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Sample records for deep upper mantle

  1. Seismic evidence for water deep in Earth's upper mantle.

    PubMed

    van der Meijde, Mark; Marone, Federica; Giardini, Domenico; van der Lee, Suzan

    2003-06-06

    Water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone. Observations of converted seismic waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a depth of 410 kilometers, most likely explained by as much as 700 parts per million of water by weight.

  2. 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.

  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.

  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. Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep-seated thermal anomaly

    NASA Astrophysics Data System (ADS)

    Benoit, Margaret H.; Nyblade, Andrew A.; Owens, Thomas J.; Stuart, Graham

    2006-11-01

    Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low-velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by ˜20-30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by ˜300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths ≥660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

  6. Rheology of fine-grained forsterite aggregate under deep upper mantle conditions

    NASA Astrophysics Data System (ADS)

    Nishihara, Y.; Ohuchi, T.; Kawazoe, T.; Spengler, D.; Tasaka, M.; Hiraga, T.; Kikegawa, T.; Suzuki, A.; Ohtani, E.

    2010-12-01

    Under the conditions of the Earth’s mantle, both diffusion creep and dislocation creep can be the dominant deformation mechanism depending on physical and chemical environments. These two mechanisms are quite different in terms of stress dependence of viscosity and development of lattice-preferred orientation. Thus it is important to understand the dominant deformation mechanism in the mantle. Previous studies on rheology of olivine under high-pressure (>3 GPa) mostly focused on dislocation creep (e.g. Kawazoe et al., 2009; Durham et al., 2009). Knowledge of diffusion creep of olivine under deep upper mantle condition (>100 km) has been quite limited. In order to clarify the dominant deformation mechanism in the upper mantle, we have conducted deformation experiments at high-pressure and high-temperature using fine-grained forsterite aggregate. Experiments were carried out using a D-DIA apparatus “D-CAP (deformation cubic-anvil press)” installed at NE7 beamline, PF-AR, High Energy Accelerator Research Institute, Tsukuba, Japan. The samples are sintered aggregate of 90%forsterite + 10%enstatite with average grain size of ~1 μm. High-pressure was generated by MA6-6 assembly (e.g. Kawazoe et al., 2010) using cubic (Mg,Co)O pressure medium and WC anvils with 5 mm truncation edge length. High-temperature was generated using graphite furnace and was monitored by WRe thermocouple. Deformation experiments were conducted at pressure of 3-5.5 GPa, temperature of 1573 K, and uniaxial strain rate of 7 x 10-6-2 x 10-4 s-1. Sample stress was measured by two-dimensional X-ray diffraction using monochromatized synchrotron X-ray (50 keV) and imaging plate detector (e.g. Nishihara et al., 2009). Sample strain was measured by X-ray radiography. H2O concentration in starting material and recovered samples was determined based on FTIR analyses (Paterson, 1982). Steady state flow stress was determined at each deformation condition. The stress-strain rate data taken at

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

    PubMed

    Matsukage, Kyoko N; Jing, Zhicheng; Karato, Shun-ichiro

    2005-11-24

    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 approximately 410 km 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.

  8. 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

  9. 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

  10. Slab dehydration and fluid migration at the base of the upper mantle: implications for deep earthquake mechanisms

    NASA Astrophysics Data System (ADS)

    Richard, Guillaume; Monnereau, Marc; Rabinowicz, Michel

    2007-03-01

    Water enters the Earth's mantle via subduction of oceanic lithosphere and sediments. A lot of this water immediately returns to the atmosphere through arc volcanism, but part, retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected to be subducted as deep as the bottom of the upper mantle (660 km depth). Then, due to its low solubility in lower mantle minerals, water is likely to be released as a hydrated fluid during the spinel-post-spinel phase change. The dynamics of this fluid phase is investigated through a 1-D model of compaction, in which a source term has been introduced to take the fluid precipitation into account. The competition between the advective transport by the descending slab and the buoyant rise of the fluid results in three distinct situations, depending on the properties of the solid and the fluid phases. Low matrix permeability and high fluid viscosity inhibit compaction and favour the entrainment of fluid towards the deep mantle. In this case, the entire slab water content would enter the lower mantle and would be mixed at large scale. However, realistic values of the fluid viscosity and matrix permeability make this possibility unlikely. When effective, compaction results in an accumulation of fluid at and below the phase boundary. Then, depending on the value of the matrix viscosity, the situation evolves differently. Above 1020 Pa s, accumulation of fluid extends below the phase boundary and the pressure difference between the fluid and the matrix increases continuously, exceeding the yield strength of rocks. As a result, cracks would form and evolve towards the formation of dykes. In case of very low mantle viscosity, possibly due to strong grain size reduction during phase change, compaction becomes very efficient and the fluid remains confined within the phase change horizon, without increasing pressure. In the long term, this last situation appears unstable and would also

  11. 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.

  12. 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.

  13. 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.

  14. Upper mantle diapers, lower crustal magmatic underplating, and lithospheric dismemberment of the Great Basin and Colorado Plateau regions, Nevada and Utah; implications from deep MT resistivity surveying

    NASA Astrophysics Data System (ADS)

    Wannamaker, P. E.; Doerner, W. M.; Hasterok, D. P.

    2005-12-01

    In the rifted Basin and Range province of the southwestern U.S., a common faulting model for extensional basins based e.g. on reflection seismology data shows dominant displacement along master faults roughly coincident with the main topographic scarp. On the other hand, complementary data such as drilling, earthquake focal mechanisms, volcanic occurrences, and trace indicators such as helium isotopes suggest that there are alternative geometries of crustal scale faulting and material transport from the deep crust and upper mantle in this province. Recent magnetotelluric (MT) profiling results reveal families of structures commonly dominated by high-angle conductors interpreted to reflect crustal scale fault zones. Based mainly on cross cutting relationships, these faults appear to be late Cenozoic in age and are of low resistivity due to fluids or alteration (including possible graphitization). In the Ruby Mtns area of north-central Nevada, high angle faults along the margins of the core complex connect from near surface to a regional lower crustal conductor interpreted to contain high-temperature fluids and perhaps melts. Such faults may exemplify the high angle normal faults upon which the major earthquakes of the Great Basin appear to nucleate. A larger-scale transect centered on Dixie Valley shows major conductive crustal-scale structures connecting to conductive lower crust below Dixie Valley, the Black Rock desert in NW Nevada, and in east-central Nevada in the Monitor-Diamond Valley area. In the Great Basin-Colorado Plateau transition of Utah, the main structures revealed are a series of nested low-angle detachment structures underlying the incipient development of several rift grabens. All these major fault zones appear to overlie regions of particularly conductive lower crust interpreted to be caused by recent basaltic underplating. In the GB-CP transition, long period data show two, low-resistivity upper mantle diapirs underlying the concentrated

  15. 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.

  16. Upper-mantle origin of the Yellowstone hotspot

    USGS Publications Warehouse

    Christiansen, R.L.; Foulger, G.R.; Evans, J.R.

    2002-01-01

    Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ???200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

  17. 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.

  18. Does MORB reflect upper mantle diversity?

    NASA Astrophysics Data System (ADS)

    Murton, B. J.; Smith, H.; Fitton, G.

    2013-12-01

    It is often stated that MORB provides a window into the composition of the earth's upper mantle. Although MORB displays a range of compositions, its spatial scale tends to be much longer than that of oceanic hotspots or mantle plumes, which also display greater compositional heterogeneity. Hence it is tempting to conclude that oceanic upper mantle is more homogeneous than mantle involved in hotspots and plumes. Observations from the interaction between Iceland and the adjacent Reykjanes Ridge offer a chance to test this view. A remarkable feature of this interaction is the rapid diminution of heterogeneity in basaltic lavas from onshore Iceland to off-shore along the adjacent mid-ocean spreading ridge (the Reykjanes Ridge). Young volcanic rocks on Iceland exhibit a wide range of trace-element and isotopic compositions reflecting a diversity of composition within the plume mantle beneath Iceland. The neovolcanic axis of the adjacent spreading ridge is also affected by the Iceland plume: the presence of a large diameter bathymetric swell, V-shaped ridges migrating out from Iceland, and associated enriched geochemical compositions all point to a ~1000-km diameter regional plume influence. Despite this, the diversity of lava composition decreases rapidly along the Reykanes Ridge away from Iceland. This decrease is unlikely to be an artefact of sampling as the Reykjanes Ridge has a very high density of samples acquired from dredge stations located every 2-3 km, each of which recovered a number of individual lavas. Collapsing the diversity of young lava compositions on Iceland produces a mean composition that lies on a mixing line between lavas from the northern Reykjanes Ridge and the highly enriched peripheral Icelandic volcano of Snaefjelsness. We argue that this decrease in heterogeneity is real and is evidence for either a mantle process that homogenises the outflowing Icelandic plume as it flows away from Iceland beneath the Reykjanes Ridge, or alternatively a

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

    NASA Astrophysics Data System (ADS)

    Zhang, Fengxue; Wu, Qingju; Grand, Stephen P.; Li, Yonghua; Gao, Mengtan; Demberel, Sodnomsambuu; Ulziibat, Munkhuu; Sukhbaatar, Usnikh

    2017-02-01

    Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

  5. Dehydrogenation of goethite in Earth's deep lower mantle

    NASA Astrophysics Data System (ADS)

    Hu, Qingyang; Kim, Duck Young; Liu, Jin; Meng, Yue; Yang, Liuxiang; Zhang, Dongzhou; Mao, Wendy L.; Mao, Ho-kwang

    2017-02-01

    The cycling of hydrogen influences the structure, composition, and stratification of Earth’s interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO2H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, x, in the P-phase FeO2Hx. Using a combination of theoretical simulations and high-pressure–temperature experiments, we calibrated the x dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < x < 0.81, corresponding to 19–61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower x, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.

  6. 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.

  7. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    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.

  8. 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

    2011-10-07

    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.

  9. Robustness of Global Radial Anisotropy Models of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Xing, Z.; Beghein, C.; Yuan, K.

    2014-12-01

    discrepancies, inferences on the depth of continental roots (~200-250km) based on either the extent of the dlnVS>0 anomalies or the depth at which ξ changes sign remain independent of the crustal model employed. We also note that VSV>VSH dominates the deep upper mantle except in central Pacific, which is characterized by VSH>VSV down to the transition zone.

  10. 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.

  11. 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

  12. European upper mantle tomography: adaptively parameterized models

    NASA Astrophysics Data System (ADS)

    Schäfer, J.; Boschi, L.

    2009-04-01

    We have devised a new algorithm for upper-mantle surface-wave tomography based on adaptive parameterization: i.e. the size of each parameterization pixel depends on the local density of seismic data coverage. The advantage in using this kind of parameterization is that a high resolution can be achieved in regions with dense data coverage while a lower (and cheaper) resolution is kept in regions with low coverage. This way, parameterization is everywhere optimal, both in terms of its computational cost, and of model resolution. This is especially important for data sets with inhomogenous data coverage, as it is usually the case for global seismic databases. The data set we use has an especially good coverage around Switzerland and over central Europe. We focus on periods from 35s to 150s. The final goal of the project is to determine a new model of seismic velocities for the upper mantle underlying Europe and the Mediterranean Basin, of resolution higher than what is currently found in the literature. Our inversions involve regularization via norm and roughness minimization, and this in turn requires that discrete norm and roughness operators associated with our adaptive grid be precisely defined. The discretization of the roughness damping operator in the case of adaptive parameterizations is not as trivial as it is for the uniform ones; important complications arise from the significant lateral variations in the size of pixels. We chose to first define the roughness operator in a spherical harmonic framework, and subsequently translate it to discrete pixels via a linear transformation. Since the smallest pixels we allow in our parameterization have a size of 0.625 °, the spherical-harmonic roughness operator has to be defined up to harmonic degree 899, corresponding to 810.000 harmonic coefficients. This results in considerable computational costs: we conduct the harmonic-pixel transformations on a small Beowulf cluster. We validate our implementation of adaptive

  13. 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.

  14. Topographic Expression of Deep Crustal and Mantle Processes

    NASA Astrophysics Data System (ADS)

    Whipple, K. X.; Ouimet, W. B.; Baldwin, J. A.

    2006-12-01

    The topography of mountain ranges records much information about the history of tectonic events and climatic conditions. Although this record is not easily read, as there are many imperfectly understood factors that influence landscape morphology, there is much encouraging evidence that the river steepness index (a cousin of the Hack gradient index) exhibits a robust correlation with rock uplift rate. Published, and soon to be published, examples span a wide range of field conditions from sites around the globe (e.g., Western US, Bolivia, Nepal, China, Taiwan). Differences in substrate rock properties can have a pronounced influence on channel steepness for a given rock uplift rate, however, and analyses are best limited to regional studies where climatic conditions and lithology are largely uniform. In such settings, careful analysis of river profiles and the relief structure of the landscape can reveal important information about the history of rock uplift, recording temporal changes in uplift/river incision rates. This potential adds considerably to the value of topographic analyses, especially where uplift is driven by deep crustal or mantle processes and not clearly expressed in upper crustal structures and geodetic shortening data. However, this potential comes at the price of complicating the interpretation of the relationship between topography and the modern snapshot of deep crustal/mantle conditions provided by geophysical imaging techniques. Over what timescales does topography retain a record of past events in earth's interior? Important questions remain unanswered about the age of topography in many landscapes where deep crustal or mantle dynamics have been invoked. We discuss the factors that control landscape response time, both to an increase in rock uplift rate and to the cessation of tectonic activity, and present constraints on the antiquity of topography in several field settings.

  15. 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.

  16. 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

  17. 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

  18. 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

  19. The thermal regimes of the upper mantle beneath Precambrian and Phanerozoic structures up to the thermobarometry data of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Glebovitsky, V. A.; Nikitina, L. P.; Khiltova, V. Ya.; Ovchinnikov, N. O.

    2004-05-01

    The thermal state of the upper mantle beneath tectonic structures of various ages and types (Archaean cratons, Early Proterozoic accretionary and collisional orogens, and Phanerozoic structures) is characterized by geotherms and by thermal gradients (TG) derived from data on the P- T conditions of mineral equilibria in garnet and garnet-spinel peridotite xenoliths from kimberlites (East Siberia, Northeastern Europe, India, Central Africa, North America, and Canada) and alkali basalts (Southeastern Siberia, Mongolia, southeastern China, southeastern Australia, Central Africa, South America, and the Solomon and Hawaiian islands). The use of the same garnet-orthopyroxene thermobarometer (Theophrastus Contributions to Advanced Studies in Geology. 3: Capricious Earth: Models and Modelling of Geologic Processes and Objects 2000 44) for all xenoliths allowed us to avoid discrepancies in estimation of the P- T conditions, which may be a result of the mismatch between different thermometers and barometers, and to compare the thermal regimes in the mantle in various regions. Thus, it was established that (1) mantle geotherms and geothermal gradients, obtained from the estimation of P- T equilibrium conditions of deep xenoliths, correspond to the age of crust tectonic structures and respectively to the time of lithosphere stabilization; it can be suggested that the ancient structures of the upper mantle were preserved within continental roots; (2) thermal regimes under continental mantle between the Archaean cratons and Palaeoproterozoic belts are different today; (3) the continental mantle under Neoproterozoic and Phanerozoic belts is characterized by significantly higher values of geothermal gradient compared to the mantle under Early Precambrian structures; (4) lithosphere dynamics seems to change at the boundary between Early and Mezo-Neoproterozoic and Precambrian and Phanerozoic.

  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.

  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. Localized seismic deformation in the upper mantle revealed by dense seismic arrays

    NASA Astrophysics Data System (ADS)

    Inbal, Asaf; Ampuero, Jean Paul; Clayton, Robert W.

    2016-10-01

    Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

  6. Localized seismic deformation in the upper mantle revealed by dense seismic arrays.

    PubMed

    Inbal, Asaf; Ampuero, Jean Paul; Clayton, Robert W

    2016-10-07

    Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

  7. Upper- Mantle Driven Dynamic Uplift in Central Anatolia

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Based on geological and geophysical observations and interpretations of the present-day geodynamics, we propose that mantle structures beneath the crust drive a non-isostatic component of topography in Central Anatolia. Topography residuals for the region were calculated from the isostatic component of topography according to the principle of Airy isostasy while assuming crustal block is in hydrostatic equilibrium in the mantle. For the geodynamic interpretations we ran numerous 2D thermo-mechanical models based on different temperature inputs and viscous creep strength coefficients and using available P-wave tomography data along a N-S directional profile (33oE) through Central Anatolia as an estimate on the regional mantle structure. Our models are uniformly affected by widespread NE-SW oriented mantle flow obtained in the shear-wave azimuthal anisotropy studies and predict dynamic topography based on vertical components of density-driven flow in the upper mantle mainly induced by 2D temperature variations. The dynamic topography results indicate ~1 km instantaneous uplift in concordance with the under-compensated topography in the region. The data and modelling results define the region as a plateau-like uplift, slightly inflated in southern part based on dynamic topography patterns. The dynamic topography induced by upper-mantle flow provides robust new information about the main geodynamic components by showing broad consistency with independent data sets and observables for the area; such as, asthenospheric source of volcanism, gravity data, and high surface heat flow distributions.

  8. 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

  9. 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.

  10. Upper mantle reflectors: Modelling of seismic wavefield characteristics and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hansen, T. M.; Balling, N.

    2003-04-01

    In recent years a number of deep seismic experiments have demonstrated the existence of seismic reflectors in the mantle lithosphere. The origin of the reflective structures is a matter of debate. Hypothesis and interpretations include remnant subduction zones, shear zones, fluids and seismic anisotropy. Through forward modelling studies including numerical full wavefield modelling, we have found that both upper mantle shear zones of reduced seismic velocity and density and remnant subduction slabs containing high-density eclogites may have sufficient seismic impedance contrasts to normal mantle peridotites to generate near-normal recidence reflectivity. Wide-angle energy may originate from subduction slabs containing high or low velocity eclogites whereas localized shear zones in mantle peridotite may generally not produce significant wide-angle energy. Analysis of two specific deep-seismic data sets from the North Sea (MONA LISA data) and the Baltic Sea (BABEL data) show good agreement between observations and modelling results for dipping remnant subduction slabs containing small-scale inhomogeneities associated with incomplete transformation of low velocity/low density crustal material to high velocity/high density eclogite. Our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper mantle seismic reflectors, the remnant subduction and extensional shear zone models.

  11. 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).

  12. Estimating Upper Mantle Hydration from In Situ Electrical Conductivity

    NASA Astrophysics Data System (ADS)

    Behrens, J.; Constable, S.; Heinson, G.; Everett, M.; Weiss, C.; Key, K.

    2004-12-01

    The electrical conductivity of 35-40 Ma Pacific plate has been measured in situ; one robust result is the presence of bulk anisotropy in the lithospheric upper mantle. We interpret this anisotropy to be a result of hydrothermal circulation into the upper mantle along spreading-ridge-parallel normal faults: the associated zones of serpentinized peridotite provide the pathways of enhanced electrical conductivity required by the data. Our modeling bounds the range of possible anisotropic ratios, which are then used to estimate the amount of water required to serpentinize the requisite amounts of peridotite. These data sets, however, do not indicate anisotropy in the bulk conductivity of the crust, nor in the asthenospheric mantle. This second point is significant, as recent measurements of sub-continental asthenospheric conductivity have been interpreted to indicate anisotropy aligned with present plate motion, with the diffusion of hydrogen through olivine advanced as an explanation.

  13. On the deep-mantle origin of the Deccan Traps.

    PubMed

    Glišović, Petar; Forte, Alessandro M

    2017-02-10

    The Deccan Traps in west-central India constitute one of Earth's largest continental flood basalt provinces, whose eruption played a role in the Cretaceous-Paleogene extinction event. The unknown mantle structure under the Indian Ocean at the start of the Cenozoic presents a challenge for connecting the event to a deep mantle origin. We used a back-and-forth iterative method for time-reversed convection modeling, which incorporates tomography-based, present-day mantle heterogeneity to reconstruct mantle structure at the start of the Cenozoic. We show a very low-density, deep-seated upwelling that ascends beneath the Réunion hot spot at the time of the Deccan eruptions. We found a second active upwelling below the Comores hot spot that likely contributed to the region of partial melt feeding the massive eruption.

  14. On the deep-mantle origin of the Deccan Traps

    NASA Astrophysics Data System (ADS)

    Glišović, Petar; Forte, Alessandro M.

    2017-02-01

    The Deccan Traps in west-central India constitute one of Earth’s largest continental flood basalt provinces, whose eruption played a role in the Cretaceous-Paleogene extinction event. The unknown mantle structure under the Indian Ocean at the start of the Cenozoic presents a challenge for connecting the event to a deep mantle origin. We used a back-and-forth iterative method for time-reversed convection modeling, which incorporates tomography-based, present-day mantle heterogeneity to reconstruct mantle structure at the start of the Cenozoic. We show a very low-density, deep-seated upwelling that ascends beneath the Réunion hot spot at the time of the Deccan eruptions. We found a second active upwelling below the Comores hot spot that likely contributed to the region of partial melt feeding the massive eruption.

  15. Geodetic Estimate of Water in the Wharton Basin Upper Mantle

    NASA Astrophysics Data System (ADS)

    Masuti, S. S.; Barbot, S.; Karato, S. I.; Feng, L.; Bannerjee, P.; Natawidjaja, D.

    2015-12-01

    The formation of oceans at the Earth's surface and hence the origin of life can be directly linked to the fate of water after planetary formation, but how much water is now confined in the upper mantle remains elusive. Current estimates of water in olivine from geochemistry are between 200 and 3600 H/106 Si. Here, we exploit the water-sensitive rheology of olivine to estimate the water content in the Wharton Basin asthenosphere in the Indian Ocean. We explore the role of water stratification in the upper mantle and the transient behavior of olivine flow in the context of postseismic deformation following the 2012 Mw 8.6 Wharton Basin earthquake using geodetic data from the Sumatra GPS network. We build a model that incorporates afterslip in the brittle upper mantle and viscous flow in the asthenosphere. We introduce a formulation of the transient rheology of olivine in the form of a flow law coupled to a state evolution that characterizes the internal stress of the mineral before steady-state. We find that the Wharton Basin asthenosphere contains about 1000 H/106 Si, representing 10% of water saturation of olivine at 100 km depth. If these results can be extrapolated to other depths, this indicates an equivalent of about 0.7 ocean mass is now present in the Earth's upper mantle.

  16. Volatile element content of the heterogeneous upper mantle

    NASA Astrophysics Data System (ADS)

    Shimizu, K.; Saal, A. E.; Hauri, E. H.; Forsyth, D. W.; Kamenetsky, V. S.; Niu, Y.

    2014-12-01

    The physical properties of the asthenosphere (e.g., seismic velocity, viscosity, electrical conductivity) have been attributed to either mineral properties at relevant temperature, pressure, and water content or to the presence of a low melt fraction. We resort to the geochemical studies of MORB to unravel the composition of the asthenosphere. It is important to determine to what extent the geochemical variations in axial MORB do represent a homogeneous mantle composition and variations in the physical conditions of magma generation and transport; or alternatively, they represent mixing of melts from a heterogeneous upper mantle. Lavas from intra-transform faults and off-axis seamounts share a common mantle source with axial MORB, but experience less differentiation and homogenization. Therefore they provide better estimates for the end-member volatile budget of the heterogeneous upper mantle. We present major, trace, and volatile element data (H2O, CO2, Cl, F, S) as well as Sr, Nd, and Pb isotopic compositions [1, 2] of basaltic glasses (MgO > 6.0 wt%) from the NEPR seamounts, Quebrada-Discovery-Gofar transform fault system, and Macquarie Island. The samples range from incompatible trace element (ITE) depleted (DMORB: Th/La<0.035) to enriched (EMORB: Th/La>0.07) spanning the entire range of EPR MORB. The isotopic composition of the samples correlates with the degree of trace element enrichment indicating long-lived mantle heterogeneity. Once shallow-level processes (degassing, crystallization, and crustal assimilation) have been considered, we conducted a two-component (DMORB- and EMORB-) mantle melting-mixing model. Our model reproduces the major, trace and volatile element contents and isotopic composition of our samples and suggests that (1) 90% of the upper mantle is highly depleted in ITE (DMORB source) with only 10% of an enriched component (EMORB source), (2) the EMORB source is peridotitic rather than pyroxenitic, and (3) NMORB do not represent an actual

  17. The link between Hawaiian mantle plume composition, magmatic flux, and deep mantle geodynamics

    NASA Astrophysics Data System (ADS)

    Harrison, Lauren N.; Weis, Dominique; Garcia, Michael O.

    2017-04-01

    Oceanic island basalts sample mantle reservoirs that are isotopically and compositionally heterogeneous. The Hawaiian-Emperor chain represents ∼85 Myr of volcanism supplied by a deep mantle plume. Two geographically and geochemically delineated trends, Kea and Loa, are well documented within the Hawaiian Islands. Enriched Loa compositions originate from subduction recycled or primordial material stored in deep mantle reservoirs such as the large low shear velocity province (LLSVP) below Hawai'i. Loa compositions have not been observed along the Emperor Seamounts (>50 Ma), whereas lavas on the Hawaiian Islands (<6.5 Ma) sample both Kea and Loa sources. Lead isotopes in shield lavas along the Northwest Hawaiian Ridge (NWHR) spanning ∼42 Myr between the bend in the chain and the Hawaiian Islands record the geochemical evolution of the Hawaiian mantle plume over a time period when many geophysical parameters (volcanic propagation rate, magmatic flux, mantle potential temperature) increased significantly. Along the NWHR, the Loa geochemical component appears ephemerally, which we link to the sampling of different lower mantle compositional domains by the Hawaiian mantle plume. The plume initially sampled only the deep Pacific mantle (Kea component) from outside the LLSVP during the formation of the Emperor Seamounts. Southward migration and anchoring of the plume on the LLSVP led to entrainment of increasing amounts of LLSVP material (Loa component) along the NWHR as documented by an increase in 208Pb*/206Pb* with decreasing age. The correlation between 208Pb*/206Pb* and magmatic flux suggests source composition affects the magmatic flux, and explains why the Hawaiian mantle plume has dramatically strengthened through time.

  18. Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume

    NASA Astrophysics Data System (ADS)

    Weis, Dominique; Garcia, Michael O.; Rhodes, J. Michael; Jellinek, Mark; Scoates, James S.

    2011-12-01

    Linear chains of volcanic ocean islands are one of the most distinctive features on our planet. The longest, the Hawaiian-Emperor Chain, has been active for more than 80 million years, and is thought to have formed as the Pacific Plate moved across the Hawaiian mantle plume, the hottest and most productive of Earth's plumes. Volcanoes fed by the plume today form two adjacent trends, including Mauna Kea and Mauna Loa, that exhibit strikingly different geochemical characteristics. An extensive data set of isotopic analyses shows that lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years. Seismological data suggest that the Hawaiian mantle plume, when projected into the deep mantle, overlies the boundary between typical Pacific lower mantle and a sharply defined layer of apparently different material. This layer exhibits low seismic shear velocities and occurs on the Loa side of the plume. We conclude that the geochemical differences between the Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material. Similar indications of preferential sampling at the limit of a large anomalous low-velocity zone are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. We infer that the anomalous low-velocity zones at the core-mantle boundary are storing geochemical anomalies that are enriched in recycled material and sampled by strong mantle plumes.

  19. Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle

    NASA Astrophysics Data System (ADS)

    Konrad-Schmolke, Matthias; Halama, Ralf; Manea, Vlad C.

    2016-08-01

    The subduction of hydrated slab mantle is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. The most critical unknowns are the initial hydration state and the dehydration behavior of the subducted oceanic mantle. Here we present a combined thermomechanical, thermodynamic, and geochemical model of the Kamchatka subduction zone that indicates significant dehydration of subducted slab mantle beneath Kamchatka. Evidence for the subduction of hydrated oceanic mantle comes from across-arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. Our thermodynamic-geochemical models successfully predict the complex geochemical patterns and the spatial distribution of arc volcanoes in Kamchatka assuming the subduction of hydrated oceanic mantle. Our results show that water content and dehydration behavior of the slab mantle beneath Kamchatka can be directly linked to compositional features in arc volcanic rocks. Depending on hydration depth of the slab mantle, our models yield water recycling rates between 1.1 × 103 and 7.4 × 103 Tg/Ma/km corresponding to values between 0.75 × 106 and 5.2 × 106 Tg/Ma for the entire Kamchatkan subduction zone. These values are up to one order of magnitude lower than previous estimates for Kamchatka, but clearly show that subducted hydrated slab mantle significantly contributes to the water budget in the Kamchatkan subduction zone.

  20. The Hales discontinuity and upper mantle anisotropy beneath cratons

    NASA Astrophysics Data System (ADS)

    Musacchio, G.; White, D. J.; Thomson, C. J.

    2003-04-01

    Seismic velocity discontinuities are commonly found within the upper 100 km of the mantle lithosphere, with great variability in their depth, lateral extent, and the polarity of velocity jump. Among the more commonly observed is the Hales discontinuity, identified in a variety of tectonic environments, and commonly associated with a high-velocity, highly reflective and sometimes anisotropic layer. In the Archean Western Superior Province of the Canadian Shield, long range R/WAR profiling gives a high-resolution estimate of the mantle V_P in the shallow upper mantle, providing a more certain determination of the nature of the Hales dicontinuity. Ray-based travel-time inversion of the data, have shown that Vp in the uppermost mantle is 8.0-8.3 km/s. A 15-20 km thick layer (layer-H) with >6% seismic anisotropy (N-S V_P of 8.3 km/s and E-W V_P of 8.8 km/s) dips northward at ˜10^o from a minimum depth of 48-50 km. The attitude of layer-H is consistent with the general tectonic strike; its depth range (50-75 km) falls within that of the Hales discontinuity. If a link between the Hales discontinuity and layer-H can be drawn, observations strengthen the objection that the estimated velocity contrast (0.2 to 0.4 km/s depending on the direction of wave propagation) is relatively high if layer-H represents a phase transition, and thus (re)opens the debate on the nature of shallow upper-mantle boundaries beneath continents. The high V_P and intermediate anisotropy of upper-mantle layer-H requires a harzburgite peridotitic composition with the a-axis of olivine aligned E-W. Layer-H might have emplaced during accretion (2.7 Ga Kenoran orogeny) of the North American proto-craton and be relic oceanic lithosphere. The Hales discontinuity might be an expression of continents accretion and map relic slabs in the shallow upper mantle.

  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. Toward a coherent model for the melting behavior of the deep Earth's mantle

    NASA Astrophysics Data System (ADS)

    Andrault, D.; Bolfan-Casanova, N.; Bouhifd, M. A.; Boujibar, A.; Garbarino, G.; Manthilake, G.; Mezouar, M.; Monteux, J.; Parisiades, P.; Pesce, G.

    2017-04-01

    Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth's history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle. In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure

  3. 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.

  4. Upper mantle viscosity and dynamic subsidence of curved continental margins.

    PubMed

    Sacek, Victor; Ussami, Naomi

    2013-01-01

    Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s.

  5. 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.

  6. Double Layering and Bilateral Asymmetry of a Thermochemical Plume in the Upper Mantle beneath Hawaii

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Classical plume theory describes purely thermal upwellings that rise through the entire mantle, pond beneath the lithospheric plate in a thin "pancake," and generate hotspot volcanism. High-resolution seismic velocity images obtained from the Plume-Lithosphere Undersea Melt Experiment (PLUME) 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 broad low-velocity body in the upper mantle that is much thicker and more asymmetric than a thermal pancake predicted from purely thermal plume models. Geochemical observations also argue against a purely thermal (i.e., isochemical) mantle source for Hawaiian lavas and instead indicate a heterogeneous plume involving mafic lithologies such as eclogite. To explore the dynamical and melting behavior of hot plumes that also contain eclogite, we perform three-dimensional numerical simulations of thermochemical convection. The models simulate eclogite with an excess density relative to ambient-mantle peridotite that peaks at depths of 410-300 km due to solid phase changes and lessens at depths of 250-190 km where eclogite is removed by melting. For a plume core with an eclogite content >12%, a moderately buoyant plume stem rises into the upper mantle but pools as a much wider body at depths of 450-300 km (the "deep eclogite pool", or DEP). Out of the top of the DEP rises a shallow plume that feeds hotspot volcanism and supplies material into a thin sublithospheric pancake that supports the hotspot swell. Seismic resolution tests indicate that the double layering of hot plume material (DEP and shallow pancake) can account for the thick low-velocity body as imaged by PLUME. We also find that upwelling fingers of non-eclogitic outskirt material can support magmatism comparable in volume and geographic distribution to Hawaiian rejuvenated stage and arch volcanism. In some models, thermochemical plumes with radial and

  7. 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

  8. Inherited Upper Mantle Structures in the Southeastern United States

    NASA Astrophysics Data System (ADS)

    Wagner, L. S.; Fischer, K. M.; Hawman, R. B.; Biryol, C. B.

    2014-12-01

    The southeastern United States has experienced repeated cycles of terrane accretion, continental suturing, and extensive rifting over the past 1 Ga. Recent data collected from the EarthScope Transportable Array and the EarthScope FlexArray deployment SESAME (South Eastern Suture of the Appalachian Margin Experiment) allow us to image in detail the upper mantle structures associated with these tectonic events using both Rayleigh waves and teleseismic body waves. Of particular interest is the mantle expression of the Suwannee suture. Located across southern Georgia and Alabama, the Suwannee suture represents the boundary between the Gondwanan Suwannee terrane and the Laurentian margin. Previous authors have speculated that this accretion occurred due to southward dipping subduction of the Laurentian lithosphere, though more recent work suggests a possible dextral transpressional plate boundary. We find that the mantle lithosphere across the suture may dip slightly to the south, but the dip angle is much shallower than that of crustal structures seen in scattered wave and active source images. Ongoing work will help to constrain the precise dip angle and will also investigate the nature of lower velocities located between the mantle lithosphere and the overriding crust. Intriguingly, we do image a distinct dipping high velocity feature further to the west. This slab-like structure dips to the east, and has a surface projection located near the 1 Ga Grenville front. While the provenance of this high velocity anomaly is still unclear, a few intriguing observations can be made. Given the location of this anomaly in a tectonically stable area, it is unlikely to be due to an ongoing lithospheric process. This then raises the question of how such a dipping structure can persist over time despite over-riding plate motions and mantle flow patterns. The increase in shear velocities is unlikely due to any remnant thermal effects. Our ability to image this structure at all

  9. 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.

  10. Large gem diamonds from metallic liquid in Earth's deep mantle.

    PubMed

    Smith, Evan M; Shirey, Steven B; Nestola, Fabrizio; Bullock, Emma S; Wang, Jianhua; Richardson, Stephen H; Wang, Wuyi

    2016-12-16

    The redox state of Earth's convecting mantle, masked by the lithospheric plates and basaltic magmatism of plate tectonics, is a key unknown in the evolutionary history of our planet. Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.

  11. The European Continent : Surface Expression of Upper Mantle Dynamics

    NASA Astrophysics Data System (ADS)

    Tondi, M. R.; Schivardi, R.; Molinari, I.; Morelli, A.

    2012-12-01

    images of the European upper mantle isotropic shear-wave speeds and mass densities, recently recovered by combined inversion of surface-wave information and GRACE satellite gravity data (Tondi et al., 2012) are used to select the regions where the residual topography and the residual mantle gravity anomalies are strongly correlated (correlation coefficient is equal to 1). We assume surface uplift processes with negative density anomalies and downward pull with positive anomalies. Our work shows a strong correlation among the areas where, on the basis of our assumptions, the mantle dynamics have surface expression and the areas of low values of radial anisotropy: (1) the southern margins of the East European Craton, (2) the North-Eastern edges of the Arabian Plateau, (3) the northern edge of the CEVP (Central European Volcanic Province), (4) the North-Eastern part of the Atlantic Ocean, between Greenland and Iceland.

  12. 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

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

    USGS Publications Warehouse

    Brocher, T.M.; Parsons, T.; Trehu, A.M.; Snelson, C.M.; Fisher, M.A.

    2003-01-01

    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 slap gap during the evolution of the California margin.

  14. 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.

  15. 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

  16. Upper-mantle reflectors: modelling of seismic wavefield characteristics and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hansen, T. M.; Balling, N.

    2004-05-01

    Deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle. However, reflected and refracted energy from mantle lithosphere are observed in both near-normal incidence and wide-angle data. The origin of the reflective structures is a matter of debate. Hypotheses include remnant subduction zones, shear zones, fluids and seismic anisotropy. Through analytical and numerical modelling studies, including full wavefield modelling, we investigate seismic characteristic signatures generated from a variety of geologically plausible models. We have found that both upper-mantle shear zones of reduced velocity and density and remnant subduction slabs containing high-density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near-normal incidence reflectivity. Wide-angle energy originates from subduction slabs containing either high- or low-velocity eclogites, whereas intermediate-velocity eclogites are unlikely to produce significant wide-angle phases. In general, energy of seismic phases originating from upper-mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference. Maximum wavefield anomalies are generated from sub Moho dipping slabs of incomplete transformation of low-velocity/low-density crustal material to high-velocity/high-density eclogites. Localized shear zones generated in mantle peridotite generally do not produce significant wide-angle energy. Only if highly inhomogeneous structures containing material of marked (ca 10 per cent) velocity and density reduction are present, may shear zones be observed in wide-angle data. Analyses of two specific deep-seismic data sets (MONA LISA data) from the North Sea and (BABEL data) from the Baltic Sea

  17. 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.

  18. Grain-Boundary Plasticity and the Strength of the Upper Mantle

    NASA Astrophysics Data System (ADS)

    Raterron, P.; Bollinger, C.; Hilairet, N.; Merkel, S.

    2014-12-01

    The rheology of the Earth's upper mantle is controlled by the plasticity of olivine-rich rocks. Despite considerable efforts to better understand their plasticity, experimental flow laws fail to explain important geodetic and geophysical observations, such as fast post-seismic surface displacements or the "elusiveness" of the lithosphere-asthenosphere boundary beneath cratons. The plasticity of olivine single crystals is well understood and was experimentally quantified to mantle pressures and temperatures (e.g., Bai et al., 1991, JGR, 96, 2441-2463; Raterron et al., 2012, PEPI, 200-201, 105-112). The plasticity of aggregates, however, involves additional mechanisms, and the fundamental question of the amount of strain accommodation at grain boundaries remain unanswered. In this study, we compare the plasticity of olivine aggregates deformed experimentally at mantle conditions (Durham et al., 2009, PEPI, 172, 67-73; Hilairet et al., 2012, JGR, 117, B01203 ; Bollinger et al., 2013, PEPI, 228, 211-219) to that of single crystals and demonstrate that strain at grain boundaries can be orders of magnitude larger than intracrystalline strain. We further show that the proportion of grain-boundary strain decreases with increasing temperature and stress. Applied along mantle geotherms, our results shows that grain boundary plasticity is dominant in the shallow mantle. The strength of olivine in the lithosphere can be more than one order of magnitude lower than predicted by classical flow laws (Hirth and Kohlstedt, 2003, AGU Geophys. Monogr, 138, 83-105). In the deep upper mantle, grain boundary plasticity vanishes and strain is mostly accommodated within the grains.

  19. 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

  20. The African upper mantle: the view from surface waves

    NASA Astrophysics Data System (ADS)

    Fishwick, Stewart

    2014-05-01

    Given the sparse distribution of seismic stations across significant regions of the African continent surface wave tomography is the ideal seismological technique to give a clear picture of the large scale structure for the upper mantle. An updated tomographic model is presented, and reviewed in comparison with other tomography for the region. Cratonic regions are clearly outlined with fast velocities extending to depths of >175km. Areas of slow shear velocity, at depths of 100-150km, show good correlation with long wavelength gravity highs and areas of uplifted topography. The numerous temporary deployments of seismometers along the East African rift system provide strong constraints on the structure in this region. Importantly, many of the seismological features are now converging in a range of tomographic models, adding to the confidence in interpretations. However, significant challenges remain, both for seismology and for the interpretations of the results. Pushing towards smaller features and higher resolution to understand geological problems is still difficult. For example, the mantle structure imaged beneath the Bushveld Complex remains very variable depending on the technique used. Lithospheric thickness can be estimated using a variety of proxies - comparisons of this are shown for southern Africa. But, are the seismic models actually compatible with a mineral physics view of the lithosphere? From a geodynamic perspective, how do localised regions of low velocity in the upper mantle relate to the larger patterns of whole mantle circulation? While seismic imaging is providing an increasingly clear picture of the present velocity structure more integration is still needed to answer many of the questions related to the African continent.

  1. 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.

  2. 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.

  3. 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.

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

    PubMed

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

    2015-10-01

    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 10(19.6) to 10(20.7) 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.

  5. The Upper Mantle Flow Field around South-Africa as Reflected by Isotopic Provinciality

    NASA Astrophysics Data System (ADS)

    Meyzen, C.; Blichert-Toft, J.; Ludden, J.; Humler, E.; Mevel, C.; Albarede, F.

    2006-12-01

    Isotopic studies of MORB have established the existence of broad isotopic provinces within the underlying asthenosphere, such as in the Indian Ocean (DUPAL). How these features relate to mantle circulation is, however, still unknown. The steepness of the transition between such isotopic provinces will define the geometry of the velocity field in the upper mantle. In this respect, the transition between the Indian and South Atlantic provinces, two domains that are isotopically contrasted, should be readily identifiable over this long ridge segment. Here, we present Hf isotope data for 60 samples dredged along the SWIR between 35° and 69°E. The new Hf isotope data show that the Indian asthenosphere does not spill directly into the South Atlantic upper mantle: the general decreasing southward gradient observed for ^{176}Hf/^{177}Hf down the mid- Atlantic Ridge, and also for Sr isotopes and model Th/U ratios (derived from Pb isotopes), is overprinted by material with radiogenic Sr, unradiogenic Hf and high Th/U. The Indian domain grades into the South Atlantic around Bouvet, while the South Atlantic collides with the Atlantic province around Tristan. We interpret these features to represent fronts between three adjacent isotopic provinces similar to what has been suggested for the Australian-Antarctic Discordance. The common DUPAL signature of MORB and OIB from the Indian province and the geochemistry of Gulf of Aden MORB and the Afar plume suggest that the source of this distinctive mantle component is deep and lies to the north of the province. This is also what the three-dimensional flow field computed by Behn et al. (2004) from shear-wave splitting shows with a major lower mantle upwelling radiating at the base of the asthenosphere under the Afar plume. Lower mantle gushing out from this source flows southward unimpeded along the Indian ridges, whereas it only reaches the South Atlantic ridge after first having been deflected under the deep roots of the South

  6. 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.

  7. 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.

  8. Imaging the Atlantic upper mantle with Rayleigh waves

    NASA Astrophysics Data System (ADS)

    James, E.; Dalton, C. A.; Gaherty, J. B.

    2013-12-01

    The seismic properties of the oceanic upper mantle provide important constraints on the thermal evolution of the lithosphere, the rheological contrast across the lithosphere-asthenosphere boundary, and mantle flow within the asthenosphere. Knowledge of the seismic properties of the oceanic upper mantle comes primarily from regional seismic models of the Pacific basin and the East Pacific Rise. Considerably less is known about the seismic structure beneath the Atlantic, and differences in the spreading rates, plate velocities, and proximity of hotspots to the ridge in the Atlantic and Pacific suggest that different dynamic processes may be occurring beneath the two basins. We conduct a whole-basin study to investigate the seismic velocity structure of the Atlantic upper mantle. A new data set of Rayleigh wave phase delays is measured using waveforms generated by 453 earthquakes with magnitude > 5.5 that occurred within or along the margins of the Atlantic basin between January 1992 and October 2012. We use data from 544 permanent and temporary broadband seismic stations located within or on the margins of the basin; paths with any significant length through continental regions are excluded. The complete dataset, which consists of nearly 10,000 paths, is used to explore the lateral distribution of Rayleigh wave phase velocity in the Atlantic basin. We consider two approaches for parameterizing the phase-velocity variation. One, phase velocity is assumed to vary only as a function of seafloor age, and a pure-path approach is employed to determine age-dependent velocities. Two, we utilize a more general 2-D parameterization in order to capture phase-velocity variations that cannot be incorporated into the underlying age dependence. In both scenarios, phase velocity shows a strong dependence on seafloor age, with the lowest velocities associated with the youngest seafloor and increasing velocity with seafloor age. We find that pure-path phase velocities in the Atlantic

  9. Tracking Silica in the Earth's Subduction Zone and Upper Mantle

    NASA Astrophysics Data System (ADS)

    Chen, T.; Wang, X.; Zou, Y.; Gwanmesia, G. D.; Liebermann, R. C.; Li, B.

    2014-12-01

    The X-discontinuity (~300 km) in the upper mantle has been revealed under some continental or oceanic region by a number of seismic studies, at which depth the P and S wave velocities increase by about 2%. One possible cause for this discontinuity is the coesite-stishovite phase transition. In this study, we conducted ultrasonic interferometry measurements on polycrystalline coesite and stishovite up to 12.6 GPa at ambient temperature and 14GPa 1073K, respectively. While the P wave velocities of coesite continuously increase with pressure, the S wave velocities exhibit a monotonic decrease to the peak pressure of the current experiment followed by a reversible recovery upon release of pressure. As a result, within the pressure range of 8-12 GPa (corresponding to ~250-350 km depths), the velocity contrasts between coesite and stishovite reach as high as ~38% for P wave and 48%-50% for S wave together with impedance contrasts of 71-69% and ~78% for P and S waves, respectively, the highest among all known phase transitions in mantle minerals. With such extreme contrasts, the coesite-stishovite phase transition in the MORB composition with 4-10wt% of SiO2 is sufficient to generate velocity and impedance contrasts comparable to those reported for the X-discontinuity. The current data, together with the seismic X-discontinuity, may provide a geophysical approach to track the ancient subducted oceanic slabs, and place constraints on the amount of silica in the upper mantle.

  10. Anisotropic Structure of the Upper Mantle, Imaged with Surface and S Waveform Tomography

    NASA Astrophysics Data System (ADS)

    Schaeffer, A. J.; Lebedev, S.

    2011-12-01

    The rapid recent expansion of global and regional seismic networks has paved the way for a new generation of tomographic models, with significantly improved resolution at global and regional scales. We present a new global model of shear velocity and azimuthal anisotropy in the upper mantle, down to the base of the transition zone. The model is constrained by an unprecedentedly large waveform dataset collected from over 2000 stations of GSN and affiliates, USArray, VEBSN, CNSN, PASSCAL experiments, and other networks with data available from IRIS, ORFEUS, and GFZ data centers. Applying the accurate and efficient automated multimode inversion of surface- and S-wave forms to this massive dataset, we generated linear constraints on elastic structure within approximate sensitivity volumes between individual source-receiver pairs, with respect to a 3D reference model. The full waveform inversions resulted in more than one million successful fits (one million seismograms), with structural information extracted from both the fundamental and higher modes. The linear equations were then simultaneously solved for a high-resolution, 3D model of shear velocity and azimuthal anisotropy in the upper mantle. In continental domains, clearly identifiable boundaries between different tectonic features such as basins and relic mountain ranges are readily observable, as well as the signature of deep cratonic roots versus juvenile accretionary margins. Both active and fossil subduction zones are marked by slab signatures deep in the upper mantle and extending through the transition zone. In oceanic regions, largest mid-ocean-ridge anomalies indicative of melting terminate at depths of 100-120 km, with evidence for vertical flow in the upper mantle observed through a combination of VSV, VSH, and azimuthal anisotropy. Spatio-temporal evolution (cooling and thickening) of lithosphere away from the spreading ridges matches the signature expected from geodynamic and thermal modeling. The

  11. 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

  12. 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.

  13. 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

  14. 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

  15. 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.

  16. Deep venous thrombosis of the upper extremity.

    PubMed

    Stephens, M B

    1997-02-01

    Deep venous thrombosis (DVT) of the upper extremity is a relatively uncommon but important cause of morbidity, especially in young active persons. The causes of upper extremity DVT may be categorized as catheter-related, spontaneous (effort-related) and miscellaneous (e.g., trauma, intravenous drug use). Diagnosis is based on clinical history and confirmed by either duplex ultrasonography or contrast venography. Significant controversy surrounds the optimal management of upper extremity DVT. Treatment options include conservative therapy, anticoagulation, catheter-directed thrombolysis and surgical intervention to remove intravascular clot or revise the anatomy of the costoclavicular space. Early aggressive treatment of active young patients may decrease long-term morbidity.

  17. 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

  18. Shear velocity structure of the crust and upper mantle of Madagascar derived from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Pratt, Martin J.; Wysession, Michael E.; Aleqabi, Ghassan; Wiens, Douglas A.; Nyblade, Andrew A.; Shore, Patrick; Rambolamanana, Gérard; Andriampenomanana, Fenitra; Rakotondraibe, Tsiriandrimanana; Tucker, Robert D.; Barruol, Guilhem; Rindraharisaona, Elisa

    2017-01-01

    The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until <1 Ma, but the sources of which remains uncertain. Combined analysis of three complementary surface wave methods (ambient noise, Rayleigh wave cross-correlations, and two-plane-wave) illuminate the upper mantle down to depths of 150 km. The phase-velocity measurements from the three methods for periods of 8-182 s are combined at each node and interpolated to generate the first 3-D shear-velocity model for sub-Madagascar velocity structure. Shallow (upper 10 km) low-shear-velocity regions correlate well with sedimentary basins along the west coast. Upper mantle low-shear-velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism. These anomalies appear distinct at depths <100 km, suggesting that any connection between the zones lies at depths greater than the resolution of surface-wave tomography. An additional low-shear velocity anomaly is also identified at depths 50-150 km beneath the southwest region of intraplate volcanism. We interpret these three low-velocity regions as upwelling asthenosphere beneath the island, producing high-elevation topography and relatively low-volume magmatism.

  19. Variation of the upper mantle velocity structure along the central-south Andes

    NASA Astrophysics Data System (ADS)

    Liang, X.; Sandvol, E. A.; Shen, Y.; Gao, H.; Zhang, Z.

    2013-12-01

    Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical, geological, and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the sensitivity needed to constrain the deep mantle structure. The broadband waveform data are from 393 portable stations of eight temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference wave propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measured by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and the subducting NAZCA slab. Low upper

  20. 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

  1. 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

  2. Insights on the upper mantle beneath the Eastern Alps.

    PubMed

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

    2014-10-01

    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.

  3. Magnetization of lower oceanic crust and upper mantle

    NASA Astrophysics Data System (ADS)

    Kikawa, E.

    2004-05-01

    The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a key stone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthews-Morley Hypothesis. Magnetic inversion studies indicated that the upper oceanic extrusive layer (Layer 2A of 0.5km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties of the rocks recovered from the sea floor, however, have shown that the magnetization of Layer 2A, together with the observations that this layer could record geomagnetic field reversals within a vertical section, is insufficient to give the required size of observed magnetic anomalies and that some contribution from lower intrusive rocks is necessary. Magnetization of oceanic intrusive rocks were observed to be reasonably high enough to contribute to sea-floor spreading magnetic anomalies, but were considered somewhat equivocal until late 1980Os, in part because studies had been conducted on unoriented dredged and ophiolite samples and on intermittent DSDP/ODP cores. Since ODP Leg 118 that cored and recovered continuous 500m of oceanic intrusive layer at Site 735B, Southwest Indian Ridge with an extremely high recovery of 87 percent, there have been several ODP Legs (legs 147, 153, 176, 179 and 209) that were devoted to drilling gabbroic rocks and peridotites. In terms of the magnetization intensities, all of the results obtained from these ODP Legs were supportive of the model that a significant contribution must come from gabbros and peridotites and the source of the lineated magnetic anomalies must reside in most of the oceanic crust as well as crust-mantle boundary

  4. 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

  5. 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

  6. Lattice thermal conductivity of minerals in the deep mantle condition

    NASA Astrophysics Data System (ADS)

    Dekura, H.; Tsuchiya, T.; Tsuchiya, J.

    2011-12-01

    Thermal transport property of materials under pressure and temperature is of importance for understanding the dynamics of the solid Earth and the thermal history. Both experimental and theoretical determinations of the thermal conductivity, however, still remain technically challenging particularly at the deep mantle condition. Recent progress in ab initio computational method based on the density-functional theory is now makes it possible to examine the transport phenomena including the lattice thermal conduction. The intrinsic bulk thermal conduction of insulator is caused by lattice anharmonicity owing to phonon-phonon interaction. The key parameter to predict lattice thermal conductivity is thus the anharmonic coupling constant. Earlier theoretical works calculated the lattice thermal conductivity of MgO with ab initio molecular dynamics simulation or finite difference lattice dynamics simulation (Nico de Koker, Phys. Rev. Lett. 103, 125902, 2009; X. Tang and J. Dong, Proc. Natl. Acad. Sci. U.S.A. 107, 4539, 2010). However, in these approaches, the simulation cell size could often be insufficient for accurate description of the long wavelength phonon scattering. This leads to a lack of the decay channels for the phonons. As an alternative approach, the anharmonic coupling strength between phonon modes can be evaluated within the density-functional perturbation theory. In this approach, the higher-order force tensors are calculated through a number of phonon decay channels obtained within the perturbative scheme taking care only of the primitive cell. We have been developing a technique for calculation of the phonon linewidth necessary to obtain the phonon lifetime. Then the lattice thermal conductivity is evaluated combining with additional harmonic-level of propeties. In this presentation, we show the behavior of lattice thermal conductivity in lower mantle minerals, and discuss the effects of pressure and temperature on their conductivities up to the deep

  7. Mantle Evolution Associated With the Rio Grande Rift: Geochemistry of Upper Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Kil, Y.; Wendlandt, R. F.

    2001-12-01

    Upper mantle xenoliths from three locales associated with the southern Rio Grande Rift have been investigated to determine lithosphere composition, chemical processes, and pre-eruptive pressure and temperature conditions. Sample locations, Potrillo and Elephant Butte within the rift axis and Adam's Diggings, located 50 km west of the rift axis, were specifically selected to evaluate spatial differences in mantle evolution. Xenolith suites from all locations included spinel lherzolites, harzburgites, and pyroxenites hosted in basanite and alkali basalt. Thin section, electron microprobe, and LA-ICPMS analyses were used to obtain detailed textural information, mineral compositions, and whole rock geochemistry. Xenoliths are classified as protogranular, porphyroclastic, or equigranular texture types. Equigranular texture types occur in the off-axis site. Recrystallized olivine grains are larger in xenoliths from sites along the rift axis than from the rift shoulder. Geothermal gradients based on mineral compositions, utilizing two-pyroxene and olivine-spinel geothermometers and the Ca-in olivine geothermobarometer, indicate temperatures off the rift axis at Adam's Diggings that are 75o-100oC cooler for a given pressure than under the rift axis. Whole rock chemical data and mineral modes support an early depletion event affecting xenoliths from all locations: Al2O3, CaO, Na2O, TiO2, V, Sc, Yb, and clinopyroxene content decrease with increasing MgO. The average (La/Yb)n of clinopyroxenes are 12.37, 0.95, and 1.14 for Adam's Diggings, Elephant Butte, and Potrillo xenoliths, respectively. This LREE enrichment and the occurrence of phlogopite that is interpreted to be primary in xenoliths from the off-axis site indicate both cryptic and modal metasomatic events. Both LREE-enriched and -depleted lherzolites are present at rift axis sites. Differences in recrystallized olivine size, xenolith textures, composition, and pre-eruptive pressure-temperature conditions between rift

  8. 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

  9. 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

  10. 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.

  11. Upper mantle heterogeneity below the Mid-Atlantic Ridge, 0°-15°N

    NASA Astrophysics Data System (ADS)

    Bonatti, E.; Peyve, A.; Kepezhinskas, P.; Kurentsova, N.; Seyler, M.; Skolotnev, S.; Udintsev, G.

    1992-04-01

    Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a "cold" upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (˜8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a "hot spot"-type geochemical signature. The areal association of refractory

  12. Dislocation Damping and Anisotropic Attenuation in the Earth's Upper Mantle

    NASA Astrophysics Data System (ADS)

    Jackson, I.; Farla, R. J.; Fitz Gerald, J. D.; Faul, U.; Zimmerman, M. E.

    2011-12-01

    Seismic anisotropy, attributed to olivine lattice preferred orientation, suggests that tectonic deformation in the Earth's shallow upper mantle involves dislocation creep. Reversible glide of dislocations, generated by the prevailing/fossil tectonic stress, may result in anelastic relaxation that contributes to the reduction of seismic wave speeds and associated attenuation. To test this hypothesis, polycrystalline olivine specimens were synthesised from synthetic (sol-gel) precursors and hot-pressed at high temperature. The hot-pressed material is fully dense, fine-grained and essentially dry and melt-free olivine. Other, coarser-grained material was synthesised from San Carlos olivine powders. These contrasting materials provided the opportunity to distinguish between the influences of grain size and dislocation density. Selected specimens were deformed by dislocation creep either in compression or torsion and characterised for dislocation density via oxidation and backscattered electron imaging. Additionally, the dislocation recovery rate was determined for both olivines at different temperatures and time durations. The results established that a maximum temperature of 1100C should allow a relatively stable dislocation density to be maintained during prolonged mechanical testing (> 50 hours). The shear modulus and associated strain-energy dissipation in both hot-pressed and pre-deformed specimens were subsequently measured at seismic frequencies under conditions of simultaneous high pressure and temperature with torsional forced-oscillation methods. These experiments were carried out with strain amplitudes < 10-5 to permit direct comparison with seismological models. The high-temperature dissipation background, attributed in undeformed fine-grained materials to grain-boundary sliding, and the associated partial relaxation of the shear modulus, are systematically enhanced in the pre-deformed materials - suggesting a role for the dislocations introduced during the

  13. 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.

  14. 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

  15. The Effects of Increased Thermal Conductivity and Viscosity on Mixing Rates and Convection Patterns in the Deep Lower Mantle.

    NASA Astrophysics Data System (ADS)

    Naliboff, J. B.; Kellogg, L. H.

    2004-12-01

    Changes in the spin state of iron in both magnetowustite and perovskite at lower mantle conditions may result in increases in radiative thermal transport and viscosity that could suppress convection in the lowermost mantle (Badro et al. 2003, 2004). It has been suggested that such a stagnant layer in the lower mantle could serve as a reservoir for a significant portion of the mantle's incompatible elements, accounting for the isotopic characteristics of hot spots linked to proposed deep-rooted mantle plumes. We investigate the possible effects on mantle dynamics of increases in thermal conductivity and viscosity, using finite-element models of mantle convection in 2-D. Our previous results (Naliboff et al. 2003) showed that increases in thermal conductivity in the lower mantle up to 250 times that in the upper mantle, with otherwise uniform physical properties, fail to isolate a stagnant layer beneath a mid-mantle phase change. When both the viscosity and thermal conductivity increase in the lower mantle, flow velocities through the lower layer and across the boundary decrease. To investigate the rate of mass exchange and mixing in the presence of a partially stagnant layer, we injected tracer particles into the models. We examine mixing in three different classes of models: two models have a viscosity and thermal conductivity change at the mantle mid-point; the third has a viscosity increase at 660 km and a viscosity and thermal conductivity change near 2000 km depth. In models in which the viscosity and thermal conductivity increases by a factor of 10 at the mid-mantle, multi-cell whole-mantle convection rapidly produces a marble cake mantle, leaving no isolated reservoir of material in the lower mantle. Increasing the viscosity and thermal conductivity in the lower mantle by a factor of 50 or 100 produces a relatively stable pattern of convection with a few strong upwellings and downwellings. Although mixing rates decrease and the residence time of material in

  16. 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.

  17. 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.

  18. Supercritical Clinopyroxene in Upper Mantle Peridotites and their Bearing on the Composition of Mantle Melts

    NASA Astrophysics Data System (ADS)

    Muntener, O.; Pilet, S.; Ulianov, A.; Vonlanthen, P.

    2011-12-01

    It is generally accepted that a fertile upper mantle compositions consists of the assemblage olivine, clinopyroxene, orthopyroxene and an aluminous phase, either plagioclase, spinel or garnet. Along the solidus, the composition of the constituent phases changes as a function of pressure and temperature. Several experimental studies showed that clinopyroxene-orthopyroxene phase relations are more complex than suggested by residual peridotite mineral assemblages. At pressures less than about 18 kbar and/or low Mg numbers, pigeonite might be stable at solidus temperatures of upper mantle compositions. At higher pressures, once the solidus temperature is exceeding the cpx - pigeonite solvus, the clinopyroxene becomes supercritical and displays very low CaO contents (1,2). Since these complex phase relations occur at high temperatures and pressures, and might be quenchable in high pressure experiments, they are difficult to observe in the natural rock record. In this contribution, we present results from serpentinized spinel harzburgites from ODP leg 210 from the Newfoundland margin that preserve relics of supercritical cpx coexisting with opx, spinel and olivine. Cr numbers reach up to 0.66 for spinels and up to 0.3 for clinopyroxene and are among the most refractory samples from abyssal peridotites observed worldwide. We present textural and chemical data that suggest the presence of exsolved cpx coexisting with opx in highly depleted spinel peridotite. Actual compositions indicate that the final equilibration occurred at temperatures around 870 °C as determined by two-pyroxene thermometry. Combined with geological reasoning these temperatures suggest that the drilled peridotites represent ancient refractory mantle that is unrelated to the Iberia Newfoundland rift (3). Digital images were used to recombine the primary pyroxene composition and we calculate an XCa on the M2-site in cpx of 0.24 to 0.29. Electron backscatter diffraction (EBSD) of exsolved cpx and opx

  19. Signal restoration through deconvolution applied to deep mantle seismic probes

    NASA Astrophysics Data System (ADS)

    Stefan, W.; Garnero, E.; Renaut, R. A.

    2006-12-01

    We present a method of signal restoration to improve the signal-to-noise ratio, sharpen seismic arrival onset, and act as an empirical source deconvolution of specific seismic arrivals. Observed time-series gi are modelled as a convolution of a simpler time-series fi, and an invariant point spread function (PSF) h that attempts to account for the earthquake source process. The method is used on the shear wave time window containing SKS and S, whereby using a Gaussian PSF produces more impulsive, narrower, signals in the wave train. The resulting restored time-series facilitates more accurate and objective relative traveltime estimation of the individual seismic arrivals. We demonstrate the accuracy of the reconstruction method on synthetic seismograms generated by the reflectivity method. Clean and sharp reconstructions are obtained with real data, even for signals with relatively high noise content. Reconstructed signals are simpler, more impulsive, and narrower, which allows highlighting of some details of arrivals that are not readily apparent in raw waveforms. In particular, phases nearly coincident in time can be separately identified after processing. This is demonstrated for two seismic wave pairs used to probe deep mantle and core-mantle boundary structure: (1) the Sab and Scd arrivals, which travel above and within, respectively, a 200-300-km-thick, higher than average shear wave velocity layer at the base of the mantle, observable in the 88-92 deg epicentral distance range and (2) SKS and SPdiff KS, which are core waves with the latter having short arcs of P-wave diffraction, and are nearly identical in timing near 108-110 deg in distance. A Java/Matlab algorithm was developed for the signal restoration, which can be downloaded from the authors web page, along with example data and synthetic seismograms.

  20. 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

  1. Double layering of thermochemical-plume material can reconcile upper-mantle seismic velocity structure beneath Hawaii

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Volcanism far from plate boundaries, in Hawaii and elsewhere, 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), bilaterally symmetric "pancake" beneath the overriding lithosphere. New high-resolution seismic velocity images obtained from the PLUME seismic experiment indeed support the concept of a deep-rooted mantle plume to feed Hawaiian volcanism. However, in detail these images challenge classical 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 pancake. Classical plumes are, moreover, inconsistent with geochemical aspects of Hawaiian volcanism, which indicate a heterogeneous mantle source involving mafic lithologies such as eclogite, and not an exclusively thermal (i.e., isochemical) origin. To explore the dynamical behavior and melting of plumes with a substantial fraction of eclogite (10-18%), we performed thermochemical three-dimensional numerical experiments. Relative to the ambient-mantle peridotite, eclogite is intrinsically dense. This chemical density contrast is sensitive to phase changes in the upper mantle peaking at depths of 410-300 km and fading at 250-190 km, where eclogite is removed by partial melting. For models with an eclogite content >12%, these effects cause a complex regime of plume upwelling. The thermochemical plume forms a broad and thick pool at depths of 480-300 km (deep eclogite pool, or DEP), from which one or two secondary plumes rise to feed a hot shallow pancake that supports the seafloor swell. The rising secondary plumes undergo decompression melting at their deflection points to supply shield stage and rejuvenated stage volcanism. Their transience in vigor can reconcile observations of temporal variability of Hawaiian hotspot volcanism. The double layering of hot plume material

  2. Upper mantle P velocities beneath the North America craton

    NASA Astrophysics Data System (ADS)

    Chu, R.; Helmberger, D. V.

    2010-12-01

    In this work, we determined the detailed mechanisms of three earthquakes occurring in Quebec, Texas and Idaho for use in modeling triplication data. The first event provided pure-path triplication recordings at over 400 USArray stations. Although amplitudes of the direct P waves are small, the depth phase sP is clear and displays shadow-zone characteristics indicative a low velocity layer (LVL) in the upper mantle, where the amplitude of the AB branch decreases rapidly at a distance of 16 degrees. Another feature of the LVL is that the AB branch can be seen at distances larger than 23 degrees. Similar to the Canadian Shield velocity model S25 (LeFevre and Helmberger, 1989), we found a LVL between 160 km and 215 km and obtained excellent fits assuming 1D model. The other two events are located near the craton margins and have been recorded by the MOMA array (Texas event) and CANOE array (Idaho event). These mixed paths are mostly craton with modified 1D models producing good fits. We, also, produced 2D modeling results that use tomographic images for correcting the source structures.

  3. Illuminating the upper mantle beneath the Newer Volcanics province, southeast Australia, using seismic body wave tomography

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Sandiford, M.

    2012-12-01

    model shows a clear zone of low velocity underlying the NVP (maximum perturbation of -4% relative to AK135). It clearly extends to a depth of just over 200 km, before terminating, with no evidence of reduced velocities down to approximately 300 km, the maximum depth resolution of the seismic data. Furthermore, nearer the surface (~100km depth), there appears to be three distinct regions of low velocity that are distributed E-W between central Victoria and Mt. Gambier near the South Australian border. The lack of evidence for a deep seated anomaly is consistent with the hypothesis that the source of the NVP is confined to the upper mantle, although an important caveat is that plumes are expected to be narrow as they rise through the mantle before broadening out as they encounter the base of the lithosphere; as such, it s possible that the limited spatial resolution of the data (approximately 50 km) is unable to detect narrow vertical structures at depth. However, combined with the observations discussed earlier, our results strengthen the argument for a localized upper mantle anomaly.

  4. 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

  5. Determining upper mantle structures using gravity, seismology, and GIA modelling in Fennoscandia

    NASA Astrophysics Data System (ADS)

    Root, B. C.; van der Wal, W.; Vermeersen, B. L. A.

    2015-12-01

    The 3D structure of the upper mantle plays a large role in Glacial Isostatic Adjustment (GIA). Finite-element software is able to model this 3D structure, but knowledge of the upper mantle is needed to make these models realistic. Nowadays, global maps are made of the crustal structure and temperature of the upper mantle from seismic observations. Also, satellite gravity missions, such as GOCE and GRACE, determine global gravity fields. Combining these data sets could give new insights in Glacial Isostatic Adjustment and explain some discrepancies seen in currents geological observations with 1D rheology Earth models. We obtain upper mantle models that fit gravity observations. Then, the upper mantle seismic velocities are converted to temperature profiles; that are used to determine the amount of strain according to diffusion and dislocation creep in the upper mantle. The obtained 3D rheology models are used in a finite element GIA model to observe the effect of the 3D structures during GIA. The GIA model results are compared to geological observations of the sea level change, GPS uplift rates, and ongoing gravity change in the area. This study specifically studies the effect of compositional differences in the upper mantle on the modelled remaining uplift and gravity signal. Molecular conversion relations for primitive mantle rock composition, Garnet Lherzolite rock composition, and Archon, iron depleted rock composition are used to compute the temperature and density profiles. The Fennoscandian lithosphere is believed to contain these three types of composition, yet, it is not yet known in what relative amounts and locations. An iterative approach is used to find the best compositional structure to fit the GIA observables in the Fenoscandian upper mantle.

  6. 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.

  7. 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

  8. Upper Mantle Structure beneath Afar: inferences from surface waves.

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.; Debayle, E.; Lepine, J.; Leveque, J.; Cara, M.; Ataley, A.; Sholan, J.

    2001-12-01

    The Afar hotspot is related to one of the most important plume from a geodynamic point of view. It has been advocated to be the surface expression of the South-West African Superswell. Below the lithosphere, the Afar plume might feed other hotspots in central Africa (Hadiouche et al., 1989; Ebinger & Sleep, 1998). The processes of interaction between crust, lithosphere and plume are not well understood. In order to gain insight into the scientific issue, we have performed a surface-wave tomography covering the Horn of Africa. A data set of 1404 paths for Rayleigh waves and 473 paths for Love waves was selected in the period range 45-200s. They were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment, in Tanzania and Saudi Arabia. Other data come from the broadband stations deployed in Ethiopia and Yemen in the framework of the French INSU program ``Horn of Africa''. The results presented here come from a path average phase velocities obtained with a method based on a least-squares minimization (Beucler et al., 2000). The local phase velocity distribution and the azimuthal anisotropy were simultaneously retrieved by using the tomographic technique of Montagner (1986). A correction of the data is applied according to the crustal structure of the 3SMAC model (Nataf & Ricard, 1996). We find low velocities down to 200 km depth beneath the Red Sea, the Gulf of Aden, Afars, the Ethiopian Plateau and southern Arabia. High velocities are present in the eastern Arabia and the Tanzania Craton. The anisotropy beneath Afar seems to be complex, but enables to map the flow pattern at the interface lithosphere-asthenosphere. The results presented here are complementary to those obtained by Debayle et al. (2001) at upper-mantle transition zone depths using waveform inversion of higher Rayle igh modes.

  9. 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

  10. 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.

  11. Upper mantle dynamics and quaternary climate in cratonic areas (DynaQlim)—Understanding the glacial isostatic adjustment

    NASA Astrophysics Data System (ADS)

    Poutanen, Markku; Ivins, Erik R.

    2010-07-01

    A substantial material flow, deep within the solid Earth, is caused by the periodic ocean-continent water transport of the Quaternary ice ages. That lateral transport is enormous, causing 120-135 m of equivalent global sea-level rise and fall, or about 45-50 Peta tonnes (1 Peta tonne = 10 18 kg) of surface mass transfer. The global manifestation of the slow mantle flow response to this surface load is glacial isostatic adjustment (GIA). Measurements of this phenomenon offer a unique opportunity to retrieve information pertaining to both the Earth's upper mantle and the changing mass of glaciers and ice sheets during the past. The waxing and waning of ice mass is driven by long-term variations in climate. DynaQlim (upper mantle dynamics and quaternary climate in cratonic areas), a regional coordination committee of the International Lithosphere Program (ILP) since 2007, is focused on studying the relations between upper mantle dynamics, its composition and physical properties, temperature, rheology, and Quaternary climate. Combining historical and modern terrestrial and space-borne geodetic observations with seismological investigations, studies of the postglacial faults and continuum mechanical modelling of GIA, the research goal of DynaQlim is to offer new insights into properties of the lithosphere and upper mantle. The joint inversion of different types of observational data is an important step toward providing a better understanding of GIA on all levels of Earth sciences. A primary regional focus of DynaQlim is the study of cratonic areas of northern Canada and Scandinavia. Greenland and Antarctica are also of great interest, as they represent observational examples of ice sheet dynamics and mass change in response to relatively strong present-day climate forcing.

  12. 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.

  13. A review of crust and upper mantle structure beneath the Indian subcontinent

    NASA Astrophysics Data System (ADS)

    Singh, Arun; Singh, Chandrani; Kennett, B. L. N.

    2015-03-01

    This review presents an account of the variations in crustal and upper mantle structure beneath the Indian subcontinent and its environs, with emphasis on passive seismic results supplemented by results using controlled seismic sources. Receiver function results from more than 600 seismic stations, and over 10,000 km of deep seismic profiles have been exploited to produce maps of average crustal velocities and thickness across the region. The crustal thickness varies from 29 km at the southern tip of India to 88 km under the Himalayan collision zone, and the patterns of variation show significant deviations from the predictions of global models. The average crustal shear velocity (Vs) is low in the Himalaya-Tibet collision zone compared to Indian shield. Major crustal features are as follows: (a) the Eastern Dharwar Craton has a thinner and simpler crustal structure crust than the Western Dharwar Craton, (b) Himalayan crustal thickness picks clearly follow a trend with elevation, (c) the rift zones of the Godavari graben and Narmada-Son Lineament show deeper depths of crust than their surroundings, and (d) most of the Indian cratonic fragments, Bundelkhand, Bhandara and Singhbhum, show thick crust in comparison to the Eastern Dharwar Craton. Heat flow and crustal thickness estimates do not show any positive correlations for India. Estimates of the thickness of the lithosphere show large inconsistencies among various techniques not only in terms of thickness but also in the nature of the transition to the asthenosphere (gradual or sharp). The lithosphere beneath India shows signs of attrition and preservation in different regions, with a highly heterogeneous nature, and does not appear to have been thinned on broader scale during India's rapid motion north towards Asia. The mantle transition zone beneath India is predominantly normal with some clear variations in the Himalayan region (early arrivals) and Southwest Deccan Volcanic Province and Southern Granulite

  14. Global Upper Mantle Structure from Finite-Frequency Surface-Wave Tomography

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Nolet, G.; Dahlen, F.; Laske, G.

    2004-12-01

    We report global shear-wave velocity structure and radial anisotropy in the upper mantle obtained by finite-frequency surface-wave tomography, based on complete three-dimensional Born sensitivity kernels developed by Zhou et al (2004). Because wavefront healing effects are properly taken into account, finite-frequency surface-wave tomography improves the resolution of small-scale mantle heterogeneities using long-period surface waves. The resulting S-wave velocity models fit the dispersion data better, and show stronger small-scale mantle anomalies compare to traditional ray-theory-based tomographic models. Separate inversions of Love wave (SH-type) and Rayleigh wave (SV-type) dispersion provide insight into the radial anisotropy in the upper mantle. In our model, the globally averaged radial anisotropy is positive (V SH > V SV) (horizontal flow) in the top 220 km, and becomes negative (V SV > V SH) (vertical flow) below 220 km depth. In cratons, both SH and SV velocities show strong fast anomalies down to 250 km depth, and the fast anomalies gradually diminish below 250 km. Radial anisotropy beneath cratons is positive, which largely agrees with a recent global model by Gung et.~al (2003). The old Pacific plate is characterized by strong positive anisotropy with its maximum centered west of Hawaii; this supports an earlier observation on Pacific radial anisotropy by Ekstrom & Dziewonski (1998). The depth extent of mid-ocean ridges and the primary force that drives plate tectonics has been a long-standing question. In our model, ridge anomalies are characterized by strong negative radial anisotropy (vertical flow). Ridge anomalies at fast-spreading centers are stronger than those at slow-spreading centers at shallow depth, but the amount of velocity reduction rapidly decreases below 250 km. However, at slow-spreading centers such as the north Mid-Atlantic ridge (MAR) and East Africa (Red Sea), ridge anomalies extend down at least to the top of the transition zones

  15. 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.

  16. Upper Extremity Deep Vein Thromboses: The Bowler and the Barista

    PubMed Central

    du Breuil, Anne L.; Close, Jeremy

    2016-01-01

    Effort thrombosis of the upper extremity refers to a deep venous thrombosis of the upper extremity resulting from repetitive activity of the upper limb. Most cases of effort thrombosis occur in young elite athletes with strenuous upper extremity activity. This article reports two cases who both developed upper extremity deep vein thromboses, the first being a 67-year-old bowler and the second a 25-year-old barista, and illustrates that effort thrombosis should be included in the differential diagnosis in any patient with symptoms concerning DVT associated with repetitive activity. A literature review explores the recommended therapies for upper extremity deep vein thromboses. PMID:27800207

  17. Multi-Observable Thermochemical Tomography of the lithosphere and upper mantle

    NASA Astrophysics Data System (ADS)

    Afonso, J. C.; Yang, Y.; Rawlinson, N.; Jones, A. G.; Fullea, J.; Qashqai, M.

    2015-12-01

    Current knowledge of the present-day physical state and structure of the lithosphere and upper mantle essentially derives from four independent sources: i) gravity field and thermal modelling, ii) modelling/inversion of different seismic datasets, iii) magnetotelluric studies, and iv) thermobarometric and geochemical data from exhumed mantle samples. Unfortunately, the integration of these different sources of information in modern geophysical studies is still uncommon and significant discrepancies and/or inconsistencies in predictions between these sources are still the rule rather than the exception.In this contribution we will present a thermodynamically-constrained multi-observable probabilistic inversion method capable of jointly inverting i) surface and body wave datasets, gravity anomalies, geoid height, gravity gradients, receiver functions, surface heat flow, magnetotelluric data, and elevation (static and dynamic) in 3D spherical coordinates. Key aspects of the method are: (a) it combines multiple geophysical observables with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic-geophysical framework; (b) it works with thermophysical models of the Earth rather than with parameterized structures of physical parameters (e.g. Vs, Vp, density, etc), (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) a priori compositional information relies on robust statistical analyses of a large database of natural mantle samples; (f) it provides a natural platform to estimate realistic uncertainties; (g) it handles multiscale parameterizations and complex physical models, and (h) it includes dynamic (convection) effects on surface observables by solving the complete Stokes flow using multi-dimensional decomposition methods. We will present results for both synthetic and real case studies, which serve to highlight the advantages and limitations of this new

  18. 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

    2013-11-07

    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.

  19. Upper-mantle structures beneath USArray derived from waveform complexity

    NASA Astrophysics Data System (ADS)

    Sun, Daoyuan; Helmberger, Don

    2011-01-01

    Tomographic imaging of the crust and upper mantle beneath the western United States has greatly improved with the addition of USArray data. These models display many detailed images of both fast and slow blobs penetrating into the transition zone. To study such features, we apply a newly developed technique, called MultiPath Detector analysis, to the SH waveform data. The method simulates each observed body waveform by performing a decomposition; by [S(t)+C×S(t-ΔLR)]/2, where S(t) is the synthetics for a reference model. Time separation ΔLR and amplitude ratio C are needed to obtain a high cross-correlation between a simulated waveform and data. The travel time of the composite waveform relative to the reference model synthetics is defined as ΔT. A simulated annealing algorithm is used to determine the parameters ΔLR and C. We also record the amplitude ratio (Amp) between the synthetics for the reference model relative to the data. Generally, large ΔLR values are associated with low Amp's. Whereas the conventional tomography yields a travel time correction (ΔT), our analysis yields an extra parameter (ΔLR), which describes the waveform complexity. With the array, we can construct a mapping of the gradient of ΔLR with complexity patterns. A horizontal structure introduces waveform complexity along the distance profile (in-plane multipathing). An azimuthally orientation ΔLR pattern indicates a vertical structure with out-of-plane multipathing. Using such maps generated from artificial data, we can easily recognize features produced by dipping fast structures and slow structures (DSS). Many of these features display organized waveform complexity that are distinctly directional indicative of dipping sharp-edges. Here, we process the array data for events arriving from various azimuths and construct maps of multipathing patterns. The similarity between tomographic features and complexity maps is striking. When features are dipping such as the slab structures

  20. Teleseismic Body Wave Attenuation in the Upper Mantle beneath the United States

    NASA Astrophysics Data System (ADS)

    Cafferky, S.; Schmandt, B.

    2014-12-01

    EarthScope seismic data provide opportunities to examine mantle properties on a continental scale as the Transportable Array (TA) nears the end of its traverse across the contiguous United States. We use P- and S-wave amplitude spectra from all >M5.7 deep earthquakes recorded by the TA to examine seismic attenuation patterns in the upper mantle. More than 2 million inter-station P-wave spectral ratios were inverted for maps of relative tP* variations across the U.S. in multiple frequency bands between 0.08 - 2 Hz. We plan to have corresponding S-wave results by meeting time. Maps of tP* are strongly correlated (>0.8) for frequency bands of 0.08 - 2 Hz, 0.25 - 2 Hz, 0.08 - 1 Hz, and 0.25 - 1 Hz. The broader the frequency band examined (e.g. 0.08 - 2 Hz), the lower the magnitude in variations of tP*; however, those broader frequency bands still exhibited geographic patterns similar to the narrow frequency bands. We compare our maps' tP* with seismic velocity models and constraints on crustal scattering to assess the physical origin of apparent attenuation. In the tectonically active and high heat flow domain of the western U.S., tP* variations are moderately correlated with thermal variations predicted by tomography studies of seismic velocity. However, contrast in tP* between western Cordillera and the cratonic interior is weaker than predicted by tomography. Additionally some areas of high attenuation are correlated with Precambrian tectonic boundaries within the Laurentian craton. The weak contrast between the western and eastern U.S. and correlations with Precambrian tectonics suggest that elastic scattering due to small-scale (~10 - 100 km) heterogeneity or compositional variations in the lithosphere are major contributors to tP* estimates from deep earthquake spectral ratios. Moderate correlation of tP* with estimates of mantle temperature within the western U.S. suggests deep earthquake spectral ratios do carry some evidence of intrinsic attenuation, but

  1. 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.

  2. 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling

    NASA Astrophysics Data System (ADS)

    Nataf, Henri-Claude; Ricard, Yanick

    1996-05-01

    We present an a priori three-dimensional 'tomographic' model of the upper mantle. We construct this model (called 3SMAC — three-dimensional seismological model a priori constrained) in four steps: we compile information on the thickness of 'chemical' layers in the Earth (water, sediments, upper and lower crust, etc); we get a 3D temperature distribution from thermal plate models applied to the oceans and continents; we deduce the mineralogy in the mantle from pressure and temperature and we finally get a three-dimensional model of density, seismic velocities, and attenuation by introducing laboratory measurements of these quantities as a function of pressure and temperature. The model is thus consistent with various geophysical data, such as ocean bathymetry, and surface heat flux. We use this model to compute synthetic travel-times of body waves, and we compare them with observations. A similar exercise is performed for surface waves and normal modes in a companion paper (Ricard et al., 1996, J. Geophys. Res., in press). We find that our model predicts the bulk of the observed travel-time variations. Both the amplitude and general pattern are well recovered. The discrepancies suggest that tomography can provide useful regional information on the thermal state of the continents. In the oceans, the flattening of the sea-floor beond 70 Ma seems difficult to reconcile with the seismic observations. Overall, our 3SMAC model is both a realistic model, which can be used to test various tomographic methods, and a model of the minimum heterogeneities to be expected from geodynamical modeling. Therefore, it should be a useful a priori model to be used in tomographic inversions, in order to retrieve reliable images of heterogeneities in the transition zone, which should, in turn, greatly improve our understanding of geodynamical processes in the deep Earth. 3SMAC and accompanying software can be retrieved by anonymous ftp at geoscope.ipgp.jussieu.fr.

  3. 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.

  4. 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.

  5. 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

  6. 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.

  7. 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

  8. Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Liu, Jingao; Huang, Jian; Xiao, Yan; Chu, Zhu-Yin; Zhao, Xin-Miao; Tang, Limei

    2017-02-01

    compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20 ± 0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28 ± 0.05‰, 2SD, Chen et al., 2013b; +0.30 ± 0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

  9. 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.

  10. 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

  11. Temperature Control of Continental Lithosphere Elastic Thickness: Effective Elastic Thickness Te vs Upper Mantle Velocity Vs

    NASA Astrophysics Data System (ADS)

    Hyndman, R. D.; Currie, C. A.; Mazzotti, S.; Frederiksen, A.

    2006-12-01

    The elastic thickness of continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes. Recently it has been questioned whether the elastic thickness and strength are dependent on crust and upper mantle temperatures and compositions in the way predicted by laboratory data. We test this dependence in western North America by a regional comparison of the effective elastic thickness (Te) from topography-gravity coherence, and upper mantle temperatures mapped by tomography shear wave velocities (Vs). We find a good correlation between Te and Vs of the form expected based on the thermal and laboratory data. The Te distribution is strongly bimodal as previously found globally, less than 20 km for the high temperature Cordillera and over 100 km for the adjacent cold stable Canadian Shield. Only intermediate thermal regimes have intermediate Te that suggests a weak layer in the lower crust over a stronger upper mantle. Strength envelopes based on laboratory data correspond to the observed Te for thermal regimes with temperatures at the Moho of 800-900C for the Cordillera and 400-500C for the Shield, in agreement with temperatures from Vs and other estimators. Our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory- based rheology provides a good first order estimate of the deformation behaviour of the crust and upper mantle. The Cordillera and other continental backarcs are weak enough to be deformed by plate boundary forces, whereas cratons are generally much too strong. In the Cordillera, the upper mantle is too hot for brittle failure and earthquakes occur only in the upper 10-15 km of the crust. In the cool craton, earthquakes occur rarely in the upper mantle because the total lithosphere strength is too great for significant deformation by plate tectonic forces.

  12. Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

    NASA Astrophysics Data System (ADS)

    Chalot-Prat, Françoise; Doglioni, Carlo; Falloon, Trevor

    2017-01-01

    Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at 90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere

  13. 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

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. Structure of the crust and upper mantle in the western United States

    USGS Publications Warehouse

    Pakiser, L.C.

    1963-01-01

    Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2,000 stations in the western conterminous United States as a part of the VELA UNIFORM program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The speed of compressional waves in the upper-mantle rocks ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province. In general, the speed of compressional waves in the upper-mantle rocks tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The crust tends to be thick in regions where the speed of compressional waves in the upper-mantle rocks (and presumably the density) is high, and tends to be relatively thin where the speed of compressional waves in the upper-mantle rocks (and density) is lower. With in the Basin and Range province, crustal thickness seems to vary directly with regional altitude above sea level. Evidence that a layer of intermediate compressional-wave speed exists in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper- mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic

  20. Crust and upper mantle structure beneath southeast Australia from ambient noise and teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Pilia, S.; Young, M.; Salmon, M.; Yang, Y.

    2016-10-01

    In the last decade, the lithospheric structure beneath southeast Australia has been intensively studied using passive seismic data from WOMBAT, the largest transportable seismic array in the southern hemisphere. The two primary imaging methods that have been applied are ambient noise tomography for the crust and teleseismic tomography for the upper mantle. Despite these recent studies, no attempt has yet been made to provide an integrated view of the crust-mantle system. Here, we perform teleseismic tomography using WOMBAT data that includes a detailed crustal model from ambient noise tomography in the starting model. A Moho surface from the Australian seismological reference Earth model (AuSREM) is also included. This has the dual benefit of accounting for the unresolved crustal component of the teleseismic arrival time residuals, and producing a model that reveals a high level of detail in both the crust and upper mantle. Our new integrated P-wave model contains a number of noteworthy features, including (i) low velocity anomalies in the lower crust and high velocity anomalies in the lithospheric mantle beneath the Gawler Craton and Curnamona Province, which are of Paleoproterozoic-Archean origin; (ii) a marked velocity transition in the crust and lithospheric mantle near the Moyston Fault, which we interpret as the boundary between the Lachlan and Delamerian orogens; (iii) a rapid eastward decrease in upper mantle velocity 200 km inboard of the east coast of Australia, which is consistent with a marked thinning of the lithosphere; (iv) an increase in upper mantle velocity north of the Gawler Craton and Curnamona Province, which points to the presence of thicker lithosphere associated with the Precambrian shield region of the Australian continent; (v) Cenozoic intraplate basaltic volcanic centres distributed exclusively above the zone of thinner lithosphere inboard of the east coast, with the exception of low volume leucitite volcanics.

  1. Towards imaging thermal and compositional structure in the global upper mantle

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Ruan, Y.

    2009-12-01

    Rapid progress has been made in imaging S-wave velocity structure in the global upper mantle in recent surface-wave tomographic studies. Perturbations in S-wave speed alone may have either a thermal or compositional origin, therefore, at least one additional independent observable is required to resolve mantle heterogeneities in temperature and composition. Theoretically, observations of 3-D anelasticity (Q) structure --- together with 3-D S-wave velocity structure --- can be used to constrain temperature and compositional variations in the upper mantle. Despite of their great importance, the coupling between elastic and anelastic effects in surface waves has not been well understood, and large discrepancies exits among global upper-mantle Q models. We investigate the dual dependence of seismic traveltimes and amplitudes upon 3-D velocity and 3-D Q structure by simulating seismic wave propagation in 3-D temperature earth models and show that (1) lateral variations in Q have significant effects on surface-wave traveltimes -— it accounts for about a quarter of the observed surface-wave delay times if mantle seismic anomalies are dominantly thermal; (2) surface-wave amplitude perturbations are dependent upon the geometry and lengthscale of mantle anomalies in both wavespeed and Q structure through elastic and anelastic focusing and defocusing. The common practice of inverting seismic amplitudes for ``attenuation" may have led to large discrepancies as seen among current global Q models. We develop a finite-frequency approach to simultaneously invert for lateral variations in velocity and anelasticity (Q), fully accounting for the coupling between elastic and anelastic effects in seismic traveltimes and amplitudes. This work opens the opportunity for joint diffractional tomography of high-resolution lateral heterogeneities in temperature and composition in the upper mantle.

  2. Upper Extremity Deep Vein Thrombosis: A Community-Based Perspective

    PubMed Central

    Spencer, Frederick A.; Emery, Cathy; Lessard, Darleen; Goldberg, Robert J.

    2010-01-01

    Purpose The purpose of this study was to examine the magnitude, risk factors, management strategies, and outcomes in a population-based investigation of patients with upper, as compared to lower, extremity deep vein thrombosis diagnosed in 1999. Methods The medical records of all residents from Worcester, Massachusetts (2000 census=478,000) diagnosed with ICD-9 codes consistent with possible deep vein thrombosis at all Worcester hospitals during 1999 were reviewed and validated. Results The age-adjusted attack rate (per 100,000 population) of upper extremity deep vein thrombosis was 16 (95% CI 13, 20) compared to 91 (83,100) for lower extremity deep vein thrombosis. Patients with upper extremity deep vein thrombosis were significantly more likely to have undergone recent central line placement, a cardiac procedure, or an intensive care unit admission than patients with lower extremity deep vein thrombosis. Although short and 1-year recurrence rates of venous thromboembolism and all-cause mortality were not significantly different between patients with upper, versus lower, extremity deep vein thrombosis, patients with upper extremity deep vein thrombosis were less likely to have pulmonary embolism at presentation or in follow-up. Conclusions Patients with upper extremity deep vein thrombosis represent a clinically important patient population in the community setting. Risk factors, occurrence of pulmonary embolism, and timing and location of venous thromboembolism recurrence differ between patients with upper as compared to lower extremity deep vein thrombosis. These data suggest that strategies for prophylaxis and treatment of upper extremity deep vein thrombosis need further study and refinement. PMID:17679126

  3. Deep mantle heat flow and thermal evolution of the Earth's core based on thermo-chemical mantle convection

    NASA Astrophysics Data System (ADS)

    Nakagawa, T.; Tackley, P.; Buffett, B.

    2004-12-01

    A coupled core-mantle evolution model that combines the global heat balance in the core with a fully-dynamical thermo-chemical mantle convection [Nakagawa and Tackley, 2004 published in EPSL] is used to investigate the deep mantle heat flow that is required to sustain the magnetic field generated by the geodynamo process. Effects of a radioactive heat source due to potassium in the core are also included in the global heat balance in the Earth??s core. Two important parameters are checked in this study; (1) density variation between depleted hartzbergite and basaltic material (0 to 3 percent) and (2) concentration of radioactive potassium in the core alloy (0ppm to 400ppm). The parameter set that most closely satisfies the criteria of size of the inner core (1220km at present time) is around 2 percent of density difference in a convecting mantle and 200ppm of radioactive heat source in the core. The concentration of potassium in the core is consistent with the geochemical approach [Murthy et al., 2003] but smaller than other successful thermal evolution models [Labrosse, 2003; Nimmo et al., 2004]. Heat flow through the core-mantle boundary and the contribution of radioactive heat sources in the core are consistent with theoretical estimates [e.g. Buffett, 2002] and geochemical constraints [Gessmann and Wood, 2002]. The power available to the geodynamo, based on the predicted heat flow through the core-mantle boundary, is approximately four times greater than the value predicted by numerical models of the geodynamo [Christensen and Kutzner, 2004] but closer to theoretical estimates [e.g. Buffett, 2002].

  4. Anisotropic Signature of the Afar plume in the Upper Mantle.

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.; Debayle, E.; Leveque, J.; Cara, M.; Lepine, J.

    2002-12-01

    Plumes remain enigmatic geological objects and it is still unclear how they are formed and whether they act independently from plate tectonics. The role of plumes in mantle dynamics can be investigated by studying their interaction with lithosphere and crust and their perturbations on flow pattern in the mantle. The flow pattern can be derived from seismic anisotropy. An anisotropic surface wave tomography in the Horn of Africa was performed. The choice of the experiment in the Horn of Africa is motivated by the the presence of the Afar hotspot, one of the biggest continental hotspot. In the framework of the mantle degree 2 pattern, the Afar hotspot is the antipode of the Pacific superswell, but its origin at depth and its geodynamic importance are still debated. Data were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment in Tanzania and Saudi Arabia. We completed our data base with a French deployment of portable broadband stations surrounding the Afar Hotspot. Path average phase velocities are obtained by using a method based on a least-squares minimization (Beucler et al.,2002). A correction of the data is applied according to the a priori 3SMAC model (Nataf and Ricard, 1996). 3D-models of velocity, radial and azimuthal anisotropies are inverted for. Down to 250km, low velocities are found beneath the Red Sea, the Gulf of Aden, the South East of the Tanzania Craton, the Afar hotspot. High velocities are present in the eastern Arabia and the Tanzania Craton. These results are in agreement with the isotropic model of Debayle et al. (2002). The anisotropy model beneath Afar displays a complex pattern. The azimuthal anisotropy shows that the Afar plume might be interpreted as feeding other hotspots in central Africa. Deeper in the asthenosphere, a wide stem of positive radial anisotropy (VSH > VSV) comes up, where we might expect the reverse sign. The same observation was made below Iceland (Gaherty, 2001) and Hawaii (Montagner

  5. 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.

  6. Three-dimensional shear wave velocity structure in the Atlantic upper mantle

    NASA Astrophysics Data System (ADS)

    James, Esther Kezia Candace

    Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is 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. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 3-D upper mantle shear wave speed structure beneath the South Pacific Superswell by a BBOBS array

    NASA Astrophysics Data System (ADS)

    Isse, T.; Suetsugu, D.; Shiobara, H.; Sugioka, H.; Yoshizawa, K.; Kanazawa, T.; Fukao, Y.

    2005-12-01

    Previous seismic tomography studies show a broad low velocity anomaly in the lower mantle, so-called superplume, beneath the South Pacific and there are hotspot chains and large scale topographic high at surface of this region. However, the resolution of seismic tomography is poor, especially in the upper mantle, because of limited spatial distribution of seismic stations. To improve the station coverage, we deployed an array of long-term broadband ocean bottom seismometers (BBOBS) in this region. The quality of the vertical component of seismograms recorded by the BBOBS array is comparable with those by island seismic stations. This observation has enabled us to obtain a more precise 3-D shear wave speed structure in the upper mantle of this region by analyzing Rayleigh waves. We employed a two-station method to determine phase velocity of fundamental mode Rayleigh wave recorded by the BBOBS array and island stations in the Pacific Ocean. We obtained 1025 path-average phase velocity dispersion curves including 188 dispersion curves using the BBOBS data in a period range between 40 and 140 seconds. We then inverted them to a 3-D shear wave speed structure down to a depth of 200 km. At shallow depths the eastern part of the French Polynesia region is in general slower than the western part, which indicates an age-dependence of seismic structure of the uppermost mantle. Slow speed anomalies corresponding to the hotspots are apparently superposed on this age-dependence: Slow speed anomalies can be seen from the surface to a depth of 200 km beneath the Society, Pitcairn, and Macdonald hotspots, but they are limited only to the deep part beneath the Samoa hotspot. The slow speed anomalies beneath the Pitcairn and Society hotspots apparently coalesce at a depth of 100 km, where a single anomaly extending upward from below seems to branch into two directions. A resolution analysis indicates that the BBOBS array data has improved the spatial resolution substantially.

  12. Accounting for lateral variations of the upper mantle gradient in Pn tomography studies

    NASA Astrophysics Data System (ADS)

    Phillips, W. S.; Begnaud, M. L.; Rowe, C. A.; Steck, L. K.; Myers, S. C.; Pasyanos, M. E.; Ballard, S.

    2007-07-01

    The effect of an upper mantle velocity gradient on regional arrival times has been approximated by a cubic distance term, which can be extended to two dimensions for use in tomographic studies. To demonstrate this, we add a laterally varying upper mantle gradient to the standard P n time-term tomography technique, and apply to a data set from Asia compiled using ground truth, event location criteria. We observe strong lateral variations in the gradient, ranging from -0.001 to 0.003 s-1, with high gradients associated with the Tethys convergence zone. The gradient patterns may reflect lateral variations in the thermal gradient of the mantle lid. Variance reduction is 63% with respect to P n tomography without gradients. Adding gradients allows the use of longer path lengths, improving velocity image definition in high-gradient regions with sparse station distribution, such as Tibet.

  13. Using Receiver Functions to Image the Montana Crust and Upper Mantle

    NASA Astrophysics Data System (ADS)

    Sirianni, R. T.; Russo, R. M.

    2008-12-01

    We determined receiver functions (RFs) at six permanent Advanced National Seismic System (ANSS) stations to examine crust and upper mantle structure of the Wyoming craton (WC) and Medicine Hat block (MHB). The Deep Probe & SAREX projects (Henstock et al., 1998; Clowes et al., 2002; Gorman et al., 2002) used active source seismics to model a high velocity crustal layer (the so-called 7x layer) beneath the WC. This layer exhibits P wave velocities that are high for lower continental crust (~7+ km/s) and extends from 30-55 km below the surface. Interpretations of the active source data indicate that this layer may represent wide scale crustal underplating of the WC, implying post-Archean craton modification with implications for Laurentia assembly. We used 43 earthquakes from a wide azimuthal distribution recorded at the Montana ANSS stations; high signal-to-noise ratios of 25 of these RFs were acceptable for further analysis. Receiver functions constrain crustal velocity structure beneath a seismometer by using P-to-S wave conversions at sharp velocity contrast boundaries. Preliminary results for seismic stations DGMT, EGMT, and LAO, located to the east of the Deep Probe and SAREX seismic line on the Wyoming craton/Medicine Hat block show the influence of sedimentary cover and a strong Ps phase at approximately four seconds after P. At BOZ and MSO, located in the Rocky mountains, the sedimentary cover signal previously noted is absent, and instead we observe a sharp Ps phase at about four and a half seconds after P. RFs at station RLMT (on the WC) are highly anomalous, probably reflecting complex conversions from two differently oriented dipping layers. We will use the RFs to produce suites of acceptable structural models to test for the presence and lateral extent of the 7x layer and other structural features of the Rocky Mountains-craton transition.

  14. 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.

  15. 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.

  16. The Interaction Between Supercontinent Cycles and Compositional Variations in the Deep Mantle

    NASA Astrophysics Data System (ADS)

    Lowman, J. P.; Trim, S. J.

    2015-12-01

    Earth is the only planet known to currently feature active plate tectonics. Two features that may influence the Earth's ability to sustain plate-like surface motion are the presence of continents and the inferred chemical piles lying on the core mantle boundary. In our previous study that modelled thermochemical convection in the mantle with evolving plates, it was shown that upwellings that form on top of chemical piles are relatively weak and make a diminished contribution to lithospheric stress. Yet, surface yielding is required in order to maintain plate tectonics and form new plate boundaries. Consequently an intrinsically dense layer in the lower mantle can decrease the vigour of convection and the likelihood of surface failure. In contrast to the mantle upwellings that form above the chemically dense provinces in our models, particularly vigorous plumes form where the ambient mantle lies adjacent to the core mantle boundary and at the edges of the chemically dense piles. Continents also affect surface mobility, due to their inherent buoyancy and their distinct yield strength. In this study we employ numerical models of mantle convection featuring both tectonic plates and compositional variation in the mantle and lithosphere. Plate-like surface motion is dynamically modelled using a force-balance method that determines plate velocities based upon lithospheric stresses. Oceanic and continental margins evolve in response to the plate velocities and specified lithospheric yield stresses. Compositional variations in the deep mantle are tracked using the tracer ratio method. For a range of ratios of the ambient mantle density to the density of the compositionally enriched material, we examine the the impact of mantle compositional variation on plate evolution, the effect of continents on planetary surface mobility and the frequency of supercontinent assembly versus the mobility of compositional provinces.

  17. Upper mantle shear wave velocity structure of the east Anatolian-Caucasus region

    NASA Astrophysics Data System (ADS)

    Skobeltsyn, Gleb Anatolyevich

    The Eastern Anatolian-Caucasus region is a relatively young part of the Alpine- Himalayan orogenic belt and has been formed as the result of the ongoing continental collision of Arabia and Eurasia. In spite of a number of geological studies that have been conducted in this area, there is still no consensus within the geoscience community about the regional tectonic settings and a model for the late Cenozoic tectonic evolution of the Anatolian Plateau. Knowledge of the upper mantle velocity structure in this region can provide the geological community with important constraints that are crucial for developing an understanding of the regional geology and the processes associated with early stages of mountain building. In the present dissertation, I describe two studies of the regional upper mantle S wave velocity structure. In order to derive the absolute velocity structure of the upper mantle, I have applied surface wave tomography to model Rayleigh wave phase velocities as a function of period. Then I inverted the Rayleigh phase velocities to obtain S wave velocities as a function of depth. The resulted high-resolution 3-D S wave velocity model of the regional upper mantle is characterized by a better depth resolution than any preexisting tomographic models. I also conducted an S wave splitting analysis using traditional methods and developed a two-layer grid search algorithm in order to infer the upper mantle anisotropic structure. The results of the S wave splitting analysis for the stations located in Azerbaijan are the first in the region. (Abstract shortened by ProQuest.).

  18. Deep Seismic Reflectivity at Volcanic Margins: Reflections from the Petrological Moho or from within the Mantle?

    NASA Astrophysics Data System (ADS)

    Kusznir, Nick; Roberts, Alan; Bellingham, Paul

    2015-04-01

    Advances in deep long-offset seismic-reflection acquisition and processing now frequently provide imaging of strong and laterally continuous reflectors in the TWTT range of 10 to 14 seconds. While an initial interpretation might be that these reflectors correspond to the crust-mantle interface, this interpretation may in some cases be incorrect or over-simplistic. Do these deep reflectors correspond to the petrological Moho or could they be located within the mantle? Examples of deep laterally-coherent reflectivity can be seen within the ocean-continent transition of the Argentine, Uruguayan and S Brazilian volcanic margins of the S Atlantic. An initial qualitative interpretation of the seismic data suggests the presence of deep crustal "keels" or crustal roots underlying well developed seaward dipping reflectors (SDRs). Joint inversion of the PSTM time-domain seismic reflection and gravity anomaly data has been used to determine the average interval density and seismic velocity between base sediment and the deep seismic reflectivity. Joint inversion densities and seismic velocities for this depth interval reach values in excess of 3000 kg/m3 and 7.0 km/sec for the entire thickness of the interval, substantially in excess of densities and velocities observed for normal oceanic and continental crust. The high densities determined from joint seismic-gravity inversion under the SDR regions are also consistent with results from flexural subsidence analysis. We consider two interpretations of these results. One interpretation is that the strong deep reflectivity corresponds to the base of the petrological crust and that the crust has an abnormally high average density and seismic velocity due to high-temperature mantle-plume-related magmatism. An alternative interpretation is that the deep seismic reflectivity is located within the mantle beneath the petrological Moho, and that the high density and seismic velocity result from averaging of both crustal basement (~2850

  19. Are Deep Seismic Reflections at Volcanic Margins from the Petrological Moho or from within the Mantle?

    NASA Astrophysics Data System (ADS)

    Harkin, C. J.; Kusznir, N. J.; Roberts, A. M.; Bellingham, P.; Manatschal, G.

    2015-12-01

    Deep long-offset seismic-reflection now frequently provide imaging of strong and laterally continuous reflectors in the TWTT range of 10 to 14 seconds. Examples of deep laterally-coherent reflectivity can be seen within the ocean-continent transition of the Argentine, Uruguayan and S Brazilian volcanic margins of the S Atlantic. Qualitative interpretation of the seismic data suggests the presence of deep crustal "keels" or crustal roots underlying well developed seaward dipping reflectors (SDRs). While an initial interpretation might be that these reflectors correspond to the crust-mantle interface, this interpretation may in some cases be incorrect or over-simplistic. Do these deep reflectors correspond to the petrological Moho or could they be located within the mantle? Joint inversion of the PSTM time-domain seismic reflection and gravity anomaly data has been used to determine the average interval density and seismic velocity between base sediment and the deep seismic reflectivity. Joint inversion densities and seismic velocities for this depth interval reach values in excess of 3000 kg/m3 and 7.0 km/sec for the entire thickness of the interval, substantially in excess of densities and velocities observed for normal oceanic and continental crust. The high densities determined from joint seismic-gravity inversion under the SDR regions are also consistent with results from flexural subsidence analysis. We consider two interpretations of these results. One interpretation is that the strong deep reflectivity corresponds to the base of the petrological crust and that the crust has an abnormally high average density and seismic velocity due to high-temperature mantle-plume-related magmatism. An alternative interpretation is that the deep seismic reflectivity is located within the mantle beneath the petrological Moho, and that the high density and seismic velocity result from averaging of both crustal basement (~2850 kg/m3) and mantle (~3300 kg/m3) values. In some

  20. 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

  1. 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.

  2. An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

    NASA Astrophysics Data System (ADS)

    Warren, J. M.; Shimizu, N.; Sakaguchi, C.; Dick, H. J. B.; Nakamura, E.

    2009-12-01

    Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.

  3. 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

  4. Upper Mantle Qβ Structure beneath the East Pacific Rise from Shear Wave Triplicated Waveforms

    NASA Astrophysics Data System (ADS)

    Wang, Y.; Grand, S. P.; Tang, Y.

    2012-12-01

    A consensus on how mantle flows in the transition zone region of the Earth has not been reached. Some propose a boundary to flow while others prefer a model with essentially no boundary to flow across the upper-lower mantle boundary. It is possible that an intermediate situation exists where flow is intermittent across the boundary. A situation where mantle flow is inhibited near the 660 km discontinuity will result in a thermal boundary layer at that depth and thus high temperature gradients. Such a high temperature gradient zone is difficult to detect using seismic velocities but may be detectable through measurements of Q since attenuation is highly sensitive to temperature. Models of seismic attenuation as a function of depth through the mantle are difficult to determine. Normal mode and surface waves have been used but use long wavelength waves and thus can only resolve broad scale structure. Body waves such as multiple ScS phases have been used for regional mantle attenuation studies but lack vertical resolution. S waves recorded from 15 to 28 degrees distance turn within the upper mantle and due to discontinuities near 410 and 660 km depth are triplicated with multiple arrivals sampling different depths arriving at the same station. The triplicated arrivals can also be seen in SS waves at double the distance and SSS waves at triple the distance …. Here we model dense profiles of broadband S, SS, SSS and SSSS waves recorded mainly by US-Array, Canadian seismic network and other surrounding stations in the North America. Earthquakes along the East Pacific Rise were recorded with the wave paths underneath oceanic crust younger than 15Ma. The distance range covered is from 15 to 105 degrees, consequently triplicated body waves sample the transition zone from S to SSSS. By modeling the relative amplitudes of the triplicated waveforms a model of Qβ (.01-.1 Hz) as a function of depth is determined focused on the mantle transition zone.

  5. Mapping Upper Mantle Seismic Discontinuities Using Singular Spectrum Analysis

    NASA Astrophysics Data System (ADS)

    Gu, Y. J.; Dokht, R.; Sacchi, M. D.

    2015-12-01

    Seismic discontinuities are fundamental to the understanding of mantle composition and dynamics. Their depth and impedance are generally determined using secondary seismic phases, most commonly SS precursors and P-to-S converted waves. However, the analysis and interpretation using these approaches often suffer from incomplete data coverage, high noise levels and interfering seismic phases, especially near tectonically complex regions such as subduction zones and continental margins. To overcome these pitfalls, we apply Singular Spectrum Analysis (SSA) to remove random noise, reconstruct missing traces and enhance the robustness of SS precursors and P-to-S conversions from seismic discontinuities. Our method takes advantage of the predictability of time series in frequency-space domain and performs a rank reduction using a singular value decomposition of the trajectory matrix. We apply SSA to synthetic record sections as well as observations of 1) SS precursors beneath the northwestern Pacific subduction zones, and 2) P-to-S converted waves from the Western Canada Sedimentary Basin (WCSB). In comparison with raw or interpolated data, the SSA enhanced reflectivity maps show a greater resolution and a stronger negative correlation between the depths of the 410 and 660 km discontinuities. These effects can be attributed to the suppression of incoherent noise, which tends to reduce the signal amplitude during normal averaging procedures, through rank reduction and the emphasis of principle singular values. Our new results suggest a more laterally coherent 520 km reflection in the western Pacific regions. Similar improvements in data imaging are achieved in western Canada, where strong lateral variations in discontinuity topography are observed in the craton-Cordillera boundary zone. Improvements from SSA relative to conventional approaches are most notable in under-sampled regions.

  6. Kinetics of melt migration in upper mantle-type rocks

    NASA Astrophysics Data System (ADS)

    Riley, G. N.; Kohlstedt, D. L.

    1991-08-01

    Experiments have been performed to determine the permeability of an aggregate of olivine plus a silicate melt, as well as the viscosity of the matrix. Melt migration couples were formed between discs composed of olivine with about 12% of a synthetic potassium-aluminum silicate glass and discs of polycrystalline San Carlos olivine. Four melt infiltration experiments were carried out at temperatures between 1050 and 1255°C at 300 MPa in a gas-medium apparatus; each couple was held at the experimental conditions for 2 h. At temperature, capillary forces cause the molten glass to infiltrate into the dunite along triple junctions because the dihedral angle is less than 60°. In order to analyze the resulting melt migration profiles, the coupled differential equations governing melt migration via "porous flow" driven by capillary forces and resisted by compaction/dilation of the matrix were solved numerically. The effects of dihedral angle, melt fraction exponent and amount of the melt in the source region on the spatial and temporal evolution of the melt distribution were investigated with numerical simulations for the initial and boundary conditions imposed by the experimental geometry. The permeability of the aggregate and the viscosity of the matrix were determined by comparison of the melt migration profiles obtained from the experiments with those generated from simulation. The permeability of the partially molten aggregate increased approximately linearly with increasing melt fraction and, at 1255°C, the permeability of the rock and viscosity of the olivine are about 8 × 10 -16 m 2 and 8 × 10 9 Pa s, respectively, for a grain size of 4.2 μm and a melt fraction of 0.145. These results predict a relatively high permeability at low melt fractions, indicating that only very small amounts of melt (˜ 0.1%) could be maintained in a dunitic mantle.

  7. 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.

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

    DOE PAGES

    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

  9. 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.

  10. 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.

  11. 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.

  12. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle

    NASA Astrophysics Data System (ADS)

    Ding, Shuo; Dasgupta, Rajdeep

    2017-02-01

    Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO* and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼ 10- 15% melting with bulk S of 100-150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/ melt and initial Cu in the

  13. 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.

  14. Seismogenic Shear Zones In The Upper Mantle: Some Evidences From Central Italy

    NASA Astrophysics Data System (ADS)

    Creati, N.; Boncio, P.; Lavecchia, G.

    The presence of brittle, brittle-ductile and ductile shear zones down to the upper man- tle is rather well known in the literature and testified by both geophysical (seismic lines) and geological (milonyte fault rocks in exumated lherzolites) data. Moreover, the occurrence of intermediate earthquakes (down to a depth about 150-200 km) which cannot be easily interpreted as Wadati-Benioff zones (High Atlas, Romania, Hindukush, Tibet and Burma) suggests the existence of intra-lithosphere seismogenic shear zones. In this paper, we will illustrate preliminary evidences about a possible seismogenic intra-lithosphere shear zone in Northern-Central Italy. Up to now, the oc- currence in this area of earthquakes down to a depth of about 90 km has been linked to the westward subduction of the Adriatic lithosphere. As a matter of fact, the loca- tion of the deep earthquakes does not fit well with the location of the Apennine Moho doubling zone, that, as classically recognized, would sign the location of the slab. In order to better address the problem, we have performed a detailed analysis of the deep seismicity along two regional sections coinciding with the traces of the deep crust CROP 03 (Punta Ala-Gabicce) and DSS 1978 (Piombino-Ancona) seismic profiles. In both sections, the earthquake distribution defines a seismic area 30 km wide and 100 km long, deepening westward at about 30 and reaching a maximum depth of about 75 km. This area substantially overlaps on the easternmost SW-dipping reverse shear zone shown by the DSS 78 and CROP 03 profiles. This shear zone, named Adriatic Shear Zone (ASZ), emerges in the Adriatic Sea and deepens westward dislocating the base of the crust. The ASZ along-dip distribution of the seismicity is not homogeneous and four main clusters are observed at depths of 5-10 km, 18-25 km, 35-40 km and 60-70 km. The distribution of seismicity has been compared with the rheological strat- ification and structure of the lithosphere. Adopting a not

  15. Deep mantle melting-solidifying and produced heterogeneities

    NASA Astrophysics Data System (ADS)

    Fomin, Ilya; Tackley, Paul

    2015-04-01

    Model for solid-liquid equilibrium and substance properties in lower mantle conditions is important to understand the early stages of evolution of terrestrial planets, such as core formation and magma ocean crystallization. This model is also necessary to prove theories on some modern seismic features of the Earth (e.g. ultra-low velocity zones) and petrological observations (e.g. lower mantle mineral assemblage inclusions in diamonds). Numerous experimental and numerical studies of the lower mantle phases provide sufficient amount of data to build up a thermodynamic model, which can be used in geophysical fluid dynamics research. Molecular Dynamics modeling provides data on thermodynamic properties of solids and liquids (density, heat capacity, thermal expansion, latent heat of melting etc.). Absence of minor components (iron, alkali etc.) makes it to overestimate melting temperatures significantly (up to 20-30%), so experimental data are also very important. Our model is based on MD data by [de Koker et al., 2013] with evaluation of all important parameters according to classical thermodynamic equations. Melting temperatures (especially at eutectic points) are corrected along Clausius-Clapeyron slopes to agree with modern experimental data ([Andrault et al., 2011], [Andrault et al., 2014], [Fiquet et al., 2010], [Hirose et al., 1999], [Mosenfelder et al., 2007], [Nomura et al., 2014], [Ozawa et al., 2011], [Zerr et al., 1998]). KD value for iron reported by [Andrault et al., 2012] was used. Proposed model was implemented into StagYY software (e.g. [Tackley, 2008]). It is a finite-volume discretization code for advection of solid and liquid in a planetary scale. A principal new feature of the used code modification is that we use separated variables for chemical compounds: SiO2, FeO, MgO and other (list can be extended). So it is possible to trace mantle heterogeneities produced by melting and solidifying events. Calculations predict appearing and disappearing

  16. Large gem diamonds from metallic liquid in Earth’s deep mantle

    NASA Astrophysics Data System (ADS)

    Smith, Evan M.; Shirey, Steven B.; Nestola, Fabrizio; Bullock, Emma S.; Wang, Jianhua; Richardson, Stephen H.; Wang, Wuyi

    2016-12-01

    The redox state of Earth’s convecting mantle, masked by the lithospheric plates and basaltic magmatism of plate tectonics, is a key unknown in the evolutionary history of our planet. Here we report that large, exceptional gem diamonds like the Cullinan, Constellation, and Koh-i-Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle. These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated conditions. We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.

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

    PubMed

    Mukhopadhyay, Sujoy

    2012-06-06

    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.

  18. 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.

  19. Constraining upper mantle mass structure below the oceans from seismic and geodetic data

    NASA Astrophysics Data System (ADS)

    Panet, Isabelle; Romanowicz, Barbara; Greff, Marianne; French, Scott

    2016-04-01

    We address the question of understanding the mantle convective structure below the oceans, in the sublithospheric to transition zone depth range. For that, we study how the mantle mass distribution can be constrained from a combination of global seismic tomography, gravity and bathymetry data and models. We focus on oriented patterns, that may arise from interactions between the flows and the plate motions. A directional analysis of the geodetic datasets, over the Pacific and Indian Oceans, shows the presence of elongated anomalies following the direction of the present-day absolute plate motions, correlated with the low shear velocity channels in the upper mantle from the SEMum2 model (French et al., 2013). We derive regional sensitivity kernels relating these observables to the internal mass distribution, and set up an inverse problem to determine the seismic velocity to density conversion factor. We discuss our approach and preliminary results.

  20. Petrology of lower crustal and upper mantle xenoliths from the Cima Volcanic Field, California

    USGS Publications Warehouse

    Wilshire, H.G.; McGuire, A.V.; Noller, J.S.; Turrin, B.D.

    1991-01-01

    Basaltic rocks of the Cima Volcanic Field in the southern Basin and Range province contain abundant gabbro, pyroxenite, and peridotite xenoliths. Composite xenoliths containing two or more rock types show that upper-mantle spinel peridotite was enriched by multiple dike intrusions in at least three episodes; the mantle was further enriched by intergranular and shear-zone melt infiltration in at least two episodes. Because of their high densities, the gabbros and pyroxenites can occupy the zone immediately above the present Moho (modeled on seismic data as 10-13 km thick, with Vp 6.8 km/s) only if their seismic velocities are reduced by the joints, partial melts, and fluid inclusions that occur in them. Alternatively, these xenoliths may have been derived entirely from beneath the Moho, in which case the Moho is not the local crust-mantle boundary. -from Authors

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

    NASA Astrophysics Data System (ADS)

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

    2017-03-01

    The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge - Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently, it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100 km down to a depth of 250 km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250 km the low velocity structure ramifies into narrow veins, each with a radius of ∼50 km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

  2. Melting experiments on anhydrous peridotite KLB-1: Compositions of magmas in the upper mantle and transition zone

    NASA Astrophysics Data System (ADS)

    Herzberg, Claude; Zhang, Jianzhong

    1996-04-01

    Electron microprobe results are reported for liquid and crystalline phases that were synthesized at 5-22.5 GPa in multianvil experiments on anhydrous peridotite KLB-1 [Zhang and Herzberg, 1994]. The results provide information on the partitioning of TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, Na2O, and NiO among liquid and the crystalline phases olivine, modified spinel, garnet, magnesiowustite, and magnesium perovskite. Uncertainties in these partition coefficients stem from quenching problems and from the effects of thermal migration of liquid in a temperature gradient. We have, however, exploited the temperature gradients by determining how the crystalline phase chemistry varies throughout the melting interval from the liquidus to the solidus. This has permitted new constraints to be obtained on the compositions of liquids along the anhydrous peridotite solidus at low melt fractions and at pressures in the 5-18 GPa range. It is demonstrated that the wide range of Al2O3 and CaO/Al2O3 contents in picrites and komatiites can be explained by melt segregation at upper mantle pressures that ranged from 3 to ˜10 GPa. These magmas could have formed by anhydrous melting in plumes with temperatures that were only 100°-200°C higher than ambient mantle below ridges, demonstrating that unusually hot conditions are not required to form komatiites. Primary igneous MgO contents in excess of 26% should be rare, and those that do exist in some komatiites can be explained by advanced melting during adiabatic or superadiabatic ascent, by low Na2O in the source, or by melting in hot plumes from the transition zone and lower mantle. Evidence for deep melting in hot plumes is rather conjectural, but it may be contained in some 2700 Myr komatiites that have high MgO and mantle-like CaO/Al2O3.

  3. 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

  4. Upper mantle electrical resistivity structure beneath the central Mariana subduction system

    NASA Astrophysics Data System (ADS)

    Matsuno, Tetsuo; Seama, Nobukazu; Evans, Rob L.; Chave, Alan D.; Baba, Kiyoshi; White, Antony; Goto, Tada-Nori; Heinson, Graham; Boren, Goran; Yoneda, Asami; Utada, Hisashi

    2010-09-01

    This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80-100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The

  5. Tectonic denudation of the upper mantle along passive margins: a model based on drilling results (ODP leg 103, western Galicia margin, Spain)

    NASA Astrophysics Data System (ADS)

    Boillot, G.; Recq, M.; Winterer, E. L.; Meyer, A. W.; Applegate, J.; Baltuck, M.; Bergen, J. A.; Comas, M. C.; Davies, T. A.; Dunham, K.; Evans, C. A.; Girardeau, J.; Goldberg, G.; Haggerty, J.; Jansa, L. F.; Johnson, J. A.; Kasahara, J.; Loreau, J. P.; Luna-Sierra, E.; Moullade, M.; Ogg, J.; Sarti, M.; Thurow, J.; Williamson, M.

    1987-01-01

    During ODP Leg 103, serpentinized peridotite (clinopyroxene-spinel harzburgite) was cored within the basement approximatively at the boundary between the North Atlantic oceanic curst 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 seafloor occurred at the end of the period of rifting, roughly 110 Ma ago. Ductile shear zones observed in the cored 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, we use a non-uniform stretching model for the lithosphere, set up from the Wernicke's model (1985).

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake

    NASA Astrophysics Data System (ADS)

    Masuti, Sagar; Barbot, Sylvain D.; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

    2016-10-01

    Water, the most abundant volatile in Earth’s interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight—or 1,600 H atoms per million Si atoms—in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

  11. Upper-mantle water stratification inferred from observations of the 2012 Indian Ocean earthquake.

    PubMed

    Masuti, Sagar; Barbot, Sylvain D; Karato, Shun-Ichiro; Feng, Lujia; Banerjee, Paramesh

    2016-10-20

    Water, the most abundant volatile in Earth's interior, preserves the young surface of our planet by catalysing mantle convection, lubricating plate tectonics and feeding arc volcanism. Since planetary accretion, water has been exchanged between the hydrosphere and the geosphere, but its depth distribution in the mantle remains elusive. Water drastically reduces the strength of olivine and this effect can be exploited to estimate the water content of olivine from the mechanical response of the asthenosphere to stress perturbations such as the ones following large earthquakes. Here, we exploit the sensitivity to water of the strength of olivine, the weakest and most abundant mineral in the upper mantle, and observations of the exceptionally large (moment magnitude 8.6) 2012 Indian Ocean earthquake to constrain the stratification of water content in the upper mantle. Taking into account a wide range of temperature conditions and the transient creep of olivine, we explain the transient deformation in the aftermath of the earthquake that was recorded by continuous geodetic stations along Sumatra as the result of water- and stress-activated creep of olivine. This implies a minimum water content of about 0.01 per cent by weight-or 1,600 H atoms per million Si atoms-in the asthenosphere (the part of the upper mantle below the lithosphere). The earthquake ruptured conjugate faults down to great depths, compatible with dry olivine in the oceanic lithosphere. We attribute the steep rheological contrast to dehydration across the lithosphere-asthenosphere boundary, presumably by buoyant melt migration to form the oceanic crust.

  12. 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%.

  13. New upper mantle P-velocity model of Eurasia resolves connections to main regional tectonic structures

    NASA Astrophysics Data System (ADS)

    Koulakov, I.; Kaban, M. K.; Cloetingh, S.

    2012-12-01

    We present a new model of P-velocity anomalies in the upper mantle beneath Eurasia constructed by merging several existing models and by computing new results for a number of gap areas. The models were computed based on tomographic inversions of travel-time data from the worldwide catalogues (ISC, 2001). The calculations were performed in a series of overlapping circular areas of 700-1000 km size. All data with rays corresponding to sources and/or stations in the areas traveling through the target volume were, at least partly, used in the inversions. In case of lack of stations and events, the calculations were based on PP-rays with reflection points in the target area. The new model of Eurasia resolves connections between upper mantle structures and main tectonic units. Cratonic blocks in Eurasia, such as the East-European, Siberian, Indian and Arabian cratons are detected in terms of high-velocity patterns down to 250-300 km depth. The subduction zones in the western Pacific, Burma and the Mediterranean are robustly resolved, consistent with previous studies. In zones of continental collision, we observe traces of mantle delamination as drops of high-velocity material in the mantle. Sites of intraplate volcanism in Europe, Siberia, Mongolia and Yakutia coincide with low-velocity areas, interpreted as overheated upper mantle. Digital version of the model can be downloaded at www.ivan-art.com/temp/vis_eurasia.zip. P-velocity anomalies beneath Eurasia at 100 km depth from regional tomographic inversion. Polygons indicate possible locations of cratonic lithosphere blocks; stars mark the areas of Cenozoic volcanism. P-velocity anomalies beneath Eurasia at 300 km depth from regional tomographic inversion. Polygons indicate possible locations of cratonic lithosphere blocks.

  14. 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.

  15. Hydration Mechanisms, Crystal Preferred Orientation, and Anisotropy in the Upper Mantle and Transition Zone

    NASA Astrophysics Data System (ADS)

    Smyth, J. R.; Ye, Y.; Jacobsen, S. D.

    2011-12-01

    Nominally anhydrous silicate minerals of the upper mantle and transition zone incorporate H2O into their structures at mantle temperatures and pressures as ordered hydroxyl defects. These defects can commonly be identified by single-crystal X-ray diffraction and polarized infrared spectroscopy. Regional or large-scale mantle hydration can account for two to ten times the mass of water in Earth's oceans affecting anisotropic elastic properties of the mantle's constituent minerals such that hydration causes distinct patterns of seismic anisotropy. Recent crystallographic studies indicate that the principal hydration mechanism of the nominally anhydrous minerals is by protonation of octahedral (Mg,Fe) cation vacancies. Here we show that in the most abundant minerals of the upper 660km, olivine (alpha), wadsleyite (beta), and ringwoodite (gamma) polymorphs of Mg2SiO4, hydrated point defects order to form planes of weakness that can control lattice preferred orientation and velocity anisotropy in various regions of the upper mantle and transition zone. In olivine, ordering of protonated (Mg,Fe) vacancies in the M1 octahedral site predicts that hydration will enhance c-axis alignment parallel to the direction of shear in the (010) plane, as in type-B LPO with SH > SV by up to 10% under horizontal shear. In wadsleyite, vacancies and protons order into the M3 octahedral sites which form double edge-sharing chains of octahedra parallel to a. This vacancy ordering predicts a slip vector of [100] on {011} planes and, unlike olivine, would produce SH < SV by up to 2-6% under horizontal or vertical shear. In ringwoodite, velocity anisotropy is weak, but shear velocity differences of one or two percent are possible. Velocity and electrical conductivity anisotropy may thus be useful indicators of hydration in planetary interiors.

  16. Upper mantle structure of the Tonga-Lau-Fiji region from Rayleigh wave tomography

    NASA Astrophysics Data System (ADS)

    Wei, S. Shawn; Zha, Yang; Shen, Weisen; Wiens, Douglas A.; Conder, James A.; Webb, Spahr C.

    2016-11-01

    We investigate the upper mantle seismic structure in the Tonga-Lau-Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient-noise and two-plane-wave tomography. The results suggest a wide low-velocity zone beneath the Lau Basin, with a minimum SV-velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back-arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60-80 km thick lithosphere underlain by a low-velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau-Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench-perpendicular beneath Fiji to spreading-perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

  17. 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.

  18. 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.

  19. 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.

  20. Low velocities in the oceanic upper mantle and their relation to plumes: insights from SEM-based waveform tomography

    NASA Astrophysics Data System (ADS)

    Lekic, V.; French, S. W.; Romanowicz, B. A.

    2013-12-01

    The exchange of heat, mass and momentum between tectonic plates and mantle convection controls lithospheric evolution and hotspot volcanism, and must occur at a range of spatial scales. Yet, the detailed morphology of the associated convection patterns continues to elude geophysicists. Because seismic velocities are affected by temperature, seismic tomography can be used to map the patterns of flow in the Earth's mantle. Here, we present a global-scale long-period full-waveform seismic tomographic model SEMum2 constructed using the Spectral Element Method, which can very accurately model wave propagation through highly complex structures, and account for phenomena such as scattering, (de)focusing, and wavefront healing. Notably, SEMum2 achieves more realistic amplitudes of lateral heterogeneity - particularly low velocities in the upper 250km - than previous generations of global models, while still retrieving the long-wavelength structure present in earlier tomographic models. Cluster analysis of profiles of shear velocity in the SEMum2 oceanic upper mantle, confirms the presence of a well marked shear wave low velocity zone (LVZ) beneath the lithosphere, with a velocity minimum which deepens progressively as a function of age of the plate. The LVZ minimum in SEMum2 reaches values that are lower than in previous tomographic global models and in agreement with local estimates where available. Interestingly, reaching below this "classical" low velocity zone, the model reveals a pattern of alternating lower and higher velocities organized into elongated bands in the direction of absolute plate motion (APM), with a quasi-regular spacing of ~2000 km perpendicular to the APM. This fingerlike structure, most prominent around 200-250 km and extending down to 350-400 km, is most prominent beneath the Pacific plate, but also present under the eastern Antarctic plate, in the south Atlantic and in parts of the Indian Ocean Below this depth, the low velocities appear organized

  1. Upper mantle velocity structure beneath the Cameroon Volcanic Line region and implications for the formation of mantle hot lines

    NASA Astrophysics Data System (ADS)

    Adams, A. N.; Wiens, D. A.; Euler, G. G.; Nyblade, A.; Shore, P.

    2013-12-01

    The Cameroon Volcanic Line (CVL) is a 1800km long feature, extending SW-NE from the Gulf of Guinea into Central Africa. Volcanism along the line does not display the typical age progression exhibited by hotspot-related volcanic tracks, leading to speculation over the geodynamic source of this intraplate feature. Numerous models have been proposed to explain the linear nature of the CVL in the absence of age progression, including laterally transported material from a single or multiple plumes, reactivation of the Central African Shear Zone, edge-flow convection associated with the neighboring Congo Craton, and convection driven by lithospheric instabilities at the edge of continental lithosphere. In this study, we calculate Rayleigh wave phase velocities and upper mantle shear wave velocity structure beneath the continental portion of the CVL to investigate the geodynamic source of the CVL. Rayleigh wave phase velocities are measured at periods from 20 to 182 seconds following the two-plane wave methodology developed by Forsyth and Li (2005), and using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. We find that phase velocities beneath the CVL are reduced, while velocities beneath the Congo Craton to the south are elevated. This is observed for all periods, but the difference between regions decreases at the longest periods measured in the study. Shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the CVL, extending from 50km to at least 200km depth, with a sharp vertical boundary with the faster velocities beneath the Congo Craton. These observations are most consistent with the edge convection or lithospheric instability models as the source of the continental CVL. Further study of offshore structure will aid in better characterizing

  2. A global horizontal shear velocity model of the upper mantle from multimode Love wave measurements

    NASA Astrophysics Data System (ADS)

    Ho, Tak; Priestley, Keith; Debayle, Eric

    2016-10-01

    Surface wave studies in the 1960s provided the first indication that the upper mantle was radially anisotropic. Resolving the anisotropic structure is important because it may yield information on deformation and flow patterns in the upper mantle. The existing radially anisotropic models are in poor agreement. Rayleigh waves have been studied extensively and recent models show general agreement. Less work has focused on Love waves and the models that do exist are less well-constrained than are Rayleigh wave models, suggesting it is the Love wave models that are responsible for the poor agreement in the radially anisotropic structure of the upper mantle. We have adapted the waveform inversion procedure of Debayle & Ricard to extract propagation information for the fundamental mode and up to the fifth overtone from Love waveforms in the 50-250 s period range. We have tomographically inverted these results for a mantle horizontal shear wave-speed model (βh(z)) to transition zone depths. We include azimuthal anisotropy (2θ and 4θ terms) in the tomography, but in this paper we discuss only the isotropic βh(z) structure. The data set is significantly larger, almost 500 000 Love waveforms, than previously published Love wave data sets and provides ˜17 000 000 constraints on the upper-mantle βh(z) structure. Sensitivity and resolution tests show that the horizontal resolution of the model is on the order of 800-1000 km to transition zone depths. The high wave-speed roots beneath the oldest parts of the continents appear to extend deeper for βh(z) than for βv(z) as in previous βh(z) models, but the resolution tests indicate that at least parts of these features could be artefacts. The low wave speeds beneath the mid-ocean ridges fade by ˜150 km depth except for the upper mantle beneath the East Pacific Rise which remains slow to ˜250 km depth. The resolution tests suggest that the low wave speeds at deeper depths beneath the East Pacific Rise are not solely due

  3. Deep mantle structure and the postperovskite phase transition

    PubMed Central

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

    2005-01-01

    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

  4. 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.

  5. Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Zhang, Jin S.; Bass, Jay D.

    2016-09-01

    We present the elastic properties of San Carlos olivine up to P = 12.8(8) GPa and T = 1300(200) K using Brillouin spectroscopy with CO2 laser heating. A comparison of our results with the global seismic model AK135 yields average olivine content near 410 km depth of about 37% and 43% in a dry and wet (1.9 wt % H2O) upper mantle, respectively. These olivine contents are far less than in the pyrolite model. However, comparisons of our results with regional seismic models lead to very different conclusions. High olivine contents of up to 87% are implied by seismic models of the western U.S. and eastern Pacific regions. In contrast, we infer less than 35% olivine under the central Pacific. Strong variations of olivine content and upper mantle lithologies near the 410 km discontinuity are suggested by regional seismic models.

  6. 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.

  7. 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.

  8. Calibration of Three-Dimensional Upper Mantle Structure in Eurasia Using Regional and Teleseismic Full Waveform Seismic Data

    DTIC Science & Technology

    2007-09-01

    successfully for global and regional mantle tomography at Berkeley since 1995. In the subregion of study, our “N-Born” model is parameterized at...waveforms, Geophys. J. Int. 143: 709–728. Panning, M. and B. Romanowicz (2004). Inferences on flow at the base of Earth’s mantle based on seismic...CALIBRATION OF THREE-DIMENSIONAL UPPER MANTLE STRUCTURE IN EURASIA USING REGIONAL AND TELESEISMIC FULL WAVEFORM SEISMIC DATA Barbara Romanowicz1

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. The Deep Mantle Volatile Cycle Revealed in Superdeep Diamonds and their Mineral Inclusions

    NASA Astrophysics Data System (ADS)

    Walter, Michael; Thomson, Andrew; Frost, Jennifer; Bulanova, Galina; Smith, Chris; Kohn, Simon; Burnham, Antony

    2013-04-01

    Diamonds crystallize in the mantle primarily as a consequence of fluid or melt metasomatism. In doing so they sample the fluid-melt-solid equilibria directly by incorporation of carbon and its isotopic flavours, and by entrapping other phases as they grow. Superdeep diamonds from the transition zone and lower mantle provide evidence for crystallization from melts derived from subducted materials [1, 2]. The presence of deeply subducted volatile components such as carbon and water are important because they lower the solidus of subducted materials. The source of carbon may ultimately be via deposition of biogenic or abiogenic carbon in subducted crust, and water may become available via dehydration of high-pressure hydrous phases in the slab (e.g. superhydrous B, Phase D) [3]. Foundering of slabs around 700 km due to density inversion and thermalization with surrounding mantle leads to the generation of low-degree, volatile-charged melts. Melts from subducted oceanic crust may be carbonated, and diamond crystallization occurs as a consequence of 'redox freezing' when the oxidized slab melts react with reducing mantle rocks [4]. Reaction of slab melts with mantle peridotite may precipitate phases such as Ca-perovskite, Mg-perovskite, majorite and ferropericlase. Here we will survey evidence from the chemistry of superdeep mineral inclusions for a record of this deep mantle reactive transport process, and speculate on the role of deep mantle volatiles. 1. Bulanova, G.P., et al., Contributions to Mineralogy and Petrology, 2010. 160: p. 489-510. 2. Walter, M.J., et al., Nature, 2008. 454: p. 622-U30. 3. Harte, B., Mineralogical Magazine, 2010. 74: p. 189-215. 4. Rohrbach, A. and M.W. Schmidt, Nature, 2011. 472: p. 209-212.

  15. Spin and valence dependence of iron partitioning in Earth’s deep mantle

    PubMed Central

    Piet, Hélène; Badro, James; Nabiei, Farhang; Dennenwaldt, Teresa; Shim, Sang-Heon; Cantoni, Marco; Hébert, Cécile; Gillet, Philippe

    2016-01-01

    We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth’s lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth’s mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D” layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth. PMID:27647917

  16. Spin and valence dependence of iron partitioning in Earth's deep mantle.

    PubMed

    Piet, Hélène; Badro, James; Nabiei, Farhang; Dennenwaldt, Teresa; Shim, Sang-Heon; Cantoni, Marco; Hébert, Cécile; Gillet, Philippe

    2016-10-04

    We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth's lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth's mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.

  17. Crust and upper mantle structure of the New Madrid Seismic Zone: Insight into intraplate earthquakes

    NASA Astrophysics Data System (ADS)

    Chen, Chuanxu; Zhao, Dapeng; Wu, Shiguo

    2014-05-01

    We determine a 3-D P-wave velocity model of the crust and upper mantle down to 400 km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the New Madrid Seismic Zone. We used 4871 high-quality arrival times from 187 local earthquakes and 30,846 precise travel-time residuals from 1041 teleseismic events recorded by the EarthScope/USArray Transportable Array. Our results show that, beneath the Reelfoot rift, a significant low-velocity (low-V) zone exists in the upper mantle down to 200 km depth, with a large volume of 200 × 200 × 150 km3. The origin of the low-V zone may be related to the passage of the Bermuda hotspot and the stalled ancient Farallon slab materials foundering in the mantle transition zone. This low-V zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere, being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust, leading to the large intraplate earthquakes in the New Madrid Seismic Zone.

  18. 3D density model of the upper mantle of Asia based on inversion of gravity and seismic tomography data

    NASA Astrophysics Data System (ADS)

    Kaban, Mikhail K.; Stolk, Ward; Tesauro, Magdala; El Khrepy, Sami; Al-Arifi, Nassir; Beekman, Fred; Cloetingh, Sierd A. P. L.

    2016-11-01

    We construct a new-generation 3D density model of the upper mantle of Asia and its surrounding areas based on a joint interpretation of several data sets. A recent model of the crust combining nearly all available seismic data is employed to calculate the impact of the crust on the gravity anomalies and observed topography and to estimate the residual mantle anomalies and residual topography. These fields are jointly inverted to calculate the density variations in the lithosphere and upper mantle down to 325 km. As an initial approximation, we estimate density variations using a seismic tomography model. Seismic velocity variations are converted into temperatures and then to density variations based on mineral physics constraints. In the Occam-type inversion, we fit both the residual mantle gravity anomalies and residual topography by finding deviations to the initial model. The obtained corrections improve the resolution of the initial model and reflect important features of the mantle structure that are not well resolved by the seismic tomography. The most significant negative corrections of the upper mantle density, found in the Siberian and East European cratons, can be associated with depleted mantle material. The most pronounced positive density anomalies are found beneath the Tarim and South Caspian basins, Barents Sea, and Bay of Bengal. We attribute these anomalies to eclogites in the uppermost mantle, which have substantially affected the evolution of the basins. Furthermore, the obtained results provide evidence for the presence of eclogites in the oceanic subducting mantle lithosphere.

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

    PubMed

    Duffy, Thomas S

    2008-11-28

    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)SiO3 in the CaIrO3-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).

  20. 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).

  1. 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

  2. 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

  3. 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

  4. 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

  5. Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge.

    PubMed

    Hanan, Barry B; Blichert-Toft, Janne; Pyle, Douglas G; Christie, David M

    2004-11-04

    The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

  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. 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.

  8. 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.

  9. Crust and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data (Postprint). Annual Report 1

    DTIC Science & Technology

    2012-05-10

    upper mantle velocity structure of the region expressing the continental collision between the Arabian and Eurasian plates is being investigated using a...Eurasian plates using a joint inversion of body wave arrival times and satellite gravity. The body wave data set is derived from previous and on-going work...velocity structure in the crust and upper mantle of the region expressing the continental collision between the Arabian and Eurasian plates . Our strategy

  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. 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.

  12. Variation of the upper mantle velocity structure along the central-south Andes

    NASA Astrophysics Data System (ADS)

    Liang, Xiaofeng; Sandvol, Eric; Shen, Yang; Gao, Haiying

    2014-05-01

    Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical and geological and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach with non-linear stacking to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the deeper depth-sensitivity needed to constrain the mantle structure. The broadband waveform data are from 393 portable stations of four temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference waveform propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measurement by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and

  13. The sensitivity of GNSS measurements in Fennoscandia to distinct three-dimensional upper-mantle structures

    NASA Astrophysics Data System (ADS)

    Steffen, Holger; Wu, Patrick

    2015-04-01

    This poster will present the results of Steffen & Wu (2014). The sensitivity of GNSS measurements in Fennoscandia to nearby viscosity variations in the upper mantle is investigated using a three-dimensional finite element model of glacial isostatic adjustment (GIA). Based on the lateral viscosity structure inferred from seismic tomography and the location of the ice margin at the last glacial maximum (LGM), the GIA earth model is subdivided into four layers, where each of them contains an amalgamation of about 20 blocks of different shapes in the central area. The sensitivity kernels of the three velocity components at 10 selected GNSS stations are then computed for all the blocks. We find that GNSS stations within the formerly glaciated area are most sensitive to mantle viscosities below and in its near proximity, i.e., within about 250 km in general. However, this can be as large as 1000 km if the stations lie near the center of uplift. The sensitivity of all stations to regions outside the ice margin during the LGM is generally negligible. In addition, it is shown that prominent structures in the second (250-450 km depth) and third layers (450-550 km depth) of the upper mantle may be readily detected by GNSS measurements, while the viscosity in the first mantle layer below the lithosphere (70-250 km depth) along the Norwegian coast, which is related to lateral lithospheric thickness variation there, can also be detected but with limited sensitivity. For future investigations on the lateral viscosity structure, preference should be on GNSS stations within the LGM ice margin. But these stations can be grouped into clusters to improve the inference of viscosity in a specific area. However, the GNSS measurements used in such inversion should be weighted according to their sensitivity. Such weighting should also be applied when they are used in combination with other GIA data (e.g., relative sea-level and gravity data) for the inference of mantle viscosity. Reference

  14. Seismic anisotropy beneath La Réunion hotspot track: plume spreading vs deep mantle convection

    NASA Astrophysics Data System (ADS)

    Barruol, G.; Fontaine, F. R.

    2012-12-01

    Seismic anisotropy beneath the Western Indian Ocean is analyzed from temporary and permanent seismological deployments on the Piton de la Fournaise volcano, the active place of La Réunion hotspot, and from the permanent stations in Mauritius, Rodrigues and the Maldives Islands, in order to decipher the sublithospheric spreading signature of La Réunion mantle plume and the large-scale mantle flow pattern induced by the buoyancy-driven upwelling of the African superplume. The comparison of the SKS splitting observations with geodynamic mantle flow models show that the large-scale anisotropy pattern - characterized by fast directions trending NE-SW in the north (Maldives and Seychelles) to EW in the south (Mauritius, Rodrigues and La Réunion) may be largely explained by asthenospheric flow resulting from the combined effects of plate motion and deep mantle circulation. Anisotropy observed at the seismic stations installed on La Réunion Island shows, however, complex backazimuthal variations characterized by numerous "nulls" and by fast split directions trending normal to the plate motion observed within only a small backazimuthal window, that cannot be explained by neither a single nor two anisotropic layers. By testing models of sublithospheric spreading of La Réunion mantle upwelling, we show that this complex anisotropy pattern can be explained by a parabolic asthenospheric plume spreading with a plume conduit located 100 to 200 km north of La Réunion Island. Anisotropy beneath the GEOSCOPE station recently installed in Rodrigues Island does not appear to be influenced by the La Réunion plume-spreading signature but is fully compatible with either a model of large-scale deep mantle convection pattern and/or with a channeled asthenospheric flow beneath the Rodrigues ridge.

  15. 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

  16. Helium in deep circulating groundwater in the Great Hungarian Plain: Flow dynamics and crustal and mantle helium fluxes

    NASA Astrophysics Data System (ADS)

    Stute, M.; Sonntag, C.; Deák, J.; Schlosser, P.

    1992-05-01

    Observed helium concentrations in deep circulating groundwater of the sedimentary basin of the Great Hungarian Plain (GHP), Hungary, cover a range of three orders of magnitude (≈4 ·10 -8 to 4 · 10 -5 ccSTP g-1). 3He /4He ratios and noble gas concentrations are used to separate helium components originating from the atmosphere, tritium decay, crustal production, and mantle degassing. The characteristic distribution of measured helium concentrations and isotope ratios can be reproduced qualitatively by a simple two-dimensional advection/diffusion model. Other simple models isolating parts of the regional flow domain (recharge, discharge, and horizontal flow) are discussed and applied to derive quantitative information on helium fluxes due to degassing of the Earth's crust /mantle and on the dynamics of groundwater flow. The estimated helium flux of 0.7-4.5 · 10 9 atoms 4He m -2 s -1 is lower than values derived from other deep groundwater circulation systems, probably because the relatively young upper few thousand meters of the sedimentary basin (Tertiary to Quaternary age) shield the flux from the deeper crust. The high mantle helium flux of up to 4.2 · 10 8 atoms 4He m -2 s -1 is probably related to the Miocene volcanism or to continuing intrusion accompanying extension. By fitting calculated helium depth profiles to measured data in the discharge area, vertical flow velocities of the order of 1.5 mm y -1 are estimated. Assuming that a flux of 0.7-4.5 · 10 9 atoms 4He m -2 s -1 is representative for the entire basin, the turnover time of the regional groundwater flow system is estimated to be about 10 6 y.

  17. Anisotropy in the Pacific upper mantle from inversion of a surface-wave dispersion dataset

    NASA Astrophysics Data System (ADS)

    Eddy, C. L.; Ekstrom, G.; Nettles, M.; Gaherty, J. B.

    2015-12-01

    We present work towards a three-dimensional model of the anisotropic velocity structure of the Pacific upper mantle. Models of seismic anisotropy in oceanic regions provide important constraints on the geometry of strain in the mantle, the nature of the lithosphere-asthenosphere transition, and the possible presence of partial melt in the asthenosphere. The goal of this work is to produce a three-dimensional model of isotropic and anisotropic velocities in the Pacific, which will improve constraints on olivine fabrics and strain geometries in the oceanic upper mantle. Measurements of fundamental-mode dispersion for Rayleigh and Love waves traversing oceanic paths are drawn from the waveform dataset used to construct the global dispersion model GDM52. We develop anisotropic phase-velocity maps of the Pacific basin for Rayleigh and Love waves between 25 s and 250 s and invert the phase-velocity maps for anisotropic velocity structure at depth. The resulting models are radially anisotropic and include the G parameters that are related to the azimuthal anisotropy of vSV. We compare results of these two-step inversions with direct inversions of fundamental-mode phase anomalies for three-dimensional anisotropic structure. In much of the central and western Pacific, vertical gradients in both vS and anisotropy are consistent with the transition from rigid lithosphere to viscously deforming asthenosphere. In future work we plan to incorporate waveform data providing constraints on higher-mode dispersion in the modeling of the three-dimensional anisotropic structure.

  18. Upper mantle seismic velocity anomaly beneath southern Taiwan as revealed by teleseismic relative arrival times

    NASA Astrophysics Data System (ADS)

    Chen, Po-Fei; Huang, Bor-Shouh; Chiao, Ling-Yun

    2011-01-01

    Probing the lateral heterogeneity of the upper mantle seismic velocity structure beneath southern and central Taiwan is critical to understanding the local tectonics and orogeny. A linear broadband array that transects southern Taiwan, together with carefully selected teleseismic sources with the right azimuth provides useful constraints. They are capable of differentiating the lateral heterogeneity along the profile with systematic coverage of ray paths. We implement a scheme based on the genetic algorithm to simultaneously determine the relative delayed times of the teleseismic first arrivals of array data. The resulting patterns of the delayed times systematically vary as a function of the incident angle. Ray tracing attributes the observed variations to a high velocity anomaly dipping east in the mantle beneath the southeast of Taiwan. Combining the ray tracing analysis and a pseudo-spectral method to solve the 2-D wave propagations, we determine the extent of the anomaly that best fits the observations via the forward grid search. The east-dipping fast anomaly in the upper mantle beneath the southeast of Taiwan agrees with the results from several previous studies and indicates that the nature of the local ongoing arc-continent collision is likely characterized by the thin-skinned style.

  19. Crustal and upper mantle structure beneath the NE Tibetan Plateau and its tectonic implication

    NASA Astrophysics Data System (ADS)

    Li, H.; Zheng, D.; Shen, Y.; Ouyang, L.; Li, X.; Tan, J.

    2015-12-01

    The crustal and upper mantle velocity structures in the northeastern Tibetan Plateau are obtained from joint analysis of receiver functions and Rayleigh wave dispersion curves derived from teleseismic earthquake arrivals and ambient noise seismic data. The resulting velocity model reveals a close correlation between the thick (>60 km) crust and the presence of an intra-crustal low-velocity zone, which is detected beneath the Qiangtang and Songpan-Ganzi terranes as well as the northwestern Qilian orogen. However, the high Vp/Vs ratio is found only beneath the Qiangtang and Songpan-Ganzi terranes. The crustal low-velocity zone is not observed beneath the west Qinling and southeastern Qilian orogens, which have a relatively thin (~50 km) crust, indicating that crustal channel flow is not the primary mechanism by which the northeastern Tibetan plateau grows. In contrast to the widespread low velocities in the mid-to-lower crust beneath the Qiangtang and Songpan-Ganzi terranes, the upper mantle in these two regions shows alternating high and low velocity anomalies. A continuous low-velocity zone from the mid-to-lower crust down to 140 km beneath the eastern Kunlun fault suggests an induced local mantle upwelling after the delamination of the lithosphere.

  20. Interaction of Cocos and Rivera plates with the upper-mantle transition zone underneath central Mexico

    NASA Astrophysics Data System (ADS)

    Pérez-Campos, Xyoli; Clayton, Robert W.

    2014-06-01

    Receiver functions (RFs) from 224 permanent and temporary stations in central and southern Mexico were used to characterize the upper-mantle transition zone in that region. Discontinuities at 410 and 660 km depth are both deeper compared to iasp91, which reflects a slow velocity anomaly in the upper mantle. They show topography on the interfaces that is consistent with the interaction of the subducted slab or its broken off extension. A low-velocity layer on top of the 410 is identified mainly on the continental side of where the slab pierces it (i.e. in the lee of the slab roll-back). In general the RFs show a complex behaviour where the mantle has been disturbed by the lateral motion of the subducted slab, and are simple where it has not. Complexity on the 660 coincides with the place where the broken off portion of the Farallon Plate would have penetrated this interface or is possibly lying on top of it.

  1. The Influence of Water on Seismic Wave Attenuation in the Upper Mantle

    NASA Astrophysics Data System (ADS)

    David, E. C.; Jackson, I.; Faul, U.; Berry, A.

    2014-12-01

    Trace amounts of water, present as protons structurally bound in olivine crystal defects, are inferred to significantly enhance the low-strain solid-state viscoelastic relaxation responsible for attenuation and dispersion of seismic waves in the upper mantle. This inferrence is supported by recent observation of water weakening at moderate compressive strains in synthetic, water-undersaturated aggregates (Faul et al., in preparation). In these fine-grained olivine polycrystals of Fo90 composition, doped with 0.02wt% TiO2, "water" is incorporated in the remarkably stable Ti-clinohumite defect. Such synthetic olivine specimens reproduce the infrared spectra of natural mantle olivines (Berry et al., 2005), and present the advantage of being melt-free and of low dislocation density. The water contents in such synthetic polycrystalline olivine aggregates, which can be quantitatively measured by Fourier Transform Infrared Spectroscopy (FTIR), range up to 90 ppm, and are thus representative of water-undersaturated conditions in the upper mantle. We will report here the outcome of torsional-oscillation tests,in which attenuation and shear modulus were measured at seismic frequencies (mHz-Hz) and various temperatures up to 1300C on Pt-encapsulated, Ti-doped olivine specimens, enclosed within a mild-steel jacket.

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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

  7. 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

  8. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures. Application to understanding mantle dynamics and seismic anisotropy

    SciTech Connect

    Durham, William B.

    2016-05-02

    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA), to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.-

  9. Experimental investigation of flow-induced fabrics in rocks at upper-mantle pressures: Application to understanding mantle dynamics and seismic anisotropy

    SciTech Connect

    Kohlstedt, David L.

    2016-04-26

    The goal of this collaborative research effort between W.B. Durham at the Massachusetts Institute of Technology (MIT) and D.L. Kohlstedt and S. Mei at the University of Minnesota (UMN) was to exploit a newly developed technology for high-pressure, high-temperature deformation experimentation, namely, the deformation DIA (D-DIA) to determine the deformation behavior of a number of important upper mantle rock types including olivine, garnet, enstatite, and periclase. Experiments were carried out under both hydrous and anhydrous conditions and at both lithospheric and asthenospheric stress and temperature conditions. The result was a group of flow laws for Earth’s upper mantle that quantitatively describe the viscosity of mantle rocks from shallow depths (the lithosphere) to great depths (the asthenosphere). These flow laws are fundamental for modeling the geodynamic behavior and heat transport from depth to Earth’s surface.

  10. 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

  11. Experimental halogen partitioning between earth upper mantle minerals and silicate melt

    NASA Astrophysics Data System (ADS)

    Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Burgess, Ray; Ballentine, Christopher J.

    2013-04-01

    Owing to their incompatibility, halogens have similar geochemical properties to noble gases in many systems and may therefore be used as key tracers of volatile transport processes in the earth. Halogen fractionation may occur during partial melting of the upper mantle, fractional crystallization or partitioning between immiscible fluids. Experimental determination of the halogen partitioning behaviour is the basis for the investigation of the concentration and distribution of halogens in the earth's mantle. High P-T partition experiments were performed in a piston cylinder apparatus using a model primitive mantle composition proposed by Jagoutz et al. (1979) simplified to the four components CaO, MgO, A2lO3 and SiO2 (CMAS) according to the procedure of O'Hara (1968). Defined small amounts of halogens (0.2 wt%) were added as CaF2, CaCl2 and CaBr2. All experiments were first heated up to 1720° C and then cooled slowly to the target temperature to guarantee growth of large homogeneous crystals, following the method of Beyer et al. (2011). Pressures range between 1.0 GPa and 2.5 GPa and final experimental temperatures between 1500° C and 1600° C, thus representing partial melting conditions of the earth upper mantle. Back-scattered electron images of polished samples show euhedral, almost rectangular forsterite grains or a mixture of euhedral forsterite and pyroxene grains with a side length of up to 150 μm, which are embedded in a MORB-like melt. Electron microprobe analysis reveals a homogeneous major element composition of the forsterite and pyroxene single crystals as well as of the melt. Halogen mapping, measured via Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS), shows no concentration gradients within the minerals or within the melt. These observations suggest that the experiments were performed at equilibrium conditions. The fact that we were able to produce large pyroxene and forsterite crystals at equilibrium conditions in a halogen doped

  12. Seismic Imaging of the crust and upper mantle beneath Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Ebinger, C. J.

    2009-12-01

    In March 2007 41 seismic stations were deployed in north east Ethiopia. These stations recorded until October 2009, whereupon the array was condensed to 13 stations. Here we show estimates of crustal structure derived from receiver functions and upper mantle velocity structure, derived from tomography and shear-wave splitting using the first 2.5 years of data. Bulk crustal structure has been determined by H-k stacking receiver functions. Crustal Thickness varies from ~45km on the rift margins to ~16km beneath the northeastern Afar stations. Estimates of Vp/Vs show normal continental crust values (1.7-1.8) on the rift margins, and very high values (2.0-2.2) in Afar, similar to results for the Main Ethiopian Rift (MER). This supports ideas of high levels of melt in the crust beneath the Ethiopian Rift. Additionally, we use a common conversion point migration technique to obtain high resolution images of crustal structure beneath the region. Both techniques show a linear region of thin crust (~16km) trending north-south, the same trend as the Red Sea rift. SKS-wave splitting results show a general north east-south west fast direction in the MER, systematically rotating to a more north-south fast direction towards the Red Sea. Additionally, stations close to the recent Dabbahu diking episode show sharp lateral changes over small lateral distances (40° over <30km), with fast directions overlying the Dabbahu segment aligning parallel with the recent diking. This supports ideas of melt dominated anisotropy beneath the Ethiopian rift. The magnitude of splitting in this region is smaller than that seen at the MER, suggesting a thinner region of melt, or less focused melt is causing the anisotropy. Seismic tomography inversions show that in the top 150km low velocities highlight plate boundaries. The low velocity anomalies extend from the main Ethiopian rift NE, towards Djibouti, and from Djibouti NW towards the Dabbahu segment The lowest velocities exist on the rift

  13. 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

  14. 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

  15. 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.

  16. Crust and Upper Mantle Velocity Structure of the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Nyamwandha, C. A.; Powell, C. A.; Langston, C. A.

    2014-12-01

    Detailed P wave velocity (Vp) and S wave velocity models (Vs) and Vp/Vs ratios for the crust and upper mantle associated with the New Madrid Seismic Zone (NMSZ) are presented. The specific study region spans latitude 34 to 39.5 degrees north and longitude 87 to 93 degrees west and extends to a depth of at least 500 km. The density of data from three networks - The Cooperative New Madrid Seismic Network (CNMSN) operated by CERI, the Earthscope transportable array (TA), and the FlexArray (FA) Northern Embayment Lithospheric Embayment (NELE) project stations - provides us with the opportunity to derive detailed velocity models for this region. We use arrival times from local and regional earthquakes and travel time residuals from teleseismic earthquakes recorded by the three networks from September 2011 to date. The teleseismic body wave arrival times are measured using an Automated and Interactive Measurement of Body Wave Arrival Times (AIMBAT) package (Lou et al., 2012). We perform a joint local and teleseismic inversion (Zhao et al.,1994) to determine the velocity structure. For the local events, the hypocenters are relocated iteratively in the inversion process using an efficient 3-D ray tracing technique. We image a significant low velocity anomaly in the upper mantle with a concentration at about 200 - 300 km depth and it is a consistent feature in both the Vp and Vs tomography results. Checkerboard tests show that the spatial resolution is high in the upper mantle especially for the Vp model. The spatial resolution in the crust is fairly high for most of the study area except at the edges and the southeastern part, which can be attributed to diminished local earthquake activity. We perform synthetic tests to isolate smearing effects and further confirm the features in the tomographic images. Vp/Vs ratios are determined for the portions of the model with highest resolution. Preliminary results indicate that significant Vp/Vs ratio variations are present only at

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. 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

  2. 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

  3. 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.

  4. Microstructures and Rheology of the Earth Upper Mantle Inferred from a Multiscale Approach

    SciTech Connect

    O Castelnau; P Cordier; R Lebensohn; S Merkel; P Raterron

    2011-12-31

    The strongly anisotropic rheology of olivine polycrystals, associated to their microstructure, constitutes a key feature affecting the dynamics of the Earth's upper mantle. High pressure deformation experiments carried out on olivine single crystals under synchrotron radiation, together with estimations of lattice friction based on first-principle calculations, show a transition from easy [100] to easy [001] slips as pressure and temperature (thus depth) increases. We input these data at the slip system level into the second-order extension of the self-consistent scheme to assess microstructure evolution along a typical flow pattern beneath an oceanic spreading center.

  5. Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle

    PubMed

    Moreira; Kunz; Allegre

    1998-02-20

    New experimental data on the isotopic variations of neon, argon, and xenon in a popping rock imply that the 40Ar/36Ar ratio of the upper mantle is less than 44,000 and that the 129Xe/130Xe ratio is less than 8.2. The elemental abundance pattern of rare gases is chondritic-like and is quite distinct from the solar pattern. These data imply that Earth accreted from planetesimals that probably underwent a transformation of their rare gas budget from solar- to chondritic-like, leaving the isotopic composition unchanged from the solar pattern.

  6. Electrical resistivity structure of the upper mantle in the Southern Mariana Trough

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    We show a 2-D electrical resistivity structure of the upper mantle in the Southern Mariana Trough. The Mariana Trough is an active back-arc basin in which the seafloor spreading has occurred. In the southern region of the Mariana Trough, the seafloor spreading rate is 35-45 km/Myr at present (Kato et al., 2003) that is slow, but there are characteristics of the fast spreading ridge such as an axial topographic high (Martinez et al., 2000) and a nearly constant low mantle Bouguer anomaly along the spreading axis suggesting high magmatic activity with a sheet-like mantle upwelling under the spreading axis (Kitada et al., 2006). We carried out an electromagnetic experiment along a ~120 km length profile across the spreading axis to estimate an electrical resistivity structure, and hence the physical property like temperature, water and melt content in the upper mantle. The observation was made using ten Ocean Bottom Electro-Magnetometers (OBEMs) from August to November in 2010. The data was recorded for ~85 days in two OBEMs and for ~60 days in the rest of the OBEMs. Successfully, eight OBEMs recorded time-variations of the electric and magnetic fields and two OBEMs recorded only those of the magnetic field. The magnetotelluric (MT) method is a base for the data analysis. We carried out the time-series data analysis to estimate the MT responses and correct topographic distortions in the MT responses. We have basically performed a smooth model inversion analysis using the processed MT responses to estimate a minimum electrical resistivity structure, and also have considered a prior constraint in the inversion analysis for the subducted slab inferred from a seismic research (Gudmundsson and Sambridge, 1998). The obtained 2-D electrical resistivity structure shows an asymmetry about the spreading center. The trenchward side shows higher resistivity (~300 Ohm-m), while the opposite side (the west side) shows that intermediate resistivity (~100 Ohm-m) with ~40 km thickness

  7. 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

  8. 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.

  9. 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

  10. 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

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. Upper Mantle Origin of the Newberry Hotspot Track: Evidence From Shear-Wave Splitting

    NASA Astrophysics Data System (ADS)

    Xue, M.; Allen, R. M.

    2004-12-01

    In the northwestern United States there are two hotspot tracks: the Newberry track and the Yellowstone track. Both are located on the North American Plate with the Yellowstone track parallel to plate motion and the Newberry track oblique to it. While a mantle plume is probably the most popular cause of the Yellowstone track, the Newberry track cannot be the product of plate motion over a stationary mantle source. Instead proposed causal mechanisms include upper mantle process where melt buoyancy driven convection is directed west-northwest by subduction-driven corner flow or alternatively a westward-spreading plume head. In this SKS splitting study, we collected data from the OATS (Oregon Array for Teleseismic Study) array, a deployment of the University of Wisconsin Broadband Network (UWBN) along the Newberry track from NW to SE Oregon, which was installed in May 2003 and will operate until September 2005. Measurements were made for 23 events at 12 OATS stations using Wolfe and Silver (1998)¡_s multi-event stacking procedure. A gradual rotation of fast polarization direction is observed from NE-SW at the northwest end of the array to E-W to the southeast. Most stations also exhibit null results when the event back azimuth was parallel or perpendicular to the fast direction determined from other events, strongly indicating a single layer of anisotropy. The first order observation is that the SKS splits are not aligned with the Newberry hotspot track indicating that either the splits are not sensitive to mantle flow oriented along the track or the track is not the product of asthenospheric flow. We prefer the second explanation as our null splitting observations strongly argue for one layer of anisotropy. If our continuing analysis confirms this conclusion, then the alignment of the Yellowstone track with plate motion and anisotropy may be coincidental rather than representative of the causal mechanism.

  16. Splitting intensity measurements of North America and finite-frequency modeling of upper mantle anisotropy

    NASA Astrophysics Data System (ADS)

    Hongsresawat, Sutatcha

    The central theme of this dissertation is to investigate the interaction between the overlying lithospheric plates and the hotter and more deformable asthenosphere to examine how they are coupled. The answer will have very significant implications because the coupled lithosphere-asthenosphere setting predicts shear beneath the lithosphere which drives large-scale mantle convection, a widely-accepted schematic for modern mantle convection research. In our research, the primary tool to examine this coupled interaction is anisotropy in the upper mantle observed via shear wave splitting. The first research component involves measuring splitting intensity (SI) of the core-refracted shear waves (SKS) observed at 1,436 USArray Transportable Array (USArray-TA) seismic stations which cover most of the contiguous U.S. By fitting a sinusoidal to the back-azimuthal dependence of splitting intensity, traditional splitting parameters, the polarization angle between the radial direction and the fast axis, φ, and the delay time between the fast and slow polarizations, deltat, are obtained and used in comparison with absolute plate motion (APM), geological basement provinces, magnetic, gravity anomalies and lithospheric thickness to reveal the interactions between asthenospheric flow and lithospheric anisotropic structures of several geological regions of North America. Preliminary results shows that the Rocky Mountain front has a complicated flow due to transition with thickness along APM flow, the Gulf Coast has a strong APM asthenospheric signature in region of thin lithosphere and the northern Central U.S. has complicated interactions between asthenosphere and lithosphere. We observe a notable contrast between the Superior Province vs. Trans-Hudson where lithospheric texture alignment plays an important role in adding vs. subtracting the splitting signals. The next component of our research focuses on the Idaho-Oregon (IDOR) region. This region is an assemblage of several

  17. I. Rupture properties of large subduction earthquakes. II. Broadband upper mantle structure of western North America

    NASA Astrophysics Data System (ADS)

    Melbourne, Timothy Ian

    This thesis contains two studies, one of which employs geodetic data bearing on large subduction earthquakes to infer complexity of rupture duration, and the other is a high frequency seismological study of the upper mantle discontinuity structure under western North America and the East Pacific Rise. In the first part, we present Global Positioning System and tide gauge data which record the co-seismic deformation which accompanied the 1995 Mw8.0 Jalisco event offshore central Mexico, the 1994 Mw7.5 Sanriku event offshore Northern Honshu, Japan, and the 1995 Mw8.1 Antofagasta earthquake offshore Northern Chile. In two of the three cases we find that the mainshocks were followed by significant amounts of rapid, post-seismic deformation which is best and most easily explained by continued slip near the co-seismic rupture patch. This is the first documented case of rapid slip migration following a large earthquake, and is pertinent to earthquake prediction based on precursory deformation. As the three GPS data sets represent the best observations of large subduction earthquakes to date and two of them show significant amounts of aseismic energy release, they strongly suggest silent faulting may be common in certain types of subduction zones. This, in turn, bears on estimates of global moment release, seismic coupling, and our understanding of the natural hazards associated with convergent margins. The second part of this dissertation utilizes high frequency body waves to infer the upper mantle structure of western North America and the East Pacific Rise. An uncharacteristically large Mw5.9 earthquake located in Western Texas provided a vivid topside reflection off the 410 Km velocity discontinuity ("410"), which we model to infer the fine details of this structure. We find that, contrary to conventional wisdom, the 410 is not sharp, and our results help reconcile seismic observations of 410 structure with laboratory predictions. By analyzing differences between our

  18. 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

  19. 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

  20. 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.

  1. Regionalized temperature variations in the upper 400 km of the Earth's mantle

    NASA Astrophysics Data System (ADS)

    Tralli, David M.; Ita, Joel J.

    Tectonically regionalized variations in the temperature of the upper 400 km of the Earth's mantle are estimated from analysis of global seismic travel-time data cataloged by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion. Summary phase diagrams for the olivine and pyroxene-garnet subsystems are constructed in conjunction with a thermodynamic potential formulation that allows self-consistent determination of density, bulk modulus and adiabats throughout the pressure and temperature regimes of the mantle. Perturbations in estimated seismic parameters are expressed in terms of variations in temperature using the model temperature derivatives of the bulk modulus and density at a given temperature and pressure. Confidence bounds on the velocity estimates are used to place corresponding bounds on the constructed seismic parameters. A simple differential relationship is solved iteratively to obtain a temperature variation for a given variation in seismic parameter. This approach allows the estimation of a range of seismically determined temperature variations by employing a given compositional model. Results indicate that whereas the P and S velocity variations in the upper mantle are consistent with the tectonic regionalization, variations in V p/V s ratios are irregular. This leads to unstable estimates of the seismic parameters and thus estimates of mean temperature anomalies, typically within 600°C of the weighted mean, that are inconsistent with the regionalized seismic data. A comparison of two compositional models is used to show the trade-off with estimated temperature variations. A refined regionalization and analysis of a larger ISC data set are suggested to stabilize the S velocity inversion, reduce statistical uncertainties on the seismic parameters, and thus improve constraints on estimated temperature variations.

  2. Mantle Xenoliths from the Calatrava Volcanic Province, Spain - Evidence for Carbonatite- Silicate Interaction in the Upper Mantle.

    NASA Astrophysics Data System (ADS)

    Humphreys, E. R.; Bailey, K.; Hawkesworth, C. J.; Wall, F.

    2008-12-01

    Mantle xenoliths entrained in pyroclastic tuffs from the Calatrava Volcanic Province (CVP), Spain represent a snapshot of the lithospheric mantle from the last 10 Ma. They display significant heterogeneity in modal mineralogy and mineral chemistry. The presence of carbonate as inclusions in mantle minerals and the chemical composition of the clinopyroxenes suggest interaction of the lithospheric mantle with a compositionally different melt or fluid phase. Our study details the chemical complexity of xenoliths from this province, and seeks to distinguish the effects of partial melting and carbonatite-mantle interaction. The CVP is an alkaline volcanic province (8 to 1.6 Ma) located in a failed rift in central Spain. Volcanoes are dominantly maars or cinder cones, some of which are associated with minor lava extrusions. Carbonate forms a major component in many pyroclastic deposits (Bailey et al., 2005) and xenolithic material is prolific in most pyroclastic tuff rings. Mantle xenoliths are abundant and they show an affinity to pyroclastic tuffs including a melilitite or leucitite silicate component. Nodules encompassing lherzolite, wehrlite, harzburgite, pyroxenites and a range of composite lithologies have been studied from three localities. The dominant nodule composition from the CVP is spinel lherzolite, but wehrlite is also common. Mg numbers of olivines from lherzolites and wehrlites show a strong bimodality with wehrlitic olivines being enriched in iron Fo(84.7-85.5) whereas lherzolites show mantle values of Fo(89.6-90.6). The analysed wehrlites contain phlogopite in major quantities; a rare mineral component for Cenozoic European mantle xenoliths. Texturally, wehrlites differ from lherzolites primarily as a result of reaction textures and disequilibrium features. Spongy clinopyroxene reaction halos are commonly associated with interstitial melt and spinels also show reaction textures with granular boundaries, enriched in chromium and iron. Lherzolites

  3. Crust and upper-mantle discontinuities from analysis of broadband seismological data in the Mediterranean region

    NASA Astrophysics Data System (ADS)

    van der Meijde, M.; van der Lee, S.; Giardini, D.

    2001-12-01

    We have analyzed receiver functions to derive simple crustal models for a total of 17 permanent and temporary 3-component broadband seismological stations in the Mediterranean region. The 12 studied temporary stations have been operated under the MIDSEA project. To determine an accurate Moho depth we have reduced the trade-off between crustal velocities and discontinuity depth by using a new grid search method, which is an extension of recently published methods to determine crustal thickness. The values we find for Moho depth range from around 20 km for intra-oceanic islands and extended continental margins to near 45 km in regions where the Eurasian and African continents have collided. The relatively stable north-eastern African margin shows crustal thicknesses close to a standard value of 35 km while the relatively tectonically disturbed margin of north-western Africa shows significantly thinner crust. Modeling of crustal structure shows that all stacked receiver functions can be explained within standard deviations by a 2- or 3-layer model containing a sedimentary layer and/or a mid-crustal discontinuity. Both receiver function analysis and cross-correlation are powerful tools to reveal interfaces in the upper-mantle. For studying upper-mantle discontinuities we use both these methods. We transform the processed signals to the slowness-time domain to highlight P-to-S conversions from interfaces at different depths. The tectonic complexity of the Mediterranean region is found to extend down to the transition zone.

  4. Pressure and temperature evolution of upper mantle under the Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Kil, Y.; Wendlandt, R. F.

    2004-11-01

    Spinel peridotite xenoliths associated with the Rio Grande Rift axis (Potrillo and Elephant Butte volcanic fields) and the western rift shoulder (Adam’s Diggings) have been investigated to correlate pre-eruptive pressure and temperature conditions with xenolith deformation textures and rift location. Temperatures of xenolith equilibration at the rift shoulder are 100 250°C cooler for a given pressure than the temperatures at the rift axis. Undeformed xenoliths (protogranular texture) are derived from higher temperature and higher pressure conditions than deformed xenoliths (porphyroclastic and equigranular textures) in the rift axis. Exsolution lamellae in pyroxenes, small decreases in Al contents of orthopyroxenes from core to rim, and small differences in porphyroclastic orthopyroxene compositions versus neoblastic orthopyroxene compositions indicate high temperatures followed by cooling and a larger cooling interval in deformed rocks than in undeformed rocks. These features, along with thermal histories based on calcium zoning in olivine rims, indicate that the upper mantle under Adam’s Diggings and Elephant Butte has undergone cooling from an initial high temperature state followed by a late heating event, and the upper mantle under Potrillo has undergone cooling, reheating, and late heating events.

  5. 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.

  6. The Upper Mantle Under the South Pacific Super-Swell from Multimode Surface Waveform Tomography

    NASA Astrophysics Data System (ADS)

    Maggi, A.; Debayle, E.; Priestley, K.; Barruol, G.; Fontaine, F.; Reymond, D.

    2003-12-01

    The South Pacific contains a swarm of volcanic island chains superimposed on a broad bathymetric high known as the South Pacific Superswell. The islands and swell are thought to be the surface manifestation of a ``superplume'' beneath the the region. We present a Sv-wave speed tomographic model for the South Pacific derived from multi-mode waveform inversion of more than 17,000 vertical component seismograms. Most of the data are from the Global Digital Seismic Network but we include important data from ten broadband seismographs deployed in French Polynesia as part of the PLUME experiment (Polynesia Lithosphere and Upper Mantle Experiment). We use preferentially short propagation paths (Δ < 54o) to minimize off great circle path propagation, but increase the path length in areas of insufficient coverage. We resolve the subduction zones bounding the South Pacific to the east and west with a width compatible with the smoothing used in the tomographic inversion. The slow wave speed structure associated with the East Pacific Rise is offset to the west as has previously been noted in more detailed local studies. There seems to be no pervasive low wave speed feature in the upper mantle beneath the South Pacific. However, low wave speed structures extend from shallow depth to the transition zone beneath the Society hotspot, the Austral hotspot and Easter Island. The presence of similar structures beneath other Pacific hotspots is currently masked by lack of resolution.

  7. 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.

  8. Seismic structure of the European crust and upper mantle based on adjoint tomography

    NASA Astrophysics Data System (ADS)

    Zhu, H.; Bozdag, E.; Peter, D.; Tromp, J.

    2013-12-01

    We present a new crustal and upper mantle model for the European continent and the North Atlantic Ocean, named EU60. It is constructed based on adjoint tomography and involves 3D variations in elastic wavespeeds, anelastic attenuation, and radial/azimuthal anisotropy. Long-wavelength elastic wavespeed structure of EU60 agree with previous body- and surface-wave tomographic models. Some hitherto unidentified features, such as the Adria microplate, naturally emerge from smoothed starting model. Subducting slabs, slab detachment, ancient suture zones, continental rifts and back-arc basins are well resolved in EU60. For anelastic structure, we find an anti-correlation between shear wavespeeds and anelastic attenuation at shallow depths. At greater depths, this anti-correlation becomes relatively weak, in agreement with previous attenuation studies at global scales. Consistent with radial anisotropy in 1D reference models, the European continent is dominated by features with radially anisotropic parameter xi>1, indicating the presence of horizontal flow within the upper mantle. In addition, subduction zones, such as the Apennines and Hellenic arcs, are characterized as vertical flow with xi<1 at depths greater than 150~km. For azimuthal anisotropy, we find that the direction of fast anisotropic axis is well correlated with complicated tectonic evolution in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean and counter-clockwise rotation of the Anatolian Plate. The ``point spread function'' is used to assess image quality and analyze tradeoff between different model parameters.

  9. An Evolutionary S-wave Model of the Earth Upper Mantle and Transition Zone

    NASA Astrophysics Data System (ADS)

    Debayle, E.; Dubuffet, F.; Durand, S.

    2015-12-01

    We present 3D2015_03Sv, an evolutionary S-wave model of the upper mantle. At the time of submitting this abstract, the model is based on the waveform modeling of most Rayleigh waves recorded between 1976 and March 2015, and includes 1,330,210 fundamental and higher mode Rayleigh waveforms analyzed at periods between 40 and 400 s. The use of approximate forward theory and modeling allows updating the model with new data on a regular basis, a few days after the publication of the monthly centroid moment tensor (CMT) catalog issued at the Lamont-Doherty Earth Observatory of Columbia University. 3D2015_03Sv contains azimuthal anisotropy and achieves a lateral resolution of ~600 km in the upper mantle. Comparison with other seismic models shows that in the uppermost 200 km, the use of massive datasets with large redundancies allows to average errors, so that it is possible to build models that are consistent up to degree 60. In the transition zone, the number of data decreases and the effect of modeling choices is important. The most recent seismic models agree up to degree 15, which represents an improvement compared to the previous generation of models.

  10. Deep blast

    NASA Astrophysics Data System (ADS)

    From southern New Mexico to the Great Slave Lake of Canada, scientists from the United States and Canada recently detonated 10 underground chemical explosions to generate a clearer picture of the Earth's crust and upper mantle. Called Project Deep Probe, the experiment is designed to see through the crust and into the upper mantle to a depth of 300 miles.In the United States, Earth scientists from Rice University, Purdue University, and the University of Oregon are participating in the project. “Researchers hope to get a picture of the upper mantle beneath the Rocky Mountains and the Colorado Plateau, to understand the role the mantle played in formation and uplift,” says Alan Levander of Rice. To enhance that “picture,” 750 portable seismographs were placed along a roughly north-south line extending from Crownpoint, New Mexico to Edmonton, Alberta. The seismic recordings will be used to enhance weak seismic waves that penetrated the upper mantle.

  11. 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.

  12. Upper-mantle velocities below the Scandinavian Mountains from P- and S-wave traveltime tomography

    NASA Astrophysics Data System (ADS)

    Hejrani, Babak; Balling, Niels; Jacobsen, Bo Holm; England, Richard

    2017-01-01

    The relative traveltime residuals of more than 20 000 arrival times of teleseismic P and S waves measured over a period of more than 10 yr in five separate temporary and two permanent seismic networks covering the Scandinavian (Scandes) Mountains and adjacent areas of the Baltic Shield are inverted to 3-D tomograms of P and S velocities and the VP/VS ratio. Resolution analysis documents that good 3-D resolution is available under the dense network south of 64° latitude (Southern Scandes Mountains), and patchier, but highly useful resolution is available further north, where station coverage is more uneven. A pronounced upper-mantle velocity boundary (UMVB) that transects the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre-Trøndelag Fault Complex), after which it follows closely along the coast further north. Seismic velocities in the depth interval 100-300 km change significantly across the UMVB from low relative VP and even lower relative VS on the western side, to high relative VP and even higher relative VS to the east. This main velocity boundary therefore also separates relatively high VP/VS ratio to the west and relatively low VP/VS to the east. Under the Southern Scandes Mountains (most of southern Norway), we find low relative VP, even lower relative VS and hence high VP/VS ratios. These velocities are indicative of thinner lithosphere, higher temperature and less depletion and/or fluid content in a relatively shallow asthenosphere. At first sight, this might support the idea of a mantle buoyancy source for the high topography. Under the Northern Scandes Mountains, we find the opposite situation: high relative VP, even higher relative VS and hence low VP/VS ratios, consistent with thick, dry, depleted lithosphere, similar to that in most of the Baltic Shield area. This demonstrates significant differences in upper-mantle conditions between the Southern

  13. Upper-mantle velocities below the Scandinavian Mountains from P- and S- wave traveltime tomography

    NASA Astrophysics Data System (ADS)

    Hejrani, Babak; Balling, Niels; Jacobsen, Bo Holm; England, Richard

    2016-09-01

    The relative traveltime residuals of more than 20,000 arrival-times of teleseismic P- and S-waves measured over a period of more than 10 years in five separate temporary and two permanent seismic networks covering the Scandinavian (Scandes) Mountains and adjacent areas of the Baltic Shield are inverted to 3D tomograms of P- and S- velocities and the VP/VS ratio. Resolution analysis documents that good 3D resolution is available under the dense network south of 64° latitude (Southern Scandes Mountains), and patchier, but highly useful resolution is available further north, where station coverage is more uneven. A pronounced upper-mantle velocity boundary (UMVB) that transects the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre-Trøndelag Fault Complex), after which it follows closely along the coast further north. Seismic velocities in the depth interval 100-300 km change significantly across the UMVB from low relative VP and even lower relative VS on the western side, to high relative VP and even higher relative VS to the east. This main velocity boundary therefore also separates relatively high VP/VS ratio to the west and relatively low VP/VS to the east. Under the Southern Scandes Mountains (most of southern Norway) we find low relative VP, even lower relative VS and hence high VP/VS ratios. These velocities are indicative of thinner lithosphere, higher temperature and less depletion and/or fluid content in a relatively shallow asthenosphere. At first sight, this might support the idea of a mantle buoyancy source for the high topography. Under the Northern Scandes Mountains we find the opposite situation: high relative VP, even higher relative VS and hence low VP/VS ratios, consistent with thick, dry, depleted lithosphere, similar to that in most of the Baltic Shield area. This demonstrates significant differences in upper mantle conditions between the Southern

  14. 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

  15. Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration

    USGS Publications Warehouse

    Miller, Nathaniel; Lizarralde, Daniel

    2016-01-01

    Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5–12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from ~3.5 wt % to as low as 0.33 wt % H2O.

  16. Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration

    NASA Astrophysics Data System (ADS)

    Miller, Nathaniel C.; Lizarralde, Daniel

    2016-08-01

    Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5-12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from 3.5 wt % to as low as 0.33 wt % H2O.

  17. 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

  18. 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}.

  19. 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.

  20. Strong lateral variations of S-wave velocity in the upper mantle across the western Alps

    NASA Astrophysics Data System (ADS)

    Lyu, Chao; Pedersen, Helle; Paul, Anne; Zhao, Liang

    2016-04-01

    Absolute S-wave velocity gives more insight into temperature and mineralogy than relative P-wave velocity variations (ΔV p/ V p) imaged by teleseismic traveltime tomography. Moreover, teleseismic P-wave tomography has poor vertical but good horizontal resolution. By contrast, the inversion of surface waves dispersion data gives absolute S-wave velocity with a good vertical but relatively poor horizontal resolution. However, the horizontal resolution of surface wave imaging can be improved by using closely spaced stations in mini-arrays. In this work, we use Rayleigh wave phase velocity dispersion data to measure absolute S-wave velocities beneath the CIFALPS profile across the French-Italian western Alps. We apply the array processing technique proposed by Pedersen et al. (2003) to derive Rayleigh wave phase dispersion curves between 20 s and 100 s period in 15 mini-arrays along the CIFALPS line. We estimate a 1-D S-wave velocity model at depth 50-150 km beneath each mini-array by inverting the dispersion curves jointly with receiver functions. The joint inversion helps separating the crustal and mantle contributions in the inversion of dispersion curves. Distinct lithospheric structures and marked lateral variations are revealed beneath the study region, correlating well with regional geological and tectonic features. The average S-wave velocity from 50 to 150 km depth beneath the CIFALPS area is ˜4.48km/s, almost the same as in model AK135, indicating a normal upper mantle structure in average. Lateral variations are dominated by relatively low velocities (˜4.4km/s) in the mantle of the European plate, very low velocities (4.0km/s, i.e. approximately 12% lower than AK135) beneath the Dora Maira internal crystalline massif and high velocities (˜ 5.0km/s, i.e. 12% higher than AK135) beneath the Po plain. The lateral variations of S-wave velocity perturbation show the same features as the P wave tomography (Zhao et al., submitted), but with different amplitudes

  1. 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

  2. Investigation of the upper mantle beneath the North Pacific and Aleutian Trough using PP bounce point functions

    NASA Astrophysics Data System (ADS)

    Williams, C.; Gurrola, H.

    2013-12-01

    flat on both profiles but starts to shallow at about 3 degrees south of the trough. The 100 km deep LAB appears to be weaker as it crosses beneath the CT but there is a stronger phase that shallows from 100 km to about 30 km directly beneath the CT. This looks similar to the way the LAB would look under an active mid oceanic ridge (MOR) but spreading at the CT ended 80mya. There are strong negative phases directly beneath CT that are shaped much like we might expect partial melt in a hot mantle beneath a MOR would look. The most likely explanation for the CT to resemble an active ridge is that the negative phases are the result of mantle depleted in iron due to fractionation and the old MOR related LAB is preserved. Several other lens of negative phases appear in layers starting at 7 km south of the trench and trending north to the trench. We believe these to be partial melt of slab material that has been recycled out of the transition zone. We do image the 100 km LAB as it subducts into the mantle. The 200 km discontinuity also seems to subduct into the mantle but only as a positive phase. It appears the subducting slab is composed of the entire 200 km thick portion of the upper mantle.

  3. 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.

  4. 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

  5. Upper Mantle Structure and Properties from Combined Seismological and Experimental Models (Invited)

    NASA Astrophysics Data System (ADS)

    Faul, U.; Jackson, I.; Dalton, C. A.

    2010-12-01

    Combining the interpretation of global seismic velocity and attenuation models with forward calculations based on experimental data can provide a powerful tool to investigate the physical state of the upper mantle. We have extensively re-calibrated our experimental assembly and added new data to our previously published experimental model (Faul and Jackson, 2005). In particular, this allows improved resolution at low levels of attenuation, which suggests the existence of a plateau at temperatures below 1000°C and seismic frequencies (Jackson and Faul, 2010). We combine this experimental model with the global shear velocity model of Kustowski et al. (2006) and the global shear attenuation model of Dalton et al. (2007). The velocity model contains 3-D variations in radially anisotropic shear-wave speed and is constructed from large data sets of surface-wave phase anomalies, long-period waveforms, and body-wave travel times. The attenuation model is derived from > 30,000 fundamental-mode Rayleigh wave amplitude measurements at each period in the period range from 50-250 s. The amplitudes are inverted simultaneously for the coefficients of the 3-D attenuation model as well as frequency-dependent correction factors for each source and receiver. Focusing effects due to elastic heterogeneities are accounted for from jointly determined phase velocity maps. Velocity and attenuation-depth profiles show a low velocity, high attenuation zone in the upper mantle that is broadly consistent with experimental predictions. In plots of attenuation (1/Q) vs velocity at discrete depth intervals the seismological model data form distinct trends. When the model data are separated into continental and oceanic regions, a change in slope occurs at different depth for these regions. This change in slope can be interpreted as the lithosphere-asthenosphere boundary at a (globally averaged) depth between 100 and 150 km for oceans and between 200 and 250 km for continents. Since the

  6. 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.

  7. Lateral heterogeneity scales in regional and global upper mantle shear velocity models

    NASA Astrophysics Data System (ADS)

    Meschede, Matthias; Romanowicz, Barbara

    2015-02-01

    We analyse the lateral heterogeneity scales of recent upper mantle tomographic shear velocity (Vs) global and regional models. Our goal is to constrain the spherical harmonics power spectrum over the largest possible range of scales to get an estimate of the strength and statistical distribution of both long and small-scale structure. We use a spherical multitaper method to obtain high quality power spectral estimates from the regional models. After deconvolution of the employed taper functions, we combine global and regional spectral estimates from scales of 20 000 to around 200 km (degree 100). In contrast to previous studies that focus on linear power spectral densities, we interpret the logarithmic power per harmonic degree l as heterogeneity strength at a particular depth and horizontal scale. Throughout the mantle, we observe in recent global models, that their low degree spectrum is anisotropic with respect to Earth's rotation axis. We then constrain the uppermost mantle spectrum from global and regional models. Their power spectra transfer smoothly into each other in overlapping spectral bands, and model correlation is in general best in the uppermost 250 km (i.e. the `heterosphere'). In Europe, we see good correlation from the largest scales down to features of about 500 km. Detailed analysis and interpretation of spectral shape in this depth range shows that the heterosphere has several characteristic length scales and varying spectral decay rates. We interpret these as expressions of different physical processes. At larger depths, the correlation between different models drops, and the power spectrum exhibits strong small scale structure whose location and strength is not as well resolved at present. The spectrum also has bands with elevated power that likely correspond to length scales that are enhanced due to the inversion process.

  8. Tomographic Imaging of the Crust and Upper Mantle Beneath the Western Tien Shan

    NASA Astrophysics Data System (ADS)

    Li, Z.; Roecker, S.

    2007-12-01

    We combine P and S wave arrival times from the recent MANAS deployment of broad band sensors with the prior GENGHIS deployment and analogue observations from the Kyrgyz Institute of Seismology to generate a high resolution (5-20 km) image of elastic wavespeeds in the crust and upper mantle beneath the western Tien Shan. The total data set consists of 29,006 P and 21,491 S arrivals from 2176 local events along with 27,196 P arrivals from 2631 teleseismic events recorded at 144 stations. Near surface structure is constrained by a combination of arrival times from local and regional events, Moho depths determined from receiver functions, and travel times from the 2007 MANAS active source profile. Our principal finding is a large high wavespeed region in the mantle beneath most of western Tien Shan dipping to the north to depths as great as 600 km. This region appears to be continuous to shallow depths and surfaces at the southern range front near where the Tarim Basin is being overthrust by the Kokshal range. This result suggests that the Tarim Basin continues to actively subduct beneath the Tien Shan. We do not see any similar feature associated with underthrusting of the Kazach shield to the south. We also inverted a subset of regionally located events for Pn wavespeed and anisotropy. Similar to the larger scale results, Pn wavespeeds in the uppermost mantle are 3-4 percent lower than normal under most of the Tien Shan. The fast direction of Pn anisotropy is predominantly north-south in agreement with the sparser sampling of previous SKS studies. Taken with the tomographic image, we suggest that the anisotropy is most likely due to flow in the asthenosphere induced by the subduction of the Tarim basin.

  9. Azimuthal seismic anisotropy in the Earth's upper mantle and the thickness of tectonic plates

    NASA Astrophysics Data System (ADS)

    Schaeffer, A. J.; Lebedev, S.; Becker, T. W.

    2016-11-01

    Azimuthal seismic anisotropy, the dependence of seismic wave speeds on propagation azimuth, is largely due to fabrics within the Earth's crust and mantle, produced by deformation. It thus provides constraints on the distribution and evolution of deformation within the upper mantle. Here, we present a new global, azimuthally anisotropic model of the crust, upper mantle and transition zone. Two versions of this new model are computed: the rough SL2016svAr and the smooth SL2016svA. Both are constrained by a very large data set of waveform fits (˜750 000 vertical component seismogram fits). Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms in broad period ranges (dominantly from 11 to 450 s, with the best global sampling in the 20-350 s range), yielding resolving power from the crust down to the transition zone. In our global tomographic inversion, regularization of anisotropy is implemented to more uniformly recover the amplitude and orientation of anisotropy, including near the poles. Our massive waveform data set, with complementary large global networks and high-density regional array data, produces improved resolution of global azimuthal anisotropy patterns. We show that regional scale variations, related to regional lithospheric deformation and mantle flow, can now be resolved by the global models, in particular in densely sampled regions. For oceanic regions, we compare quantitatively the directions of past and present plate motions and the fast-propagation orientations of anisotropy. By doing so, we infer the depth of the boundary between the rigid, high-viscosity lithosphere (preserving ancient, frozen fabric) and the rheologically weak asthenosphere (characterized by fabric developed recently). The average depth of thus inferred rheological lithosphere-asthenosphere boundary (LAB) beneath the world's oceans is ˜115 km. The LAB depth displays a clear dependence on the age of the oceanic

  10. Small effect of water on upper mantle rheology based on silicon self-diffusion coefficients

    NASA Astrophysics Data System (ADS)

    Fei, H.; Wiedenbeck, M.; Yamazaki, D.; Katsura, T.

    2012-12-01

    Water has been considered to significantly affect the mantle dynamics. In particular, experimental deformation studies [1-4] claimed that even small amount of water enhanced the creep in olivine by orders of magnitude. However, we note that their results are experimental artifact due to a number of limitations: e.g., unavoidable grain boundary sliding when polycrystalline samples were used; limited ranges of water contents due to the limited pressures; several orders higher stress and strain rate than those in nature. High temperature creep of silicate minerals is controlled by silicon self-diffusion. Therefore, measurement of silicon self-diffusion coefficients (DSi) in minerals, which can be performed without these limitations, is an independent way to study the mantle rheology. In this study, we measured DSi in Mg end-member of olivine, namely, forsterite, as a function of water content (CH2O) across a wide range, and concluded that effect of water on upper mantle rheology is very small. Forsterite single crystals were doped with <1 to ~800 μg/g of water at 1600 K, 8 GPa using talc+brucite water sources and graphite buffer. The CH2O in the samples were controlled by the ratio of water sources to graphite. The water doped samples were polished, deposited with 500 nm 29Si enriched Mg2SiO4 thin films, and annealed at 8 GPa, 1600 or 1800 K for diffusion with the same proportion of water sources, which successfully made constant values of CH2O during diffusion annealing. The diffusion profiles were obtained by SIMS. CH2O in the samples were determined by FT-IR before and after diffusion, and also examined by SIMS. Our results yield a relationship: DSi ∝ (CH2O)1/3. This is explained by defect chemistry, where DSi∝[VSi‧‧‧‧]×[VO●●]∝(CH2O)2/3×(CH2O)-1/3=(CH2O)1/3 under the charge neutrality condition of [(OH)O●]=2[VMg‧‧] because both Si and O vacancies are needed for Si ions to diffuse. The water contents exponent (1/3) determined in this study

  11. Receiver Function Analysis of Crustal and Upper Mantle Layering Across the Western Superior Province

    NASA Astrophysics Data System (ADS)

    Olaleye, Morounkeji

    2011-12-01

    The Superior Province is the Earth's largest Archean craton. Its western portion in Canada represents the nucleus of the North American continent, and has a lineated structure with well-preserved supracrustal rock sequences, mineral resources, and greenstone-granite terranes. Its strong east-west tectonic fabric is most commonly attributed to the formation and widespread accretion of island arcs and accretionary prisms ˜ 2.6 Ga ago (Lucas et al., 1998). The Superior Province is underlain by lithospheric mantle that exhibits strong regional variations in anisotropy and velocity structure (Darbyshire et al., 2007). The stratigraphy, velocity structure and thickness of the crust and upper-mantle beneath the western Superior Province, were examined through the analysis of seismic discontinuities on the radial and transverse components of P-wave receiver functions. Global earthquakes that occurred between 2003 and 2008 and recorded by 13 broadly spaced FedNor/POLARIS and CNSN three-component broadband seismic stations across western Ontario were used to create receiver functions. Receiver functions were calculated using a panel deconvolution approach (using inter-trace regularization constraints) to improve signal-to-noise ratio. Inversion for lithospheric parameters was carried out through a directed Monte-Carlo search method that uses the neighbourhood algorithm of Sambridge (1999). The receiver function data show indications of crustal and mantle layering. Generally, it was observed that in the western Superior Province, seismic stations in the southern portion of the study area (south of ˜ 51° N): LDIO, EPLO, PNPO, TIMO and NANO reveal a uniform crust, but a complicated and layered mantle; whereas stations in the northern portion of the study area (north of ˜ 51° N): KASO, RLKO, SILO, VIMO and PKLO reveal a more uniform mantle layer, but a stratified crust. The only exception is ATKO, which displays dominant crustal layering, but is located south of ˜ 51° N

  12. 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.

  13. 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

    2009-05-07

    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.

  14. Experimental determination of melt interconnectivity and electrical conductivity in the upper mantle

    NASA Astrophysics Data System (ADS)

    Laumonier, Mickael; Farla, Robert; Frost, Daniel J.; Katsura, Tomoo; Marquardt, Katharina; Bouvier, Anne-Sophie; Baumgartner, Lukas P.

    2017-04-01

    The presence of a small fraction of basaltic melt is a potential explanation for mantle electrical conductivity anomalies detected near the top of the oceanic asthenosphere. The interpretation of magnetotelluric profiles in terms of the nature and proportion of melt, however, relies on mathematical models that have not been experimentally tested at realistically low melt fractions (<0.01). In order to address this, we have performed in situ electrical conductivity measurements on partially molten olivine aggregates. The obtained data suggest that the bulk conductivity follows the conventional Archie's law with the melt fraction exponents of 0.75 and 1.37 at melt fractions greater and smaller than 0.5 vol.% respectively at 1350 °C. Our results imply multiple conducting phases in melt-bearing olivine aggregate and a connectedness threshold at ∼0.5 vol.% of melt. The model predicts that the conductive oceanic upper asthenosphere contains 0.5 to 1 vol.% of melt, which is consistent with the durable presence of melt at depths over millions years while the oceanic plates spread apart at the mid-ocean ridge. Beneath ridges a minimum permeability may allow mid-ocean ridge basalts to rise out of the mantle, where our model indicates that melt is present in proportions of up to 4 vol.%.

  15. High-resolution teleseismic tomography of upper-mantle structure using an a priori three-dimensional crustal model

    NASA Astrophysics Data System (ADS)

    Waldhauser, Felix; Lippitsch, Regina; Kissling, Edi; Ansorge, Jörg

    2002-08-01

    The effect of an a priori known 3-D crustal model in teleseismic tomography of upper-mantle structure is investigated. We developed a 3-D crustal P-wave velocity model for the greater Alpine region, encompassing the central and western Alps and the northern Apennines, to estimate the crustal contribution to teleseismic traveltimes. The model is constructed by comparative use of published information from active and passive seismic surveys. The model components are chosen to represent the present large-scale Alpine crustal structure and for their significant effect on the propagation of seismic wavefields. They are first-order structures such as the crust-mantle boundary, sedimentary basins and the high-velocity Ivrea body. Teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite-difference technique to the IASP91 traveltime tables. Residuals are produced for a synthetic upper-mantle model featuring two slab structures and the 3-D crustal model on top of it. The crustal model produces traveltime residuals in the range between -0.7 and 1.5 s that vary strongly as a function of backazimuth and epicentral distance. We find that the non-linear inversion of the synthetic residuals without correcting for the 3-D crustal structure erroneously maps the crustal anomalies into the upper mantle. Correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle. We conclude that with the increasing amount of high-quality seismic traveltime data, correction for near-surface structure is essential for increasing resolution in tomographic images of upper-mantle structure.

  16. 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

  17. 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.

  18. 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

  19. The Sidi Mohamed peridotites (Edough Massif, NE Algeria): Evidence for an upper mantle origin

    NASA Astrophysics Data System (ADS)

    Zobir, Soraya Hadj; Oberhänsli, Roland

    2013-12-01

    The Hercynian Edough massif is the easternmost crystalline massif of the Algerian coast. It consists of two tectonically superposed units composed of micaschists, gneisses, and peridotite. This study concentrates on the small and isolated Sidi Mohamed peridotite outcrop area (0.03 km2). The Sidi Mohamed peridotite is composed mainly of harzburgites (Mg-rich olivine and orthopyroxene as major minerals). The Ni (2051-2920 ppm), Cr (2368-5514 ppm) and MgO (~28-35 wt.%) whole-rock composition and the relative depletion in Nb make these harzburgites comparable to depleted peridotites related to a subduction zone. We suggest that the Sidi Mohamed ultramafic body was derived directly from the upper mantle and tectonically incorporated into the gneiss units of the Edough metamorphic core complex in a subduction environment.

  20. 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.

  1. 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.

  2. Can We Probe the Conductivity of the Lithosphere and Upper Mantle Using Satellite Tidal Magnetic Signals?

    NASA Technical Reports Server (NTRS)

    Schnepf, N. R.; Kuvshinov, A.; Sabaka, T.

    2015-01-01

    A few studies convincingly demonstrated that the magnetic fields induced by the lunar semidiurnal (M2) ocean flow can be identified in satellite observations. This result encourages using M2 satellite magnetic data to constrain subsurface electrical conductivity in oceanic regions. Traditional satellite-based induction studies using signals of magnetospheric origin are mostly sensitive to conducting structures because of the inductive coupling between primary and induced sources. In contrast, galvanic coupling from the oceanic tidal signal allows for studying less conductive, shallower structures. We perform global 3-D electromagnetic numerical simulations to investigate the sensitivity of M2 signals to conductivity distributions at different depths. The results of our sensitivity analysis suggest it will be promising to use M2 oceanic signals detected at satellite altitude for probing lithospheric and upper mantle conductivity. Our simulations also suggest that M2 seafloor electric and magnetic field data may provide complementary details to better constrain lithospheric conductivity.

  3. 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.

  4. 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).

  5. 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

  6. Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling.

    PubMed

    Rowley, David B; Forte, Alessandro M; Rowan, Christopher J; Glišović, Petar; Moucha, Robert; Grand, Stephen P; Simmons, Nathan A

    2016-12-01

    Earth's tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth's dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region.

  7. Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

    PubMed Central

    Rowley, David B.; Forte, Alessandro M.; Rowan, Christopher J.; Glišović, Petar; Moucha, Robert; Grand, Stephen P.; Simmons, Nathan A.

    2016-01-01

    Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region. PMID:28028535

  8. 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

  9. 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.

  10. 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

  11. An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Ismaı̈l, Walid Ben; Mainprice, David

    1998-10-01

    We present a unique database of 110 olivine petrofabrics and their calculated seismic properties. The samples come from a variety of the upper mantle geodynamic environments (ophiolites, subduction zones, and kimberlites) with a wide range of micro-structures. A phenomenological relationship is established between P- and S-wave seismic anisotropy and the degree of crystal alignment (fabric strength). Seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20% for P-waves and 10 to 15% for S-waves. Despite a large variation in the symmetry of fabric patterns, the average seismic properties of the different fabric, micro-structural and geodynamic settings have similar anisotropies in both magnitude and symmetry. Hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known, but not geodynamic environment or details of the petrofabric pattern. A simple pattern of seismic anisotropy characterises the average sample of the database, which has the following features: the polarisation plane of the fastest S-wave is parallel or sub-parallel to the foliation plane; the maximum shear wave splitting is parallel to the Y structural direction (in foliation plane and normal to the lineation); the maximum of the P-wave velocity is parallel to the high concentration of [100] axes, which is sub-parallel to the lineation. The [100] orientation distribution has the greatest influence on the P-wave seismic anisotropy. The [100] and [001] orientation distributions have the greatest influence on the symmetry of S-wave anisotropy, although the magnitude of anisotropy is influenced by the distribution of all three principal axes. Although the database only contains olivine petrofabrics, this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle.

  12. Upper extremity deep vein thrombosis in a military patient.

    PubMed

    Bullock, Charlotte; Johnston, A McD

    2016-08-01

    We describe the case of a 23-year-old serviceman on overseas deployment who presented with a painful, swollen arm. Investigations showed an upper extremity deep vein thrombosis (UEDVT) of the right arm with an associated asymptomatic pulmonary embolism, which was treated with warfarin anticoagulation. Further investigation identified positional obstruction at the thoracic outlet, and the patient was diagnosed with Paget-Schroetter syndrome. He underwent elective resection of the first rib, and has now returned to normal duties. After review of the literature on UEDVT, it is suggested that in this military patient, the occurrence of an anatomical variant put him at risk of upper limb venous thrombosis, which was probably potentiated by the occupational factor of carrying a rifle. The external compression of the subclavian vein from the rifle butt and hypertrophied muscles, in addition to the anatomical variation, caused repetitive microtrauma of the vessel intima, which precipitated venous thrombosis.

  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. 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

  15. 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

  16. 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

  17. A radial anisotropy model of the upper mantle from surface wave observations

    NASA Astrophysics Data System (ADS)

    Ho, T. M.; Debayle, E.; Priestley, K. F.; Chapman, C. H.

    2014-12-01

    Radial anisotropy within the upper mantle was first encountered in the early 1960's based on studies using surface waves. A disrepancy between the Love and Rayleigh wave data was observed which could not be explained using a simple isotropic model. This was later reconconciled using a transversely isotropic model now assumed in many modern day anisotropic models such as SAW642AN and S362ANI. Radial anisotropy is attributed to the lattice preferred orientation (LPO) of the anisotropic crystals believed to be organised by the flow in the upper mantle. These models are therefore important for analysing the geometry of the flow and the deformation of the mantle. Surface wave observations offer a unique way of studying the radial anisotropy. Rayleigh waves are sensitive to the vertical shear velocity (SV) and Love waves are sensitive to the horizontal shear velocity (SH). The ratio of these give the radial anisotropy parameter ξ. Although radial anisotropy models exist, they are usually limited to the fundamental mode measurments with poor path coverage due to the noise on the horizontal components. Higher mode Love wave measurements are difficult for oceanic paths. This is because group velocities of the higher modes are similar to the fundamental mode between a period range of 50-100 s. These therefore arrive and interferre with each other. The higher mode information therefore cannot be extracted easily. We modify the method of Debayle and Ricard 2012 which allow the extraction of information up to the 5th overtone by mimicking the interferrence from the arrival of the fundamental and higher modes. Synthetic tests show an excellent recovery of the fundamental and higher mode information from the Love waves. The inclusion of the the higher modes greatly increases the resolution to the SH velocity and increases the sampling to deeper structures. We apply this modified method to a large dataset and construct an SH model. This is then combined with an equivalent SV

  18. 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

  19. 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

  20. Chemical layering in the upper mantle of Mars: Evidence from olivine-hosted melt inclusions in Tissint

    NASA Astrophysics Data System (ADS)

    Basu Sarbadhikari, A.; Babu, E. V. S. S. K.; Vijaya Kumar, T.

    2017-02-01

    Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine-hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine-phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo76-70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo66-55). REE-plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole-rock. Model calculations indicate two-stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in 10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole-rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE-rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.

  1. 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

  2. Crustal and upper mantle anisotropy associated with fossilized transpression along the Denali Fault, northern Canadian Cordillera

    NASA Astrophysics Data System (ADS)

    Rasendra, N.; Bonnin, M.; Marechal, A.; Tiberi, C.; Mazzotti, S.

    2013-12-01

    The Denali Fault is a major, ~1200 km-long, continental strike-slip fault that participates to the plate boundary system of western North America since the Early Cenozoic. In southwest Yukon, it accommodated ~400 km of dextral displacement in a late Cretaceous - Eocene transpression phase during which allochtonous terranes were accreted to the North America margin. Smaller strike-slip and thrust faults mark a 50 - 100 km wide tectonic corridor between the St. Elias Mountains and the central Yukon plateau, with the Denali Fault along its eastern edge. We examine the crustal and upper mantle structure in SW Yukon using a combination of receiver function (RF) and shear-wave splitting (SKS) analysis on a network of eleven seismic stations deployed in the Denali Fault region. To first order, crustal thickness (35 - 41 km) correlates well with the topography, indicating a ~5 km crustal root beneath the eastern side of the St. Elias Mountains. RFs display a strong P-to-S conversion within the crust, which systematically varies with back-azimuth. Stacking and inversion of RF according the two complementary back-azimuth ranges show a strong Vs anisotropy (> 10%) at mid-crustal depths (15 - 20 km) for a subset of stations within 5 - 25 km of the Denali Fault and inside its tectonic corridor. Other stations, further away or in a different geological setting, show a weaker (< 5%) anisotropy. This Vs anisotropy occurs in a low-velocity zone with the slow velocity axis perpendicular to the Denali Fault trend. Similarly, SKS splitting measurements indicate a strong Vs anisotropy in the upper mantle, with the fast direction parallel to the Denali Fault trend at all but one station within +/- 25 km from the fault. Present-day tectonics is well constrained by an array of campaign and permanent GPS stations in SE Alaska and SW Yukon. These data indicate that the Denali fault and its associated fault array currently accommodate less than 2 mm/yr of transpressional, dextral motion (< 5

  3. 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

  4. Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

    USGS Publications Warehouse

    Mukasa, S.B.; Wilshire, H.G.

    1997-01-01

    Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and

  5. Effects of iron enrichment on the chemistry and physical properties of deep lower mantle silicates

    NASA Astrophysics Data System (ADS)

    De Pasquale, Antonella

    Variations in seismic wave speed and density in the Earth's deep lower mantle have been linked to chemical heterogeneities. In order to identify the compositions of these regions and determine their roles in Earth history and dynamics, experimental measurements are needed of the effects of compositional variation, particularly major elements Fe and Al, on phase equilibria and physical properties of mantle minerals. The experiments that comprise this dissertation provide new constraints on the chemistry and compressibility of mantle silicates. Experiments were conducted at mantle pressure-temperature conditions using the laser-heated diamond anvil cell. Determination of pressure in the diamond anvil cell requires internal pressure calibrants which suffer from uncertainty as high as 10% at Mbar pressures. A series of experiments were performed to test the reliability and agreement of pressure scales for Au, Mo, MgO, NaCl B2, Ne and Pt. These data were used to determine a new comprehensive pressure scale for use in experiments on mantle materials. The lower mantle's dominant phase is (Mg,Fe,Al)(Fe,Al,Si)O3 perovskite. At pressure-temperature conditions comparable to the deep lower mantle, perovskite undergoes a transition to a post-perovskite phase. I synthesized perovskites and post-perovskites from a series of Fe-rich (enstatite--ferrosilite, (Mg1--x,Fex)SiO 3, 0 < x < 74) and Fe,Al-rich (pyrope--almandine, (Mg1--x,Fex) 3Al2Si3O12, 0 < x < 100) compositions. These experiments have shown that as much as 75% FeSiO 3 is soluble in perovskite at 70--80 GPa. Fe was observed to lower and broaden the pressure range of the post-perovskite transition. Volume data were collected over a range of pressures for all compositions to constrain the effects of Fe and Al on the equations of state of these phases. Fe and Al incorporation were observed to increase the unit cell volume of perovskite but have a weak effect on its compressibility. The electronic behavior of Fe in

  6. Tibetan and Indian lithospheres in the upper mantle beneath Tibet: Evidence from broadband surface-wave dispersion

    NASA Astrophysics Data System (ADS)

    Agius, Matthew R.; Lebedev, Sergei

    2013-10-01

    Broadband seismic experiments over the last two decades have produced dense data coverage across Tibet. Yet, the mechanism of the India-Asia lithospheric convergence beneath it remains a puzzle, with even its basic features debated and with very different end-member models advocated today. We measured highly accurate Rayleigh- and Love-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of interstation measurements made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by extensive series of nonlinear inversions of the data, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy. Temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships. Our results reveal strong heterogeneity in the upper mantle beneath Tibet. Very large high-velocity anomalies in the upper mantle are consistent with the presence of underthrust (beneath southwestern Tibet) and subducted (beneath central and eastern Tibet) Indian lithosphere. The corresponding thermal anomalies match those estimated for subducted Indian lithosphere. In contrast to the Indian lithosphere, Tibetan lithosphere and asthenosphere display low-to-normal shear speeds; Tibetan lithosphere is thus warm and thin. Radial anisotropy in the upper mantle is weak in central and strong in northeastern Tibet, possibly reflecting asthenospheric flow above the subducting Indian lithospheric slab.

  7. Constrain the crust and upper mantle structure beneath the equatorial Eastern Pacific Rise from ambient noise and earthquake surface waves

    NASA Astrophysics Data System (ADS)

    Gao, C.; Yao, H.; Gouedard, P.; Collins, J. A.; McGuire, J. J.; van der Hilst, R.

    2012-12-01

    In this study we combine ambient noise and earthquake surface waves to jointly constrain the crust and upper mantle shear velocity structure beneath the equatorial eastern Pacific Rise using data from ocean bottom seismometers deployed in 2008. We measure the inter-station Rayleigh-wave phase velocity dispersion curves of the fundamental mode in the period band 2 - 30 s and the first higher mode in the period band 3 - 7 s from vertical component ambient noise cross-correlation functions. We also determine the inter-station phase velocity dispersion curves in the period band 20 - 100 s from an earthquake-based surface-wave two-station method. The average dispersion data from both ambient noise and earthquake surface waves are used to determine the average shear velocity structure in the crust and upper mantle using a global searching Neighborhood algorithm. Our results reveal a pronounced low velocity zone in the upper mantle with the shear wave speed as slow as 3.85 km/s beneath the equatorial eastern Pacific Rise, possibly caused by a combination of high temperature and the presence of partial melt beneath the mid-ocean ridges. We will also measure Love wave dispersion curves from transverse component ambient noise cross-correlation functions and earthquake surface waves. Together with Rayleigh-wave dispersion measurements, we will determine the radial anisotropy to constrain the deformation of the crust and upper mantle beneath the equatorial eastern Pacific Rise.

  8. Isotropic and anisotropic shear velocity model of the NA upper mantle using EarthScope data

    NASA Astrophysics Data System (ADS)

    Leiva, J.; Clouzet, P.; French, S. W.; Yuan, H.; Romanowicz, B. A.

    2013-12-01

    The EarthScope TA deployment has provided dense array coverage across the continental US and with it, the opportunity for high resolution 3D seismic velocity imaging of both lithosphere and asthenosphere in the continent. Building upon our previous work, we present a new 3D isotropic, radially and azimuthally anisotropic shear wave model of the North American (NA) lithospheric mantle, using full waveform tomography and shorter-period (40 s) waveform data. Our isotropic velocity model exhibits pronounced spatial correlation between major tectonic localities of the eastern NA continent, as evidenced in the geology, and seismic anomalies, suggesting recurring episodes of tectonic events not only are well exposed at the surface, but also leave persistent scars in the continental lithosphere mantle, marked by isotropic and radially anisotropic velocity anomalies that reach as deep as 100-150 km. In eastern North America, our Vs images distinguish the fast velocity cratonic NA from the deep rooted large volume high velocity blocks which are east of the continent rift margin and extend 200-300 km offshore into Atlantic. In between is a prominent narrow band of low velocities that roughly follows the south and eastern Laurentia rift margin and extends into New England. The lithosphere associated with this low velocity band is thinned likely due to combined effects of repeated rifting processes along the rift margin and northward extension of the Bermuda low-velocity channel across the New England region. Deep rooted high velocity blocks east of the Laurentia margin are proposed to represent the Proterozoic Gondwanian terranes of pan-African affinity, which were captured during the Rodinia formation but left behind during the opening of the Atlantic Ocean. The anisotropy model takes advantage of the up-to-date SKS compilation in the continent and new splitting results from Greenland. The new joint waveform and SKS splitting data inversion is carried out with a 2

  9. Ultra-depleted isotopic compositions in fertile asthenosphere-derived peridotites: constraints on the composition of the upper mantle

    NASA Astrophysics Data System (ADS)

    Byerly, B. L.; Lassiter, J. C.

    2012-12-01

    Recent studies of abyssal peridotites (AP) and OIB xenoliths have reported refractory, isotopically ultra-depleted domains within the convecting upper mantle with Nd- and Hf-isotope compositions that extend far beyond the MORB field. These results have important implications regarding the average composition of the depleted upper mantle and the genetic relationship between MORB and AP. However, the abundance of ultra-depleted domains in the mantle is unclear. In addition, recent melt extraction processes at mid-ocean ridges make it difficult to evaluate the compositions of ultra-depleted domains prior to exhumation and thus evaluate their role in melt generation. To better constrain the abundance and composition of typical convecting upper mantle, we examined a suite of spinel peridotite xenoliths from the central Rio Grande Rift (RGR) where most of the preexisting lithosphere has been convectively removed and replaced with depleted upper mantle. Seismic tomography indicates that the lithosphere beneath the RGR has been substantially removed (Gao, 2004), and geochemical evidence supports this. Two distinct populations of xenoliths are observed from Elephant Butte, central RGR. One population, interpreted to derive from residual Proterozoic lithospheric mantle, is refractory (bulk Al2O3 <2.3 wt.%), LREE- and LILE-enriched, has enriched Sr, Nd, and Pb isotopic compositions and along with xenoliths from the Eastern Colorado Plateau define a strong Lu/Hf-176Hf/177Hf "pseudo-isochron" with an apparent age of ~1.6 Ga. In contrast, the majority of the RGR xenoliths have fertile major element compositions (bulk Al2O3 ~ 4.0 wt %), low spinel Cr# (~10), and LREE-depleted trace element patterns, and overlap with composition estimates for the depleted mantle (Workman & Hart, 2005). We interpret these xenoliths to reflect recent replacement of the pre-existing lithosphere with material from the convecting upper mantle. The fertile xenoliths have cpx Sr-, Nd-, and Hf

  10. Melting curve of the deep mantle applied to properties of early magma ocean and actual core-mantle boundary

    NASA Astrophysics Data System (ADS)

    Andrault, Denis; Lo Nigro, Giacomo; Bolfan-Casanova, Nathalie; Bouhifd, Mohamed A.; Garbarino, Gaston; Mezouar, Mohamed

    2010-05-01

    Our planet experienced partial melting early in its history as a consequence of energy release due to accretion. Partial mantle melting could still happen today in the lowermost mantle. Occurrence of melting is primordial for the chemical segregation between the different Earth's reservoirs and for the dynamics of the whole planet. Melting of iron-alloys is relatively easy to achieve, but the silicated mantle happens to be more refractory. We investigated experimentally melting properties of two starting material, forsterite and chondritic-mantle, at pressures ranging from 25 to 140 GPa, using laser-heated diamond anvil cell coupled with synchrotron radiation. We show that partial melting in the lowermost mantle, as suggested by seismology on the basis of the ultra-low velocity zones (ULVZ), requires temperatures above 4200 K at the core-mantle boundary. At low pressures, our curve plots significantly lower than previous reports. Compared to recent estimates of mantle geotherm, while this temperature remains possible if the Earth's core is very hot, it is more likely that ULVZs correspond to high concentration of incompatible elements driven down to the D"-layer by subducting slabs or extracted out from the outer core. When our chondritic melting curve is coupled with recent isentropic temperature profiles for a magma ocean, we obtain a correlation between magma ocean depth and the potential temperature (Tp) at its surface; an ocean depth of 1000 km (equivalent to ~40 GPa) corresponds to Tp=2000 K, which happens to be significantly hotter than the estimated surface temperature of a sustained magma ocean. It emphasizes the importance of a lid at the magma ocean surface at an epoch as early as that of core-mantle segregation.

  11. The Effect of Plate Motion History on the Longevity of Deep Mantle Heterogeneities

    NASA Astrophysics Data System (ADS)

    Bull, Abigail; Domeier, Mathew; Torsvik, Trond

    2014-05-01

    Numerical studies of mantle convection have attempted to explain tomographic observations that reveal a lower mantle dominated by broad regional areas of lower-than-average shear-wave speeds beneath Africa and the Central Pacific. The anomalous regions, termed LLSVPs ("large low shear velocity provinces"), are inferred to be thermochemical structures encircled by regions of higher-than-average shear-wave speeds associated with Mesozoic and Cenozoic subduction zones. The origin and long-term evolution of the LLSVPs remains enigmatic. It has been proposed that the LLSVP beneath Africa was not present before 240 Ma, prior to which time the lower mantle was dominated by a degree-1 convection pattern with a major upwelling centred close to the present-day Pacific LLSVP and subduction concentrated mainly in the antipodal hemisphere. The African LLSVP would thus have formed during the time-frame of the supercontinent Pangea as a result of return flow in the mantle due to circum-Pacific subduction. An opposing hypothesis, which propounds a more long-term stability for both the African and Pacific LLSVPs, is suggested by recent palaeomagnetic plate motion models that propose a geographic correlation between the surface eruption sites of Phanerozoic kimberlites, major hotspots and Large Igneous Provinces to deep regions of the mantle termed "Plume Generation Zones" (PGZs), which lie at the margins of the LLSVPs. If the surface volcanism was sourced from the PGZs, such a link would suggest that both LLSVPs may have remained stationary for at least the age of the volcanics. i.e., 540 Myr. To investigate these competing hypotheses for the evolution of LLSVPs in Earth's mantle, we integrate plate tectonic histories and numerical models of mantle dynamics and perform a series of 3D spherical thermochemical convection calculations with Earth-like boundary conditions. We improve upon previous studies by employing a new, TPW-corrected global plate motion model to impose surface

  12. Seismological observations in Northwestern South America: Evidence for two subduction segments, contrasting crustal thicknesses and upper mantle flow

    NASA Astrophysics Data System (ADS)

    Yarce, Jefferson; Monsalve, Gaspar; Becker, Thorsten W.; Cardona, Agustín; Poveda, Esteban; Alvira, Daniel; Ordoñez-Carmona, Oswaldo

    2014-12-01

    The cause of tectonic deformation in northwestern South America and its link to upper mantle structure and flow are debated. We use a combination of broadband and short period travel time seismic data for P-waves to show that observations are consistent with the presence of two subduction segments in Colombia and contrasting values of crustal thickness. In Northern Colombia, at latitudes greater than 6°N, most of the seismic stations are associated with negative teleseismic travel time residuals, relative to a regional mean, suggesting that the upper mantle is seismically faster than predicted from global models. In particular, for the Caribbean coastal plains there are no signs of significant anomalies in the upper mantle, evidenced by the small magnitude of the travel time delays and subdued Pn speeds (~ 7.97 km/s). To the southeast of such plains there is an increase in magnitude of the negative travel time residuals, including the Northern Eastern Cordillera, the Perija Range and the Merida Andes. An analysis of non-isostatic residual topography, based on a model of crustal thickness in northwestern South America, is consistent with a slab-associated upper mantle flow beneath the region just east of the Bucaramanga Nest. We interpret these results to indicate the presence of a Caribbean slab, initially flat beneath the Caribbean coastal plains, and steepening sharply in the southeast, including the area of Bucaramanga. For most of the western Andean region and the Pacific coast, south of 6°N, teleseismic differential travel time residuals are predominantly positive, indicating that the upper mantle is in general seismically slower than the reference model. Beneath the Central Cordillera, just to the east of this area, the residuals become smaller and predominantly negative; residual non-isostatic topography is negative as well. These features are probably related to the effect of the Nazca subduction developing an asthenospheric wedge.

  13. Helium Isotopic Compositions of Antarctic High-Mg Rocks Related to the Karoo Continental Flood Basalts: Evidence for a Depleted Upper Mantle Source?

    NASA Astrophysics Data System (ADS)

    Heinonen, J. S.; Kurz, M. D.

    2014-12-01

    The isotopic composition of helium is often considered to be one of the key elements in resolving deep mantle plume vs. upper mantle origin of hotspot-related volcanic rocks. High 3He/4He values, greater than 10 times atmospheric (Ra), are generally thought to indicate plume-related sources in the lower mantle. The use of helium isotopes in continental flood basalt (CFB) provinces has been limited by the lack of fresh rock material, poor exposures, time-integrated ingrowth of radiogenic 4He, and strong lithospheric overprinting. Vestfjella mountain range at western Dronning Maud Land, Antarctica, is comprised of lava flows and intrusive rocks that belong to the Jurassic (~180 Ma) Karoo continental flood basalt province, the bulk of which is exposed in southern Africa. The Karoo CFBs and related rocks show strong lithospheric influence in their geochemistry in general, but some high-Mg dikes from Vestfjella show geochemical evidence of derivation from sublithospheric sources. In an attempt to determine the first estimate for the helium isotopic composition of the Karoo mantle sources, we performed He isotopic measurements on six primitive Vestfjella dike samples collected from variably exposed nunataks. Helium was extracted by in-vacuo stepwise crushing and melting of separated and carefully hand-picked olivine phenocrysts (Ø = 0.25-1 mm; ~10 000 grains in total; abraded and unabraded fractions). The results of coupled crushing and melting measurements show evidence of both cosmogenic and radiogenic helium contributions within the olivines (i.e. by having high He contents and anomalously low or high 3He/4He released by melting), which complicates interpretation of the data. As a best estimate for the mantle isotopic composition, we use the sample with the highest amount of He released (> 50%) during the first crushing step of an abraded coarse fraction, which gave 3He/4He of 7.03 ± 0.23 (2σ) Ra. This value is indistinguishable from those measured from Southwest

  14. Point defects and diffusion in the upper mantle minerals - Experimental and theoretical insights

    NASA Astrophysics Data System (ADS)

    Dohmen, Ralf

    2010-05-01

    Solid-state diffusion controls a variety of dynamic processes within the Earth, e.g., the rheological behavior of rocks as well as the element and isotopic exchange on different spatial scales. Modeling these processes requires well-defined diffusion coefficients along with appropriate physical models. In the last decade a significant increase in number and improvement of quality of diffusion data for the major mantle minerals olivine, clinopyroxene, and orthopyroxene have been achieved mainly due to improved experimental and analytical techniques. A reliable application of these data to various conditions within the Earth is ultimately linked to a basic and quantitative understanding of the diffusion mechanisms and the parameters affecting the concentration of the relevant point defects. These have to be identified by a combination of experimental studies and point defect thermodynamic models. The availability of a large body of systematic diffusion data makes olivine the mineral of choice to explore this avenue. Diffusion coefficients for monovalent (Li, H), divalent (Fe, Mg, Mn, Ni, Co, Ca, Sr), trivalent (REE, Cr) and tetravalent (Si, Hf) cations as well as O are known typically as a function of temperature, and often as a function of other variables such as pressure, oxygen fugacity or water fugacity. The large amount of experimental data can be reproduced using a quantitative point defect model, which explicitly considers the various minor and trace elements in olivine (Dohmen and Chakraborty, 2007). This method was further developed to consider also H related defects and this provides now a parameterized equation to predict Fe-Mg diffusion (and potentially also Ca, Mn, Ni, etc.) in olivine over the whole range of conditions in the Earth's upper mantle. The approach is perfectly general and can be extended to any other mineral provided enough data are available. Much less is known about diffusion mechanisms and point defects in other major mantle minerals

  15. Lithosphere structure and upper mantle characteristics below the Bay of Bengal

    NASA Astrophysics Data System (ADS)

    Rao, G. Srinivasa; Radhakrishna, M.; Sreejith, K. M.; Krishna, K. S.; Bull, J. M.

    2016-07-01

    The oceanic lithosphere in the Bay of Bengal (BOB) formed 80-120 Ma following the breakup of eastern Gondwanaland. Since its formation, it has been affected by the emplacement of two long N-S trending linear aseismic ridges (85°E and Ninetyeast) and by the loading of ca. 20-km of sediments of the Bengal Fan. Here, we present the results of a combined spatial and spectral domain analysis of residual geoid, bathymetry and gravity data constrained by seismic reflection and refraction data. Self-consistent geoid and gravity modelling defined by temperature-dependent mantle densities along a N-S transect in the BOB region revealed that the depth to the lithosphere-asthenosphere boundary (LAB) deepens steeply from 77 km in the south to 127 km in north, with the greater thickness being anomalously thick compared to the lithosphere of similar-age beneath the Pacific Ocean. The Geoid-Topography Ratio (GTR) analysis of the 85°E and Ninetyeast ridges indicate that they are compensated at shallow depths. Effective elastic thickness (Te) estimates obtained through admittance/ coherence analysis as well as the flexural modelling along these ridges led to the conclusions: (i) 85°E Ridge was emplaced in off-ridge environment (Te = 10-15 km); (ii) the higher Te values of ˜25 km over the Afanasy Nikitin Seamount (ANS) reflect the secondary emplacement of the seamount peaks in off-ridge environment, (iii) that the emplacement of the Ninetyeast Ridge north of 2°N occurred in an off-ridge environment as indicated by higher Te values (25-30 km). Furthermore, the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths >800 km are compensated by processes related to upper mantle convection.

  16. Upper mantle structure beneath the Alpine orogen from high-resolution teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Lippitsch, Regina; Kissling, Edi; Ansorge, JöRg

    2003-08-01

    To understand the evolution of the Alpine orogen, knowledge of the actual structure of the lithosphere-asthenosphere system is important. We perform high-resolution teleseismic tomography with manually picked P wave arrival times from seismograms recorded in the greater Alpine region. The resulting data set consists of 4199 relative P wave arrivals and 499 absolute P wave arrivals from 76 teleseismic events, corrected for the contribution of the Alpine crust to the travel times. The three-dimensional (3-D) crustal model established from controlled-source seismology data for that purpose represents the large-scale Alpine crustal structure. Absolute P wave arrival times are used to compute an initial reference model for the inversion. Tests with synthetic data document that the combination of nonlinear inversion, high-quality teleseismic data, and usage of an a priori 3-D crustal model allows a reliable resolution of cells at 50 km × 50 km × 30 km. Hence structures as small as two cells can be resolved in the upper mantle. Our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km. Along strike of the Alps, the inversion reveals a high-velocity structure that dips toward the SE beneath the Adriatic microplate in the western and central Alps. In the eastern Alps we observe a northeastward dipping feature, subducting beneath the European plate. We interpret this feature in the western and central Alps as subducted, mainly continental European lower lithosphere. For the east, we propose that parts of the Vardar oceanic basin were subducted toward the NE, forcing continental Adriatic lower lithosphere to subduct northeastward beneath the European plate.

  17. The Mono Arch, eastern Sierra region, California: Dynamic topography associated with upper-mantle upwelling?

    USGS Publications Warehouse

    Jayko, A.S.

    2009-01-01

    A broad, topographic flexure localized east of and over the central and southern Sierra Nevada, herein named the Mono Arch, apparently represents crustal response to lithospheric and/or upper-mantle processes, probably dominated by mantle upwelling within the continental interior associated Pacific-North American plate-boundary deformation. This zone of flexure is identified through comparison between the topographic characteristics of the active Cascade volcanic arc and backarc regions with the analogous former arc and backarc in the Sierra Nevada and eastern Sierra Nevada. Serial topographic profiles measured normal to the modern Cascade backarc reveal an accordance of topographic lows defined by valley floors with an average minimum elevation of ???1400-1500m for over 175km to the southeast. Although the accordance drops in elevation slightly to the south, the modern Cascade backarc region is remarkably level, and is characterized by relief up to ???750m above this baseline elevation. By contrast, serial topographic profiles over the former arc and backarc transitions of the eastern Sierra region exhibit a regional anticlinal warping defined by accordant valley floors and by a late Miocene-early Pliocene erosion surface and associated deposits. The amplitude of this flexure above regionally flat baseline elevations to the east varies spatially along the length of the former Sierran arc, with a maximum of ???1000m centred over the Bridgeport Basin. The total zone of flexure is approximately 350km long N-S and 100km wide E-W, and extends from Indian Wells Valley in the south to the Sonora Pass region in the north. Previous geophysical, petrologic, and geodetic studies suggest that the Mono Arch overlies a zone of active mantle upwelling. This region also represents a zone crustal weakness formerly exploited by the middle-to-late Miocene arc and is presently the locus of seismic and volcanic activities. This seismic zone, which lies east of the Sierra Nevada block

  18. Rheologic effects of crystal preferred orientation in upper mantle flow near plate boundaries

    NASA Astrophysics Data System (ADS)

    Blackman, Donna; Castelnau, Olivier; Dawson, Paul; Boyce, Donald

    2016-04-01

    Observations of anisotropy provide insight into upper mantle processes. Flow-induced mineral alignment provides a link between mantle deformation patterns and seismic anisotropy. Our study focuses on the rheologic effects of crystal preferred orientation (CPO), which develops during mantle flow, in order to assess whether corresponding anisotropic viscosity could significantly impact the pattern of flow. We employ a coupled nonlinear numerical method to link CPO and the flow model via a local viscosity tensor field that quantifies the stress/strain-rate response of a textured mineral aggregate. For a given flow field, the CPO is computed along streamlines using a self-consistent texture model and is then used to update the viscosity tensor field. The new viscosity tensor field defines the local properties for the next flow computation. This iteration produces a coupled nonlinear model for which seismic signatures can be predicted. Results thus far confirm that CPO can impact flow pattern by altering rheology in directionally-dependent ways, particularly in regions of high flow gradient. Multiple iterations run for an initial, linear stress/strain-rate case (power law exponent n=1) converge to a flow field and CPO distribution that are modestly different from the reference, scalar viscosity case. Upwelling rates directly below the spreading axis are slightly reduced and flow is focused somewhat toward the axis. Predicted seismic anisotropy differences are modest. P-wave anisotropy is a few percent greater in the flow 'corner', near the spreading axis, below the lithosphere and extending 40-100 km off axis. Predicted S-wave splitting differences would be below seafloor measurement limits. Calculations with non-linear stress/strain-rate relation, which is more realistic for olivine, indicate that effects are stronger than for the linear case. For n=2-3, the distribution and strength of CPO for the first iteration are greater than for n=1, although the fast seismic

  19. The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

    NASA Astrophysics Data System (ADS)

    Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

    2016-04-01

    The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S

  20. 3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle

    NASA Astrophysics Data System (ADS)

    Yuan, Huaiyu; Romanowicz, Barbara; Fischer, Karen M.; Abt, David

    2011-03-01

    Using a combination of long period seismic waveforms and SKS splitting measurements, we have developed a 3-D upper-mantle model (SAWum_NA2) of North America that includes isotropic shear velocity, with a lateral resolution of ˜250 km, as well as radial and azimuthal anisotropy, with a lateral resolution of ˜500 km. Combining these results, we infer several key features of lithosphere and asthenosphere structure. A rapid change from thin (˜70-80 km) lithosphere in the western United States (WUS) to thick lithosphere (˜200 km) in the central, cratonic part of the continent closely follows the Rocky Mountain Front (RMF). Changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere, corresponding to the fast-to-slow discontinuity found in receiver functions. Below the lithosphere, azimuthal anisotropy manifests a maximum, stronger in the WUS than under the craton, and the fast axis of anisotropy aligns with the absolute plate motion, as described in the hotspot reference frame (HS3-NUVEL 1A). In the WUS, this zone is confined between 70 and 150 km, decreasing in strength with depth from the top, from the RMF to the San Andreas Fault system and the Juan de Fuca/Gorda ridges. This result suggests that shear associated with lithosphere-asthenosphere coupling dominates mantle deformation down to this depth in the western part of the continent. The depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study, although it is peaked around 270 km, a robust result. Radial anisotropy is such that, predominantly, ξ > 1, where ξ= (Vsh/Vsv)2, under the continent and its borders down to ˜200 km, with stronger ξ in the bordering oceanic regions. Across the continent and below 200 km, alternating zones of weaker and stronger radial anisotropy, with predominantly ξ < 1, correlate with zones of small lateral changes in the fast axis direction of

  1. Spin crossover and iron-rich silicate melt in the Earth's deep mantle (Invited)

    NASA Astrophysics Data System (ADS)

    Hirose, K.; Nomura, R.; Ozawa, H.; Tateno, S.; Hernlund, J. W.

    2010-12-01

    spin crossover of iron in silicate melt. These results imply that (Mg,Fe)SiO3 liquid becomes more dense than coexisting solid at ~1800-km depth in the lower mantle. Soon after Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ~1,000 km in thickness. We also infer that (Mg,Fe)SiO3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved toward a Fe-rich/Si-poor composition, consistent with seismic inferences of structures in the CMB region.

  2. Upper Mantle Structure Beneath the Galápagos Archipelago From Body Wave Data

    NASA Astrophysics Data System (ADS)

    Toomey, D. R.; Hooft Toomey, E. E.; Hooft Toomey, E. E.; Solomon, S. C.; James, D. E.; Hall, M. L.

    2001-12-01

    We report on the initial results from an ongoing broadband seismic experiment conducted in the Galápagos Islands. The Galápagos offer an advantageous site for a reconnaissance seismic study of an oceanic hotspot, because the islands cover nearly the entire bathymetric swell or surface signature of the presumed underlying plume. In September 1999 we deployed 10 three-component broadband seismometers, which augment an existing GSN site and a telemetered array of high-frequency seismometers. The combined seismic networks record data suitable for imaging upper mantle structure, constraining crustal thickness variations, and characterizing local seismic activity associated with tectonic and volcanic processes. The resulting elliptically shaped array is 300 km by 200 km in aperture (east-west/north-south, respectively); stations are spaced 50-70 km apart. A primary goal of the experiment is to define the first-order characteristics of the upper mantle structure beneath the Galápagos hotspot in order to ascertain if the hotspot is associated with a plume-like feature in the upper mantle. To do so, we are inverting the delay times of teleseismic P and S body waves. The Galápagos hotspot is well situated with respect to regional and teleseismic earthquake sources, and the region displays remarkably low seismic noise, as it is located in the doldrums with calm winds (<5 m/s) over 60% of the time (90% of the time from February through April). Examination of our data and plots of magnitude versus epicentral distance indicates that teleseismic earthquakes with mb>5.5 and regional earthquakes with mb>4.2 provide useful data. P waves often show excellent signal-to-noise ratios in the frequency band 0.3 to 2 Hz. These phases can be picked, using waveform alignment, to an accuracy of 20 ms. The spectral content of the larger events is broad, with teleseismic earthquakes generating signals at frequencies up to 5 Hz, as well as at lower frequencies (< 0.1 Hz). The S wave data are

  3. Upper mantle structure of the Congo Craton and the East African Rift from full wave ambient noise tomography

    NASA Astrophysics Data System (ADS)

    Emry, E.; Shen, Y.; Nyblade, A.; Bao, X.; Flinders, A. F.

    2015-12-01

    The relationship between lithospheric structure, mantle flow, and continental rifting along the East African Rift is the subject of ongoing discussion. The upper mantle beneath the Main Ethiopian Rift and the East African Rift farther south has been seismically imaged following the deployment of several temporary regional arrays. However, due to uneven distribution of seismic arrays, key questions regarding a connection between these upper mantle anomalies at the Turkana Depression and the effect that the thick Congo Craton has on diverting upwelling material towards the East African Rift are poorly resolved. We use overlapping records from several temporary and permanent broadband seismic arrays (1980-2014) located throughout the African continent and surrounding regions in order to image the upper mantle beneath the East African Rift and the Congo Craton where regional seismic arrays have not been deployed. We do this by seismic ambient noise tomography using the recently developed frequency-time normalization (FTN) method to extract empirical Green's functions (EGFs) at periods of 7-250 seconds. We cross correlate the normalized continuous records and stack them to obtain EGFs for each temporally coincident station-station pair. We simulate wave propagation through a spherical Earth using a finite-difference method, measure phase delay times between synthetics and EGFs, and invert them for velocity perturbations with 3D Rayleigh wave sensitivity kernels. We will present results from full-wave ambient noise inversions that illuminate upper mantle structure throughout the continent, with particular focus on the Congo Craton and northern sections of the East African Rift System.

  4. Large-scale shear velocity structure of the upper mantle beneath Europe and surrounding regions

    NASA Astrophysics Data System (ADS)

    Legendre, C. P.; Meier, T. M.; Lebedev, S.; Friederich, W.

    2009-12-01

    The automated multimode waveform inversion technique developed by Lebedev et al. (2005) was applied to available data of broadband stations in Europe and surrounding regions. It performs a fitting of the complete waveform starting from the S-wave onset to the surface wave. Assuming the location and focal mechanism of a considered earthquake as known, the first basic step is to consider each available seismogram separately and to find the 1D-model that can explain the filtered seismogram best. In a second step, each 1D-model serves as a linear constraint in an inversion for a 3D S-wave velocity model of the upper mantle. We collected data for the years from 1990 to 2006 from all permanent stations for which data were available via the data centers of ORFEUS, GEOFON amd IRIS, and from others that build the Virtual European Seismological Network (VEBSN). In addition, we incorporated data from temporary experiments like SVEKALAPKO, TOR and the Eifel plume project as well as permanent stations in France. Just recently we were also able to add the data recorded by the temporary broadband EGELADOS network in the southern Aegean. In this way, a huge data set of about 500000 seismograms came about from which about 60000 1D-models could be constructed. The resulting models exhibit an overwhelming structural detail in relation to the size of the region considered in the inversion. They are to our knowledge the most detailed models of shear wave velocity currently available for the European upper mantle and surroundings. Most prominent features are an extremely sharp demarcation of the East European platform from Western Europe. Narrow high velocity regions follow the Hellenic arc and the Ionian trench toward the north. Whereas high velocities are found beneath the western Alps between about 100 km to 200 km depth, the eastern Alps show a low velocity anomaly at these depths. Low velocity zones are found at depths around 150 km in the Pannonian basin, the back-arc of the

  5. Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Janutyte, I.; Majdanski, M.; Voss, P. H.; Kozlovskaya, E.; Passeq Working Group

    2014-07-01

    The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ) which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment PASSEQ 2006-2008 within the study area from Germany to Lithuania was used to compile the dataset of manually picked 6008 top quality arrivals of P waves from teleseismic earthquakes. We used the non-linear teleseismic tomography algorithm TELINV to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% compared to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger Western Europe. We find that the seismic lithosphere-asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift the negative anomalies are observed from the depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ the asthenosphere is at a depth of 150-180 km, which is an intermediate value between that of the EEC and Western Europe. The results imply that the seismic LAB in the northern part of the TESZ is of a shape of a ramp dipping to the NE direction. In the southern part of the TESZ the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the

  6. Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Janutyte, I.; Majdanski, M.; Voss, P. H.; Kozlovskaya, E.; Passeq Working Group

    2015-01-01

    The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ), which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment (PASSEQ) 2006-2008 within the study area from Germany to Lithuania was used to compile the data set of manually picked 6008 top-quality arrivals of P waves from teleseismic earthquakes. We used the TELINV nonlinear teleseismic tomography algorithm to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% with respect to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger western Europe. We find that the seismic lithosphere-asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift, the negative anomalies are observed from a depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ, the asthenosphere is at a depth of 150-180 km, which is an intermediate value between that of the EEC and western Europe. The results imply that the seismic LAB in the northern part of the TESZ is in the shape of a ramp dipping to the northeasterly direction. In the southern part of the TESZ, the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and

  7. Mantle transition zone structure beneath the Changbai volcano: Insight into deep slab dehydration and hot upwelling near the 410 km discontinuity

    NASA Astrophysics Data System (ADS)

    Tian, You; Zhu, Hongxiang; Zhao, Dapeng; Liu, Cai; Feng, Xuan; Liu, Ting; Ma, Jincheng

    2016-08-01

    We study the detailed mantle transition zone structure beneath the active Changbai intraplate volcano in Northeast China by using a receiver-function method. A total of 3005 teleseismic receiver functions recorded by 70 broadband stations are obtained by using a common-conversion-point stacking method. For conducting the time-to-depth conversion, we use a three-dimensional velocity model of the study region so as to take into account the influence of structural heterogeneities. Our results reveal significant depth variations of the 410, 520, and 660 km discontinuities. A broad depression of the 410 km discontinuity and a low-velocity anomaly are revealed beneath the Changbai volcano, which may reflect a large-scale hot mantle upwelling around the 410 km discontinuity with a positive Clapeyron slope. The 520 km discontinuity is identified clearly, and its uplift occurs above the stagnant Pacific slab. We also find a prominent depression of the 660 km discontinuity, which is elongated along the trend of deep earthquake clusters in a range of 39°N-44°N latitude, and the depression area has a lateral extent of about 400 km. Because the 520 and 660 km discontinuities correspond to positive and negative Clapeyron slopes, respectively, we think that the 520 uplift and the 660 depression are caused by the cold subducting Pacific slab. A part of the Pacific slab may have penetrated into the lower mantle and so caused the large-scale 660 depression in front of the deep earthquake clusters. Our results also reveal a part of the upper boundary of the subducting Pacific slab in the mantle transition zone.

  8. Experimental petrology of peridotites, including effects of water and carbon on melting in the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Green, David H.

    2015-02-01

    For over 50 years, the use of high-pressure piston/cylinder apparatus combined with an increasing diversity of microbeam analytical techniques has enabled the study of mantle peridotite compositions and of magmas derived by melting in the upper mantle. The experimental studies have been guided by the petrology and geochemistry of peridotites from diverse settings and by the remarkable range of mantle-derived magma types. Recent experimental study using FTIR spectroscopy to monitor water content of minerals has shown that fertile lherzolite (MORB-source upper mantle) at ~1,000 °C can store ~200 ppm H2O in defect sites in nominally anhydrous minerals (olivine, pyroxenes, garnet and spinel). Water in excess of 200 ppm stabilizes amphibole (pargasite) at P < 3 GPa up to the lherzolite solidus. However, at P > 3 GPa, water in excess of 200 ppm appears as an aqueous vapour phase and this depresses the temperature of the upper mantle solidus. Provided the uppermost mantle (lithosphere) has H2O < 4,000 ppm, the mantle solidus has a distinctive P, T shape. The temperature of the vapour- undersaturated or dehydration solidus is approximately constant at 1,100 °C at pressures up to ~3 GPa and then decreases sharply to ~1,010 °C. The strongly negative d T/d P of the vapour-undersaturated solidus of fertile lherzolite from 2.8 to 3 GPa provides the basis for understanding the lithosphere/asthenosphere boundary. Through upward migration of near-solidus hydrous silicate melt, the asthenosphere becomes geochemically zoned with the `enriched' intraplate basalt source (>500 ppm H2O) overlying the `depleted' MORB source (~200 ppm H2O). From the study of primitive MOR picrites, the modern mantle potential temperature for MORB petrogenesis is ~1,430 °C. The intersection of the 1,430 °C adiabat with the vapour-saturated lherzolite solidus at ~230 km suggests that upwelling beneath mid-ocean ridges begins around this depth. In intraplate volcanism, diapiric upwelling begins from

  9. [Upper extremity deep vein thrombosis following ovarian stimulation].

    PubMed

    Bar-On, Shikma; Cohen, Aviad; Levin, Ishai; Avni, Amiran; Lessing, Joseph B; Atmog, Benny

    2011-11-01

    Upper extremity deep venous thrombosis (UEDVT) is uncommon as a spontaneous event in the general population and is associated with well-defined risk factors. Thromboembotic events are serious, but fortunately rare, complications following ovarian stimulation for IVF. A review of the Literature indicates that thromboembolic events after ovarian stimulation are usually associated with ovarian hyperstimulation syndrome (OHSS). The incidence of UEDVT is higher in women undergoing assisted reproductive technology (ART) compared to the general population. The incidence of this condition is estimated to be 0.08%-0.11% of treatment cycles. While lower extremity DVT may be considered a natural consequence of OHSS, given the diminished venous return secondary to enlarged ovaries and ascites, it is unclear why there appears to be a predilection for thrombi in the upper extremities in women undergoing ART. Early diagnosis and treatment is crucial for both maternal and fetal well-being. Since infertility treatment is becoming commonplace in today's society, women undergoing treatment and their clinicians should be better informed of the presentation and clinical course of UEDVT to enable early diagnosis and start treatment. Consideration must be given to screening patients at risk for OHSS for thrombophilias, as well as administrating prophylactic anticoagulation therapy to patients who develop OHSS.

  10. Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Sheehan, Anne Francis

    1991-01-01

    Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

  11. The upper mantle beneath the Cascade Range: A comparison with the Gulf of California

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1984-01-01

    Seismograms from 22 earthquakes along the northeast Pacific rim recorded in southern California form the data set for investigation of the upper mantle beneath the Cascade Range-Juan de Fuca region, a transitional area encompassing both very young ocean floor and a continental margin. These data consist of 853 seismograms (6 deg delta 42 deg) which produce 1068 travel times and 40 ray parameter estimates. These data are compared directly to another large suite of records representative of structure beneath the Gulf of California, an active spreading center. The spreading center model, GCA, was used as a starting point in WKBJ synthetic seismogram modeling and perturb GCA until the northeast Pacific data are matched. Application of wave field continuation to these two groups of data provides checks on model's consistency with the data as well as an estimate of the resolvability of differences between the two areas. Differences between the models derived from these two data sets are interpretable in terms of lateral structural variation beneath the two regimes.

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

    NASA Astrophysics Data System (ADS)

    Bonnin, Mickael; Nolet, Guust; Thomas, Christine; Villaseñor, Antonio; Gallart, Josep; Levander, Alan

    2013-04-01

    In this study we take advantage of the dense broadband-station networks available in western Mediterranean region (IberArray, PICASSO and MOROCCO-MUENSTER networks) to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone. This model is based on teleseismic arrival times recorded between 2008 and 2012 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. Such a tomography is required to scrutinize the nature and extent of the thermal anomalies inferred beneath Northern Africa, especially in the Atlas ranges region and associated to sparse volcanic activities. Tomography is notably needed to help in determining the hypothetical connection between those hot anomalies and the Canary Island hotspot as proposed by geochemistry studies. It also provides new insights on the geometry of the subducting slab previously inferred from tomography, GPS measurements or shear-wave splitting patterns beneath the Alboran Sea and the Betic ranges and is indispensable for deciphering the complex geodynamic history of the Western Mediterranean region. We shall present the overall statistics of the delays, their geographical distribution, as well as the first inversion results.

  13. Upper mantle structure of the southern Arabian margin: insights from teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Leroy, Sylvie; Keir, Derek; Ahmed, Abdulhakim; Boschi, Lapo; Rolandone, Frédérique; Stuart, Graham W.; Khanbari, Khaled; El Hussain, Issa

    2015-04-01

    We image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted continental margin with 59 broadband stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. We use teleseismic tomography to compute a relative P wave velocity model in eastern Yemen and southern Oman down to 400 km depth. Our model shows low velocity anomalies located in the vicinity of five major fracture zones and regions of recent volcanism. These low velocity anomalies are likely caused by localized asthenospheric upwelling and partial melting, caused by small-scale convection promoted by gradients in LAB (lithosphere-asthenosphere boundary) topography near the fracture zones. In addition, low velocities underlie regions of elevated topography in between major sedimentary basins. We suggest locally buoyant mantle creates uplift and dynamic topography on the rift mar- gin that impacts the course of seasonal rivers and the sedimentation at the mouth of those rivers. Our new P wave velocity model suggests that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones.

  14. Titanium-hydroxyl defect-controlled rheology of the Earth's upper mantle

    NASA Astrophysics Data System (ADS)

    Faul, Ulrich H.; Cline, Christopher J.; David, Emmanuel C.; Berry, Andrew J.; Jackson, Ian

    2016-10-01

    Experiments were conducted with hydrous olivine to investigate the defect responsible for the influence of water (hydrogen structurally incorporated as hydroxyl) on the olivine rheology. Solution-gelation derived Fo90 olivine doped with nominally 0.04-0.1 wt.% TiO2 was first hot-pressed and then deformed in platinum capsules at 300 MPa confining pressure and temperatures from 1200- 1350°C. The water content was not buffered so that deformation occurred at water-undersaturated conditions. Due to the enhanced grain growth under hydrous conditions, the samples were at least a factor of three more coarse-grained than their dry counterparts and deformed in powerlaw creep at differential stresses as low as a few tens of MPa. Since all experiments were conducted at the same confining pressure, the essentially linear relationship between strain rate and water content was for the first time determined independently of an activation volume. Infrared spectra are dominated by absorption bands at 3572 and 3525 cm-1. These bands also predominate in infrared spectra of natural olivine, and can only be reproduced experimentally in the presence of titanium. In contrast to the previous interpretation of the hydrous rheology in terms of intrinsic point defects, the experiments show that extrinsic defects (impurities) in natural olivine play the dominant role for water weakening at the water contents expected for most of the upper mantle.

  15. Tomographic Imaging of Upper Mantle P- and S-wave Velocity Heterogeneity Beneath the Arabian Peninsula

    SciTech Connect

    Park, Y; Nyblade, A; Rodgers, A; Al-Amri, A

    2005-08-30

    We report the estimates of three-dimensional P- and S-wave velocity structure beneath the Arabian Peninsula estimated from travel time delay tomography. We have completed travel time measurements and inversion of a partial data set provided by King Abdulaziz City for Science and Technology (KACST). This study builds on previous work by Benoit et al. (2003) following the methods of VanDecar and Crosson (1990) and VanDecar (1991). Data were collected from the Saudi Arabian National Digital Seismic Network (SANDSN) operated by KACST. The network consists of 38 stations (27 broadband and 11 short-period). We augmented the KACST data with delay times measured from permanent Incorporated Research Institutions for Seismology (IRIS) stations in the region (RAYN, EIL and MRNI) and the 1996 Saudi Arabian PASSCAL Experiment. This study shows the inverted P- and S-wave models computed with the combined data with all three different seismic networks (KASCST, IRIS, and the 1996 Saudi Arabian PASSCAL experiment) with best coverage beneath the Arabian Shield. Tomographic images reveal low velocity features in the upper mantle along a north-south line from the southern Asir region to the northeastern portion of the Arabian Shield.

  16. Upper Mantle Structure Beneath The Arabian Peninsula From body and Surface Wave Tomography

    NASA Astrophysics Data System (ADS)

    Park, Y.; Nyblade, A.; Rodgers, A.; Al-Amri, A.

    2006-05-01

    We have imaged tomographically the three-dimensional velocity structure of the upper mantle beneath the Arabian Peninsula using teleseismic P- and S-waves and Rayleigh wave phase velocities. The data came from the Saudi Arabian National Digital Seismic Network (SANDSN) operated by King Abdulaziz City for Science and Technology (KACST: 21 broadband stations and 4 short-period stations). We augmented the KACST data with delay times measured from permanent Incorporated Research Institutions for Seismology (IRIS) stations in the region (RAYN, EIL and MRNI) and the 1996 Saudi Arabian PASSCAL Experiment (9 broadband stations). The P- and S wave models were inverted from 401 earthquakes resulting in 3416 ray paths with P- and PKP-wave arrivals, and 1602 ray paths with S- and SKS-wave arrivals came from 201 earthquakes, respectively. The P and S wave models yield consistent results. The models show strong low velocity regions beneath the southeastern Arabian Shield and the mid-eastern edge of Arabian Shield. The low velocity anomaly in the southeastern part of the Arabian Shield does not extend north of 21°N and dips to south. It likely represents the northeastern edge of the Afar hotspot. Surface wave tomography is being performed using fundamental mode Rayleigh wave phase velocities measured across the SANDSN. Preliminary phase velocity maps will be provided and compared to the body wave tomographic results.

  17. VP and VS structure of the Yellowstone hot spot from teleseismic tomography: Evidence for an upper mantle plume

    USGS Publications Warehouse

    Waite, Gregory P.; Smith, Robert B.; Allen, Richard M.

    2006-01-01

    The movement of the lithosphere over a stationary mantle magmatic source, often thought to be a mantle plume, explains key features of the 16 Ma Yellowstone–Snake River Plain volcanic system. However, the seismic signature of a Yellowstone plume has remained elusive because of the lack of adequate data. We employ new teleseismic P and S wave traveltime data to develop tomographic images of the Yellowstone hot spot upper mantle. The teleseismic data were recorded with two temporary seismograph arrays deployed in a 500 km by 600 km area centered on Yellowstone. Additional data from nearby regional seismic networks were incorporated into the data set. The VP and VS models reveal a strong low-velocity anomaly from ∼50 to 200 km directly beneath the Yellowstone caldera and eastern Snake River Plain, as has been imaged in previous studies. Peak anomalies are −2.3% for VP and −5.5% for VS. A weaker, anomaly with a velocity perturbation of up to −1.0% VP and −2.5% VS continues to at least 400 km depth. This anomaly dips 30° from vertical, west-northwest to a location beneath the northern Rocky Mountains. We interpret the low-velocity body as a plume of upwelling hot, and possibly wet rock, from the mantle transition zone that promotes small-scale convection in the upper ∼200 km of the mantle and long-lived volcanism. A high-velocity anomaly, 1.2%VP and 1.9% VS, is located at ∼100 to 250 km depth southeast of Yellowstone and may represent a downwelling of colder, denser mantle material.

  18. The lithosphere-asthenosphere and crust-mantle boundaries in the region of the Upper Rhine Graben as seen by S-wave receiver functions

    NASA Astrophysics Data System (ADS)

    Ritter, J. R. R.; Seiberlich, C.; Wawerzinek, B.

    2012-04-01

    The Upper Rhine Graben is a branch of the European Cenozoic Rift System and is characterised by a clear rift structure which stretches more than 300 km from Basel to Frankfurt. Since 2004 we study the deep structure of the Upper Rhine Graben within the TIMO project, using the mobile seismic broadband stations of the KArlsruhe BroadBand Array (KABBA). The data are complemented with recordings from permanent stations (BFO, ECH, STU, TNS and WLF). Here we present the results from shear wave receiver function (S-RF) modelling. S-RF are waveforms which should contain only S-to-P converted phases which were generated at seismic discontinuities inside the Earth. The stacked S-RF contain clear signals from the crust-mantle boundary (Moho) under the study region. After a depth migration the Moho topography varies between 25 km and 28 km underneath the Upper Rhine Graben region; within the error limits of 5 km there is no difference between the graben itself and its shoulders. In the southern part of the graben there is an indication for a thinning of the crust to about 23 km. After the Moho signals there is a second phase with opposite polarity in the S-RF. We interpret this signal as conversion from the lithosphere-asthenosphere boundary (LAB). A depth migration results in LAB depths of 70-80 km under the Upper Rhine Graben; the graben itself does not show a specific anomaly. The most shallow LAB depths are found in the region of the Eifel (about 60 km), where a small mantle plume is active.

  19. Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle

    NASA Astrophysics Data System (ADS)

    Shimizu, Kei; Saal, Alberto E.; Myers, Corinne E.; Nagle, Ashley N.; Hauri, Erik H.; Forsyth, Donald W.; Kamenetsky, Vadim S.; Niu, Yaoling

    2016-03-01

    We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65% to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in

  20. Crustal and upper mantle structure beneath south-western margin of the Arabian Peninsula from teleseismic tomography

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Basuyau, Clémence; Leroy, Sylvie; Tiberi, Christel; Ahmed, Abdulhakim; Stuart, Graham W.; Keir, Derek; Rolandone, Frédérique; Ganad, Ismail; Khanbari, Khaled; Boschi, Lapo

    2014-07-01

    image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south-western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low-velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off-axis volcanic events (from 15 Ma to present). This low-velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small-scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred.

  1. The characteristics of lower crust and upper mantle in the Cima volcanic field deduced from xenolith studies

    NASA Astrophysics Data System (ADS)

    Cardon, K. P.; Anthony, E.

    2015-12-01

    A lithospheric model based on mineral chemistry, textures, and temperatures is used to interpret the seismic structure of the upper mantle and lower crust observed under the Cima Volcanic Field, CA. Seismic velocities calculated from xenolith compositions are used in conjunction with petrologic information to interpret geophysical models of the area. The lower crust is composed of mafic compositions and contains a high percentage of quenched partial melt. The combination of quenched partial melt and mafic composition explains the relatively low seismic velocities observed in seismic models. The mafic composition is consistent with a rift environment. Melt compositions, some with > 60 wt% SiO2 are found in all types of Cima xenoliths, although pyroxenites and gabbros contain the largest amount. Pyroxenite from the uppermost mantle transitions into gabbroic compositions and plagioclase rich lithologies in the crust. Temperatures calculated for peridotite xenoliths range from ~ 950 to 1030˚ C. Plagioclase bearing samples have the lowest temperatures and are interpreted as residing in the immediate sub-Moho mantle. Plagioclase bearing lherzolite structurally overlies spinel bearing peridotite. Strain accumulation is most prevalent in plagioclase bearing peridotite and virtually absent from pyroxenites and gabbros. Seismic velocities calculated for peridotite xenoliths are faster than pyroxenite and gabbroic samples. Despite the chemical heterogeneity and complex history of the Moho transitional are most mantle is composed dominantly by peridotite. Very little lithosphere, rhelologically speaking, remains under the volcanic field. We interpret lithospheric dismemberment to be caused by hot mantle working northward from the Gulf of California.

  2. Crust and upper mantle structure associated with extension in the Woodlark Rift, Papua New Guinea from Rayleigh-wave tomography

    NASA Astrophysics Data System (ADS)

    Jin, Ge; Gaherty, James B.; Abers, Geoffery A.; Kim, Younghee; Eilon, Zachary; Buck, W. Roger

    2015-11-01

    The Woodlark seafloor spreading center is propagating westward into the Australian plate near the D'Entrecasteaux Islands (DI), Papua New Guinea, generating an active transition zone from continental rifting to seafloor spreading. From March 2010 to July 2011, we deployed 31 on-shore and 8 offshore broadband seismic stations around the DI region, to explore the dynamic processes of the lithosphere extension and the exhumation of the high-pressure terranes exposed on those islands. We measure the multiband (10-60 s) Rayleigh-wave phase velocities from both ambient noise and earthquake signals. These measurements are then inverted for a three-dimensional shear-velocity model for the crust and upper mantle. The results indicate that the lithosphere extension is localized near the rift axis beneath the DI, with a shear-velocity structure in the upper mantle that is similar to mid-ocean ridges. Beneath the Kiribisi Basin west of DI, an ultraslow shear-velocity anomaly (˜4.0 km/s) is observed at shallow mantle depth (30-60 km), which can be interpreted either by the presence of excess partial melt due to slow melt extraction, or by the existence of felsic crustal material buried to mantle depth and not yet exhumed.

  3. Messengers from the deep: Fossil wadsleyite-chromite microstructures from the Mantle Transition Zone

    NASA Astrophysics Data System (ADS)

    Satsukawa, Takako; Griffin, William L.; Piazolo, Sandra; O'Reilly, Suzanne Y.

    2015-11-01

    Investigations of the Mantle Transition Zone (MTZ; 410-660 km deep) by deformation experiments and geophysical methods suggest that the MTZ has distinct rheological properties, but their exact cause is still unclear due to the lack of natural samples. Here we present the first direct evidence for crystal-plastic deformation by dislocation creep in the MTZ using a chromitite from the Luobusa peridotite (E. Tibet). Chromite grains show exsolution of diopside and SiO2, suggesting previous equilibration in the MTZ. Electron backscattered diffraction (EBSD) analysis reveals that olivine grains co-existing with exsolved phases inside chromite grains and occurring on chromite grain boundaries have a single pronounced crystallographic preferred orientation (CPO). This suggests that olivine preserves the CPO of a high-pressure polymorph (wadsleyite) before the high-pressure polymorph of chromite began to invert and exsolve. Chromite also shows a significant CPO. Thus, the fine-grained high-pressure phases were deformed by dislocation creep in the MTZ. Grain growth in inverted chromite produced an equilibrated microstructure during exhumation to the surface, masking at first sight its MTZ deformation history. These unique observations provide a window into the deep Earth, and constraints for interpreting geophysical signals and their geodynamic implications in a geologically robust context.

  4. Messengers from the deep: Fossil wadsleyite-chromite microstructures from the Mantle Transition Zone.

    PubMed

    Satsukawa, Takako; Griffin, William L; Piazolo, Sandra; O'Reilly, Suzanne Y

    2015-11-13

    Investigations of the Mantle Transition Zone (MTZ; 410-660 km deep) by deformation experiments and geophysical methods suggest that the MTZ has distinct rheological properties, but their exact cause is still unclear due to the lack of natural samples. Here we present the first direct evidence for crystal-plastic deformation by dislocation creep in the MTZ using a chromitite from the Luobusa peridotite (E. Tibet). Chromite grains show exsolution of diopside and SiO2, suggesting previous equilibration in the MTZ. Electron backscattered diffraction (EBSD) analysis reveals that olivine grains co-existing with exsolved phases inside chromite grains and occurring on chromite grain boundaries have a single pronounced crystallographic preferred orientation (CPO). This suggests that olivine preserves the CPO of a high-pressure polymorph (wadsleyite) before the high-pressure polymorph of chromite began to invert and exsolve. Chromite also shows a significant CPO. Thus, the fine-grained high-pressure phases were deformed by dislocation creep in the MTZ. Grain growth in inverted chromite produced an equilibrated microstructure during exhumation to the surface, masking at first sight its MTZ deformation history. These unique observations provide a window into the deep Earth, and constraints for interpreting geophysical signals and their geodynamic implications in a geologically robust context.

  5. Messengers from the deep: Fossil wadsleyite-chromite microstructures from the Mantle Transition Zone

    PubMed Central

    Satsukawa, Takako; Griffin, William L.; Piazolo, Sandra; O’Reilly, Suzanne Y.

    2015-01-01

    Investigations of the Mantle Transition Zone (MTZ; 410–660 km deep) by deformation experiments and geophysical methods suggest that the MTZ has distinct rheological properties, but their exact cause is still unclear due to the lack of natural samples. Here we present the first direct evidence for crystal-plastic deformation by dislocation creep in the MTZ using a chromitite from the Luobusa peridotite (E. Tibet). Chromite grains show exsolution of diopside and SiO2, suggesting previous equilibration in the MTZ. Electron backscattered diffraction (EBSD) analysis reveals that olivine grains co-existing with exsolved phases inside chromite grains and occurring on chromite grain boundaries have a single pronounced crystallographic preferred orientation (CPO). This suggests that olivine preserves the CPO of a high-pressure polymorph (wadsleyite) before the high-pressure polymorph of chromite began to invert and exsolve. Chromite also shows a significant CPO. Thus, the fine-grained high-pressure phases were deformed by dislocation creep in the MTZ. Grain growth in inverted chromite produced an equilibrated microstructure during exhumation to the surface, masking at first sight its MTZ deformation history. These unique observations provide a window into the deep Earth, and constraints for interpreting geophysical signals and their geodynamic implications in a geologically robust context. PMID:26563583

  6. From the Surface Topography to the Upper Mantle, Seismic constraints on the Crustal structure Across Morocco

    NASA Astrophysics Data System (ADS)

    Carbonell, Ramon; Díaz, Jordi; Gallart, Josep; Gil, Alba; Ayarza, Puy; Palomeras, Immaculada; Levander, Alan; Marti, David; Harnafi, Mimoun

    2015-04-01

    the root zone and is less than 35 km elsewhere. Approaching the Rif, the controlled source data reveals a prominent crustal root over 50 km depth which is located where the topography does not exceed 1400 m. These features indicate that complex structure and processes beneath the crust play a key role in supporting the particular geometry of the surface topography of this part of the western Mediterranean. On one hand the Atlas is being supported by the mantle, on the other the abrupt change in crustal thickness at the Nekkor fault and the deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African Neo-Tethys passive margin.

  7. Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone

    USGS Publications Warehouse

    Benz, H.M.; McCarthy, J.

    1994-01-01

    A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a Pn velocity of 7.95 km s−1, suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

  8. Upper-mantle shear-wave structure under East and Southeast Asia from Automated Multimode Inversion of waveforms

    NASA Astrophysics Data System (ADS)

    Legendre, C. P.; Zhao, L.; Chen, Q.-F.

    2015-10-01

    We present a new Sv-velocity model of the upper mantle under East and Southeast Asia constrained by the inversion of seismic waveforms recorded by broad-band stations. Seismograms from earthquakes occurred between 1977 and 2012 are collected from about 4786 permanent and temporary stations in the region whenever and wherever available. Automated Multimode Inversion of surface and multiple-S waveforms is applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations are then solved for the seismic velocity perturbations in the crust and upper mantle with respect to a three-dimensional (3-D) reference model and a realistic crust. Major features of the lithosphere-asthenosphere system in East and Southeast Asia are identified in the resulting model. At lithospheric depth, low velocities can be seen beneath Tibet, whereas high velocities are found beneath cratonic regions, such as the Siberian, North China, Yangtze,) Tarim, and Dharwarand cratons. A number of microplates are mapped and the interaction with neighbouring plates is discussed. Slabs from the Pacific and Indian Oceans can be seen in the upper mantle. Passive marginal basins and subduction zones are also properly resolved.

  9. Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan (Guinea)

    NASA Astrophysics Data System (ADS)

    Palot, M.; Pearson, D. G.; Stern, R. A.; Stachel, T.; Harris, J. W.

    2014-08-01

    Sublithospheric diamonds that sample the transition zone and uppermost lower mantle provide a unique view into the deep Earth. In order to investigate the origin of diamond-forming C-H-O-N fluids within the deep mantle, within the framework of the terrestrial deep volatile cycle, we conducted a δ13C-δ15N-[N] micro-analytical study, by secondary ion mass spectrometry, of five Kankan diamonds from the asthenosphere/transition zone and the lower mantle. Abrupt and large changes in δ13C within KK-99 (up to 10.2‰) and KK-200A (up to 6.9‰) illustrate distinct episodes of diamond growth, involving different fluids, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. Despite limited variability of δ13C within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic δ13C-δ15N-[N] co-variations which can be modelled as a single diamond growth episode in a Rayleigh process from fluids/melts. These data constrain the carbon isotopic fractionation factors to be both negative (ΔC = -0.9‰ for KK-200B and -2.0‰ for both KK-203 and -207) and positive (ΔC = +1.0‰ for KK-204), consistent with equilibrium between diamond and oxidised (CO2 or carbonate) and reduced (CH4 or carbide) fluids respectively. The modelling of δ15N-[N] systematics suggests that the diamonds are depleted by ∼4‰ (KK-200B) and ∼0‰ (KK-204) relative to the oxidised and reduced sources respectively. Modelling the co-variation indicates a compatible behaviour of nitrogen in diamond relative to the growth medium (KN = 4-16), independent of the redox state. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (δ13C-δ15N-[N]-KN-ΔC-ΔN) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism of diamond genesis. The metaperidotitic parageneses and the slightly negative δ13C signatures for both KK-204 and KK-207 are consistent with

  10. Long Term Seismic Observation in Mariana by OBSs : Double Seismic Zone and Upper Mantle Structure

    NASA Astrophysics Data System (ADS)

    Shiobara, H.; Sugioka, H.; Mochizuki, K.; Oki, S.; Kanazawa, T.; Fukao, Y.; Suyehiro, K.

    2005-12-01

    In order to obtain the deep arc structural image of Mariana, a large-scale seismic observation by using 58 long-term ocean bottom seismometers (LTOBS) had been performed from June 2003 until April 2004, which is a part of the MARGINS program funded by the NSF. Prior to this observation, a pilot long-term seismic array observation was conducted in the same area by using 10 LTOBSs from Oct. 2001 until Feb. 2003. At that time, 8 LTOBSs were recovered but one had no data. Recently, 2 LTOBSs, had troubles in the releasing, were recovered by the manned submersible (Shinkai 6500, Jamstec) for the research of the malfunction in July 2005. By using all 9 LTOBS's data, those are about 11 months long, hypocenter determination was performed and more than 3000 local events were found. Even with the 1D velocity structure based on the iasp91 model, double seismic zones and a systematic shift of epicenters between the PDE and this study were observed. To investigate the detail of hypocenter distribution and the 3D velocity structure, the DD inversion (tomoDD: Zhang and Thurber, 2003) was applied for this data set with the 1D structure initial model except for the crust, which has been surveyed by using a dense airgun-OBS system (Takahashi et al., 2003). The result of relocated hypocenters shows clear double seismic zones until about 200 km depth, a high activity area around the fore-arc serpentine sea-mount, the Big Blue, and a lined focuses along the current ridge axis in the back-arc basin, and the result of the tomography shows a image of subducting slab and a low-Vs region below the same sea-mount mentioned. The wedge mantle structure was not clearly resolved due to the inadequate source-receiver coverage, which will be done in the recent experiment.