245 recording stations along a northeast-trending, 360 km long profile extending from the shores of the Azov Sea in the south, across the Azov Massif (Ukrainian Shield), the DF, ending at the Ukraine-Russia border in the Voronezh Massif of the EEC. Particular scientific targets included the nature of the crust-mantle transition and the geometry of crustal/upper mantle structures related to rifting and subsequent basin inversion. Tomographic inversion, as well as, ray-trace based velocity modelling has been carried out. The velocity signature of the sedimentary basin itself is well resolved, indicating an asymmetric form (basement surface dipping more gently towards the center of the basin from the north than from the south) and a total thickness of about 20-km, comparable to estimates derived from previous seismic studies and geological interpretations. A thick (>10-km), high-velocity (>6.9 km/s), lower crustal body lies beneath the rift basin itself. This layer forms a domal structure that is offset slightly to the north compared to the main basin depocenter. A thinner (~5-km) high velocity layer is inferred beneath the southern margin of the Donbas foldbelt and Azov Massif. The former could be related to Permian uplift with the latter being due to

  20. New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin

    NASA Astrophysics Data System (ADS)

    Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba

    2016-04-01

    Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries

  1. Upper mantle and crustal structure of the East Greenland Caledonides: New geophysical evidence and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Schiffer, C.; Balling, N.; Jacobsen, B. H.; Hejrani, B.; Nielsen, S. B.

    2013-12-01

    The East Greenland and Scandinavian Caledonides once formed a major coherent mountain range, as a consequence of the collision of the continents of Laurentia and Baltica. The crustal and upper mantle structure was furthermore influenced by several geodynamic processes leading to the formation of the North Atlantic passive margins, including the gravitational collapse, extension, rifting and a possible influence by volcanism related to the Iceland hot spot. The landscape and topography were finally shaped by extensive erosion, finding its peak in the quaternary glaciations. Seismological data were acquired in the East Greenland Caledonides by the Ella-Ø-array for a period of two years (2009-2011). The array containing 11 broadband seismometers was situated at approximately 73 °N covering a distance of 270 km from the Greenland Ice Sheet to the Atlantic coast, north of the Iceland Ridge. A Receiver Function study based on an average of 36 events per station reveals a clear eastward dipping high-velocity structure underneath the study area. The geophysical character, supported by synthetic modelling, is consistent with a 10 km thick subducted slab of eclogitized oceanic crust. This might be the key for unravelling of a complex geodynamic setting and development leading to the formation of the Caledonides. The distinct preservation of structures in the upper mantle to depths of 100 km or more, limits the impact of subsequent collision and extension related deformation. In support of this interpretation, we present selected results from on-going detailed studies of the crustal and upper mantle, including a Receiver Function inversion, seismic P-wave travel time tomography and gravity modelling.

  2. The depth distribution of azimuthal anisotropy in the continental upper mantle.

    PubMed

    Marone, Federica; Romanowicz, Barbara

    2007-05-10

    The most likely cause of seismic anisotropy in the Earth's upper mantle is the lattice preferred orientation of anisotropic minerals such as olivine. Its presence reflects dynamic processes related to formation of the lithosphere as well as to present-day tectonic motions. A powerful tool for detecting and characterizing upper-mantle anisotropy is the analysis of shear-wave splitting measurements. Because of the poor vertical resolution afforded by this type of data, however, it has remained controversial whether the splitting has a lithospheric origin that is 'frozen-in' at the time of formation of the craton, or whether the anisotropy originates primarily in the asthenosphere, and is induced by shear owing to present-day absolute plate motions. In addition, predictions from surface-wave-derived models are largely incompatible with shear-wave splitting observations. Here we show that this disagreement can be resolved by simultaneously inverting surface waveforms and shear-wave splitting data. We present evidence for the presence of two layers of anisotropy with different fast-axis orientations in the cratonic part of the North American upper mantle. At asthenospheric depths (200-400 km) the fast axis is sub-parallel to the absolute plate motion, confirming the presence of shear related to current tectonic processes, whereas in the lithosphere (80-200 km), the orientation is significantly more northerly. In the western, tectonically active, part of North America, the fast-axis direction is consistent with the absolute plate motion throughout the depth range considered, in agreement with a much thinner lithosphere. PMID:17495924

  3. Sensitivity of seismic measurements to frequency-dependent attenuation and upper mantle structure: An initial approach

    NASA Astrophysics Data System (ADS)

    Bellis, C.; Holtzman, B.

    2014-07-01

    This study addresses the sensitivity of seismic attenuation measurements to dissipative mechanisms and structure in the Earth's upper mantle. The Andrade anelastic model fits experimental attenuation data with a mild power law frequency dependence and can be scaled from laboratory to Earth conditions. We incorporate this anelastic model into 400km 1-D thermal profiles of the upper mantle. These continuous-spectrum models are approximated by multiple relaxation mechanisms that are implemented within a finite-difference scheme to perform wave propagation simulations in 1-D domains. In two sets of numerical experiments, we evaluate the measurable signature of the intrinsic attenuation structure. The two sets are defined by thermal profiles with added step functions of temperature, varying in (i) amplitude and depth or (ii) amplitude and sharpness. The corresponding synthetic data are processed using both the conventional t* approach, i.e., a linear regression of the displacement frequency spectrum, and an alternative nonlinear fit to identify the integrated value of attenuation and its frequency dependence. The measured sensitivity patterns are analyzed to assess the effects of the anelastic model and its spatial distribution on seismic data (in the absence of scattering effects). We have two straightforward results: (1) the frequency dependence power law is recoverable from the measurements; (2) t* is sensitive to both the depth and the amplitude of the step, and it is insensitive to the sharpness of the step, in the 0.25 to 2 Hz band. There is much potential for gaining information about the upper mantle thermodynamic state from careful interpretation of attenuation.

  4. Upper mantle electrical resistivity structure beneath back-arc spreading centers

    NASA Astrophysics Data System (ADS)

    Seama, N.; Shibata, Y.; Kimura, M.; Shindo, H.; Matsuno, T.; Nogi, Y.; Okino, K.

    2011-12-01

    We compare four electrical resistivity structure images of the upper mantle across back-arc spreading centers (Mariana Trough at 18 N and 13 N, and the Eastern Lau at 19.7 S and 21.3 S) to provide geophysical constraints on issues of mantle dynamics beneath the back-arc spreading system related to the subducting slab. The central Mariana Trough at 18 N has the full spreading rate of 25 km/Myr, and shows characteristic slow-spreading features; existence of median valley neovolcanic zone and "Bull's eyes" mantle Bouguer anomaly (MBA) along the axes. On the other hand, the southern Mariana Trough at 13 N shows an EPR type axial relief in morphology and lower MBA than that in the central Mariana Trough (Kitada et al., 2006), suggesting abundance of magma supply, even though the full spreading rate is 35 km/Myr that is categorized as a slow spreading ridge. At the Eastern Lau spreading center, crustal thickness and morphology vary systematically with arc proximity and shows the opposed trends against spreading rate: The full spreading rate increases from 65 km/Myr at 21.3 S to 85 km/Myr at 19.7 S, while the crustal thicknesses decrease together with morphology transitions from shallow peaked volcanic highs to a deeper flat axis (Martinez et al., 2006). Matsuno et al. (2010) provides a resistivity structure image of the upper mantle across the central Mariana subduction system, which contains several key features: There is an uppermost resistive layer with a thickness of 80-100 km beneath the central Mariana Trough, suggesting dry residual from the plate accretion process. But there is no evidence for a conductive feature beneath the back-arc spreading center at 18 N, and this feature is clearly independent from the conductive region beneath the volcanic arc below 60 km depth that reflects melting and hydration driven by water release from the subducting slab. The resultant upper mantle resistivity structure well support that the melt supply is not abundant, resulting in

  5. Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle

    USGS Publications Warehouse

    Artemieva, I.M.; Billien, M.; Leveque, J.-J.; Mooney, W.D.

    2004-01-01

    Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, QS-1, and seismic velocity, VS, we compare global maps of the thermal structure of the continental upper mantle with global QS-1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 S. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between VS and T and of <0.47 between QS and T at any depth. Such low correlation coefficients can partially be attributed to modelling arrefacts; however, they also suggest that not all of the VS and QS anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, VS and QS usually inversely correlate with lithospheric temperatures: most cratonic regions show high VS and QS and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero QS anomalies approximately correspond to the 1000 ??C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 ??C. East-west profiles of VS, QS and T where continental data coverage is best (50??N latitude for North America and 60??N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in

  6. Upper-mantle seismic discontinuities and the thermal structure of subduction zones

    USGS Publications Warehouse

    Vidale, J.E.; Benz, H.M.

    1992-01-01

    The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410 km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.

  7. Improving Earthquake-Explosion Discrimination using Attenuation Models of the Crust and Upper Mantle

    SciTech Connect

    Pasyanos, M E; Walter, W R; Matzel, E M; Rodgers, A J; Ford, S R; Gok, R; Sweeney, J J

    2009-07-06

    In the past year, we have made significant progress on developing and calibrating methodologies to improve earthquake-explosion discrimination using high-frequency regional P/S amplitude ratios. Closely-spaced earthquakes and explosions generally discriminate easily using this method, as demonstrated by recordings of explosions from test sites around the world. In relatively simple geophysical regions such as the continental parts of the Yellow Sea and Korean Peninsula (YSKP) we have successfully used a 1-D Magnitude and Distance Amplitude Correction methodology (1-D MDAC) to extend the regional P/S technique over large areas. However in tectonically complex regions such as the Middle East, or the mixed oceanic-continental paths for the YSKP the lateral variations in amplitudes are not well predicted by 1-D corrections and 1-D MDAC P/S discrimination over broad areas can perform poorly. We have developed a new technique to map 2-D attenuation structure in the crust and upper mantle. We retain the MDAC source model and geometrical spreading formulation and use the amplitudes of the four primary regional phases (Pn, Pg, Sn, Lg), to develop a simultaneous multi-phase approach to determine the P-wave and S-wave attenuation of the lithosphere. The methodology allows solving for attenuation structure in different depth layers. Here we show results for the P and S-wave attenuation in crust and upper mantle layers. When applied to the Middle East, we find variations in the attenuation quality factor Q that are consistent with the complex tectonics of the region. For example, provinces along the tectonically-active Tethys collision zone (e.g. Turkish Plateau, Zagros) have high attenuation in both the crust and upper mantle, while the stable outlying regions like the Indian Shield generally have low attenuation. In the Arabian Shield, however, we find that the low attenuation in this Precambrian crust is underlain by a high-attenuation upper mantle similar to the nearby Red

  8. Thermal and mechanical structure of the upper mantle: A comparison between continental and oceanic models

    NASA Technical Reports Server (NTRS)

    Froidevaux, C.; Schubert, G.; Yuen, D. A.

    1976-01-01

    Temperature, velocity, and viscosity profiles for coupled thermal and mechanical models of the upper mantle beneath continental shields and old ocean basins show that under the continents, both tectonic plates and the asthenosphere, are thicker than they are beneath the oceans. The minimum value of viscosity in the continental asthenosphere is about an order of magnitude larger than in the shear zone beneath oceans. The shear stress or drag underneath continental plates is also approximately an order of magnitude larger than the drag on oceanic plates. Effects of shear heating may account for flattening of ocean floor topography and heat flux in old ocean basins.

  9. Developing Tools to Test the Thermo-Mechanical Models, Examples at Crustal and Upper Mantle Scale

    NASA Astrophysics Data System (ADS)

    Le Pourhiet, L.; Yamato, P.; Burov, E.; Gurnis, M.

    2005-12-01

    Testing geodynamical model is never an easy task. Depending on the spatio-temporal scale of the model, different testable predictions are needed and no magic reciepe exist. This contribution first presents different methods that have been used to test themo-mechanical modeling results at upper crustal, lithospheric and upper mantle scale using three geodynamical examples : the Gulf of Corinth (Greece), the Western Alps, and the Sierra Nevada. At short spatio-temporal scale (e.g. Gulf of Corinth). The resolution of the numerical models is usually sufficient to catch the timing and kinematics of the faults precisely enough to be tested by tectono-stratigraphic arguments. In active deforming area, microseismicity can be compared to the effective rheology and P and T axes of the focal mechanism can be compared with local orientation of the major component of the stress tensor. At lithospheric scale the resolution of the models doesn't permit anymore to constrain the models by direct observations (i.e. structural data from field or seismic reflection). Instead, synthetic P-T-t path may be computed and compared to natural ones in term of rate of exhumation for ancient orogens. Topography may also help but on continent it mainly depends on erosion laws that are complicated to constrain. Deeper in the mantle, the only available constrain are long wave length topographic data and tomographic "data". The major problem to overcome now at lithospheric and upper mantle scale, is that the so called "data" results actually from inverse models of the real data and that those inverse model are based on synthetic models. Post processing P and S wave velocities is not sufficient to be able to make testable prediction at upper mantle scale. Instead of that, direct wave propagations model must be computed. This allows checking if the differences between two models constitute a testable prediction or not. On longer term, we may be able to use those synthetic models to reduce the residue

  10. Investigating Transition Zone Thickness Variation under the Arabian Plate: Evidence Lacking for Deep Mantle Upwellings

    NASA Astrophysics Data System (ADS)

    Juliá, J.; Tang, Z.; Mai, P. M.; Zahran, H.

    2014-12-01

    Cenozoic volcanic outcrops in Arabia - locally known as harrats - span more than 2000 km along the western half of the Arabian plate, from eastern Yemen to southern Syria. The magmatism is bimodal in character, with older volcanics (30 to 20 My) being tholeiitic-to-transitional and paralleling the Red Sea margin, and younger volcanics (12 Ma to Recent) being transitional-to-strongly-alkalic and aligning in a more north-south direction. The bimodal character has been attributed to a two-stage rifting process along the Red Sea, where the old volcanics would have produced from shallow sources related to an initial passive rifting stage, and young volcanics would have originated from one or more deep-seated mantle plumes driving present active rifting. Early models suggested the harrats would have resulted from either lateral flow from the Afar plume in Ethiopia, or more locally from a separate mantle plume directly located under the shield. Most recently, tomographic images of the Arabian mantle have suggested the northern harrats could be resulting from flow originating at a deep plume under Jordan. In this work, we investigate the location of deep mantle plumes under the Arabian plate by mapping transition zone thickness with teleseismic receiver functions. The transition zone is bounded by seismic discontinuities, nominally at 410 and 660 km depth, originating from phase transitions in the olivine-normative component of the mantle. The precise depth of the discontinuities is strongly dependent on temperature and, due to the opposing signs of the corresponding Clapeyron slopes, positive temperature anomalies are expected to result in thinning of the transition zone. Our dataset consists of ~5000 low-frequency (fc < 0.25 Hz) receiver function waveforms obtained at ~110 broadband stations belonging to a number of permanent and temporary seismic networks in the region. The receiver functions were migrated to depth and stacked along a ~2000 km long record section

  11. What lies beneath: Unveiling the fine-scale 3D compositional and thermal structure of the lithosphere and upper mantle

    NASA Astrophysics Data System (ADS)

    Afonso, Juan Carlos

    2013-04-01

    The lithosphere and sublithospheric upper mantle (above 410d) are highly heterogeneous in their chemistry, thermal structure and physical properties. Since most of the upper mantle is inaccessible to direct observation, we must rely on indirect methods to estimate its thermochemical structure. Lateral discontinuities (i.e. sharp changes in the thermal and/or compositional structure) in these regions are known to correlate with the location of seismically active zones, oil producing basins, foci of magma intrusion/production, and giant ore deposits. Understanding the fine-scale thermochemical structure of the lithosphere and sublithospheric upper mantle is therefore one of the most important goals in Geosciences. A detailed knowledge of the thermal and compositional structure of the upper mantle is also an essential prerequisite to understanding the formation, deformation and destruction of continents, the physical and chemical interactions between the lithosphere and the convective sublithospheric upper mantle, the long-term stability of ancient lithosphere, and the evolution of surface topography. Unfortunately, with current geophysical methods, such a holistic and detailed characterisation remains a technically and conceptually challenging problem. In this talk, I will discuss recent advancements in thermodynamically-constrained multi-observable probabilistic inversions, which have the potential to overcome the problems affecting other inversions schemes and provide realistic estimates of the present-day thermochemical structure of the lithosphere and upper mantle. I will present results for both synthetic and real case studies, which serve to highlight the advantages and limitations of our approach compared to others. I will also discuss future work towards the incorporation of such an approach into global thermo-mechanical simulations/inversions to study the intricate connections between the thermochemical structure of the upper mantle and the evolution of

  12. Precursors to ScS Phases and dipping interface in the upper mantle beneath southwestern Japan

    NASA Astrophysics Data System (ADS)

    Nakanishi, Ichiro

    1980-10-01

    Longitudinally polarized precursors to ScS phases observed in the Shikoku and Chugoku districts, southwestern Japan, are interpreted as ScSp arrivals, resulting from ScSto-P conversions at a dipping interface in the upper mantle. An ScSp phase recorded in the Tohoku district, northeastern Japan, also is examined. The location of the conversion interface, beneath the Shikoku district, determined from the ScSp observations agrees with the upper boundary of the descending Philippine Sea plate inferred from the seismicity pattern of subcrustal earthquakes. It has been proposed on the basis of no seismic activity in the upper mantle that the leading edge of the downgoing Philippine Sea plate has not reached the upper mantle beneath the Chugoku district. The ScSp observations, however, present a possibility of the existence of an aseismic continuation of the Philippine Sea plate in the upper mantle beneath the Chugoku district. An alternative interpretation of the conversion interface may be possible. The interface inferred in the present study may correspond to a boundary between the asthenosphere and an aseismic dead slab which had descended from the Nankai trough at the previous cycle of plate convergence. This ScS-to-P conversion interface may be closely related to the late Quaternary volcanism in the Chugoku district. Low-velocity zones are required in the vicinity of the inclined ScS-to-P conversion interfaces in the upper mantle beneath southwestern and northeastern Japan in order to explain the observed relative polarity between the ScSp and ScS phases. The first-order discontinuity of the velocity contrast of about 6%, which has been suggested between the lithosphere and overlying asthenosphere, cannot simultaneously explain all of the observed amplitudes, periods, and polarity of the ScSp phases. The simplest model which explains these observations is the low-velocity zone with the sharp upper and transitional lower boundaries. This low-velocity zone may be

  13. Inferring upper-mantle flow from seismic anisotropy: An experimental perspective

    NASA Astrophysics Data System (ADS)

    Skemer, Philip; Hansen, Lars N.

    2016-02-01

    Patterns of mantle flow are most directly inferred from observations of seismic anisotropy, which is mainly caused by the crystallographic preferred orientation (CPO) of olivine, the most abundant mineral in the upper mantle. CPO is generated by high temperature ductile deformation, which often yields predictable relationships between the elastic or seismic properties of a material and the kinematics of flow. Over the last 15 years there has been a wealth of new data describing the how olivine CPO forms and evolves as a function of deformation conditions and strain magnitude. In this review, we explore the relationships between deformation, the evolution of CPO, and the development of seismic anisotropy, from the perspective of experimental rock mechanics. We first review the experimental basis for the study of olivine CPO evolution from the formative studies in the early nineteen sixties through recent advances. We then review some emerging complications to the study of CPO evolution, such as the long-lived transient CPOs that arise from changes in deformation kinematics, mechanisms, and conditions. Finally we discuss the origins of seismic anisotropy and the challenges of interpreting seismic anisotropy in terms of mantle flow.

  14. Upper and lower mantle anisotropy inferred from comprehensive SKS and SKKS splitting measurements from India

    NASA Astrophysics Data System (ADS)

    Roy, Sunil K.; Ravi Kumar, M.; Srinagesh, D.

    2014-04-01

    In this study, we investigate the upper mantle anisotropy beneath India using high quality SKS and SKKS waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations. In addition, we present evidence for anisotropy in the D″ layer beneath southeast Asia using SKS and SKKS splitting discrepancies on the same seismogram. During this exercise, we obtain 200 new splitting measurements from 35 stations recently deployed in the Indo-Gangetic plains (IGP), central India and northeast India. While the delay times between the fast and slow axes of anisotropy (δt) range from 0.3 to 1.7 s, the fast polarization azimuths (Φ) at a majority of stations in the IGP and central India coincide with the absolute plate motion of India implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy. However, stations in NE India reveal fast polarization azimuths mainly in the ENE-WSW direction suggestive of lithospheric strain induced by the ongoing Indo-Eurasian collision. Our analysis for D″ anisotropy yielded a total of 100 SKS-SKKS pairs, which can be categorized into those exhibiting (I) null measurements for one phase and significant splitting for the other phase, (II) null measurement for both the phases, (III) significant splitting for both the phases. A pair is considered to be anomalous if the splitting difference between SKS and SKKS is ⩾0.5 s and the individual split time is ⩾0.5 s. Using this criterion, we obtain 12 measurements under category III and 9 under category I that show a null measurement for SKS and large splitting for the SKKS phase. Further, we quantify the strength of the lower mantle anisotropy by correcting the SKKS measurement for the upper mantle anisotropy obtained by the SKS phase on the same seismogram. The SKS delay times are found to be consistently less than SKKS times, suggesting that the SKS phases do not capture the lower mantle anisotropy in comparison to their SKKS counterparts

  15. Progressive removal of an upper-mantle KREEP component by TTG magmatism through the Archean

    NASA Astrophysics Data System (ADS)

    Guitreau, M.; Blichert-Toft, J.; Herve, M.; Mojzsis, S. J.; Albarede, F.

    2010-12-01

    uncertainties on the 176Lu decay constant, we consider this feature to be significant. The increase of the most radiogenic ɛHf value points to mantle depletion by crust formation. Decreasing Lu/Hf ratios attest to either a decline in the degree of melt production reflective both of the cooling of the mantle and the progressive removal of its fertile component, or an increased felsic/mafic rock ratio in the crust segments. The gradual demise through the Archean of TTGs from the geological record indicates that these rocks were the igneous carrier responsible for the progressive transfer of an enriched component from the upper mantle to the crust. The maximum ɛHf value of the juvenile crust at 4.2 Ga is clearly negative, establishing the enriched character of the upper mantle at that time. That a widespread KREEP component in the upper mantle was left behind by the differentiation of the terrestrial magma ocean as proposed by [1], and was evacuated by TTG magmatism over time, is supported by Hf isotopes in Archean rocks and zircons. [1] Blichert-Toft, J., Albarède, F., 2008. EPSL 265, 686-702 [2] Kemp, A.I.S. et al., 2010. EPSL 296, 45-56.

  16. Three-dimensional Model of Azimuthal Anisotropy in the Upper Mantle and Transition Zone

    NASA Astrophysics Data System (ADS)

    Yuan, K.; Beghein, C.

    2011-12-01

    Because it can be caused by the lattice preferred orientation (LPO) of elastically anisotropic minerals, seismic anisotropy plays a key role in understanding mantle deformation. It is well documented in the uppermost mantle, where it is caused by the LPO of olivine, but its presence is more controversial at larger depths as the resolution of commonly used seismic data decreases. Determining its location and depth extent is, however, essential to constrain mantle flow. In this study, we obtained a three-dimensional (3-D) global model of azimuthal anisotropy for the upper 800km of the mantle. We used anisotropic global phase velocity maps [Visser, et al., 2008] obtained for Love wave fundamental modes and overtones (up to n=5) between 35s and 174s period. Overtone data are sensitive to structure down to much larger depths than fundamental modes, and have greater depth resolution than shear wave-splitting data. We inverted the 2Ψ terms of the overtone maps to model 3-D variations in azimuthal anisotropy for vertically polarized shear-waves (Vsv), and the 4Ψ terms of the fundamental modes and overtones to model horizontally polarized shear-waves (Vsh) azimuthal anisotropy. To account for nonlinear effects due to changes in Moho depth, we calculated local sensitivity kernels based on CRUST2.0 [Bassin, et al., 2000] and PREM [Dziewonski and Anderson, 1981]. While parameter E (Vsh anisotropy) displays one main peak in the uppermost mantle and little amplitude in the transition zone, the average amplitude of parameter G (Vsv anisotropy) displays two main, stable maxima: one in the uppermost mantle and, most remarkably, one in the lower transition zone. Statistical F-tests determined that the presence of 2Ψ anisotropy in the transition zone is required to improve the fit of the third, fourth, and fifth overtones. However, because of trade-offs among parameters characterizing transition zone anisotropy, we cannot exclude that this anisotropy is located in the upper

  17. High pressure and temperature deformation experiments on San Carlos olivine and implications for upper mantle anisotropy

    NASA Astrophysics Data System (ADS)

    Shekhar, Sushant; Frost, Daniel J.; Walte, Nicolas; Miyajima, Nobuyoshi; Heidelbach, Florian

    2010-05-01

    Crystallographic preferred orientation developed in olivine due to shearing in the mantle is thought to be the prominent reason behind seismic anisotropy in the upper mantle. Seismic anisotropy in upper mantle can be observed up to a depth of 350 km with a marked drop in the strength of anisotropy seen around 250 km. Studies on natural rock samples from the mantle and deformation experiments performed on olivine have revealed that olivine deforms mainly through dislocation creep with Burgers vectors parallel to the [100] crystallographic axis under low pressure conditions (up to 3 GPa). Under similar pressures, evidence of [001] slip has been reported due to the presence of water. In order to understand the deformation mechanism in olivine at pressures greater than 3 GPa, we have performed experiments using the deformation DIA multi-anvil apparatus. The DIA consist of 6 square faceted anvils that compress a cubic high-pressure assembly. The deformation DIA possesses two vertically acting opposing inner rams, which can be operated independently of the main compressive force to deform the sample assembly. The experimental setup consists of a hot-pressed sample of polycrystalline dry San Carlos olivine 0.2 mm cut from a 1.2 mm diameter core at 45° . This slice is sandwiched between alumina pistons also cut at 45° in simple shear geometry. Experiments have been performed at 3, 5 and 8 GPa at a deformation anvil strain rate of 1.0x10-4 s-1and temperatures between 1200-1400° C. Deformed samples were cut normal to the shear plane and parallel to the shear direction. Then the sample was polished and analyzed using electron back scattered diffraction (EBSD) to identify the crystallographic preferred orientation (CPO). The fabric that developed in olivine deformed at 3 GPa mainly resulted from the [100] slip on the (010) plane. Samples deformed at 5 GPa showed both [100] and [001] slip. On the other hand, samples deformed at 8 GPa and 1200° C, show deformation mainly

  18. Toward global waveform tomography with the SEM: Improving upper-mantle images

    NASA Astrophysics Data System (ADS)

    French, S. W.; Lekic, V.; Romanowicz, B. A.

    2011-12-01

    Over the past three decades, advances in theory and improved quality and coverage of global seismic data have lead to progressively higher-resolution global images of earth structure. While long-wavelength velocity structure correlates well across recent global models, notable differences remain - particularly in the amplitudes and gradients of velocity anomalies, crucial to characterizing the respective roles of temperature and composition in mantle dynamics. Establishing better constraints on these features represents a critical open problem, toward which advanced full-waveform modeling using finite-frequency approaches may be applied. Over the last 20 years, we have developed a full-waveform inversion methodology based on asymptotic normal mode coupling theory (Li and Romanowicz, 1995) and applied it to global elastic and anelastic tomography (e.g. Mégnin and Romanowicz, 2000; Gung and Romanowicz, 2004). The approximate synthetic waveforms thus computed can now easily be replaced by more accurate numerical synthetics, albeit at a considerably higher computational cost. As a first step toward this goal, the SEMum upper-mantle VS model of Lekic and Romanowicz (2011) was developed using the spectral element method (SEM: Komatitsch and Vilotte, 1998) to invert fundamental and higher mode surface waves (T ≥ 60s) of over 200 well-distributed global events. A coupled-SEM scheme was used to speed computation (cSEM: Capdeville, et al. 2003), combining SEM in the mantle with an efficient modal solution in a 1D core. The crust was implemented as a radially-anisotropic equivalent smooth model (e.g. Backus, 1962), adjusted to fit a global surface-wave dispersion dataset (Shapiro and Ritzwoller, 2002). A uniform 60km crustal thickness allowed us to take large time steps, further speeding computation. The lower mantle was fixed to 3D model SAW24B16 (Mégnin and Romanowicz, 2000). SEMum exhibits stronger VS anomalies in the uppermost 200km, while also attaining good

  19. Volcanology. The Yellowstone magmatic system from the mantle plume to the upper crust.

    PubMed

    Huang, Hsin-Hua; Lin, Fan-Chi; Schmandt, Brandon; Farrell, Jamie; Smith, Robert B; Tsai, Victor C

    2015-05-15

    The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we revealed a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 cubic kilometers, ~4.5 times that of the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment. PMID:25908659

  20. The Yellowstone magmatic system from the mantle plume to the upper crust

    NASA Astrophysics Data System (ADS)

    Huang, H. H.; Lin, F. C.; Schmandt, B.; Farrell, J.; Smith, R. B.; Tsai, V. C.

    2015-12-01

    The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we unveil a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 km3, ~4.5 times larger than the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment.

  1. The p-wave upper mantle structure beneath an active spreading centre - The Gulf of California

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1984-01-01

    Over 1400 seismograms of earthquakes in Mexico are analyzed and data sets for the travel time, apparent phase velocity, and relative amplitude information are utilized to produce a tightly constrained, detailed model for depths to 900 km beneath an active oceanic ridge region, the Gulf of California. The data are combined by first inverting the travel times, perturbing that model to fit the p-delta data, and then performing trial and error synthetic seismogram modelling to fit the short-period waveforms. The final model satisfies all three data sets. The ridge model is similar to existing upper mantle models for shield, tectonic-continental, and arc-trench regimes below 400 km, but differs significantly in the upper 350 km. Ridge model velocities are very low in this depth range; the model 'catches up' with the others with a very large velocity gradient from 225 to 390 km.

  2. The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation

    NASA Astrophysics Data System (ADS)

    Liu, Jingao; Scott, James M.; Martin, Candace E.; Pearson, D. Graham

    2015-08-01

    The role played by ancient melt-depleted lithospheric mantle in preserving continental crust through time is critical in understanding how continents are built, disrupted and recycled. While it has become clear that much of the extant Archean crust is underpinned by Archean mantle roots, reports of Proterozoic melt depletion ages for peridotites erupted through Phanerozoic terranes raise the possibility that ancient buoyant lithospheric mantle acts as a "life-raft" for much of the Earth's continental crust. Here we report the largest crust-lithospheric mantle age decoupling (∼2.4 Ga) so far observed on Earth and examine the potential cause for such extreme age decoupling. The Phanerozoic (<300 Ma) continental crust of West Otago, New Zealand, is intruded by Cenozoic diatremes that have erupted cratonic mantle-like highly depleted harzburgites and dunites. These peridotites have rhenium depletion Os model ages that vary from 0.5 to 2.7 Ga, firmly establishing the record of an Archean depletion event. However, the vast range in depletion ages does not correlate with melt depletion or metasomatic tracer indices, providing little support for the presence of a significant volume of ancient mantle root beneath this region. Instead, the chemical and isotopic data are best explained by mixing of relict components of Archean depleted peridotitic mantle residues that have cycled through the asthenosphere over Ga timescales along with more fertile convecting mantle. Extensive melt depletion associated with the "docking" of these melt residues beneath the young continental crust of the Zealandia continent explains the decoupled age relationship that we observe today. Hence, the newly formed lithospheric root incorporates a mixture of ancient and modern mantle derived from the convecting mantle, cooled and accreted in recent times. We argue that in this case, the ancient components played no earlier role in continent stabilization, but their highly depleted nature along with

  3. Lithium partitioning between olivine and diopside at upper mantle conditions: An experimental study

    NASA Astrophysics Data System (ADS)

    Yakob, Jessica L.; Feineman, Maureen D.; Deane, James A.; Eggler, David H.; Penniston-Dorland, Sarah C.

    2012-05-01

    Experiments were conducted at 1.5 GPa and temperatures between 700 °C and 1100 °C in order to assess the equilibrium distribution of lithium between olivine and diopside in the upper mantle. Lithium in olivine and diopside from natural mantle xenoliths displays a broad array of apparent partition coefficients ranging from ~ 0.2 to 10. In addition, a strikingly large range of lithium isotope ratios is observed in olivine and diopside from mantle xenoliths, with ∆7Liol-di (= δ7Liol - δ7Lidi) ranging from nearly zero to greater than 20‰. Both of these observations might be explained if the distribution of Li between olivine and diopside is strongly temperature dependent at mantle conditions such that a change in temperature, i.e. cooling upon exhumation, initiates diffusive re-equilibration of Li between phases in the xenolith. Accompanying dynamic fractionation of 6Li from 7Li due to differing diffusion rates of the two isotopes could then be permanently recorded in the xenolith if its temperature drops below the closure temperature before a new equilibrium is reached. The results of this study indicate a partition coefficient for Li between olivine and diopside (DLiol/di) of 2.0 ± 0.2 that is independent of temperature (within the error of our analyses) over the range 700 °C to 1100 °C. This lack of temperature dependence holds true when data from previous experiments at temperatures as high as 1375 °C are considered. Thus it appears that closed-system diffusion of Li between olivine and diopside in response to changing temperature is not an appropriate explanation for the observed range of elemental and isotopic distributions in natural xenoliths. Other possible explanations include Li redistribution in response to changing oxygen fugacity in the system, or diffusive addition or subtraction of Li during open-system interaction with an infiltrating melt or fluid.

  4. Electrical conductivity of partially-molten olivine aggregate and melt interconnectivity in the oceanic upper mantle

    NASA Astrophysics Data System (ADS)

    Laumonier, Mickael; Frost, Dan; Farla, Robert; Katsura, Tomoo; Marquardt, Katharina

    2016-04-01

    A consistent explanation for mantle geophysical anomalies such as the Lithosphere-Astenosphere Boundary (LAB) relies on the existence of little amount of melt trapped in the solid peridotite. Mathematical models have been used to assess the melt fraction possibly lying at mantle depths, but they have not been experimentally checked at low melt fraction (< 2 vol. %). To fill this gap, we performed in situ electrical conductivity (EC) measurement on a partially-molten olivine aggregate (Fo92-olivine from a natural peridotite of Lanzarote, Canary Islands, Spain) containing various amount of basaltic (MORB-like composition) melt (0 to 100%) at upper mantle conditions. We used the MAVO 6-ram press (BGI) combined with a Solartron gain phase analyser to acquire the electrical resistance of the sample at pressure of 1.5 GPa and temperature up to 1400°C. The results show the increase of the electrical conductivity with the temperature following an Arrhenius law, and with the melt fraction, but the effect of pressure between 1.5 and 3.0 GPa was found negligible at a melt fraction of 0.5 vol.%. The conductivity of a partially molten aggregate fits the modified Archie's law from 0.5 to 100 vol.%. At melt fractions of 0.25, 0.15 and 0.0 vol.%, the EC value deviates from the trend previously defined, suggesting that the melt is no longer fully interconnected through the sample, also supported by chemical mapping. Our results extend the previous results obtained on mixed system between 1 and 10% of melt. Since the melt appears fully interconnected down to very low melt fraction (0.5 vol.%), we conclude that (i) only 0.5 to 1 vol.% of melt is enough to explain the LAB EC anomaly, lower than previously determined; and (ii) deformation is not mandatory to enhance electrical conductivity of melt-bearing mantle rocks.

  5. Os isotope heterogeneity of the upper mantle: Evidence from the Mayarí Baracoa ophiolite belt in eastern Cuba

    NASA Astrophysics Data System (ADS)

    Frei, R.; Gervilla, F.; Meibom, A.; Proenza, J. A.; Garrido, C. J.

    2006-01-01

    In an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle, 27 chromite separates from the 90 My old chromite deposits in the Mayarí-Baracoa ophiolite belt in eastern Cuba have been investigated for platinum group element (PGE) concentrations and Re-Os isotopic systematics. The samples are characterized by systematically subchondritic initial 187Os/ 188Os ratios and substantial heterogeneity. The initial 187Os/ 188Os ratios vary with chromite chemistry and with geographical distribution, reflecting differences in the Os isotopic evolution for the different upper mantle sections represented by the ophiolite. Accordingly, the Os isotope data might be divided into three groups. In the Moa-Baracoa district, where the chromite bodies are located in the mantle-crust transition zone, the calculated initial γOs values average - 0.97 ± 0.69 ( n = 13). In the Sagua de Tanamo district, where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear, the initial γOs values are intermediate, with an average of - 1.77 ± 0.80 ( n = 7). In the Mayarí district, where the chromite bodies are located in the lower part of the mantle sequence, initial γOs values average - 2.66 ± 0.29 ( n = 7). These subchondritic (i.e. negative) initial γOs values are most simply explained by Re depletion during ancient partial melting and/or melt percolation events. The Os isotope heterogeneity documented here indicates a high degree of geochemical complexity on small to intermediate length scales in the upper mantle. Our results, in combination with data on chromites from the literature, show that an "average present-day Os isotopic composition" for the hypothetical depleted MORB mantle (DMM) reservoir cannot be precisely established beyond the statement that it is "broadly chondritic". Indeed, the upper mantle cannot be considered a sufficiently homogeneous geochemical

  6. A global tomographic model of shear attenuation in the upper mantle

    NASA Astrophysics Data System (ADS)

    Romanowicz, B.

    1995-07-01

    We present a global three-dimensional model of shear attenuation in the upper mantle, based on the measurement of amplitudes of low-frequency (100-300s) Rayleigh waves observed at stations of the Geoscope and Iris networks. Attenuation coefficients are measured on R1 and R2 paths using a method which minimizes the effects of focussing due to propagation in a three-dimensional elastic Earth. Through a series of tests which, in particular, involve the computation of synthetic models of attenuation and focussing, we demonstrate that long wavelength lateral variations in attenuation in the first 400-500 km of the mantle can indeed be resolved. The model is obtained in a two-step procedure. The first step consists in the computation of maps of Rayleigh wave attenuation at different periods, using an inversion method without a priori parametrisation, which involves the introduction of a correlation length, chosen here at 3000 km to optimize the trade-off between resolution and variance in the model. In the second step, after corrections for shallow structure, an inversion with depth is performed, assuming lateral heterogeneity is confined to depths between 80 and 650 km. The resulting model presents lateral variations in Qβ that are correlated with tectonic features, in particular ridges and shields in the first 250 km of the upper mantle. Below that depth the pattern shifts and becomes correlated with the hotspot distribution, particularly so if the buoyancy strength of hotspots is taken into account. Two major low-velocity zones appear to be located in the central pacific and beneath northern Africa, in the depth range 300-500 km. This pattern seems to continue at greater depth, but resolution becomes insufficient below 500 km to draw definitive conclusions. The smooth lateral variations retrieved are on the order of ±50% down to 400 km. We propose an interpretation in terms of plume/lithosphere/ridge interaction in the upper mantle, arguing for deflection of the

  7. Hydrogen in the upper mantle: Diffusion and effects on olivine transformation kinetics

    NASA Astrophysics Data System (ADS)

    Du Frane, Wyatt Louis

    Olivine is the most abundant mineral in Earth's upper mantle and can host significant amounts of hydrogen within its crystal structure. The presence of hydrogen affects many of olivine's physical properties such as electrical conductivity, viscosity, sound speed, transformation kinetics, phase equilibrium, and generally speaking the physics governing the interior of the earth. Understanding how hydrogen affects olivine is integral to understanding the Earth's interior. In this work olivine was experimentally hydrated and reacted at high pressure and temperature, to simulate upper mantle conditions. The physical properties measured in this work are used to understand seismic and magnetotelluric observations of the Earth. In the first project the effects of hydrogen on olivine transformation kinetics were examined. Growth rates for olivine's high pressure polymorphs, wadsleyite and ringwoodite, to determine if olivine can persist metastably inside cold subducting slabs in the mantle transition zone. Hydrogen significantly enhances the growth rates of olivine into ringwoodite. For olivine containing ˜75 (or higher) ppmw H2O At 18 GPa and 900°C the growth rate for ringwoodite rims is 1.0x10-9 m/s with activation enthalpy of 235 +/- 30 kJ/mol, which is too high for persistence of metastable olivine into the transition zone. Confirmation of the existence of metastable olivine by seismologists would constrain H2O contents at such locations to be < 75 ppmw H2O. In the second project deuterium-hydrogen interdiffusion coefficients were measured to help understand electrical conductivity, point defect populations, chemical transport, and defect dominated properties in olivine. For the fastest H-diffusing [100] orientation DD-H, [100] = 10(-5.04 +/- 1.43)*e(-137 +/- 31 kJ/mol)/(RT) m²/s at 2 GPa and 750--900°C. Comparison of DD-H to chemical diffusion coefficients allows us to calculate diffusivity of intrinsic defects. Olivine electrical conductivity is calculated from DD

  8. Seismic structure of the upper mantle beneath the southern Kenya Rift from wide-angle data

    NASA Astrophysics Data System (ADS)

    Byrne, G. F.; Jacob, A. W. B.; Mechie, J.; Dindi, E.

    1997-09-01

    In February 1994, the Kenya Rift International Seismic Project carried out two wide-angle reflection and refraction seismic profiles between Lake Victoria and Mombasa across southern Kenya. Our investigation of the data has revealed evidence for the presence of two upper mantle reflectors beneath southwestern Kenya, sometimes at short range, from seven shotpoints. Two-dimensional forward modelling of these reflectors using a pre-existing two-dimensional velocity-depth model for the crust [Birt, C.S., Maguire, P.H.K., Khan, M.A., Thybo, H., Keller, G.R., Patel, J., 1997. The influence of pre-existing structures on the evolution of the Southern Kenya Rift Valley — evidence from seismic and gravity studies. Tectonophysics 278, 211-242], has shown them to lie at depths of approximately 51 and 63 km. The upper reflector, denoted d 1, shallows by about 5-10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reflector, denoted d 2, are sparse and more difficult to determine, so it was not possible to define any shallowing corresponding to the surface expression of the rift. Only limited control exists over the upper mantle velocities used in the modelling. Immediately beneath the Moho we use a value of P n calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d 1 reflector a reasonable velocity contrast was introduced to produce a reflector for modelling purposes. Beneath the d 1 reflector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across d 1 and again, decreases to the average value over the next 3 km. At the d 2 reflector a similar model is used. This model accounts for the presence of the mantle reflectors seen in the data by using layers of thin higher velocity in a lower background velocity. Due to the uncertainty in the velocities the absolute position of both d 1 and d 2

  9. The Electrical Structure of Upper Mantle Beneath 70Ma Pacific Seafloor Constrained by Seafloor Magnetotelluric Data

    NASA Astrophysics Data System (ADS)

    Tursack, E. K.; Evans, R. L.; Elsenbeck, J.; Lizarralde, D.; Collins, J. A.; Gaherty, J. B.; Hirth, G.

    2013-12-01

    The NOMELT experiment focused on understanding the structure of 70Ma oceanic lithosphere and underlying asthenosphere. The experiment used a combination of seismic and magnetotelluric (MT) techniques to image the lithosphere-asthenosphere boundary (LAB) and determine if partial melt is present beneath the lithosphere, or if another mechanism is responsible for the transition between lithosphere and asthenosphere at this intermediate plate age. We inverted the seafloor MT data from 4 stations to constrain the electrical structure of the LAB. We conducted two-dimensional regularized isotropic and anisotropic inversions. The resulting electrical resistivity model was then averaged into a one-dimensional profile and compared with data from the Marianas subduction zone [1], the Phillipine Sea [2], the MELT Experiment at 17°S on the EPR [3], and the Middle America Trench [4]. The preferred electrical resistivity model for the NOMELT region is isotropic and does not contain a highly conductive layer under the 70-80 km thick resistive lithosphere. This lack of a conductive layer suggests that partial melt is not present in a well-connected network within the lithosphere-asthenosphere boundary of 70Ma oceanic plate, in contrast to other regions [2, 4]. The lack of anisotropy within the upper asthenosphere is also in contrast to previous electromagnetic studies of oceanic settings that invoked a more hydrous asthenosphere [3, 5]. [1] Matsuno T, N Seama, RL Evans, AD Chave, K Baba, A White, T Goto, G Heinson, G Boren, A Yoneda, H Utada (2010) Upper mantle electrical resistivity structure beneath the central Mariana subduction system. Geochem. Geophys. Geosys., 11: Q09003, doi:10.1029/2010GC003101. [2] Baba K, H Utada, T Goto, T Kasaya, H Shimizu, N Tada (2010) Electrical conductivity imagine of the Philippine Sea upper mantle using seafloor magnetotelluric data. Phys. Earth and Planet. Int. 183: 44-62. [3] Evans RL, G Hirth, K Baba, D Forsyth, A Chave, R Mackie (2005

  10. Global distribution of azimuthal anisotropy within the upper mantle and the crust

    NASA Astrophysics Data System (ADS)

    Schaeffer, Andrew; Lebedev, Sergei

    2014-05-01

    We present our new global, azimuthally anisotropic model of the upper mantle and the crust. We compare two versions of this new model, the rough SL2013svAr and smooth SL2013svA, which are constrained by a larger, updated waveform fit dataset (>900, 000 vertical component seismogram fits) than that used in the construction of the isotropic model SL2013sv (Schaeffer and Lebedev, 2013). These two anisotropy models are computed using a more precise regularization of anisotropy, which is tuned to honour the both the amplitude and orientation of the anisotropic terms uniformly, including near the poles. Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms, yielding resolving power from the crust down to the transition zone. Our unprecedentedly large waveform dataset, with complementary high-resolution regional arrays in additional to global networks, produces improved resolution of global azimuthal anisotropy patterns. The model also reveals smaller scale patterns of 3D anisotropy variations related to regional lithospheric deformation and mantle flow, in particular in densely sampled regions. In oceanic regions, we examine the strength of azimuthal anisotropy, as a function of depth, spatial position with respect to the spreading ridge, and deviation in fast axis orientation from the current and fossil spreading directions. In continental regions, azimuthal anisotropy is more complex. Reconciling complementary observations given by shear wave splitting, surface-wave array analysis, and large-scale, global 3D models offers new insights into the mechanisms of continental deformation and the architecture and evolution of the lithosphere. Finally, quantitative comparisons with other recently published models demonstrate which features are consistently resolved across the different models, and therefore provide a means to estimate the robustness of anisotropic patterns and amplitudes. Reference: Schaeffer, A. J

  11. IRIS and the S-velocity structure of the North American upper mantle

    NASA Astrophysics Data System (ADS)

    van der Lee, S.; Frederiksen, A. W.

    2004-12-01

    Owing to its US-based origin and resulting seismogram holdings the Data Management Center (DMC) of the Incorporated Research Institutions for Seismology (IRIS) has greatly facilitated waveform tomographic studies worldwide, and for North America in particular. We report on one such undertaking, in which nearly one and a half thousand seismograms from the IRIS DMC and the Canadian National Seismic Network have been interactively analyzed and used in a Partitioned Waveform Inversion for a tomographic model for the three-dimensional S-velocity structure of the North American upper mantle. A predecessor (NA95) of this new model is consistent with global tomographic models and revealed additional detail such as an upper-mantle component of subducted Farallon lithosphere, an enigmatic structure for the Wyoming lithosphere, and a V-shaped dent in the new England cratonic lithosphere. These details in turn helped spark additional IRIS activity in the form of further analyses of the data holdings of the DMC and PASSCAL experiments addressing these details. The new model provides relatively high-resolution images of the high-velocity rigid root beneath the Canadian shield and central US, which extends to depths of 200-300 km, the low velocities beneath the tectonically active Cordillera and the continent west of it, which also reach depths of 200-300 km, and details herein such as those mentioned above. Below these structures, high-velocity features in the transition zone are not as steep as but in line with the dipping high-velocity Farallon slab imaged in the lower mantle with tomographic methods that include teleseismic body waves. Increased accuracy in the new model, relative to its predecessors, is largely a result of extending the data base that constrains it. We checked the effects of using sensitivity kernels that cover elliptical areas around the great circles and found that they do not lead to better a posteriori data fits.

  12. Q tomography of the upper mantle using three-component long-period waveforms

    NASA Astrophysics Data System (ADS)

    Gung, Y.; Romanowicz, B.

    2004-05-01

    We present a degree-8 3-D Q model (QRLW8) of the upper mantle, derived from three-component surface waveform data in the period range 60-400 s. The inversion procedure involves two steps. In the first step, 3-D whole-mantle velocity models are derived separately for elastic SH (transverse component) and SV (vertical and longitudinal component) velocity models, using both surface and body waveforms and the non-linear asymptotic coupling theory (NACT) approach. In the second step, the surface waveforms thus aligned in phase are inverted to obtain a 3-D Q model in the depth range 80-670 km. Various stability tests are performed to assess the quality of the resulting Q model and, in particular, to assess possible contamination from focusing effects. We find that the 3-D patterns obtained are stable, but the amplitude of the lateral variations in Q is not well constrained, because large damping is necessary to extract the weak Q signal from data. The model obtained agrees with previous results in that a strong correlation of Q with tectonics is observed in the first 250 km of the upper mantle, with high attenuation under oceanic regions and low attenuation under continental shields. It is gradually replaced by a simpler pattern at larger depth. At the depths below 400 km, the Q distribution is generally dominated by two strong minima, one under the southern Pacific and one under Africa, yielding a strong degree-2 pattern. Most hotspots are located above regions of low Q at this depth. Ridges are shallow features in both velocity and Q models.

  13. Lithospheric and Upper-Mantle Structure of the Red Sea and Arabian Peninsula

    NASA Astrophysics Data System (ADS)

    Hansen, S. E.; Schwartz, S. Y.; Rodgers, A. J.; Gaherty, J. B.; Al-Amri, A. M.

    2007-12-01

    Using broadband seismic data recorded by various networks, a variety of techniques have been employed to investigate the lithospheric and upper-mantle structure of the Red Sea and Arabian Peninsula. This presentation will summarize our findings and conclusions about the tectonic evolution and current state of the Arabian Plate. S-wave receiver functions provide constraints on the lithospheric thickness and reveal very thin lithosphere (40-80 km) along the Red Sea coast, which thickens rapidly toward the interior of the Arabian Shield (100-120 km). A step of 20-40 km in lithospheric thickness is also observed at the Shield-Platform boundary. Mantle anisotropy has been analyzed using shear-wave splitting of teleseismic SKS waveforms. The consistent north-south oriented fast directions are not adequately explained by end-member models of fossilized anisotropy and present-day plate motion and have instead been explained by a combination of plate- and density-driven flow in the asthenosphere. Further constraints on the upper mantle velocity and anisotropy have been obtained by jointly inverting the receiver function constraints with frequency dependent surface wave phase delays. The results demonstrate that the thin lithospheric lid is underlain by a pronounced low-velocity zone and that anisotropy is required in both the lithosphere and asthenosphere. Attenuation and thermal estimates are also being explored and preliminary results will be presented. The combined results of these studies support a two-stage rifting history for the Red Sea, where extension and erosion by asthenospheric flow are responsible for variations in the lithospheric thickness. These lithospheric variations guide asthenospheric flow beneath western Arabia and the Red Sea, leading to a large-scale thermal anomaly that is associated with Cenozoic uplift and volcanism. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National

  14. Three-Dimensional Model for the Crust and Upper Mantle in the Barents Sea Region

    NASA Astrophysics Data System (ADS)

    Bungum, H.; Ritzmann, O.; Maercklin, N.; Faleide, J.-I.; Mooney, W. D.; Detweiler, S. T.

    2005-04-01

    The Barents Sea and its surroundings is an epicontinental region which previously has been difficult to access, partly because of its remote Arctic location (Figure 1) and partly because the region has been politically sensitive. Now, however, this region, and in particular its western parts, has been very well surveyed with a variety of geophysical studies, motivated in part by exploration for hydrocarbon resources. Since this region is interesting geophysically as well as for seismic verification, a major study [Bungum et al., 2004] was initiated in 2003 to develop a three-dimensional (3-D) seismic velocity model for the crust and upper mantle, using a grid density of 50 km. This study, in cooperation between NORSAR, the University of Oslo (UiO), and the United States Geological Survey (USGS), has led to the construction of a higher-resolution, regional lithospheric model based on a comprehensive compilation of available seismological and geophysical data. Following the methodology employed in making the global crustal model CRUST5.1 [Mooney et al., 1998], the new model consists of five crustal layers: soft and hard sediments, and crystalline upper, middle, and lower crust. Both P- and S-wave velocities and densities are specified in each layer. In addition, the density and seismic velocity structure of the uppermost mantle, essential for Pn and Sn travel time modeling, are included.

  15. Crustal and upper mantle structure of stable continental regions in North America and northern Europe

    USGS Publications Warehouse

    Masse, R.P.

    1987-01-01

    From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. The P wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thick P wave low velocity channel beginning at a depth of about 94 km is also present. A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. The S wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95-100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere. ?? 1987 Birkha??user Verlag.

  16. Crustal and upper mantle structures beneath Cenozoic volcanoes on the board of China and North Korea.

    NASA Astrophysics Data System (ADS)

    Rhie, J.; Kim, S.

    2015-12-01

    The Cenozoic-to-recent volcanoes on the border of China and North Korea are recognized as continental intraplate volcanoes. Despite of much work, the origin and mechanism of the volcanoes remain as an issue of debate, due to their complex and long-lived volcanic activities and lack of detailed information for the crust and upper mantle structures. In this work, ambient noise analysis is performed to image lithospheric structures beneath the volcanoes and surrounding regions using continuous broadband recordings of two temporary networks (1998-1999 PASSCAL array and a part of the 2009-2011 NECASSArray). To better constrain the entire depths of lithosphere in the estimated 3-D velocity structure, we utilize the spectral auto-correlation (SPAC) method and a Bayesian inversion technique to measure phase velocity dispersion data and to obtain shear-wave velocity structures, respectively. We developed a novel grid-search technique for more stable SPAC measurements, and obtained phase velocity data are compared and combined with group and phase velocity data from the conventional frequency-time analysis. Hierarchical and trans-dimensional techniques are implemented in the Bayesian method to estimate more rigorous models and associated uncertainties. The estimated 3-D model shows slower velocity (~0.3 km/s) at the bottom of lithosphere (>60 km) and less modified thick-crust beneath the volcanoes compared to other regions in the model. This suggests our model favors the theory of magma underplating, crustal assimilation, and less volume of magma supply from upper mantle.

  17. The electrical conductivity of the upper mantle as estimated from satellite magnetic field data

    NASA Technical Reports Server (NTRS)

    Didwall, E. M.

    1984-01-01

    The electrical conductivity of the upper mantle is estimated from low-latitude magnetic field variations caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land-based observatories. The procedures of analysis consist of: (1) separation of the disturbance field into internal and external parts relative to the surface of the earth, (2) estimation of an electromagnetic response function Q(omega) which relates the internally generated magnetic field variations to the external variations due to the ring current, and (3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(omega) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 0.01 S/m.

  18. Deep mantle structure as a reference frame for movements in and on the Earth

    PubMed Central

    Torsvik, Trond H.; van der Voo, Rob; Doubrovine, Pavel V.; Burke, Kevin; Steinberger, Bernhard; Ashwal, Lewis D.; Trønnes, Reidar G.; Webb, Susan J.; Bull, Abigail L.

    2014-01-01

    Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core–mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth’s axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth’s mantle may have operated throughout the entire Phanerozoic. PMID:24889632

  19. Deep mantle structure as a reference frame for movements in and on the Earth.

    PubMed

    Torsvik, Trond H; van der Voo, Rob; Doubrovine, Pavel V; Burke, Kevin; Steinberger, Bernhard; Ashwal, Lewis D; Trønnes, Reidar G; Webb, Susan J; Bull, Abigail L

    2014-06-17

    Earth's residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core-mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth's surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core-mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth's axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth's mantle may have operated throughout the entire Phanerozoic. PMID:24889632

  20. The granite-upper mantle connection in terrestrial planetary bodies: an anomaly to the current granite paradigm?

    NASA Astrophysics Data System (ADS)

    Bonin, Bernard; Bébien, Jean

    2005-03-01

    Granite formed in the terrestrial planets very soon after their accretion. The oldest granite-forming minerals (4.4 Ga zircon) and granite (4.0 Ga granodiorite) indicate conditions resembling the present-day ones, with the presence of oceans and external processes related to liquid water. As a result, the current granite paradigm states that granite is not issued directly from the melting of the mantle. However, a granite-upper mantle connection is well established from several pieces of evidence. Tiny micrometre- to millimetre-sized enclaves of granite-like glassy and crystalline materials in Earth's mantle rocks are known in oceanic and continental areas. Earth's mantle-forming minerals, such as olivine, pyroxene, and chromite, can contain silicic materials, either as glass inclusions or as crystallised products (quartz or tridymite, sanidine, K-feldspar, and/or plagioclase close to albite end-member). Importantly, the same evidence is amply found in some types of meteorites, whether they are primitive, such as ordinary chondrites, or differentiated, such as IIE irons, howardite eucrite diogenite (HED), and Martian shergottite nakhlite chassignite (SNC) achondrites. Although constituting apparently an anomaly, the granite-upper mantle connection can be reconciled with the current granite paradigm by recognising that the conditions prevailing in the formation of granite are not only necessarily crustal but can occur also at depths in mantle rocks. Unresolved problems to be explored further include whether tiny amounts of granitic material within terrestrial mantles may be hints of greater abundances and more direct mantle involvement, and what role can be played by granite trapped within the upper mantle in lithosphere buoyancy.

  1. Shear wave splitting hints at dynamical features of mantle convection: a global study of homogeneously processed source and receiver side upper mantle anisotropy

    NASA Astrophysics Data System (ADS)

    Walpole, J.; Wookey, J. M.; Masters, G.; Kendall, J. M.

    2013-12-01

    The asthenosphere is embroiled in the process of mantle convection. Its viscous properties allow it to flow around sinking slabs and deep cratonic roots as it is displaced by intruding material and dragged around by the moving layer above. As the asthenosphere flows it develops a crystalline fabric with anisotropic crystals preferentially aligned in the direction of flow. Meanwhile, the lithosphere above deforms as it is squeezed and stretched by underlying tectonic processes, enabling anisotropic fabrics to develop and become fossilised in the rigid rock and to persist over vast spans of geological time. As a shear wave passes through an anisotropic medium it splits into two orthogonally polarised quasi shear waves that propagate at different velocities (this phenomenon is known as shear wave splitting). By analysing the polarisation and the delay time of many split waves that have passed through a region it is possible to constrain the anisotropy of the medium in that region. This anisotropy is the key to revealing the deformation history of the deep Earth. In this study we present measurements of shear wave splitting recorded on S, SKS, and SKKS waves from earthquakes recorded at stations from the IRIS DMC catalogue (1976-2010). We have used a cluster analysis phase picking technique [1] to pick hundreds of thousands of high signal to noise waveforms on long period data. These picks are used to feed the broadband data into an automated processing workflow that recovers shear wave splitting parameters [2,3]. The workflow includes a new method for making source and receiver corrections, whereby the stacked error surfaces are used as input to correction rather than a single set of parameters, this propagates uncertainty information into the final measurement. Using SKS, SKKS, and source corrected S, we recover good measurements of anisotropy beneath 1,569 stations. Using receiver corrected S we recover good measurements of anisotropy beneath 470 events. We compare

  2. Inversion of gravity and bathymetry in oceanic regions for long-wavelength variations in upper mantle temperature and composition

    NASA Technical Reports Server (NTRS)

    Solomon, Sean C.; Jordan, Thomas H.

    1993-01-01

    Long-wavelength variations in geoid height, bathymetry, and SS-S travel times are all relatable to lateral variations in the characteristic temperature and bulk composition of the upper mantle. The temperature and composition are in turn relatable to mantle convection and the degree of melt extraction from the upper mantle residuum. Thus the combined inversion of the geoid or gravity field, residual bathymetry, and seismic velocity information offers the promise of resolving fundamental aspects of the pattern of mantle dynamics. The use of differential body wave travel times as a measure of seismic velocity information, in particular, permits resolution of lateral variations at scales not resolvable by conventional global or regional-scale seismic tomography with long-period surface waves. These intermediate scale lengths, well resolved in global gravity field models, are crucial for understanding the details of any chemical or physical layering in the mantle and of the characteristics of so-called 'small-scale' convection beneath oceanic lithosphere. In 1991 a three-year project to the NASA Geophysics Program was proposed to carry out a systematic inversion of long-wavelength geoid anomalies, residual bathymetric anomalies, and differential SS-S travel time delays for the lateral variation in characteristic temperature and bulk composition of the oceanic upper mantle. The project was funded as a three-year award, beginning on 1 Jan. 1992.

  3. The CIFALPS seismic experiment: first high-resolution data on the crust and upper mantle structures of the southwestern Alps

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Paul, A.; Solarino, S.; Aubert, C.; Zheng, T.; Salimbeni, S.; Guillot, S.; Zhu, R.; Wang, Q.

    2013-12-01

    The Alpine belt, being a most studied mountain belt by geologists, is an ideal natural laboratory for understanding the processes and mechanism of orogeny. A number of questions on the dynamics of the Alps, however, remain open due to the lack of detailed data on its lithospheric and sublithospheric structures. This is particular true for the very arcuate southwestern part of the belt. In order to improve images of the crust and upper mantle beneath the southwestern Alps, we have installed a temporary broadband seismic array across the belt from the Rhone valley (France) to the Po plain (Italy). The main sub-array of the CIFALPS (China-Italy-France Alps seismic survey) project is a 350-km long roughly linear profile of 46 stations trending WSW-ENE from Bollène (France) to the north of Alessandria (Italy). Its average station spacing is smaller than 10 km, with a densification to 5 km in the internal Alps. Nine additional temporary stations located ~40 km to the north and south of the main profile complement the adjacent permanent broadband networks to improve the 3-D constraints on the deep structures. Most stations are equipped with three-component broadband sensors (50Hz -120s). The array was installed in the summer of 2012 and will be operated till to September 2013. We used available dataset to compute receiver functions and analyze shear-wave splitting from SKS phases. A common-conversion point migrated receiver function section displays a strong Moho P-to-S conversion beneath the western end of the profile, which fluctuates in amplitude and depth from beneath the Vocontian basin to the Penninic front, and becomes hardly distinguishable beneath the internal zones. This image is consistent with the results of the controlled-source ECORS-CROP profile in the northwestern Alps. The lateral change in Moho signature on the ECORS-CROP line was interpreted as the result of signal attenuation across the very heterogeneous upper crust of the internal zones. We however

  4. A seismic discontinuity in the upper mantle between the Eastern Alps and the Western Carpathians: Constraints from wide angle reflections and geological implications

    NASA Astrophysics Data System (ADS)

    Oeberseder, T.; Behm, M.; Kovács, I.; Falus, G.

    2011-05-01

    Seismic investigation of the lithosphere by means of active source experiments is mostly confined to the crust and the Moho. Structures in the upper mantle are more likely to be discovered by analyses of teleseismic data, although these methods are restricted in their resolution capabilities. The relatively rare evidence for upper mantle refractors or reflectors in active source data enables challenging and interesting studies of the lower and not so well known part of the lithosphere. We present such an example from the tectonically complex region between the Eastern Alps and the Western Carpathians. This area was covered by several extensive 3D wide-angle reflection/refraction experiments within the last decade, and their layout was designed to illuminate the crustal structure and in particular the Moho discontinuity. In some areas, reflections from below the Moho are also recorded. These reflections occur at recording offsets between 200 and 500 km, and they are particularly strong in cross line recordings. We derive a set of travel times from the data and perform a tomographic inversion for the depth and shape of the reflecting interface. The inversion makes use of an existing 3D crustal model which also includes the Moho topography. Since the upper mantle velocities are poorly constrained and the azimuthal distribution of the rays is biassed, several tests are applied to investigate the reliability of possible solutions. The results from the tomographic inversion indicate an overall horizontal and radially dipping reflector. The average depth of the reflector is 55 km, which is about 25 km below the crust-mantle transition, and amplitude modelling suggests that the reflecting interface represents a velocity increase. The investigated area is further characterised by deep sedimentary basins, high heat flow, high velocities in the lower crust, diffuse Moho signature and a strong positive Bouguer anomaly. Nearby xenolith outcrops exhibit a pronounced change in

  5. Geology of the Crust and Mantle, Western United States: Geophysical data reveal a thin crust and anomalous upper mantle characteristic of active regions.

    PubMed

    Thompson, G A; Talwani, M

    1964-12-18

    Seismic refraction, gravity, phase velocity, and magnetic data, coupled with the geologic record, are all approximately satisfied by the structure shown in Fig. 9. A 20-kilometer crust under the Coast Ranges and Great Valley thickens to more than 30 kilometers under the Sierra Nevada and parts of the Basin and Range province; this whole area is underlain by an anomalous upper mantle with a velocity and density about 3 percent less than normal. It is not likely that the anomalous mantle extends much deeper than 50 kilometers, and the lower boundary may be gradational. The thicker crust or "root" under the Sierran highland region (Sierra Nevada and western Basin Ranges) is not limited to the Sierra Nevada proper. The root and the voluminous plustonic rocks originated in the Mesozoic era, and they constitute the now consolidated core of the Cordilleran eugeosyncline. But it must not be supposed that the root has persisted unchanged. The great mountain-building uplifts in the Cenozoic era must have been accompanied by large changes in the root and adjacent mantle. A zone of positive gravity and magnetic anomalies extending the length of the Great Valley is associated with mafic rocks of the western Sierra greenstone belt, an element of the Cordilleran eugeosyncline. Belts of maficto-intermediate lavas, accompanied by mafic and ultramafic intrusions, are marked by similar anomalies in other ancient geosynclines. An anomalous upper mantle of plagioclase peridotite, an expanded phase of the normal mantle, could explain about 1 kilometer of the uplift that took place over much of the region in Cenozoic time. To explain all of the Cenozoic uplift in the Sierra Nevada and Basin Ranges by this means would require the hypothesis of a separation of the anomalous mantle into crust and normal mantle fractions, followed by a renewal of the anomalous mantle through the action of regional convection currents or local overturning in the upper mantle. The low-velocity zones for

  6. The Oxidation State of Fe in MORB Glasses and the Oxygen Fugacity of the Upper Mantle

    SciTech Connect

    E Cottrell; K Kelley

    2011-12-31

    Micro-analytical determination of Fe{sup 3+}/{Sigma}Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure ({mu}-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe{sup 3+}/{Sigma}Fe ratios of 0.16 {+-} 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the quartz-fayalite-magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe{sup 3+}/{Sigma}Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or < 10%) of the difference between Fe{sup 3+}/{Sigma}Fe ratios determined by micro-colorimety and XANES can be attributed to the Moessbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe{sup 3+}/{Sigma}Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe{sup 3+} behaving incompatibly in shallow MORB magma chambers. MORB Fe{sup 3+}/{Sigma}Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na{sub 2}O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe{sup 3+} may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at {approx} QFM. Both explanations, in combination with the measured MORB Fe{sup 3+}/{Sigma}Fe ratios, point to a fertile MORB source with greater than 0.3 wt.% Fe{sub 2}O{sub 3}.

  7. Global variations in azimuthal anisotropy of the Earth's upper mantle and crust

    NASA Astrophysics Data System (ADS)

    Schaeffer, A. J.; Lebedev, S.

    2013-12-01

    Deformation within the Earth's crust and mantle often results in crystallographic preferred orientations that produce measurable seismic anisotropy. Shear wave splitting measurements have the benefit of excellent lateral resolution and are an unambiguous indicator of the presence of seismic anisotropy; however, they suffer from poor depth resolution (integrated measurement from CMB to surface), in addition to being geographically limited (measurements only made at seismometer locations). The analysis of surface wave propagation also provides insight into the azimuthal variations in wave-speed, but with significantly better depth resolution. Thanks to the rapid increase in the number of seismic stations around the world, increasingly accurate, high-resolution 3D models of azimuthal anisotropy can be calculated using surface-wave tomography. We present our new global, azimuthally anisotropic model of the upper mantle and the crust. Compared to its recent predecessor, SL2013sv (Schaeffer and Lebedev, 2013), it is constrained by an even larger waveform fit dataset (>900,000 versus 712,000 vertical-component seismograms, respectively) and was computed using a more precise regularization of anisotropy, tuned to honour the amplitude and orientation of the anisotropic terms uniformly, including near the poles. Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms, yielding resolving power from the crust down to the transition zone. Our unprecedentedly large waveform dataset, with complementary high-resolution regional arrays (including USArray) and global network sub-sets within it, produces improved resolution of global azimuthal anisotropy patterns. The model also reveals smaller scale patterns of 3D anisotropy variations related to regional lithospheric deformation and mantle flow, in particular in densely sampled regions. In oceanic regions, we examine the strength of azimuthal anisotropy, as a function of

  8. Towards the next generation of global 3D upper mantle Q models

    NASA Astrophysics Data System (ADS)

    Gung, Y.; Romanowicz, B.; Capdeville, Y.

    2003-12-01

    Global anelastic tomography can bring important constraints on the thermal structure of the mantle and therefore is dynamics, complementing those provided by elastic tomography. Progress in anelastic tomography has been slow, because of the inherent technical difficulties encountered in discriminating anelastic signal from elastic effects on amplitude data. It has been shown that while the elastic focusing/defocusing effects are not significant at low degrees ( ˜ 8) (e.g. Selby and Woodhouse, 2002; Gung and Romanowicz, 2003), they need to be included to achieve a higher resolution Q model. Ideally, one would use an exact method, such as the Spectral Element Method (SEM) for predicting the focusing effects. SEM is however very heavy computationally. We present a procedure to better constrain the 3D upper mantle Q from 3 component long-period seismic waveforms. In this procedure, the amplitude and phase perturbations due to the 3D elastic structure are corrected for using higher order normal mode asymptotic theory, and applying it to current elastic models. We first evaluate the normal mode asymptotic approach by comparing the corresponding 3D synthetics with those computed using the coupled spectral element/normal mode method (CSEM). 3 normal mode based asymptotic approaches are compared: path average approximation (PAVA), non-linear asymptotic coupling theory (NACT) and NACT+F, an extension of NACT with focusing terms computed using higher order asymptotic theory. Systematic waveform comparison and inversion experiments are implemented. We find that (1) when the anomaly lies on the source-receiver great circle path, the 3 techniques are fairly accurate for fundamental mode surface waves, but NACT and NACT+F provide much better fit for overtone phases and are therefore more powerful in resolving 3D structure in the mid and lower mantle; and (2) the off-great-circle effects, which result in focusing/defocusing and not seen by PAVA or NACT, are well explained by NACT

  9. Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications

    USGS Publications Warehouse

    Wang, Chun-Yong; Chan, W.W.; Mooney, W.D.

    2003-01-01

    Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a three-dimensional crustal and upper mantle velocity structure of southwestern China (21??-34??N, 97??-105??E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large-scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50-km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50-km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a high-conductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

  10. The crustal and upper-mantle structure of the interior Arabian platform

    NASA Astrophysics Data System (ADS)

    Al-Amri, Abdullah M. S.

    1999-02-01

    The crustal and upper-mantle velocity structure of the interior Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is utilized to obtain crustal transfer functions using the Thomson-Haskell matrix formulation for horizontally layered crustal models. 20 earthquakes recorded at the long-period station RYD between azimuths N20 degW and N150 degE were selected for the analysis based on the following criteria: focal depths in the range 5 to 215 km, body-wave magnitudes greater than 5.0, and epicentral distances in the range 7 deg to 97 deg. A careful quality check of the data left us with six events, out of 29, that had short epicentral distances (<20 deg) to be analysed. The selection criterion for the final model in the forward modelling process was based on the correlation coefficient between observed and theoretical transfer function. The model suggested that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km s^- ^1 and is about 3 km thick. The second layer has a velocity of about 6.3 km s^- ^1 and is 10 km thick. The third layer has a velocity of 6.6 km s^- ^1 and is 8 km thick. The fourth layer has a velocity of 6.9 km s^- ^1 and is 15 km thick. The lower layer has a velocity of about 7.6 km s^- ^1 and is 10 km thick. For the Mohorovicic discontinuity, a velocity of 8.3 km s^- ^1 for the upper mantle and 46 km depth are indicated.

  11. Shear wave velocity structure of the southern African upper mantle with implications for the uplift of southern Africa

    NASA Astrophysics Data System (ADS)

    Adams, Aubreya; Nyblade, Andrew

    2011-08-01

    Broad-band seismic data from the southern African seismic experiment and the AfricaArray network are used to investigate the seismic velocity structure of the upper mantle beneath southern Africa, and in particular beneath the Kaapvaal Craton. A two-plane approximation method that includes a finite frequency sensitivity kernel is employed to measure Rayleigh wave phase velocities, which are inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find phase velocities for the Kaapvaal Craton and surrounding mobile belts that are comparable to those reported by previous studies, and we find little evidence for variation from east to west across the Namaqua-Natal Belt, a region not well imaged in previous studies. A high-velocity upper-mantle lid is found beneath the Kaapvaal Craton and most of southern Africa. For the Kaapvaal Craton, the thickness of the lid (˜150-200 km) is consistent with the lid thicknesses reported in many previous studies. The cratonic lid is underlain by a ˜100-km thick low-velocity zone with a 3.9 per cent maximum velocity reduction. By comparing the velocity model to those published for other Archean cratons, we find few differences, and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern African plateau is supported by an upper-mantle thermal anomaly.

  12. Deep Mantle Large Low Shear-Wave Velocity Provinces: Principally Thermal Structures?

    NASA Astrophysics Data System (ADS)

    Davies, R.; Goes, S. D. B.

    2014-12-01

    The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth's thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical `piles' comes from seismic constraints, including: (i) their long-wavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear- and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an isochemical, well-mixed mantle. In addition, it has been proposed that hotspots and the reconstructed eruption sites of large igneous provinces correlate in location with LLSVP margins. Here, we review recent results, which indicate that the majority of these constraints do not require thermo-chemical piles: they are equally well (or poorly) explained by thermal heterogeneity alone. Our analyses and conclusions are largely based on comparisons between imaged seismic structure and synthetic seismic structures from a set of thermal and thermo-chemical mantle convection models, which are constrained by 300 Myr of plate motion histories. Modelled physical structure (temperature, pressure and composition) is converted into seismic velocities via a thermodynamic approach that accounts for elastic, anelastic and phase contributions and, subsequently, a tomographic resolution filter is applied to account for the damping and geographic bias inherent to seismic imaging. Our results indicate that, in terms of

  13. Structure of the Lithosphere and Upper Mantle Across the Arabian Peninsula

    SciTech Connect

    Al-Amri, A; Rodgers, A

    2007-01-05

    Analysis of modern broadband (BB) waveform data allows for the inference of seismic velocity structure of the crust and upper mantle using a variety of techniques. This presentation will report inferences of seismic structure of the Arabian Plate using BB data from various networks. Most data were recorded by the Saudi Arabian National Digital Seismic Network (SANDSN) which consists of 38 (26 BB, 11 SP) stations, mostly located on the Arabian Shield. Additional data were taken from the 1995-7 Saudi Arabian IRIS-PASSCAL Deployment (9 BB stations) and other stations across the Peninsula. Crustal structure, inferred from teleseismic P-wave receiver functions, reveals thicker crust in the Arabian Platform (40-45 km) and the interior of the Arabian Shield (35-40 km) and thinner crust along the Red Sea coast. Lithospheric thickness inferred from teleseismic S-wave receiver functions reveals very thin lithosphere (40-80 km) along the Red Sea coast which thickens rapidly toward the interior of the Arabian Shield (100-120 km). We also observe a step of 20-40 km in lithospheric thickness across the Shield-Platform boundary. Seismic velocity structure of the upper mantle inferred from teleseismic P- and S-wave travel time tomography reveals large differences between the Shield and Platform, with the Shield being underlain by slower velocities, {+-}3% for P-waves and {+-}6% for S-waves. Seismic anisotropy was inferred from shear-wave splitting, using teleseismic SKS waveforms. Results reveal a splitting time of approximately 1.4 seconds, with the fast axis slightly east of north. The shear-wave splitting results are consistent across the Peninsula, with a slight clockwise rotation parallel for stations near the Gulf of Aqaba. In summary, these results allow us to make several conclusions about the tectonic evolution and current state of the Arabian Plate. Lithospheric thickness implies that thinning near the Red Sea has accompanied the rupturing of the Arabian

  14. Imaging 3D anisotropic upper mantle shear velocity structure of Southeast Asia using seismic waveform inversion

    NASA Astrophysics Data System (ADS)

    Chong, J.; Yuan, H.; French, S. W.; Romanowicz, B. A.; Ni, S.

    2011-12-01

    Southeast Asia as a special region in the world which is seismically active and is surrounded by active tectonic belts, such as the Himalaya collision zone, western Pacific subduction zones and the Tianshan- Baikal tectonic belt. Seismic anisotropic tomography can shade light on the complex crust and upper mantle dynamics of this region, which is the subject of much debate. In this study, we applied full waveform time domain tomography to image 3D isotropic and anisotropic upper mantle shear velocity structure of Southeast Asia. Three component waveforms of teleseismic and far regional events (15 degree ≤ Δ≤ 165 degree) with magnitude ranges from Mw6.0 to Mw7.0 are collected from 91 permanent and 438 temporary broadband seismic stations in SE Asia. Wavepackets of both fundamental and overtone modes, filtered between 60 and 400 sec, are selected automatically according to the similarity between data and synthetic waveforms (Panning & Romanowicz, 2006). Wavepackets corresponding to event-station paths that sample the region considered are weighted according to path redundancy and signal to noise ratio. Higher modes and fundamental mode wavepackets are weighted separately in order to enhance the contribution of higher modes which are more sensitive to deeper structure compared to the fundamental mode. Synthetic waveforms and broadband sensitivity kernels are computed using normal mode asymptotic coupling theory (NACT, Li & Romanowicz, 1995). As a starting model, we consider a global anisotropic upper mantle shear velocity model based on waveform inversion using the Spectral Element Method (Lekic & Romanowicz, 2011), updated for more realistic crustal thickness (French et al., 2011) as our starting model, we correct waveforms for the effects of 3D structure outside of the region, and invert them for perturbations in the 3D structure of the target region only. We start with waveform inversion down to 60sec and after several iterations, we include shorter period

  15. Volatile Reservoirs Below The Upper Lunar Mantle And Their Incompatibility With The Giant Impact Hypothesis

    NASA Astrophysics Data System (ADS)

    Schmitt, H. H.

    2011-12-01

    Separate accretion and capture is a physically plausible alternative to the giant Earth impact hypothesis for the origin of the Moon. Like the capture hypothesis, giant impact relies on gravitational interaction of the Earth and a smaller planetesimal under very specific orbital encounter conditions. In addition to questions about the physics of debris re-aggregation in Earth-orbit, the giant impact hypothesis, as currently formulated by computer models, fails a critical reality check; namely, the Moon contains reservoirs of volatiles that would have been dispersed at ejection temperatures predicted by the models... Researchers have previously noted that concentrations of many volatile elements in both the Apollo 17 and Apollo 15 pyroclastic glass samples indicate the existence of volatile reservoirs at depth. Both the orange and green glasses are enriched over mare basalts by factors >100 in Cl, F, Br, Zn, Ge, Dc, Tl, and Ag and by factors >10 in Pb, Ga, Sb, Bi, In, Au, Ni, Se, Te, and Cu. These elements exist almost entirely in the non-glass components of the pyroclastic samples. The recent identification of significant water within the Apollo 17 orange pyroclastic glasses further emphasizes the existence of volatile reservoirs in the lunar mantle... No evidence exists that the volatiles in the vesicles of mare basalts, derived by partial melting of a differentiated and solidified lunar magma ocean (upper mantle), were comparable to those in the pyroclastic glasses. Accretionary thermal effects that produced the magma ocean, combined with low lunar gravity, to would have depleted primordial volatiles. Any remaining magma ocean water would be converted to hydrogen and FeO by migration of early-formed, broadly disseminated, immiscible FexNiySz liquid. The reservoirs for the pyroclastic volatiles, therefore, would be below about 500km (lower mantle), that is, below the base of the original magma ocean. Relatively inert hydrogen and carbon monoxide probably made up

  16. Global anisotropic tomography of the upper mantle: mapping lateral variations in lithospheric thickness

    NASA Astrophysics Data System (ADS)

    Romanowicz, B.; Gung, Y.; Panning, M.

    2003-04-01

    We have previously developed a global waveform tomography method, which utilizes information from the entire long period seismogram (body wave and surface wave energy, including overtones, diffracted waves and multiply reflected and converted phases). Our approach is based on an asymptotic normal mode formalism (NACT, Non-Linear Asymptotic Coupling Theory) which includes coupling across mode branches and thus produces accurate 2D broadband kernels for body waveforms. Previously, we worked under the asumption of isotropic structure, and derived several elastic S velocity models (SAW12D, Li and Romanowicz, 1996; SAW24B16, Megnin and Romanowicz, 2000; as well as a 3D Q model of the upper mantle (Romanowicz and Gung, 2002). We have recently extended this approach to include radial isotropy with a vertical axis of symmetry. We use three component data (˜ 85,000 surface wave and overtone wave-packets and ˜ 50,000 body wave packets) and consider the six parameters VSiso (isotropic S velocity), ξ≡(N-L)/N, η≡ F/(A-2L), VPiso(isotropic V_P), φ≡ C/A, and ρ, with appropriate mode kernels for weak radial anisotropy.To reduce the number of parameters, we introduce scaling relations for VPiso, ρ, η and φ, as inferred from laboratory experiments (Montagner and Anderson,1989), and invert for VSiso and ξ. We confirm the existence of significant anisotropy with SH>SV under the central Pacific and Indian oceans in the depth range 100-200km (Montagner and Tanimoto, 1991; Ekström and Dziewonski, 1998). At greater depths (200-400km), this signal is replaced by SH>SV under most continental cratons (Montagner, 1994). Because at these depths frozen lithospheric anisotropy cannot be sustained, we infer that both the oceanic (shallow) and continental (deep) SH>SV signal indicates a strong component of horizontal flow in the asthenospheric channel beneath the lithosphere. This is in agreement with results from recent regional studies which infer two layers of anisotropy, one

  17. The effect of topography of upper-mantle discontinuities on SS precursors

    NASA Astrophysics Data System (ADS)

    Koroni, Maria; Trampert, Jeannot

    2016-01-01

    Using the spectral-element method, we explored the effect of topography of upper-mantle discontinuities on the traveltimes of SS precursors recorded on transverse component seismograms. The latter are routinely used to infer the topography of mantle transition zone discontinuities. The step from precursory traveltimes to topographic changes is mainly done using linearised ray theory, or sometimes using finite-frequency kernels. We simulated exact seismograms in 1-D and 3-D elastic models of the mantle. In a second simulation, we added topography to the discontinuities. We compared the waveforms obtained with and without topography by cross correlation of the SS precursors. Since we did not add noise, the precursors are visible in individual seismograms without the need of stacking. The resulting time anomalies were then converted into topographic variations and compared to the original topographic models. Based on the correlation between initial and inferred models, and provided that ray coverage is good, we found that linearised ray theory gives a relatively good idea on the location of the uplifts and depressions of the discontinuities. It seriously underestimates the amplitude of the topographic variations by a factor ranging between 2 and 7. Real data depend on the 3-D elastic structure and the topography. All studies to date correct for the 3-D elastic effects assuming that the traveltimes can be linearly decomposed into a structure and a discontinuity part. We found a strong non-linearity in this decomposition which cannot be modelled without a fully non-linear inversion for elastic structure and discontinuities simultaneously.

  18. Global upper mantle structure from long-period differential travel times

    SciTech Connect

    Woodward, R.L.; Masters, G. )

    1991-04-10

    The authors have made over ten thousand measurements of PP-P and SS-S differential travel times from long-period Global Digital Seismograph Network recordings of all events with m{sub b} {ge} 5.5 which occurred during the years 1976 and 1986. The experiments indicate that lower-mantle structure and source-receiver structure can each contribute approximately {plus minus}0.5 s to the measured PP-P residuals so there is considerable signal to be explained. The pattern observed in the PP-P measurements is similar to the pattern observed in the SS-S measurements, with the SS-S residuals 2 to 4 times larger in magnitude. Comparisons of measured residuals to those predicted by the upper-mantle models of Woodhouse and Dziewonski show that the overall patterns are quite similar but the amplitude of the model residuals is roughly a factor of 2 too small. Comparisons with the predictions of a whole-mantle model of Tanimoto again shows that the predicted pattern of residuals is reasonably consistent with the observations but now the predicted residuals are too large by about a factor of 2. They have also binned the measurements according to the tectonic regionalization GTR1 of Jordan and find a qualitative correlation of average residual with tectonic region. In particular, Precambrian shields show a strong anomaly, and there is a correlation of residual size with the age of oceanic crust at the bounce point. For all tectonic regions the ratio of SS-S to PP-P residuals is approximately 2. This ratio is consistent with a thermal origin for the observed signal. Finally, measurements show no compelling evidence for azimuthal anisotroph which might be related to fossil spreading direction or the direction of absolute plate motion.

  19. Geophysical study of the crust and upper mantle beneath the central Rio Grande rift and adjacent Great Plains and Colorado Plateau

    SciTech Connect

    Ander, M.E.

    1981-03-01

    As part of the national hot dry rock (HDR) geothermal program conducted by Los Alamos Scientific Laboratory, a regional deep magnetotelluric (MT) survey of Arizona and New Mexico was performed. The main objective of the MT project was to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau, Basin and Range Province, and Rio Grande rift. Three MT profiles cross the Jemez lineament. Preliminary one-dimensional analysis of the data suggest the lineament is associated with anomalously high electrical conductivity very shallow in the crust. An MT/audiomagnetotelluric (AMT) study of a 161 km/sup 2/ HDR prospect was performed on the Zuni Indian Reservation, New Mexico. Two-dimensional gravity modeling of a 700-km gravity profile at 34/sup 0/30'N latitude was used to study the crust and upper mantle beneath the Rio Grande rift. Several models of each of three consecutive layers were produced using all available geologic and geophysical constraints. Two short-wavelength anomalies along the gravity profile were analyzed using linear optimization techniques.

  20. Management of catheter-associated upper extremity deep venous thrombosis.

    PubMed

    Crawford, Jeffrey D; Liem, Timothy K; Moneta, Gregory L

    2016-07-01

    Central venous catheters or peripherally inserted central catheters are major risk factors for upper extremity deep venous thrombosis (UEDVT). The body and quality of literature evaluating catheter-associated (CA) UEDVT have increased, yet strong evidence on screening, diagnosis, prevention, and optimal treatment is limited. We herein review the current evidence of CA UEDVT that can be applied clinically. Principally, we review the anatomy and definition of CA UEDVT, identification of risk factors, utility of duplex ultrasound as the preferred diagnostic modality, preventive strategies, and an algorithm for management of CA UEDVT. PMID:27318061

  1. Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites.

    PubMed

    Fischer, T P; Burnard, P; Marty, B; Hilton, D R; Füri, E; Palhol, F; Sharp, Z D; Mangasini, F

    2009-05-01

    Carbonatite lavas are highly unusual in that they contain almost no SiO(2) and are >50 per cent carbonate minerals. Although carbonatite magmatism has occurred throughout Earth's history, Oldoinyo Lengai, in Tanzania, is the only currently active volcano producing these exotic rocks. Here we show that volcanic gases captured during an eruptive episode at Oldoinyo Lengai are indistinguishable from those emitted along mid-ocean ridges, despite the fact that Oldoinyo Lengai carbonatites occur in a setting far removed from oceanic spreading centres. In contrast to lithophile trace elements, which are highly fractionated by the immiscible phase separation that produces these carbonatites, volatiles (CO(2), He, N(2) and Ar) are little affected by this process. Our results demonstrate that a globally homogenous reservoir exists in the upper mantle and supplies volatiles to both mid-ocean ridges and continental rifts. This argues against an unusually C-rich mantle being responsible for the genesis of Na-rich carbonatite and its nephelinite source magma at Oldoinyo Lengai. Rather, these carbonatites are formed in the shallow crust by immiscibility from silicate magmas (nephelinite), and are stable under eruption conditions as a result of their high Na contents. PMID:19424154

  2. Oxidation state of the Earth's upper mantle during the last 3800 million years: Implications for the origin of life

    NASA Technical Reports Server (NTRS)

    Delano, J. W.

    1993-01-01

    A popular, as well as scientifically rigorous, scenario for the origin of life on Earth involves the production of organic molecules by interaction of lightning (or other forms of energy) with a chemically reducing atmosphere in the early history of Earth. Experiments since the 1950's have convincingly demonstrated that the yield of organic molecules is high when the atmosphere contains molecular hydrogen, methane, ammonia, and water vapor. Additional work has also shown that such a highly reducing atmosphere might not, however, have been sufficiently long-lived in the presence of intense solar ultraviolet radiation for life to have formed from it. One way of maintaining such an atmosphere would be to have a continual replenishment of the reduced gases by prolonged volcanic outgassing from a reducing of Earth's interior. The length of time that this replenishment might need to continue is in part constrained by the flux of asteroids onto the Earth's surface containing sufficient energy to destroy most, if not all, life that had developed up to that point in time. If a reducing atmosphere is a key ingredient for the origin of life on Earth, the time of the last environmental sterilization due to large impacts would be an important constraint. In a deep marine setting (e.g., hydrothermal vent), the last global sterilization might have occurred at 4200-4000 Ma. On the Earth's surface, the last global sterilization event might have occurred at 4000-3700 Ma. If these are meaningful constraints, how likely is it that a reducing atmosphere could have survived on the Earth until about 3800 Ma ago? Due to the importance of replenishing this atmosphere with reducing components by volcanic outgassing from the mantle, geochemical information on the history of the mantle's oxidation state would be useful for addressing this question. Geochemical and experimental data discussed in this abstract suggest that extrusive mafic volcanics derived from the upper mantle have had

  3. A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico

    NASA Technical Reports Server (NTRS)

    Spence, William; Gross, Richard S.

    1990-01-01

    In this study, the lateral variations of the P wave velocity as a function of depth were examined for the regions of the Rio Grande rift and the Jemez lineament, to infer spatial distributions of partial melt in the upper mantle source zones for the rift and the lineament. The method involved measurements of teleismic P wave delays at a 22-station network followed by performing a damped least-squares three-dimensional inversion for these lateral variations. Results indicate that, directly beneath the Jemez lineament (but not beneath the Rio Grande rift), there is a 100-km-wide 1-2-percent low-P-wave-velocity feature in the depth range of 50-160 km. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift.

  4. Spectral-element global waveform tomography: A second-generation upper-mantle model

    NASA Astrophysics Data System (ADS)

    French, S. W.; Lekic, V.; Romanowicz, B. A.

    2012-12-01

    The SEMum model of Lekic and Romanowicz (2011a) was the first global upper-mantle VS model obtained using whole-waveform inversion with spectral element (SEM: Komatitsch and Vilotte, 1998) forward modeling of time domain three component waveforms. SEMum exhibits stronger amplitudes of heterogeneity in the upper 200km of the mantle compared to previous global models - particularly with respect to low-velocity anomalies. To make SEM-based waveform inversion tractable at global scales, SEMum was developed using: (1) a version of SEM coupled to 1D mode computation in the earth's core (C-SEM, Capdeville et al., 2003); (2) asymptotic normal-mode sensitivity kernels, incorporating multiple forward scattering and finite-frequency effects in the great-circle plane (NACT: Li and Romanowicz, 1995); and (3) a smooth anisotropic crustal layer of uniform 60km thickness, designed to match global surface-wave dispersion while reducing the cost of time integration in the SEM. The use of asymptotic kernels reduced the number of SEM computations considerably (≥ 3x) relative to purely numerical approaches (e.g. Tarantola, 1984), while remaining sufficiently accurate at the periods of interest (down to 60s). However, while the choice of a 60km crustal-layer thickness is justifiable in the continents, it can complicate interpretation of shallow oceanic upper-mantle structure. We here present an update to the SEMum model, designed primarily to address these concerns. The resulting model, SEMum2, was derived using a crustal layer that again fits global surface-wave dispersion, but with a more geologically consistent laterally varying thickness: approximately honoring Crust2.0 (Bassin, et al., 2000) Moho depth in the continents, while saturating at 30km in the oceans. We demonstrate that this approach does not bias our upper mantle model, which is constrained not only by fundamental mode surface waves, but also by overtone waveforms. We have also improved our data-selection and

  5. Implications of regional gravity for state of stress in the earth's crust and upper mantle.

    USGS Publications Warehouse

    McNutt, M.

    1980-01-01

    Topography is maintained by stress differences within the earth. Depending on the distribution of the stress we classify the support as either local or regional compensation. In general, the stresses implied in a regional compensation scheme are an order of magnitude larger than those corresponding to local isostasy. Gravity anomalies, a measure of the earth's departure from hydrostatic equilibrium, can be used to distinguish between the two compensation mechanisms and thus to estimate the magnitude of deviatoric stress in the crust and upper mantle. Topography created at an ocean ridge crest or in a major contiental orogenic zone appears to be locally compensated. Such features were formed on weak crust incapable of maintaining stress differences much greater than the stress from the applied load. Oceanic volcanoes formed on an already cooled, thickened lithosphere are regionally supported with elastic stresses. -Author

  6. Composition of the earth's upper mantle. I - Siderophile trace elements in ultramafic nodules

    NASA Technical Reports Server (NTRS)

    Morgan, J. W.; Wandless, G. A.; Petrie, R. K.; Irving, A. J.

    1981-01-01

    The considered investigation is concerned with a reexamination of the question of the distribution of siderophile elements in the earth's upper mantle, taking into account a more unified data base which is now available. A comprehensive suite of ultramafic inclusions was collected as part of the Basaltic Volcanism Study Project and has been analyzed by instrument neutron activation analysis for major, minor, and some lithophile trace elements. In addition, 18 of these rocks and the important sheared garnet lherzolite PHN 1611 have been analyzed by means of radiochemical neutron activation analysis for 7 siderophile elements (Au, Ge, Ir, Ni, Os, Pd, and Re) and 9 volatile elements (Ag, Bi, Cd, In, Sb, Se, Te, Tl, and Zn). The siderophile element data reveal interesting inter-element correlations, which were not apparent from the compiled abundance tables of Ringwood and Kesson (1976) and Chou (1978).

  7. Upper mantle electrical conductivity for seven subcontinental regions of the Earth

    USGS Publications Warehouse

    Campbell, W.H.; Schiffmacher, E.R.

    1988-01-01

    Spherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq) separated for the 7 continental regions of the observatories have been used to determine conductivity profiles to depths of about 600 km by the Schmucker equivalent substitute conductor method. The profiles give evidence of increases in conductivity between about 150 and 350 km depth, then a general increase in conductivity thereafter. For South America we found a high conductivity at shallow depths. The European profile showed a highly conducting layer near 125 km. At the greater depths, Europe, Australia and South America had the lowest values of conductivity. North America and east Asia had intermediate values whereas the African and central Asian profiles both showed the conductivities rising rapidly beyond 450 km depth. The regional differences indicate that there may be considerable lateral heterogeneity of electrical conductivity in the Earth's upper mantle. -Authors

  8. Synthesis of regional crust and upper-mantle structure from seismic and gravity data

    NASA Technical Reports Server (NTRS)

    Alexander, S. S.; Lavin, P. M. (Principal Investigator)

    1982-01-01

    Analyses of regional gravity and magnetic patterns, LANDSAT images and geological information revealed two major lineaments crossing western Pennsylvania and parts of surrounding states. These lineaments are inferred to be expressions of fracture zones which penetrare deeply into the crust and possibly the upper mantle. The extensions of the Tyron-Mt. Union and the Pittsburgh-Washington lineaments bound a distinct crustal block (Lake Erie-Maryland block) over 100 km wide and probably more than 600 km in length. Evidence exists for the lateral displacement of this block at least 60 km northwestward during late Precambrian to Lower Ordovician time. Subsequent movements have been mainly vertical with respect to neighboring blocks. A possible crustal block that passes through eastern Kentucky, proposed by a TVA study on tectonics in the southern Appalachians, was also investigated. Finally, the use of regional gravity and magnetic data in identifying major crustal structures beneath western Pennsylvania is discussed.

  9. Synthesis of regional crust and upper-mantle structure from seismic and gravity data

    NASA Technical Reports Server (NTRS)

    Alexander, S. S.; Lavin, P. M.

    1979-01-01

    Available seismic and ground based gravity data are combined to infer the three dimensional crust and upper mantle structure in selected regions. This synthesis and interpretation proceeds from large-scale average models suitable for early comparison with high-altitude satellite potential field data to more detailed delineation of structural boundaries and other variations that may be significant in natural resource assessment. Seismic and ground based gravity data are the primary focal point, but other relevant information (e.g. magnetic field, heat flow, Landsat imagery, geodetic leveling, and natural resources maps) is used to constrain the structure inferred and to assist in defining structural domains and boundaries. The seismic data consists of regional refraction lines, limited reflection coverage, surface wave dispersion, teleseismic P and S wave delay times, anelastic absorption, and regional seismicity patterns. The gravity data base consists of available point gravity determinations for the areas considered.

  10. Analysis of upper mantle structure using wave field continuation of P waves

    NASA Technical Reports Server (NTRS)

    Walck, M. C.; Clayton, R. W.

    1984-01-01

    Wave field continuation theory, which allows transformation of the seismic record section data directly into velocity-depth space, is tested for upper mantle analysis using a large array-recorded data set obtained at the 200-station Caltech-USGS Southern California Seismic Network that is representative of the structure beneath the gulf of California. The method's resolution capability is illustrated by the comparison of the slant stacks and downward continuation of both synthetic and data record sections. It is stressed that when high-quality, densely sampled digital data are available, the technique is easy to implement, provides an inversion which contains all the data in the global format, and produces an objective estimate of depth resolution as a function of ray parameter.

  11. Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients.

    PubMed

    Fei, Hongzhan; Wiedenbeck, Michael; Yamazaki, Daisuke; Katsura, Tomoo

    2013-06-13

    Water has been thought to affect the dynamical processes in the Earth's interior to a great extent. In particular, experimental deformation results suggest that even only a few tens of parts per million of water by weight enhances the creep rates in olivine by orders of magnitude. However, those deformation studies have limitations, such as considering only a limited range of water concentrations and very high stresses, which might affect the results. Rock deformation can also be understood as an effect of silicon self-diffusion, because the creep rates of minerals at temperatures as high as those in the Earth's interior are limited by self-diffusion of the slowest species. Here we experimentally determine the silicon self-diffusion coefficient DSi in forsterite at 8 GPa and 1,600 K to 1,800 K as a function of water content CH2O from less than 1 to about 800 parts per million of water by weight, yielding the relationship, DSi ≈ (CH2O)(1/3). This exponent is strikingly lower than that obtained by deformation experiments (1.2; ref. 7). The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. We conclude that the effect of water on upper-mantle rheology is very small. Hence, the smooth motion of the Earth's tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And finally, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions. PMID:23765497

  12. Anisotropic Shear-wave Velocity Structure of East Asian Upper Mantle from Waveform Tomography

    NASA Astrophysics Data System (ADS)

    Chong, J.; Yuan, H.; French, S. W.; Romanowicz, B. A.; Ni, S.

    2012-12-01

    East Asia is a seismically active region featuring active tectonic belts, such as the Himalaya collision zone, western Pacific subduction zones and the Tianshan- Baikal tectonic belt. In this study, we applied full waveform time domain tomography to image 3D isotropic, radially and azimuthally anisotropic upper mantle shear velocity structure of East Asia. High quality teleseismic waveforms were collected for both permanent and temporary stations in the target and its adjacent regions, providing good ray path coverage of the study region. Fundamental and overtone wave packets, filtered down to 60 sec, were inverted for isotropic and radially anisotropic shear wave structure using normal mode asymptotic coupling theory (NACT: Li and Romanowicz, 1995). Joint inversion of SKS measurements and seismic waveforms was then carried out following the methodology described in (Marone and Romanowicz, 2007). The 3D velocity model shows strong lateral heterogeneities in the target region, which correlate well with the surface geology in East Asia. Our model shows that Indian lithosphere has subducted beneath Tibet with a different northern reach from western to eastern Tibet,. We also find variations of the slab geometry in Western Pacific subduction zones. Old and stable regions, such as, Indian shield, Siberia platform, Tarim and Yangtze blocks are found to have higher shear wave velocity in the upper mantle. Lower velocity anomalies are found in regions like Baikal rift, Tienshan, Indochina block, and the regions along Japan island-Ryukyu Trench and Izu-bonin Trench. The dominant fast and slow velocity boundaries in the study region are well correlated with tectonic belts, such as the central Asian orogenic belt and Alty/Qilian-Qinling/Dabie orogenic belt. Our radially anisotropic model shows Vsh> Vsv in oceanic regions and at larger depths(>300km), and Vsv > Vsh in some orogenic zones.. We'll show preliminary results of azimuthally anisotropic joint inversion of SKS

  13. Effect of earthquakes on ambient noise surface wave tomography in upper mantle studies

    NASA Astrophysics Data System (ADS)

    Yanovskaya, Tatiana; Koroleva, Tatiana; Lyskova, Eugenia

    2016-03-01

    Application of the ambient noise surface wave tomography method (ANT) for determination of the upper mantle structure requires data on long-periodic noise (T > 40 sec). The ANT technique implies that noise sources are distributed almost uniformly over the surface. This is practically true for short-periodic noise, however it is not so in the case of long periods. In this paper we show that the main contribution to noise at long periods is caused by signals from earthquakes. In some cases they may strongly distort noise cross-correlation. This leads to an incorrect determination of surface wave velocity dispersion curves. To minimize such a distortion we propose two means: (1) to use records of noise for the periods when there is no clustering of earthquakes, such as aftershocks of strong events; (2) to stack cross-correlation functions for a period of at least three years in order to achieve sufficient uniformity of earthquake locations. Validity of this approach is demonstrated by ANT results for Europe. Tomographic reconstruction of Rayleigh wave group velocities for 10-100 sec measured along interstation paths was carried out in a central part of Western Europe where resolving power of the data was the highest. Locally averaged dispersion curves were inverted to vertical S-wave velocity sections in this area. The results correspond closely to known features of the structure of the region, namely: strong difference of the crust and upper mantle structure at the opposite sides from the Tornquist-Teisseyre Line down to ˜ 250 km, penetration of high velocity material of EEP lithosphere under Carpathians, as well as penetration of low velocity asthenospheric layer from the Carpathian region toward the northeast.

  14. Towards Multi-resolution Adjoint Tomography of the European Crust and Upper Mantle

    NASA Astrophysics Data System (ADS)

    Basini, P.; Nissen-Meyer, T.; Boschi, L.; Schenk, O.; Verbeke, J.; Hanasoge, S.; Giardini, D.

    2010-12-01

    Thanks to continuously improved instrument coverage, and the growth of high-performance computational infrastructure, it is now possible to enhance the resolution at which seismologists image the Earth's interior. While most algorithms in global seismic tomography today are grounded on the ray-theory approximation, however, resolution and model complexity can effectively be enhanced only through the application of more advanced techniques accounting for the many complexities of the partial derivatives relating seismic data and Earth structure. These include full-wave forward modelling methods and adjoint algorithms, which together set a framework for iterative, nonlinear inversion upon complex 3D structures. We take advantage of these methodological improvements using a newly developed, flexible spectral-element method (SPECFEM3D) with embedded adjoint capabilities to devise new tomographic models of the European crust and upper mantle. We chose a two-scale strategy, in which we use global surface wave data to first constrain the large-scale structures, and simultaneously invert for high-resolution, regional structures based on measurements of ambient noise in central and southern Europe. By its very nature, and as a result of the dense station coverage over the continent, the ambient-noise method affords us a particularly uniform seismic coverage. To define surface-wave sensitivity kernels, we construct a flexible, global mesh of the upper mantle only (i.e., a spherical shell) honoring all global discontinuities, and include 3D starting models down to periods of 30 seconds. The noise data are cross-correlated to obtain station-to-station Green's functions. We will present examples of sensitivity kernels computed for these noise-based Green's functions and discuss the data-specific validity of the underlying assumptions to extract Green's functions. The local setup, which is constructed using the same software as in the global case, needs to honor internal and

  15. Upper mantle structure beneath the Japan Islands using single-scattering analysis

    NASA Astrophysics Data System (ADS)

    Tonegawa, T.; Hirahara, K.; Shibutani, T.; Shiomi, K.

    2006-12-01

    In the Japan subduction zone where the Pacific plate is subducting, we can investigate the dynamics of the upper mantle beneath the subduction zone, such as the structure of the descending slab, the undulation of the seismic discontinuities and its relation, by imaging the detailed structure beneath the Japan Islands. The high sensitivity accelerometer (hereafter tiltmeter) network has recently been deployed with high-density spacing (700 stations in Japan) by NIED, and these tiltmeters can be used as long-period seismometer. In this study, we used these recordings to image upper mantle discontinuities, including the subducting Pacific slab from the Japan trench and the 410 and 660 km discontinuities, beneath the Japanese Islands by calculating receiver functions with teleseismic waves. For deconvolution, since tiltmeter seismograms have just horizontal component, we obtained source-time function by stacking all of vertical components observed at F-net broadband stations, that is, we consider these stacked waveforms as source-time functions. We applied a low- pass filter of 0.16 Hz. Assuming all later phases of P-coda in receiver function are the P-to-S scattered phases, we migrated time-domain receiver functions to depth section using 1-D JMA velocity model. As a result, the seismic discontinuities correspoding to the oceanic Moho and lower boundary of the descending slab can be imaged down to 400 km and 500 km depth, respectively. The 410 km and 660 km discontinuities are clearly imaged, and our result also shows a depression of the 660 km discontinuity affected by the stagnant slab.

  16. Effect of earthquakes on ambient noise surface wave tomography in upper-mantle studies

    NASA Astrophysics Data System (ADS)

    Yanovskaya, Tatiana; Koroleva, Tatiana; Lyskova, Eugenia

    2016-05-01

    Application of the ambient noise surface wave tomography method (ANT) for determination of the upper-mantle structure requires data on long-periodic noise (T > 40 s). The ANT technique implies that noise sources are distributed almost uniformly over the surface. This is practically true for short-periodic noise, however, it is not so in the case of long periods. In this paper we show that the main contribution to noise at long periods is caused by signals from earthquakes. In some cases, they may strongly distort noise cross-correlation. This leads to an incorrect determination of surface wave velocity dispersion curves. To minimize such a distortion we propose two means: (1) to use records of noise for the periods when there is no clustering of earthquakes, such as aftershocks of strong events; (2) to stack cross-correlation functions for a period of at least three years in order to achieve sufficient uniformity of earthquake locations. Validity of this approach is demonstrated by ANT results for Europe. Tomographic reconstruction of Rayleigh wave group velocities for 10-100 s measured along interstation paths was carried out in a central part of Western Europe where resolving power of the data was the highest. Locally averaged dispersion curves were inverted to vertical S-wave velocity sections in this area. The results correspond closely to known features of the structure of the region, namely: strong difference of the crust and upper-mantle structure at the opposite sides from the Tornquist-Teisseyre Line down to ˜ 250 km, penetration of high-velocity material of East European Platform lithosphere under Carpathians, as well as penetration of low-velocity asthenospheric layer from the Carpathian region towards the northeast.

  17. Upper mantle temperatures and lithospheric thickness of North China inferred from S-wave tomography

    NASA Astrophysics Data System (ADS)

    Xiong, X.; Yang, S.; Zheng, Y.

    2013-12-01

    Temperature is one of the most important key parameters which control the density, viscosity, and rheology of the earth's material and hence the dynamic process of the mantle. Based on the correlation between mineral temperature and seismic velocity structure, we derive the upper mantle temperatures of North China in the depth range of 50 to 300 km by high-resolution S-wave tomography model. Defining the depth where the geotherm intersects the mantle adiabat with a potential temperature of 1300°C as the lithosphere-asthenosphere boundary, we estimate the correspondent lithospheric thickness in the north China. The calculated heat flows agree well with the observed data at the surface, and the misfits for most regions of North China are within the uncertainty of the heat flow measurements. Three main characteristics can be observed from the distribution of temperature: (1) lithospheric temperature at shallow depth is in consistent with the tectonic settings. At depth shallower than 170 km, temperature under the active tectonic eastern part of North China is higher than that in the stable cratonic regions in west. The regions with high mantle temperature include the Hehuai Basin, the Bohai Bay Basin, the boundary between North China Plain and the central North China; hotter lithosphere can also be found at the northern margin of Ordos Plateau, including the Yinchuan-Hetao Rift Zone and the Yinshan Orogen. (2) The lithospheric thickness in the regions with warmer lithosphere is about 80 -100 km thick, which is relatively thinner than the stable areas. The lowest temperature is located under the Ordos Plateau in western North China, which is about 200 to 400°C lower than that in the eastern North China. The lithosphere in the Ordos block is also the thickest in the north China, with thickness ranges 140-150 km on average and about 160 km in the thickest areas; (3) At the depth between 170 and 280 km, the distribution pattern of the thermal structure is almost reverse

  18. Upper Mantle Structure and Azimuthal Anisotropy Beneath Hudson Bay From Rayleigh Wave Tomography

    NASA Astrophysics Data System (ADS)

    Darbyshire, F.

    2009-05-01

    Hudson Bay is a large intracratonic basin situated within the northern part of the Canadian Shield. The region is surrounded by Archean cratons, notably the Superior to the south and east and the Rae-Hearne to the north and west, and is largely underlain by the Paleoproterozoic Trans-Hudson orogen. Global and continental scale tomographic images of North America suggest that the seismological lithosphere of the Canadian Shield is at its thickest and fastest beneath Hudson Bay, but a regional-scale seismic study is necessary to provide sufficient detail to comprehend the structure and evolution of the region. Broadband seismograph stations were installed around the Hudson Bay region in 2006 and 2007 as part of the HuBLE (Hudson Bay Lithospheric Experiment) project. The combination of these stations with existing permanent and temporary seismograph deployments has resulted in a high-quality teleseismic data set for which detailed multi- disciplinary seismic studies of the region can be carried out. Fundamental-mode Rayleigh wave dispersion curves are calculated for >100 two-station paths. The data span a range of periods ˜15-- 220~seconds, corresponding to depths from the middle crust to the sublithospheric mantle. Comparison of the dispersion curves with those derived from global reference models and from the Canadian Shield average 'CANSD' suggests that the upper mantle beneath Hudson Bay can be characterised by a typical cratonic signature of anomalously high phase velocities. Many Hudson Bay dispersion curves exhibit higher phase velocities than those of 'CANSD', suggesting a seismically faster and thicker lithosphere than average for the Canadian Shield. 1D mantle models estimated from the two-station dispersion measurements show an average lithospheric thickness of ˜225~km, with velocities ˜4--6% above global reference values. A tomographic inversion is carried out to solve simultaneously for isotropic phase velocity heterogeneity and azimuthal anisotropy at

  19. Joint seismic-geodynamic-mineral physical modelling of African geodynamics: A reconciliation of deep-mantle convection with surface geophysical constraints

    SciTech Connect

    Forte, A M; Quere, S; Moucha, R; Simmons, N A; Grand, S P; Mitrovica, J X; Rowley, D B

    2008-08-22

    Recent progress in seismic tomography provides the first complete 3-D images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the African continent. With these latest tomography results we predict flow patterns under Africa that reveal a large-scale, active hot upwelling, or superplume, below the western margin of Africa under the Cape Verde Islands. The scale and dynamical intensity of this West African superplume (WASP) is comparable to that of the south African superplume (SASP) that has long been assumed to dominate the flow dynamics under Africa. On the basis of this new tomography model, we find the dynamics of the SASP is strongly controlled by chemical contributions to deep mantle buoyancy that significantly compensate its thermal buoyancy. In contrast, the WASP appears to be entirely dominated by thermal buoyancy. New calculations of mantle convection incorporating these two superplumes reveal that the plate-driving forces due to the flow generated by the WASP is as strong as that due to the SASP. We find that the chemical buoyancy of the SASP exerts a strong stabilising control on the pattern and amplitude of shallow mantle flow in the asthenosphere below the southern half of the African plate. The asthenospheric flow predictions provide the first high resolution maps of focussed upwellings that lie below the major centres of Late Cenozoic volcanism, including the Kenya domes and Hoggar massif that lies above a remnant plume head in the upper mantle. Inferences of sublithospheric deformation from seismic anisotropy data are shown to be sensitive to the contributions of chemical buoyancy in the SASP.

  20. Structure of the upper mantle boundaries in North Eurasia and their origin

    NASA Astrophysics Data System (ADS)

    Pavlenkova, Ninel

    2016-04-01

    The seismic profiling with Peace Nuclear Explosions (PNE) shows unusual velocity stratification of the North Eurasia upper mantle. The asthenosphere is not traced as a low velocity layer, on the contrary, the 10-20 km thick velocity inversion zones are revealed at depth around 100 km. Several seismic boundaries are traced along the profiles. The most regular boundaries are at depths of 80-120 km (N boundary or 80 boundary) and 200-250 km (L boundary). The reflections from these boundaries are complicate many phase groups which may be formed by the reflective zones with alternating of the high- and low-velocity layers. These boundaries and the low velocity layers were unexpected results of the seismic profiling because it appeared unrealistic to find the regular and strong velocity contrasts in the upper mantle whose velocities are insensitive to the material composition, and no phase transitions were revealed at the boundary depths. The interpretation of the low velocity layers at depth around 100 km as zones of partial melting is invalid in the old platform areas where the melting ("thermal asthenosphere") was supposed at the depths of 250-300 km. The revealed boundaries might appear as the physical boundaries marked by the sharp changes in the different physical or mechanical properties of the material (porosity, permeability, fissuring and others). The increase or decrease in porosity is invariably followed by the change in the fluid content, which can initiate the different physicochemical transformations of the material, such as the new degrees of metamorphism, and initiate partial melting and mobility of the material at relatively low temperature. The laboratory study of the fluids transportation through the mantle rocks at the high pressure and temperature confirm such transformation. The matter flow along these weak zones may assist the formation of the anisotropic high velocity intermediate layers. Comparison of the seismic data with other geophysical data

  1. Experimental derivation of nepheline syenite and phonolite liquids by partial melting of upper mantle peridotites

    NASA Astrophysics Data System (ADS)

    Laporte, Didier; Lambart, Sarah; Schiano, Pierre; Ottolini, Luisa

    2014-10-01

    Piston-cylinder experiments were performed to characterize the composition of liquids formed at very low degrees of melting of two fertile lherzolite compositions with 430 ppm and 910 ppm K2O at 1 and 1.3 GPa. We used the microdike technique (Laporte et al., 2004) to extract the liquid phase from the partially molten peridotite, allowing us to analyze liquid compositions at degrees of melting F down to 0.9%. At 1.3 GPa, the liquid is in equilibrium with olivine + orthopyroxene + clinopyroxene + spinel in all the experiments; at 1 GPa, plagioclase is present in addition to these four mineral phases up to about 5% of melting (T≈1240 °C). Important variations of liquid compositions are observed with decreasing temperature, including strong increases in SiO2, Na2O, K2O, and Al2O3 concentrations, and decreases in MgO, FeO, and CaO concentrations. The most extreme liquid compositions are phonolites with 57% SiO2, 20-22% Al2O3, Na2O + K2O up to 14%, and concentrations of MgO, FeO, and CaO as low as 2-3%. Reversal experiments confirm that low-degree melts of a fertile lherzolite have phonolitic compositions, and pMELTS calculations show that the amount of phonolite liquid generated at 1.2 GPa increases from 0.3% in a source with 100 ppm K2O to 3% in a source with 2000 ppm K2O. The enrichment in silica and alkalis with decreasing melt fraction is coupled with an increase of the degree of melt polymerization, which has important consequences for the partitioning of minor and trace elements. Thus Ti4+ in our experiments and, by analogy with Ti4+, other highly charged cations, and rare earth elements become less incompatible near the peridotite solidus. Our study brings a strong support to the hypothesis that phonolitic lavas or their plutonic equivalents (nepheline syenites) may be produced directly by partial melting of upper mantle rock-types at moderate pressures (1-1.5 GPa), especially where large domains of the subcontinental lithospheric mantle has been enriched in

  2. A database of crystal preferred orientation of olivine in upper mantle rocks

    NASA Astrophysics Data System (ADS)

    Mainprice, D.

    2012-12-01

    Olivine is the most volumetrically abundant mineral in the Earth's upper mantle, as such it dominates the mechanical and physical properties and has a controlling influence of the geodynamics of plate tectonics. Since the pioneering work of Hess and others we know that seismic anisotropy of the shallow mantle is related to olivine and it's crystal preferred orientation (CPO). With advent of plate tectonics the understanding of the key role of peridotite rocks became a major scientific objective and the measurement CPO of olivine in upper mantle samples became an important tool for studying the kinematics of these rocks. Our group originally lead by Adolphe Nicolas introduced the systematic use of CPO measured by U-stage for field studies all over the world for over 30 years, this tradition was extended in last 15 years by the use of electron back-scattered diffraction (EBSD) to study of CPO and the associated digital microstructure. It is an appropriate time to analysis this significant database of olivine CPO, which represents the work of our group, both present and former members, as well as collaborating colleagues. It is also interesting to compare the natural record as illustrated by our database in the light of recent experimental results stimulated by the extended ranges in temperature, pressure and finite strain, as well as intrinsic olivine variables such as hydrogen content. To analysis the database, which is heterogeneous because it is constructed from the individual work of many people over a 45 year period containing U-stage data and EBSD measurements (manual indexing point per grain, automatic indexing one point per grain, automatic indexing gridded mapping data) of various formats, we need a flexible software tool that can handle large volumes of data in consistent way. We have used the state-of-art open source MTEX toolbox for quantitative texture analysis. MTEX is a scriptable MATLAB toolbox, which permits all aspects of quantitative texture

  3. Influence of Grain-size and Water on the Seismic Structure of the Oceanic Upper Mantle

    NASA Astrophysics Data System (ADS)

    Elsenbeck, J. R.; Behn, M.; Hirth, G.

    2006-12-01

    Grain size is an important material property that has significant effects on the viscosity [1], dominant deformation mechanism [2], attenuation, and shear wave velocity [3] of the upper mantle. Many studies have investigated grain size evolution, but have yet to incorporate the evolution equations into a steady-state flow model for the oceanic upper mantle. We construct self-consistent 1.5-D steady-state Couette flow models motivated by [4] to constrain how grain size evolves with depth in a composite diffusion-dislocation creep rheology. We run cases for a dry, wet, and dehydrated mantle (with dehydration above ~60-70 km depth). The calculated grain size profiles are input into a Burger's model system [3] to calculate seismic quality factor (Q) and shear wave velocity (Vs). For ages older than 50 Myrs, Q and Vs predicted by the dehydration case best match the reference models PREM and SW02 for Q [5,6], and the low seismic shear wave velocity zone (LVZ) observed [7,8]. Faul & Jackson [3] demonstrated that to fit observed values of Q and Vs in the LVZ, a grain size of 1 mm is required. Extrapolation of olivine creep data predicts that for a grain size of 1 mm, viscosity is on the order of 1016 Pa s. In addition, a grain size of 1 mm would result in deformation being dominated by diffusion creep. Our dehydration model for 75 Myrs shows a grain size of ~1 cm, a viscosity of ~1019 Pa s at a depth of ~150 km (the center of the LVZ), and dislocation creep as the dominant deformation mechanism. We also find that the characteristic timescale for grain size to reach steady-state is less than ~10% of plate age for depths equivalent to the LVZ. These results indicate that a combination of grain size evolution and dehydration are a plausible explanation for the LVZ. [1] Hall & Parmentier, G3, 2003; [2] Montési & Hirth, EPSL, 2003; [3] Faul & Jackson, EPSL, 2005; [4] Podolefsky et al., GJI, 2004; [5] Dziewonski & Anderson, PEPI, 1981; [6] Selby & Woodhouse, JGR, 2002; [7

  4. Silicate melts density, buoyancy relations and the dynamics of magmatic processes in the upper mantle

    NASA Astrophysics Data System (ADS)

    Sanchez-Valle, Carmen; Malfait, Wim J.

    2016-04-01

    Although silicate melts comprise only a minor volume fraction of the present day Earth, they play a critical role on the Earth's geochemical and geodynamical evolution. Their physical properties, namely the density, are a key control on many magmatic processes, including magma chamber dynamics and volcanic eruptions, melt extraction from residual rocks during partial melting, as well as crystal settling and melt migration. However, the quantitative modeling of these processes has been long limited by the scarcity of data on the density and compressibility of volatile-bearing silicate melts at relevant pressure and temperature conditions. In the last decade, new experimental designs namely combining large volume presses and synchrotron-based techniques have opened the possibility for determining in situ the density of a wide range of dry and volatile-bearing (H2O and CO2) silicate melt compositions at high pressure-high temperature conditions. In this contribution we will illustrate some of these progresses with focus on recent results on the density of dry and hydrous felsic and intermediate melt compositions (rhyolite, phonolite and andesite melts) at crustal and upper mantle conditions (up to 4 GPa and 2000 K). The new data on felsic-intermediate melts has been combined with in situ data on (ultra)mafic systems and ambient pressure dilatometry and sound velocity data to calibrate a continuous, predictive density model for hydrous and CO2-bearing silicate melts with applications to magmatic processes down to the conditions of the mantle transition zone (up to 2773 K and 22 GPa). The calibration dataset consist of more than 370 density measurements on high-pressure and/or water-and CO2-bearing melts and it is formulated in terms of the partial molar properties of the oxide components. The model predicts the density of volatile-bearing liquids to within 42 kg/m3 in the calibration interval and the model extrapolations up to 3000 K and 100 GPa are in good agreement

  5. Study of the Upper Mantle Structure beneath China by Multimode Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Pandey, Shantanu; Yuan, Xiaouhui; Priestley, Keith; Kind, Rainer; Li, Xueqing

    2010-05-01

    China is an assembly of ancient continental fragments separated by fold belts accreted from late Proterozoic to Cenozoic. China being sitting on the triple junction of three major plates: Eurasian plate, Indian plate and Philippine sea plate resulted in the tectonic feature of todays like mountain ranges, fold belts, sedimentary basins and high plateaus. This has been the cause of many intraplate earthquakes also. In the Northern part this region is supposed to get some resistance from the Siberian shield. But the major tectonic feature in this region imprinted by two main tectonic events. First, the subduction of the of West Pacific plate and Philippine Sea plate to the west in the last 250 Ma. Second, the collision of Indian and Eurasian plate started about 50 Ma. It was this collision responsible for the uplift of Himalayan mountain and Tibetan Plateau. This event left there imprints on the upper mantle structure. It is generally agreed that the lithosphere is thick in west China while much of the lithospheric root was lost beneath some cratons in east China. Still it's an open debate whether the lithosphere beneath the Tibetan plateau has doubled its thickness as did the crust above or much of the thickened lithosphere was removed by mantle convection and delamination. In our study we try to determine the three dimensional Sv wave speed and azimuthal anisotropy model by analyzing the vertical component multimode Rayleigh wave seismogram. The data which we used are from approx. 40 broadband station from Chinese Seismic network and seismograms recorded by some temporary broadband seismic experiment in China. We construct the three dimensional model in two step procedure. In the first step we use the automated version of the Cara and Leveque [1987] waveform inversion technique in terms of secondary observables for modeling each multimode Rayleigh waveform to determine the path-average mantle Sv wave speed structure. In the second stage we combine the 1-D velocity

  6. The Upper Mantle Shear Boundary Layer Is The Source Of Midplate Volcanoes

    NASA Astrophysics Data System (ADS)

    Anderson, D. L.

    2011-12-01

    The lithosphere, lid, low-velocity layer (LVL) and the shallow part of the asthenosphere are all part of the upper boundary layer (BL) of the mantle, which generally overlies the canonical "convecting" upper mantle source (DMM) of ridge basalts. This global BL, Gutenberg's Region B (=BL), extends to ~200-250 km depth under cratons, which is well known, and to comparable depths under oceans, which is not generally appreciated because lid, plate, lithosphere and BL are often (erroneously) equated. A new BL is superposed on top of the pre-existing older one in oceans. The region above 220±20 km depth supports a high thermal gradient and is the most anisotropic and heterogeneous part of the mantle, indicators of thermal and shear BLs. The magnitude of the anisotropy and the velocity drop into the LVL, plus internal reflections, imply a laminated structure probably with refractory harzburgite lamellae coexisting with melt-rich sills, both normally less dense than DMM. This structure is sheared by plate motions causing shear-driven melt segregation into parallel fine-grained shear-bands, shear-driven upwellings, and decoupling and long-term isolation from DMM. The BL is twice as thick and is hotter at the base than canonical petrological and geochemical models based on McKenzie-Bickle-Steins thin-plate assumptions. The lower part of the shear layer (>150 km depth) is almost stationary with respect to plate motions and is ~200 K hotter than plate boundary magmas, features that are often attributed to mantle plumes. The refractory lamellae preserve ancient isotope signatures such as high 3He/4He ala Albarede; the melt-rich lamellae explain the volumes, compositions and locations of midplate volcanoes. BL is the largest (4x larger than D") and most accessible of all proposed geochemical reservoirs and has the required chemical, spatial, scale and thermal attributes. It resolves the Hart-Hanan conundrum concerning the Common Component FOZO; this resides in the shallowest

  7. Stability of carbonated basaltic melt at the base of the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Ghosh, S.; Litasov, K.; Ohtani, E.; Suzuki, A.

    2006-12-01

    Seismological observations of low velocity zones (LVZ) at the top of the 410-km discontinuity reveal possible existence of dense melt at this boundary (e.g. Reveanugh and Sipkin, 1994). Density measurements of anhydrous basaltic melts indicate that it is denser than surrounding mantle near 410-km depth (Ohtani and Maeda, 2001). However, melting temperature of peridotite is much higher than about 1400°C, estimated at 410-km depth. It has been shown recently that hydrous basaltic melt containing up to 2 wt.% H2O is denser than peridotite atop 410-km and therefore can be accumulated at the base of the upper mantle (Sakamaki et al., 2006). CO2 is another major volatile component in the mantle and it could be also important for explanation of LVZ near 410 km. In the present study, we have measured the density of carbonated basaltic melt at high pressures and high temperatures and discussed its possible stability at the base of the upper mantle. The density of the melt was determined using sink/float technique. The starting material was synthetic MORB glass. 5 and 10 wt.% CO2 was added to the glass as CaCO3 and Na2CO3, adjusting to proportions of related oxides. Experiments were carried out at 16-22 GPa and 2200-2300°C using a multianvil apparatus at Tohoku University, Japan. We observed neutral buoyancy of diamond density marker in MORB + 5 wt.% CO2 at 18 GPa and 2300°C, whereas, diamond was completely dissolved in the carbonated MORB melt containing 10 wt.% CO2 in 0.5-1 minute experiments. Based on the buoyancy test, the density of the carbonated basaltic melt, containing 5 wt.% CO2, is 3.56 g/cm3 at 18 GPa and 2300°C using an equation of state of diamond. To calculate the bulk modulus we assume that the pressure derivative of the isothermal bulk modulus is the same as that of the dry MORB melt, dKT/dP=5.0 and zero-pressure partial molar volume of CO2 is 32 cm3/mol (based on low-pressure experiments on carbonated basaltic melts and carbonatites, e.g. Dobson et al

  8. High pressure experimental constraints on the fate of water during subduction of oceanic crustal material into the deep mantle

    NASA Astrophysics Data System (ADS)

    Rosenthal, Anja; Frost, Daniel J.

    2014-05-01

    Knowledge of the abundance and distribution of H2O in the Earth's deep mantle remains highly controversial. The chief means of replenishment of the Earth's interior with volatiles over much of geological time is subduction, but constraints are very poor as natural samples from the deep Earth's interior subduction zones are inaccessible. High pressure experimental investigations can overcome that problem by simulating deep mantle conditions and processes. We aim to experimentally determine the maximum storage capacity, substitution mechanism and behaviour of H2O in hydrous and nominally anhydrous minerals (NAMs) during subduction of hydrated oceanic crustal material into the deep upper mantle. A particular interest is to determine the H2O content of NAMs at the conditions where nominally hydrous phases (such as phengite) are breaking down to release H2O that would then leave the slab. By applying a novel experimental approach formerly used for peridotite mantle compositions [1, 2], small amounts of H2O in eclogitic NAMs such as garnet, clinopyroxene, coesite/stishovite etc. will be determined for the first time in high pressure experiments as a function of pressure, temperature and bulk composition by using interlayers of the NAMs and volatile-rich oceanic crustal material of MORB composition. Here we present the first results of experimentally determined melting and phase relations of an altered oceanic basalt composition GA1 [3] containing varying amounts of H2O (up to 7wt.%) at varying temperatures (sub-solidus to near solidus) and pressures (6-10 GPa; i.e. from ~200 to ~330 km depth) using multi anvil apparatuses at University of Bayreuth, Germany. Experiments yield well-crystallised assemblages of garnet ± clinopyroxene ± coesite/stishovite ± rutile ± phengite ± vapour. Similar to previous studies [e.g. 4-8], the stability of phengite varies as a function of pressure, temperature, buffering mineral paragenesis and bulk H2O concentration. In addition, K2O

  9. Deep-tow magnetic survey above large exhumed mantle domains of the eastern Southwest Indian ridge

    NASA Astrophysics Data System (ADS)

    Bronner, A.; Munschy, M.; Carlut, J. H.; Searle, R. C.; Sauter, D.; Cannat, M.

    2011-12-01

    The recent discovery of a new type of seafloor, the "smooth seafloor", formed with no or very little volcanic activity along the ultra-slow spreading Southwest Indian ridge (SWIR) shows an unexpected complexity in processes of generation of the oceanic lithosphere. There, detachment faulting is thought to be a mechanism for efficient exhumation of deep-seated mantle rocks. We present here a deep-tow geological-geophysical survey over smooth seafloor at the eastern SWIR (62-64°N) combining magnetic data, geology mapping from side-scan sonar images and results from dredge sampling. We introduce a new type of calibration approach for deep-tow fluxgate magnetometer. We show that magnetic data can be corrected from the magnetic effect of the vehicle with no recourse to its attitude (pitch, roll and heading) but only using the 3 components recorded by the magnetometer and an approximation of the scalar intensity of the Earth magnetic field. The collected dredge samples as well as the side-scan images confirm the presence of large areas of exhumed mantle-derived peridodites surrounded by a few volcanic constructions. This allows us to hypothesis that magnetic anomalies are caused by serpentinized peridotites or magmatic intrusions. We show that the magnetic signature of the smooth seafloor is clearly weaker than the surrounding volcanic areas. Moreover, the calculated magnetization of a source layer as well as the comparison between deep-tow and sea-surface magnetic data argue for strong East-West variability in the distribution of the magnetized sources. This variability may results from fluid-rocks interaction along the detachment faults as well as from the repartition of the volcanic material and thus questions the seafloor spreading origin of the corresponding magnetic anomalies. Finally, we provide magnetic arguments, as calculation of block rotation or spreading asymmetry in order to better constrain tectonic mechanisms that occur during the formation of this

  10. Control of slab width on subduction-induced upper mantle flow and associated upwellings: Insights from analog models

    NASA Astrophysics Data System (ADS)

    Strak, Vincent; Schellart, Wouter P.

    2016-06-01

    The impact of slab width W (i.e., trench-parallel extent) on subduction-induced upper mantle flow remains uncertain. We present a series of free subduction analog models where W was systematically varied to upscaled values of 250-3600 km to investigate its effect on subducting plate kinematics and upper mantle return flow around the lateral slab edges. We particularly focused on the upwelling component of mantle flow, which might promote decompression melting and could thereby produce intraplate volcanism. The models show that W has a strong control on trench curvature and on the trench retreat, subducting plate, and subduction velocities, generally in good agreement with previous modeling studies. Upper mantle flow velocity maps produced by means of a stereoscopic particle image velocimetry technique indicate that the magnitude of the subduction-induced mantle flow around the lateral slab edges correlates positively with the product of W and trench retreat velocity. For all models an important upwelling component is always produced close to the lateral slab edges, with higher magnitudes for wider slabs. The trench-parallel lateral extent of this upwelling component is the same irrespective of W, but its maximum magnitude gets located closer to the subducting plate in the trench-normal direction and it is more focused when W increases. For W ≤ 2000 km the upwelling occurs laterally (in the trench-parallel direction) next to the subslab domain and the mantle wedge domain, while for W ≥ 2000 km it is located only next to the subslab domain and focuses closer to the trench tip, because of stronger poloidal flow in the mantle wedge extending laterally.

  11. A New Comprehensive Model for Crustal and Upper Mantle Structure of the European Plate

    NASA Astrophysics Data System (ADS)

    Morelli, A.; Danecek, P.; Molinari, I.; Postpischl, L.; Schivardi, R.; Serretti, P.; Tondi, M. R.

    2009-12-01

    We present a new comprehensive model of crustal and upper mantle structure of the whole European Plate — from the North Atlantic ridge to Urals, and from North Africa to the North Pole — describing seismic speeds (P and S) and density. Our description of crustal structure merges information from previous studies: large-scale compilations, seismic prospection, receiver functions, inversion of surface wave dispersion measurements and Green functions from noise correlation. We use a simple description of crustal structure, with laterally-varying sediment and cristalline layers thickness and seismic parameters. Most original information refers to P-wave speed, from which we derive S speed and density from scaling relations. This a priori crustal model by itself improves the overall fit to observed Bouguer anomaly maps, as derived from GRACE satellite data, over CRUST2.0. The new crustal model is then used as a constraint in the inversion for mantle shear wave speed, based on fitting Love and Rayleigh surface wave dispersion. In the inversion for transversely isotropic mantle structure, we use group speed measurements made on European event-to-station paths, and use a global a priori model (S20RTS) to ensure fair rendition of earth structure at depth and in border areas with little coverage from our data. The new mantle model sensibly improves over global S models in the imaging of shallow asthenospheric (slow) anomalies beneath the Alpine mobile belt, and fast lithospheric signatures under the two main Mediterranean subduction systems (Aegean and Tyrrhenian). We map compressional wave speed inverting ISC travel times (reprocessed by Engdahl et al.) with a non linear inversion scheme making use of finite-difference travel time calculation. The inversion is based on an a priori model obtained by scaling the 3D mantle S-wave speed to P. The new model substantially confirms images of descending lithospheric slabs and back-arc shallow asthenospheric regions, shown in

  12. Documenting the importance of coupled isotropic-anisotropic seismic tomography of the upper mantle beneath Northern Apennines subduction zone

    NASA Astrophysics Data System (ADS)

    Munzarova, Helena; Plomerova, Jaroslava; Kissling, Eduard

    2013-04-01

    The upper mantle velocity anisotropy together with the velocity heterogeneities affect significantly propagation of seismic waves. Velocity perturbations both due to isotropic heterogeneities and due to anisotropy are probably comparable in their amplitudes. Standard methods of imaging velocity perturbations in the upper mantle consider only isotropic propagations, in spite of the fact that seismic anisotropy has been undoubtedly proven within the whole of upper mantle. Neglecting anisotropy can cause significant artefacts in isotropic tomography results (e.g., wrong amplitudes of the heterogeneities, and/or, seriously distorted or false heterogeneities altogether). In addition, anisotropy yields unparalleled information on subsurface fabric and thus strongly enhances tectonic interpretation capabilities. The region of Northern Apennines (Italy) can serve as an example of an upper mantle volume where both a strong isotropic velocity heterogeneity and significant seismic anisotropy are present. The distinct velocity heterogeneity is represented by the subducting Adriatic slab. Strength and orientation of seismic anisotropy, both fossil one in the mantle lithosphere and anisotropy in the sub-lithospheric mantle flow, are evaluated from teleseismic P-wave travel times and shear-wave splitting (Plomerova et al., EPSL 2006). Anisotropic models of the upper mantle fabrics beneath the Northern Apennines were derived by joint analysis of anisotropic parameters evaluated from two independent body-wave data sets recorded during the RETREAT experiment (2003-2006; Munzarova et al., G-Cubed 2012, submitted). To evaluate effects of the well-known trade-off between anisotropy and heterogeneity, we calculated synthetic P travel time residual spheres, showing azimuth and incidence-angle dependent parts of the P-wave relative residuals, for the most recent tomographic model of isotropic velocity perturbations in the upper mantle beneath the Northern Apennines (Benoit et al., G

  13. Platinum group elements in a 3.5 Ga nickel-iron occurrence - Possible evidence of a deep mantle origin

    NASA Technical Reports Server (NTRS)

    Tredoux, Marian; Hart, Rodger J.; Lindsay, Nicholas M.; De Wit, Maarten J.; Armstrong, Richard A.

    1989-01-01

    This paper reports the results of new field observations and the geochemical analyses for the area of the Bon Accord (BA) (the Kaapvaal craton, South Africa) Ni-Fe deposit, with particular consideration given to the trace element, platinum-group element, and isotopic (Pb, Nd, and Os) compositions. On the basis of these data, an interpretation of BA is suggested, according to which the BA deposit is a siderophile-rich heterogeneity remaining in the deep mantle after a process of incomplete core formation. The implications of such a model for the study of core-mantle segregation and the geochemistry of the lowermost mantle are discussed.

  14. Upper Mantle Structure Beneath the Galápagos Hotspot from Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Villagomez, D. R.; Toomey, D. R.; Hooft, E. E.; Solomon, S. C.

    2004-12-01

    To understand plume-lithosphere interaction in a near-ridge setting, we present a surface wave tomographic study of the upper mantle beneath the Galápagos Archipelago. We use Rayleigh waves recorded by a network of 10 broadband seismometers deployed from 1999 to 2003 for the IGUANA experiment and the GSN station PAYG. We analyze waves in 12 separate frequency bands (8-50 mHz), which are sensitive to shear wave velocity (Vs) structure in the upper 150 km. To account for non-great-circle propagation caused by multipathing we use the two-plane-wave approximation of Forsyth and others. Two-dimensional models of phase velocity obtained at each frequency are inverted for three-dimensional variations in Vs. Average one-dimensional phase velocities are 1-2% slower than for 0-4 My-old Pacific mantle, and phase velocities vary laterally by ±3%. Inversions of phase velocities reveal that Vs varies regionally from 3.7 to 4.1 km/s, 3-15% slower than predicted along a 1300° C adiabat, and that there are two volumes of pronounced low velocity (>10% Vs reduction). Neither anomaly can be attributed to temperature alone; instead they require increased amounts of partial melt. The first anomaly, located beneath the volcanoes of the southwestern archipelago that erupt large volumes of enriched magmas, is most pronounced above 40 km depth and its magnitude increases toward the surface. This anomaly lies above an area of thinner-than-normal mantle transition zone and a cylindrical low-velocity body imaged by P and S wave tomography at depths of 100 to 250 km. This first anomaly may be the result of melt accumulation above a region of decompression melting driven by plume upwelling. The second low-velocity volume underlies the central archipelago, including the islands of Santiago and Marchena, and appears to be concentrated between 50 and 80 km depth. This anomaly is less pronounced near the surface, underlies a region that produces MORB, and coincides with a region of apparent

  15. Constraining the rheology of the lithosphere and upper mantle with geodynamic inverse modelling

    NASA Astrophysics Data System (ADS)

    Kaus, Boris; Baumann, Tobias

    2016-04-01

    The rheology of the lithosphere is of key importance for the physics of the lithosphere. Yet, it is probably the most uncertain parameter in geodynamics as experimental rock rheologies have to be extrapolated to geological conditions and as existing geophysical methods such as EET estimations make simplifying assumptions about the structure of the lithosphere. In many geologically interesting regions, such as the Alps, Andes or Himalaya, we actually have a significant amount of data already and as a result the geometry of the lithosphere is quite well constrained. Yet, knowing the geometry is only one part of the story, as we also need to have an accurate knowledge on the rheology and temperature structure of