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Sample records for afar mantle plume

  1. Comparing the composition of the earliest basalts erupted by the Iceland and Afar mantle plumes.

    NASA Astrophysics Data System (ADS)

    Stuart, Finlay M.

    2013-04-01

    The first basalts erupted by mantle plumes are typically generated by mantle melting at temperatures 200-300°C higher than average ambient mantle. This is consistent with the derivation of from a thermal boundary layer at the core-mantle boundary. Mantle plume temperatures decrease with time, likely as large plume heads give way to thin plume conduits. Consequently the early, hot plume basalts are a window into the deep mantle. At it's simplest they provide a test of whether the discrete plume source regions are primordial mantle that have been isolated since soon after Earth accretion, or have substantial contributions from subducted slabs. Here I present new isotopic and trace element determinations of the earliest picritic basalts from the ~30 Ma Afar plume in Ethiopia. They will be compared with similar material from the ~60 Ma proto-Iceland plume (PIP) in an effort to test prevailing models regarding the source of mantle plumes. The extremely primordial nature of the helium in the PIP picrites (3He/4He ~ 50 Ra) contrasts with much lower values of the Ethiopian flood basalt province (~21 Ra). The Iceland plume 3He/4He has decreased (linearly) with time, mirroring the secular cooling of the Iceland mantle plume identified by decreasing MgO and FeO in primary melts. In 60 million years the Iceland plume 3He/4He is still higher than the maximum Afar plume value. The Sr-Nd-Pb isotopic composition of the high 3He/4He Ethiopian flood basalt province picrites are remarkably homogenous (e.g. 87Sr/86Sr = 0.70396-0.70412; 206Pb/204Pb = 18.82-19.01). In comparison the PIP picrites have ranges that span nearly the global range of E-MORB and N-MORB. The Afar and proto-Iceland mantle plumes are clearly not initiated in a single deep mantle domain with the same depletion/enrichment and degassing histories, and the same scale of heterogeneity. This implies that there is more than one plume source region/mechanism that is capable of generating comparable volumes of basalt melt

  2. The mantle transition zone beneath the Afar Depression and adjacent regions: Implications for mantle plumes and hydration

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.

    2016-03-01

    The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broadband seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14,500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper mantle low-velocity zone (LVZ) beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and is interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

  3. The mantle transition zone beneath the Afar Depression and adjacent regions: implications for mantle plumes and hydration

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Gao, S. S.; Liu, K. H.; Yu, Y.

    2016-06-01

    The Afar Depression and its adjacent areas are underlain by an upper mantle marked by some of the world's largest negative velocity anomalies, which are frequently attributed to the thermal influences of a lower-mantle plume. In spite of numerous studies, however, the existence of a plume beneath the area remains enigmatic, partially due to inadequate quantities of broad-band seismic data and the limited vertical resolution at the mantle transition zone (MTZ) depth of the techniques employed by previous investigations. In this study, we use an unprecedented quantity (over 14 500) of P-to-S receiver functions (RFs) recorded by 139 stations from 12 networks to image the 410 and 660 km discontinuities and map the spatial variation of the thickness of the MTZ. Non-linear stacking of the RFs under a 1-D velocity model shows robust P-to-S conversions from both discontinuities, and their apparent depths indicate the presence of an upper-mantle low-velocity zone beneath the entire study area. The Afar Depression and the northern Main Ethiopian Rift are characterized by an apparent 40-60 km depression of both MTZ discontinuities and a normal MTZ thickness. The simplest and most probable interpretation of these observations is that the apparent depressions are solely caused by velocity perturbations in the upper mantle and not by deeper processes causing temperature or hydration anomalies within the MTZ. Thickening of the MTZ on the order of 15 km beneath the southern Arabian Plate, southern Red Sea and western Gulf of Aden, which comprise the southward extension of the Afro-Arabian Dome, could reflect long-term hydration of the MTZ. A 20 km thinning of the MTZ beneath the western Ethiopian Plateau is observed and interpreted as evidence for a possible mantle plume stem originating from the lower mantle.

  4. Seismically imaging the Afar plume

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Bastow, I. D.; Stuart, G. W.; Keir, D.; Ayele, A.; Ogubazghi, G.; Ebinger, C. J.; Belachew, M.

    2011-12-01

    Plume related flood basalt volcanism in Ethiopia has long been cited to have instigated continental breakup in northeast Africa. However, to date seismic images of the mantle beneath the region have not produced conclusive evidence of a plume-like structure. As a result the nature and even existence of a plume in the region and its role in rift initiation and continental rupture are debated. Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie northeast Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 6 other regional experiments and global network stations across Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S- wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that the top 100km is dominated by focussed low velocity zones, likely associated with melt in the lithosphere/uppermost asthenosphere. Below these depths a broad SW-NE oriented sheet like upwelling extends down to the top of the transition zone. Within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. The nature of the transition zone anomalies suggests that small upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. Our images of secondary upwellings

  5. Which mantle below the active rift segments in Afar?

    NASA Astrophysics Data System (ADS)

    Pik, Raphael; Stab, Martin; Ancellin, Marie-Anne; Sarah, Medynski; Cloquet, Christophe; Vye-Brown, Charlotte; Ayalew, Dereje; Chazot, Gilles; Bellahsen, Nicolas; Leroy, Sylvie

    2014-05-01

    The evolution of mantle sources beneath the Ethiopian volcanic province has long been discussed and debated with a long-lived controversy in identifying mantle reservoirs and locating them in the mantle. One interpretation of the isotopic composition of erupted lavas considers that the Afar mantle plume composition is best expressed by recent lavas from Afar and Gulf of Aden (e.g. Erta Ale, Manda Inakir and the 45°E torus anomaly on the Gulf of Aden) implying that all other volcanics (including other active segments and the initial flood basalt province) result from mixing of this plume component with additional lithospheric and asthenospheric components. A completely opposite view considers that the initial Oligocene continental flood basalts best represent the isotopic composition of the Afar mantle plume, which is subsequently mixed in various proportions with continental lithospheric mantle for generating some of the specific signature of Miocene and Quaternary volcanics. The precise and correct identification of mantle components involved in the generation of magmas is of particular importance because this is the only way to document the participation of mantle during extension and its potential role in break-up processes. In this contribution we provide new isotopic data for central Afar and we revisit the whole data set of the Ethiopian volcanic province in order to: (i) precisely identify the distinct mantle components implicated and (ii) discuss their location and evolution not only considering geochemical mixings, but also taking into account additional characteristics of erupted magmatic suites (volumes, location and relationships with amount of extension and segmentation). This new interpretation of geochemical data allows reconsidering the evolution of mantle in the course of rift evolution. In terms of mantle sources, two populations of active segments are frontally opposed in the volcanic province: those that share exactly the same composition with

  6. Plume- Ridge Lithospheric Interactions: Cases of Afar (Africa)

    NASA Astrophysics Data System (ADS)

    Montagner, J. P.; Stutzmann, E.; Sicilia, D.; Sebai, A.; Beucler, E.; Silveira, G.; Cara, M.; Debayle, E.; Leveque, J. J.

    2003-04-01

    Detection of mantle plumes in geophysical and geochemical data is controversial and trigger vigorous debates. It remains unclear how plumes are formed, their origin at depth, and whether they act independently from plate tectonics. We may learn about the role of plumes in mantle dynamics by studying their interactions with lithosphere and crust below ridges and the way in which they perturb the flow pattern in the uppermost mantle. Several regional tomographic studies of seismic velocity and anisotropy around several hotspots were obtained during the last 2 years. Their lateral resolution is smaller than 1000km and they enable to make qualitative intercomparison between Afar (Horn of Africa Program), Azores (COSEA project) in the Atlantic, La Reunion in the Indian Ocean and Pacific provinces hotspots. These models demonstrate that there is not only one family of plumes but several ones. Some plumes are confined in the uppermost 200km but a few can originate in the transition zone and even at the Core-mantle Boundary for superplumes. Seismic anisotropy which is a good marker of deformation processes and mantle flow pattern, shows that the interaction between a plume and a ridge below the lithosphere can occur over distances larger than 1000km, via sublithospheric channels. The existence of LACs (Low Anisotropy Channels) below the Pacific plate seems to be intimately related to the active hotspots in Central Pacific and indicate a future reorganization of plate boundaries. Another important consequence of the interaction between plume and ridge is the triggering of secondary convection in the asthenosphere, which will be discussed during the presentation.

  7. The mantle sources beneath the Afar volcanic province and their interplay with extension

    NASA Astrophysics Data System (ADS)

    Pik, Raphael; Stab, Martin; Ancellin, Marie-Anne; Medynski, Sarah; Cloquet, Christophe; Ayalew, Dereje; Yirgu, Gezahegn; Chazot, Gilles; Vye-Brown, Charlotte; Bellahsen, Nicolas; Leroy, Sylvie

    2015-04-01

    The evolution of mantle sources beneath the Ethiopian volcanic province has long been discussed and debated with a long-lived controversy in identifying mantle reservoirs and locating them in the mantle. One interpretation of the isotopic composition of erupted lavas considers that the Afar mantle plume composition is best expressed by recent lavas from Afar and Gulf of Aden (e.g. Erta Ale, Manda Inakir and the 45°E torus anomaly on the Gulf of Aden) implying that all other volcanics (including other active segments and the initial flood basalt province) result from mixing of this plume component with additional lithospheric and asthenospheric components. A completely opposite view considers that the initial Oligocene continental flood basalts best represent the isotopic composition of the Afar mantle plume, which is subsequently mixed in various proportions with continental lithospheric mantle for generating some of the specific signature of Miocene and Quaternary volcanics. The precise and correct identification of mantle components involved in the generation of magmas is of particular importance because this is the only way to document the participation of mantle during extension and its potential role in break-up processes. In this contribution we provide new isotopic data for central Afar and we revisit the whole data set of the Ethiopian volcanic province and African/Arabian intraplate volcanics in order to: (i) precisely identify the distinct mantle components implicated, (ii) discuss their location and evolution in space and time, and (3) link the evolution of mantle with extensional processes beneath the Afar province. This new interpretation of geochemical data allows reconsidering the evolution of mantle in the course of rift evolution. In terms of mantle sources, two populations of active segments are frontally opposed in the volcanic province: those that share exactly the same composition with plume related CFBs (e.g. the Manda Hararo and the Main

  8. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

    2013-02-01

    The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin

  9. Upper mantle temperature and the onset of extension and break-up in Afar, Africa

    NASA Astrophysics Data System (ADS)

    Armitage, John J.; Ferguson, David J.; Goes, Saskia; Hammond, James O. S.; Calais, Eric; Rychert, Catherine A.; Harmon, Nicholas

    2015-05-01

    It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere-asthenosphere seismic structure can be reconciled with observations. The rare-earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450 °C, rifting started around 22-23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting.

  10. Evidence for mantle plumes?

    PubMed

    Anderson, Don L; Natland, James H

    2007-11-22

    Geophysical hotspots have been attributed to partially molten asthenosphere, fertile blobs, small-scale convection and upwellings driven by core heat. Most are short-lived or too close together to be deeply seated, and do not have anomalous heat flow or temperature; many are related to tectonic features. Bourdon et al. investigate the dynamics of mantle plumes from uranium-series geochemistry and interpret their results as evidence for thermal plumes. Here we show why alternative mechanisms of upwelling and melting should be considered. PMID:18033248

  11. Mantle convection in the Middle East: Reconciling Afar upwelling, Arabia indentation and Aegean trench rollback

    NASA Astrophysics Data System (ADS)

    Faccenna, Claudio; Becker, Thorsten W.; Jolivet, Laurent; Keskin, Mehmet

    2013-08-01

    The Middle East region represents a key site within the Tethyan domain where continental break-up, collision, backarc extension and escape tectonics are kinematically linked together. We perform global mantle circulation computations to test the role of slab pull and mantle upwellings as driving forces for the kinematics of the Arabia-Anatolia-Aegean (AAA) system, evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models. Model results are compared with geodetically inferred crustal motions, residual topography, and shear wave splitting measurements. The AAA velocity field with respect to Eurasia shows an anti-clockwise toroidal pattern, with increasing velocities toward the Aegean trench. The best match to these crustal motions can be obtained by combining the effect of slab pull exerted in the Aegean with a mantle upwelling underneath Afar and, more generally, with the large-scale flow associated with a whole mantle, Tethyan convection cell. Neogene volcanism for AAA is widespread, not only in the extensional or subduction settings, but also within plates, such as in Syria-Jordan-Israel and in Turkey, with geochemical fingerprints similar of those of the Afar lava. In addition, morphological features show large uplifting domains far from plate boundaries. We speculate that the tectonic evolution of AAA is related to the progressive northward entrainment of upwelling mantle material, which is itself associated with the establishment of the downwelling part of a convection cell through the segmented Tethyan slab below the northern Zagros and Bitlis collision zone. The recently established westward flow dragged Anatolia and pushed the Aegean slab south-westward, thus accelerating backarc extension. Our model reconciles Afar plume volcanism, the collision in the Bitlis mountains and northern Zagros, and the rapid increase of Aegean trench rollback in a single coherent frame of large scale mantle

  12. Multiple mantle upwellings through the transition zone beneath the Afar Depression?

    NASA Astrophysics Data System (ADS)

    Hammond, J. O.; Kendall, J. M.; Stuart, G. W.; Thompson, D. A.; Ebinger, C. J.; Keir, D.; Ayele, A.; Goitom, B.; Ogubazghi, G.

    2012-12-01

    Previous seismic studies using regional deployments of sensors in East-Africa show that low seismic velocities underlie Africa, but their resolution is limited to the top 200-300km of the Earth. Thus, the connection between the low velocities in the uppermost mantle and those imaged in global studies in the lower mantle is unclear. We have combined new data from Afar, Ethiopia with 7 other regional experiments and global network stations across Kenya, Ethiopia, Eritrea, Djibouti and Yemen, to produce high-resolution models of upper mantle P- and S-wave velocities to the base of the transition zone. Relative travel time tomographic inversions show that within the transition zone two focussed sharp-sided low velocity regions exist: one beneath the Western Ethiopian plateau outside the rift valley, and the other beneath the Afar depression. Estimates of transition zone thickness suggest that this is unlikely to be an artefact of mantle discontinuity topography as a transition zone of normal thickness underlies the majority of Afar and surrounding regions. However, a low velocity layer is evident directly above the 410 discontinuity, co-incident with some of the lowest seismic velocities suggesting that smearing of a strong low velocity layer of limited depth extent may contribute to the tomographic models in north-east Afar. The combination of seismic constraints suggests that small low temperature (<50K) upwellings may rise from a broader low velocity plume-like feature in the lower mantle. This interpretation is supported by numerical and analogue experiments that suggest the 660km phase change and viscosity jump may impede flow from the lower to upper mantle creating a thermal boundary layer at the base of the transition zone. This allows smaller, secondary upwellings to initiate and rise to the surface. These, combined with possible evidence of melt above the 410 discontinuity can explain the seismic velocity models. Our images of secondary upwellings suggest that

  13. Seismic Imaging of Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Nataf, Henri-Claude

    The mantle plume hypothesis was proposed thirty years ago by Jason Morgan to explain hotspot volcanoes such as Hawaii. A thermal diapir (or plume) rises from the thermal boundary layer at the base of the mantle and produces a chain of volcanoes as a plate moves on top of it. The idea is very attractive, but direct evidence for actual plumes is weak, and many questions remain unanswered. With the great improvement of seismic imagery in the past ten years, new prospects have arisen. Mantle plumes are expected to be rather narrow, and their detection by seismic techniques requires specific developments as well as dedicated field experiments. Regional travel-time tomography has provided good evidence for plumes in the upper mantle beneath a few hotspots (Yellowstone, Massif Central, Iceland). Beneath Hawaii and Iceland, the plume can be detected in the transition zone because it deflects the seismic discontinuities at 410 and 660 km depths. In the lower mantle, plumes are very difficult to detect, so specific methods have been worked out for this purpose. There are hints of a plume beneath the weak Bowie hotspot, as well as intriguing observations for Hawaii. Beneath Iceland, high-resolution tomography has just revealed a wide and meandering plume-like structure extending from the core-mantle boundary up to the surface. Among the many phenomena that seem to take place in the lowermost mantle (or D''), there are also signs there of the presence of plumes. In this article I review the main results obtained so far from these studies and discuss their implications for plume dynamics. Seismic imaging of mantle plumes is still in its infancy but should soon become a turbulent teenager.

  14. The He isotope composition of the earliest picrites erupted by the Ethiopia plume, implications for mantle plume source

    NASA Astrophysics Data System (ADS)

    Stuart, Finlay; Rogers, Nick; Davies, Marc

    2016-04-01

    The earliest basalts erupted by mantle plumes are Mg-rich, and typically derived from mantle with higher potential temperature than those derived from the convecting upper mantle at mid-ocean ridges and ocean islands. The chemistry and isotopic composition of picrites from CFB provide constraints on the composition of deep Earth and thus the origin and differentiation history. We report new He-Sr-Nd-Pb isotopic composition of the picrites from the Ethiopian flood basalt province from the Dilb (Chinese Road) section. They are characterized by high Fe and Ti contents for MgO = 10-22 wt. % implying that the parent magma was derived from a high temperature low melt fraction, most probably from the Afar plume head. The picrite 3He/4He does not exceed 21 Ra, and there is a negative correlation with MgO, the highest 3He/4He corresponding to MgO = 15.4 wt. %. Age-corrected 87Sr/86Sr (0.70392-0.70408) and 143Nd/144Nd (0.512912-0.512987) display little variation and are distinct from MORB and OIB. Age-corrected Pb isotopes display a significant range (e.g. 206Pb/204Pb = 18.70-19.04) and plot above the NHRL. These values contrast with estimates of the modern Afar mantle plume which has lower 3He/4He and Sr, Nd and Pb isotope ratios that are more comparable with typical OIB. These results imply either interaction between melts derived from the Afar mantle plume and a lithospheric component, or that the original Afar mantle plume had a rather unique radiogenic isotope composition. Regardless of the details of the origins of this unusual signal, our observations place a minimum 3He/4He value of 21 Ra for the Afar mantle plume, significantly greater than the present day value of 16 Ra, implying a significant reduction over 30 Myr. In addition the Afar source was less degassed than convecting mantle but more degassed than mantle sampled by the proto-Iceland plume (3He/4He ~50 Ra). This suggests that the largest mantle plumes are not sourced in a single deep mantle domain with a

  15. The geochemical and Sr-Nd-Pb-He isotopic characterization of the mantle source of Rungwe Volcanic Province: comparison with the Afar mantle domain

    NASA Astrophysics Data System (ADS)

    Castillo, P. R.; Hilton, D. R.; Halldorsson, S. A.; Wang, R.

    2012-12-01

    The ultimate source of heat and magmatism associated with continental rifting in the East African Rift System (EARS) is generally viewed to be the African Superplume, but there is continuing debate on the surface expression of this large anomalous feature, which originates in the lower mantle. Previous studies have demonstrated an insignificant role for crustal contamination thereby identifying a single mantle plume signature in Quaternary basalts from the Main Ethiopian Rift in the northern EARS. This is designated to be the Afar plume and is characterized by, e.g., 3He/4He >15 RA, 206Pb/204Pb = 19.5 and 87Sr/86Sr = 0.7035 [Rooney et al., J. Pet. 53, 2012]. In contrast, the signature of plume(s) in the southern EARS is less constrained. Rogers et al. [EPSL 176, 2000] proposed a plume in the sub-lithospheric Kenyan mantle with characteristically lower 43Nd/144Nd than the Afar plume whereas Furman [JAES 48, 2007] advocated a high μ [HIMU] plume based primarily on the high 206Pb/204Pb ratios of lavas in all areas within and south of the Turkana Depression: both models assume a 3He/4He lower than the Afar plume. Here we report the trace element and Sr-Nd-Pb isotopic composition of basaltic lavas from the Rungwe Volcanic Province (RVP) in the southern extreme of the Western Rift previously identified as a high 3He/4He locality (~15 RA; [Hilton et al., GRL 38, 2011]). Trace element analyses are within the previously reported range of lava compositions that include a relatively large lithospheric component. More importantly, we identify correlations among incompatible trace element and isotopic ratios (e.g., 3He/4He vs 206Pb/204Pb, Rb/Sr, Nb/Ta; 87Sr/86Sr vs 208Pb/204Pb). Our new results suggest the presence of a distinct, high 3He/4He mantle source beneath RVP that is more radiogenic (e.g., 206Pb/204Pb up to ~19.8; 87Sr/86Sr up to 0.7055) than the Afar mantle plume. There is also very little or no HIMU signature in RPV basalts based on their high Sr and low Nd isotopic

  16. Revisiting Hotspots and Mantle Plumes: Some Phenomenology

    NASA Astrophysics Data System (ADS)

    King, S. D.; White-Gaynor, A. L.

    2012-12-01

    Sleep (1990) used gravity, topography and heat flow from 37 hotspots to ``constrain the mechanism for swell uplift and to obtain fluxes and excess temperatures of mantle plumes,'' complementing a previous analysis by Davies (1988). We repeat that analysis for the same 37 hotspots using gravity from EGM2008 and topography from ETOPO1 (Amante and Eakins, 2009). EGM2008 is complete to spherical harmonic degree and order 2159, or roughly 20 km spatial resolution (Pavlis et al., 2012). The vertical accuracy of ETOPO1 is on the order of 10 meters. With these new models we hope to improve the uplift and subsidence rates along all 37 hotspot tracks--one of the major limitations the previous work. For example, of the 37 hotspots considered Sleep ranked only 7 with good reliability while 14 were fair and 16 were poor. With this new information we can compare and contrast hotspots with various other groupings of hotspots based on tomographic images of mantle structure (Montelli et al, 2003), primary versus secondary hotspots (Courtillot et al., 2003) or relationship to cratonic boundaries (King, 2008). One encounters some puzzles when attempting to reconcile buoyancy fluxes with other groupings of hotspots and/or observations. For example, Coutillot et al.'s seven primary hotspots include: Afar, Easter, Hawaii, Iceland, Louisville, Réunion, and Tristan. Sleep (1990) categorized the reliability of the buoyancy flux calculated by from Afar, Hawaii, Iceland, and Réunion as good, while Tristan and Easter were fair and Louisville was poor. The calculated buoyancy fluxes from Macdonald and Marqueses (both listed as fair) are twice as large as those from Iceland, Tristan, and Réunion. While we recognize that these observations cannot uniquely constrain the origin of these anomalies, better observations should help test various hypotheses.

  17. Redox conditions for mantle plumes

    NASA Astrophysics Data System (ADS)

    Heister, L. E.; Lesher, C. E.

    2005-12-01

    The vanadium to scandium ratio (V/Sc) for basalts from mid-ocean ridge (MOR) and arc environments has been proposed as a proxy for fO2 conditions during partial melting (e.g. [1] and [2]). Contrary to barometric measurements of the fO2 of primitive lavas, the V/Sc ratio of the upper mantle at mid-ocean ridges and arcs is similar, leading previous authors to propose that the upper mantle has uniform redox potential and is well-buffered. We have attempted to broaden the applicability of the V/Sc parameter to plume-influenced localities (both oceanic and continental), where mantle heterogeneities associated with recycled sediments, mafic crust, and metasomatized mantle, whether of shallow or deep origin, exist. We find that primitive basalts from the North Atlantic Igneous Province (NAIP), Hawaii (both the Loa and Kea trends), Deccan, Columbia River, and Siberian Traps show a range of V/Sc ratios that are generally higher (average ~9) than those for MOR (average ~ 6.7) or arc (average ~7) lavas. Based on forward polybaric decompression modeling, we attribute these differences to polybaric melting and melt segregation within the garnet stability field rather than the presence of a more oxidized mantle in plume-influenced settings. Like MORB, the V/Sc ratios for plume-influenced basalts can be accounted for by an oxidation state approximately one log unit below the Ni-NiO buffer (NNO-1). Our analysis suggests that source heterogeneities have little, if any, resolvable influence on mantle redox conditions, although they have significant influence on the trace element and isotopic composition of mantle-derived melts. We suggest that variations in the redox of erupted lavas is largely a function of shallow lithospheric processes rather than intrinsic to the mantle source, regardless of tectonic setting. [1] Li and Lee (2004) EPSL, [2] Lee et al. (2005) J. of Petrology

  18. Upper mantle structure of shear-waves velocities and stratification of anisotropy in the Afar Hotspot region

    NASA Astrophysics Data System (ADS)

    Sicilia, D.; Montagner, J.-P.; Cara, M.; Stutzmann, E.; Debayle, E.; Lépine, J.-C.; Lévêque, J.-J.; Beucler, E.; Sebai, A.; Roult, G.; Ayele, A.; Sholan, J. M.

    2008-12-01

    The Afar area is one of the biggest continental hotspots active since about 30 Ma. It may be the surface expression of a mantle "plume" related to the African Superswell. Central Africa is also characterized by extensive intraplate volcanism. Around the same time (30 Ma), volcanic activity re-started in several regions of the African plate and hotspots such as Darfur, Tibesti, Hoggar and Mount Cameroon, characterized by a significant though modest volcanic production. The interactions of mantle upwelling with asthenosphere, lithosphere and crust remain unclear and seismic anisotropy might help in investigating these complex interactions. We used data from the global seismological permanent FDSN networks (GEOSCOPE, IRIS, MedNet, GEO- FON, etc.), from the temporary PASSCAL experiments in Tanzania and Saudi Arabia and a French deployment of 5 portable broadband stations surrounding the Afar Hotspot. A classical two-step tomographic inversion from surface waves performed in the Horn of Africa with selected Rayleigh wave and Love wave seismograms leads to a 3D-model of both S V velocities and azimuthal anisotropy, as well as radial SH/ SV anisotropy, with a lateral resolution of 500 km. The region is characterized by low shear-wave velocities beneath the Afar Hotspot, the Red Sea, the Gulf of Aden and East of the Tanzania Craton to 400 km depth. High velocities are present in the Eastern Arabia and the Tanzania Craton. The results of this study enable us to rule out a possible feeding of the Central Africa hotspots from the "Afar plume" above 150-200 km. The azimuthal anisotropy displays a complex pattern near the Afar Hotspot. Radial anisotropy, although poorly resolved laterally, exhibits S H slower than S V waves down to about 150 km depth, and a reverse pattern below. Both azimuthal and radial anisotropies show a stratification of anisotropy at depth, corresponding to different physical processes. These results suggest that the Afar hotspot has a different and

  19. Uppermost mantle (Pn) velocity model for the Afar region, Ethiopia: an insight into rifting processes

    NASA Astrophysics Data System (ADS)

    Stork, A. L.; Stuart, G. W.; Henderson, C. M.; Keir, D.; Hammond, J. O. S.

    2013-04-01

    The Afar Depression, Ethiopia, offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland. Using traveltime tomography and data from a temporary seismic deployment, we describe the first regional study of uppermost mantle P-wave velocities (VPn). We find two separate low VPn zones (as low as 7.2 km s-1) beneath regions of localized thinned crust in northern Afar, indicating the existence of high temperatures and, potentially, partial melt. The zones are beneath and off-axis from, contemporary crustal magma intrusions in active magmatic segments, the Dabbahu-Manda-Hararo and Erta'Ale segments. This suggests that these intrusions can be fed by off-axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting, thought to characterize segmentation of the uppermost mantle at seafloor spreading centres, are initiated during the final stages of break-up.

  20. Mantle structure beneath the Afar triple junction derived from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Gallacher, R. J.; Keir, D.; Harmon, N.; Stuart, G. W.; Leroy, S. D.; Hammond, J. O.; Kendall, J. M.; Wondem, A. A.; Gezahegn, B. G.; Ogubazghi, G.

    2013-12-01

    Continental breakup in Afar is generally magma-rich and occurs near the triple junction of the Gulf of Aden (GOA), Red Sea rift and the Main Ethiopian rift (MER). Hypotheses for the source of magmatism associated with this rifting include elevated mantle temperatures resulting from northward migration of hot African Superplume material, and also due to phases of increased decompression melting from rapid plate thinning. To evaluate these hypotheses we conducted a surface wave tomographic experiment using 571 events and 290 stations from 15 seismic networks deployed over the past 12 years. From these data we produced a 3D shear velocity model which constrains the upper 350 km of the Earth, including the lithosphere and uppermost asthenosphere where melt is produced. At 30-100 second periods, our images show a significant (~0.1 km/s) decrease in velocity from the rift flanks into the Afar depression, showing the signature of breakup between Africa and Arabia is still present throughout the mantle. Within Afar, seismic velocities are low, with particularly localised slow anomalies at the 40-second period, beneath the Asal rift (3.57 km/s), Ayelu segment of MER (3.63 km/s) and Dabbahu rift (3.63 km/s) and fast velocities on the rift flanks (3.70-3.80 km/s). These slow anomalies show localised decompression melting and intrusion beneath the rift axis of Afar. Low velocities are also present throughout the mantle beneath the northern section of the MER and in the GOA. Our results show that the mantle beneath Afar still preserves structure from rifting 30 Ma. In addition our results show that localised plate thinning beneath zones of strain focus magmatism to a narrow rift axis.

  1. Mantle plumes on Venus revisited

    NASA Technical Reports Server (NTRS)

    Kiefer, Walter S.

    1992-01-01

    The Equatorial Highlands of Venus consist of a series of quasicircular regions of high topography, rising up to about 5 km above the mean planetary radius. These highlands are strongly correlated with positive geoid anomalies, with a peak amplitude of 120 m at Atla Regio. Shield volcanism is observed at Beta, Eistla, Bell, and Atla Regiones and in the Hathor Mons-Innini Mons-Ushas Mons region of the southern hemisphere. Volcanos have also been mapped in Phoebe Regio and flood volcanism is observed in Ovda and Thetis Regiones. Extensional tectonism is also observed in Ovda and Thetis Regiones. Extensional tectonism is also observed in many of these regions. It is now widely accepted that at least Beta, Atla, Eistla, and Bell Regiones are the surface expressions of hot, rising mantel plumes. Upwelling plumes are consistent with both the volcanism and the extensional tectonism observed in these regions. The geoid anomalies and topography of these four regions show considerable variation. Peak geoid anomalies exceed 90 m at Beta and Atla, but are only 40 m at Eistla and 24 m at Bell. Similarly, the peak topography is greater at Beta and Atla than at Eistla and Bell. Such a range of values is not surprising because terrestrial hotspot swells also have a side range of geoid anomalies and topographic uplifts. Kiefer and Hager used cylindrical axisymmetric, steady-state convection calculations to show that mantle plumes can quantitatively account for both the amplitude and the shape of the long-wavelength geoid and topography at Beta and Atla. In these models, most of the topography of these highlands is due to uplift by the vertical normal stress associated with the rising plume. Additional topography may also be present due to crustal thickening by volcanism and crustal thinning by rifting. Smrekar and Phillips have also considered the geoid and topography of plumes on Venus, but they restricted themselves to considering only the geoid-topography ratio and did not

  2. Constraining the source of mantle plumes

    NASA Astrophysics Data System (ADS)

    Cagney, N.; Crameri, F.; Newsome, W. H.; Lithgow-Bertelloni, C.; Cotel, A.; Hart, S. R.; Whitehead, J. A.

    2016-02-01

    In order to link the geochemical signature of hot spot basalts to Earth's deep interior, it is first necessary to understand how plumes sample different regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the 'vortex ring bubble' is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from different depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coefficient, sc. High-sc plumes are composed of material which originated from very deep in the fluid domain, while low-sc plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coefficient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260-750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this unsampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle.

  3. Interaction of the Upper-Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Sharapov, V.; Perepechko, Y.

    2006-12-01

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

  4. The Axum-Adwa basalt-trachyte complex: a late magmatic activity at the periphery of the Afar plume

    NASA Astrophysics Data System (ADS)

    Natali, C.; Beccaluva, L.; Bianchini, G.; Siena, F.

    2013-08-01

    CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

  5. Influence of the Afar plume on the deep structure of Aden and Red Sea margins - Insight from teleseismic tomography in western Yemen

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Basuyau, Clémence; Leroy, Sylvie; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham; Rolandone, Frédérique; Ganad, Ismail Al; Khanbari, Khaled

    2013-04-01

    Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Afar plume has been largely investigated in Ethiopia to study early stages of continental break-up. Here we imaged the lithospheric structure of western continental Yemen to evaluate the role of the Afar plume on the evolution of the continental margin and its extent towards the East. A part of the YOCMAL project (YOung Conjugate MArgins Laboratory) permitted the deployment of twenty-three broadband stations in Yemen (from 2009 to 2010). Using a classical teleseismic tomography (Aki et al., 1974) on these stations together with a permanent GFZ station, we image the relative velocity variations of P-waves in the crust and lithosphere down to 300 km depth, with a maximum lateral resolution of about ~20 km. The model thus obtained shows (1) a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden (2) the presence of magmatic underplating related to seaward dipping reflectors under those two volcanic margins (3) two granitic syn-rift intrusions on the border of the great escarpment (4) a low velocity anomaly in which with evidence of partial melting, just below thick Oligocene trapps series and other 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. (5) Finally, we infer the presence of hot material under the Southwestern corner of Yemen that could be related to Miocene volcanism in Jabal an Nar.

  6. The geochemical geometry of mantle plumes

    NASA Astrophysics Data System (ADS)

    Jackson, Matthew

    2015-04-01

    Lavas erupted at oceanic hotspot volcanoes exhibit tremendous isotopic variability, which indicates that the mantle sources of the hotspots are highly heterogeneous geochemically. A key question is how the surface expression of hotspot lavas relates to the spatial distribution of the geochemical components within upwelling mantle plumes. Significant progress has been made in recent years relating the geographic distribution of geochemical heterogeneities in hotspot lavas to parallel volcanic lineaments that define the traces of oceanic hotspot tracks. For example, a well known geographic separation of parallel volcanic lineaments at Hawaii - the Loa and Kea trends - are also isotopically resolved. In addition to the Hawaiian example, clear patterns relating the geographic distribution of geochemical components along hotspot tracks are emerging from a suite of global hotspots, and these patterns suggest that geochemical heterogeneities are highly organized within upwelling mantle plume conduits. At the Samoan hotspot, the Pb-isotopic compositions measured in lavas reveal several geochemical groups, and each group corresponds to a different geographic lineament of volcanoes. Each group has a geochemical signature that relates to each of the canonical low 3He/4He mantle endmembers: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high U/Pb) and DM (depleted mantle). In Pb-isotopic space, the four geochemical groups each form an array that trends toward a common component (thus forming an "X-shape" in Pb-isotopic space). The region of isotope space where the 4 Pb-isotopic array intersect is defined by the highest 3He/4He (up to 34 Ra, or ratio to atmosphere) in the Samoan hotspot. In Pb-isotopic space, 3He/4He decreases monotonically along each of the Pb-isotopic groups away from the common region of convergence. In order to quantify the relationship between He and Pb isotopes, 3He/4He is plotted versus distance from the common component in Pb-isotopic space

  7. Rhyolites associated to Ethiopian CFB: Clues for initial rifting at the Afar plume axis

    NASA Astrophysics Data System (ADS)

    Natali, Claudio; Beccaluva, Luigi; Bianchini, Gianluca; Siena, Franca

    2011-12-01

    A comprehensive tectono-magmatic model based on new geochemical and field data is discussed in order to highlight the significance of the high-TiO 2 bimodal picrite basalt/rhyolite association in the north-eastern sector of the Ethiopian Plateau, which is considered to be the axial zone of the 30 Ma Continental Flood Basalt activity related to the Afar plume (Beccaluva et al., 2009). In this area the volcanic sequence consists of approximately 1700 m of high TiO 2 (4-6.5%) picrite basalts, covered by rhyolitic ignimbrites and lavas, with an average thickness of 300 m, which discontinuously extend over an area of nearly 13,500 km 2 (ca. 3600 km 3). Petrogenetic modelling, using rock and mineral chemical data and phase equilibria calculations by PELE and MELTS, indicates that: 1) picrite basalts could generate rhyolitic, sometimes peralkaline, residual melts with persistently high titanium contents (TiO 2 0.4-1.1%; Fluorine 0.2-0.3%; H 2O 2-3%; density ca. 2.4) corresponding to liquid fractions 9-16%; 2) closed system fractional crystallisation processes developed at 0.1-0.3 GPa pressure and 1390-750 °C temperature ranges, under QFM fO 2 conditions; 3) the highest crystallisation rate - involving 10-13% of Fe-Ti oxide removal - in the temperature range 1070-950 °C, represents a transitory (short-lived) fractionation stage, which results in the absence of erupted silica intermediate products (Daly gap). The eruption of low aspect ratio fluorine-rich rhyolitic ignimbrites and lavas capping the basic volcanics implies a rapid change from open- to closed-system tectono-magmatic conditions, which favoured the trapping of parental picrite basalts and their fractionation in upwardly zoned magma chambers. This evolution resulted from the onset of continental rifting, which was accompanied by normal faulting and block tilting, and the formation of shallow - N-S elongated - fissural chambers parallel to the future Afar Escarpment. The eruption of large volumes of rhyolitic

  8. Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept.

    PubMed

    Dannberg, Juliane; Sobolev, Stephan V

    2015-01-01

    The Earth's biggest magmatic events are believed to originate from massive melting when hot mantle plumes rising from the lowermost mantle reach the base of the lithosphere. Classical models predict large plume heads that cause kilometre-scale surface uplift, and narrow (100 km radius) plume tails that remain in the mantle after the plume head spreads below the lithosphere. However, in many cases, such uplifts and narrow plume tails are not observed. Here using numerical models, we show that the issue can be resolved if major mantle plumes contain up to 15-20% of recycled oceanic crust in a form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. We demonstrate that, despite their low buoyancy, large enough thermochemical plumes can rise through the whole mantle causing only negligible surface uplift. Their tails are bulky (>200 km radius) and remain in the upper mantle for 100 millions of years. PMID:25907970

  9. Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept

    PubMed Central

    Dannberg, Juliane; Sobolev, Stephan V.

    2015-01-01

    The Earth's biggest magmatic events are believed to originate from massive melting when hot mantle plumes rising from the lowermost mantle reach the base of the lithosphere. Classical models predict large plume heads that cause kilometre-scale surface uplift, and narrow (100 km radius) plume tails that remain in the mantle after the plume head spreads below the lithosphere. However, in many cases, such uplifts and narrow plume tails are not observed. Here using numerical models, we show that the issue can be resolved if major mantle plumes contain up to 15–20% of recycled oceanic crust in a form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. We demonstrate that, despite their low buoyancy, large enough thermochemical plumes can rise through the whole mantle causing only negligible surface uplift. Their tails are bulky (>200 km radius) and remain in the upper mantle for 100 millions of years. PMID:25907970

  10. Determining resolvability of mantle plumes with synthetic seismic modeling

    NASA Astrophysics Data System (ADS)

    Maguire, R.; Van Keken, P. E.; Ritsema, J.; Fichtner, A.; Goes, S. D. B.

    2014-12-01

    Hotspot volcanism in locations such as Hawaii and Iceland is commonly thought to be associated with plumes rising from the deep mantle. In theory these dynamic upwellings should be visible in seismic data due to their reduced seismic velocity and their effect on mantle transition zone thickness. Numerous studies have attempted to image plumes [1,2,3], but their deep mantle origin remains unclear. In addition, a debate continues as to whether lower mantle plumes are visible in the form of body wave travel time delays, or whether such delays will be erased due to wavefront healing. Here we combine geodynamic modeling of mantle plumes with synthetic seismic waveform modeling in order to quantitatively determine under what conditions mantle plumes should be seismically visible. We model compressible plumes with phase changes at 410 km and 670 km, and a viscosity reduction in the upper mantle. These plumes thin from greater than 600 km in diameter in the lower mantle, to 200 - 400 km in the upper mantle. Plume excess potential temperature is 375 K, which maps to seismic velocity reductions of 4 - 12 % in the upper mantle, and 2 - 4 % in the lower mantle. Previous work that was limited to an axisymmetric spherical geometry suggested that these plumes would not be visible in the lower mantle [4]. Here we extend this approach to full 3D spherical wave propagation modeling. Initial results using a simplified cylindrical plume conduit suggest that mantle plumes with a diameter of 1000 km or greater will retain a deep mantle seismic signature. References[1] Wolfe, Cecily J., et al. "Seismic structure of the Iceland mantle plume." Nature 385.6613 (1997): 245-247. [2] Montelli, Raffaella, et al. "Finite-frequency tomography reveals a variety of plumes in the mantle." Science 303.5656 (2004): 338-343. [3] Schmandt, Brandon, et al. "Hot mantle upwelling across the 660 beneath Yellowstone." Earth and Planetary Science Letters 331 (2012): 224-236. [4] Hwang, Yong Keun, et al

  11. Interaction of a mantle plume and a segmented mid-ocean ridge: Results from numerical modeling

    NASA Astrophysics Data System (ADS)

    Georgen, Jennifer E.

    2014-04-01

    Previous investigations have proposed that changes in lithospheric thickness across a transform fault, due to the juxtaposition of seafloor of different ages, can impede lateral dispersion of an on-ridge mantle plume. The application of this “transform damming” mechanism has been considered for several plume-ridge systems, including the Reunion hotspot and the Central Indian Ridge, the Amsterdam-St. Paul hotspot and the Southeast Indian Ridge, the Cobb hotspot and the Juan de Fuca Ridge, the Iceland hotspot and the Kolbeinsey Ridge, the Afar plume and the ridges of the Gulf of Aden, and the Marion/Crozet hotspot and the Southwest Indian Ridge. This study explores the geodynamics of the transform damming mechanism using a three-dimensional finite element numerical model. The model solves the coupled steady-state equations for conservation of mass, momentum, and energy, including thermal buoyancy and viscosity that is dependent on pressure and temperature. The plume is introduced as a circular thermal anomaly on the bottom boundary of the numerical domain. The center of the plume conduit is located directly beneath a spreading segment, at a distance of 200 km (measured in the along-axis direction) from a transform offset with length 100 km. Half-spreading rate is 0.5 cm/yr. In a series of numerical experiments, the buoyancy flux of the modeled plume is progressively increased to investigate the effects on the temperature and velocity structure of the upper mantle in the vicinity of the transform. Unlike earlier studies, which suggest that a transform always acts to decrease the along-axis extent of plume signature, these models imply that the effect of a transform on plume dispersion may be complex. Under certain ranges of plume flux modeled in this study, the region of the upper mantle undergoing along-axis flow directed away from the plume could be enhanced by the three-dimensional velocity and temperature structure associated with ridge

  12. Evidence for Little Shallow Entrainment in Starting Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Lohmann, F. C.; Phipps Morgan, J.; Hort, M.

    2005-12-01

    Basalts from intraplate or hotspot ocean islands show distinct geochemical signatures. Their diversity in composition is generally believed to result from the upwelling plume entraining shallow mantle material during ascent, while potentially also entraining other deep regions of the mantle. Here we present results from analogue laboratory experiments and numerical modelling that there is evidence for little shallow entrainment into ascending mantle plumes, i.e. most of the plume signature is inherited from the source. We conducted laboratory experiments using glucose syrup contaminated with glass beads to visualize fluid flow and origin. The plume is initiated by heating from below or by injecting hot, uncontaminated syrup. Particle movement is captured by a CCD camera. In our numerical experiments we solve the Stokes equations for a viscous fluid at infinite Prandtl number with passive tracer particles being used to track fluid flow and entrainment rates, simulating laboratory as well as mantle conditions. In both analogue experiments and numerical models we observe the classical plume structure being embedded in a `sheath' of material from the plume source region that retains little of the original temperature anomaly of the plume source. Yet, this sheath ascends in the `slipstream' of the plume at speeds close to the ascent speed of the plume head, and effectively prevents the entrainment of surrounding material into the plume head or plume tail. We find that the source region is most effectively sampled by an ascending plume and that compositional variations in the source region are preserved during plume ascent. The plume center and plume sheath combined are composed of up to 85% source material. However, there is also evidence of significant entrainment of up to 30% of surrounding material into the outer layers of the plume sheath. Entrainment rates are found to be influenced by mantle composition and structure, with the radial viscosity profile of the

  13. 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.

  14. The 2016 Case for Mantle Plumes and a Plume-Fed Asthenosphere (Augustus Love Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Morgan, Jason P.

    2016-04-01

    The process of science always returns to weighing evidence and arguments for and against a given hypothesis. As hypotheses can only be falsified, never universally proved, doubt and skepticism remain essential elements of the scientific method. In the past decade, even the hypothesis that mantle plumes exist as upwelling currents in the convecting mantle has been subject to intense scrutiny; from geochemists and geochronologists concerned that idealized plume models could not fit many details of their observations, and from seismologists concerned that mantle plumes can sometimes not be 'seen' in their increasingly high-resolution tomographic images of the mantle. In the place of mantle plumes, various locally specific and largely non-predictive hypotheses have been proposed to explain the origins of non-plate boundary volcanism at Hawaii, Samoa, etc. In my opinion, this debate has now passed from what was initially an extremely useful restorative from simply 'believing' in the idealized conventional mantle plume/hotspot scenario to becoming an active impediment to our community's ability to better understand the dynamics of the solid Earth. Having no working hypothesis at all is usually worse for making progress than having an imperfect and incomplete but partially correct one. There continues to be strong arguments and strong emerging evidence for deep mantle plumes. Furthermore, deep thermal plumes should exist in a mantle that is heated at its base, and the existence of Earth's (convective) geodynamo clearly indicates that heat flows from the core to heat the mantle's base. Here I review recent seismic evidence by French, Romanowicz, and coworkers that I feel lends strong new observational support for the existence of deep mantle plumes. I also review recent evidence consistent with the idea that secular core cooling replenishes half the mantle's heat loss through its top surface, e.g. that the present-day mantle is strongly bottom heated. Causes for

  15. The chemical structure of the Hawaiian mantle plume.

    PubMed

    Ren, Zhong-Yuan; Ingle, Stephanie; Takahashi, Eiichi; Hirano, Naoto; Hirata, Takafumi

    2005-08-11

    The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes--even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume. PMID:16100780

  16. Capture of the Canary mantle plume material by the Gibraltar arc mantle wedge during slab rollback

    NASA Astrophysics Data System (ADS)

    Mériaux, C. A.; Duarte, J. C.; Duarte, S. S.; Schellart, W. P.; Chen, Z.; Rosas, F.; Mata, J.; Terrinha, P.

    2015-06-01

    Recent evidence suggests that a portion of the Canary plume travelled northeastwards below the lithosphere of the Atlas Mountains in North Africa towards the Alboran domain and was captured ˜10 Ma ago by the Gibraltar subduction system in the Western Mediterranean. The capture would have been associated with the mantle return flow induced by the westward-retreating slab that would have dragged and trapped a portion of the plume material in the mantle wedge of the Gibraltar subduction zone. Such material eventually contaminated the subduction related volcanism in the Alboran region. In this work, we use scaled analogue models of slab-plume interaction to investigate the plausibility of the plume capture. An upper-mantle-scaled model combines a narrow (400 km) edge-fixed subduction plate with a laterally offset compositional plume. The subduction dominated by slab rollback and toroidal mantle flow is seen to increasingly impact on the plume dynamics as the area of influence of the toroidal flow cells at the surface is up to 500 × 1350 km2. While the plume head initially spreads axisymmetrically, it starts being distorted parallel to the plate in the direction of the trench as the slab trench approaches the plume edge at a separation distance of about 500 km, before getting dragged towards mantle wedge. When applied to the Canary plume-Gibraltar subduction system, our model supports the observationally based conceptual model that mantle plume material may have been dragged towards the mantle wedge by slab rollback-induced toroidal mantle flow. Using a scaling argument for the spreading of a gravity current within a channel, we also show that more than 1500 km of plume propagation in the sublithospheric Atlas corridor is dynamically plausible.

  17. Mantle plume interaction with an endothermic phase change

    NASA Technical Reports Server (NTRS)

    Schubert, Gerald; Anderson, Charles; Goldman, Peggy

    1995-01-01

    High spatial resolution numerical simulations of mantle plumes impinging from below on the endothermic phase change at 660-km depth are used to investigate the effects of latent heat release on the plume-phase change interaction. Both axisymmetric and planar upflows are considered, and the strong temperature dependence of mantle viscosity is taken into account. For plume strengths considered, a Clapeyron slope of -4 MPa/K prevents plume penetration of the phase change. Plumes readily penetrate the phase change for a Clapeyron slope of -2 MPa/K and arrive in the upper mantle considerably hotter than if they had not traversed the phase change. For the same amount of thermal drive, i.e., the same excess basal temperature, axisymmetric plumes are hotter upon reaching the upper mantle than are planar upwellings. Heating of plumes by their passage through the spinel-perovskite endothermic phase change can have important consequences for the ability of the plume to thermally thin the lithosphere and cause melting and volcanism.

  18. Mantle Plume Dynamics Constrained by Seismic Tomography and Geodynamics

    NASA Astrophysics Data System (ADS)

    Glisovic, P.; Forte, A. M.

    2012-12-01

    We construct a time-dependent, compressible mantle convection model in three-dimensional spherical geometry that is consistent with tomography-based instantaneous flow dynamics, using an updated and revised pseudo-spectral numerical method [Glisovic et al., Geophys. J. Int. 2012]. We explored the impact of two end-member surface boundary conditions, for a rigid and plate-like surface, along with geodynamically-inferred radial viscosity profiles. In each case we find that deep-mantle hot upwellings are durable and stable features in the mantle-wide convective circulation. These deeply-rooted mantle plumes show remarkable longevity over very long geological time spans (several hundred million years), mainly owing to the high viscosity in the lower mantle. Our very-long time convection simulations suggest that the deep-mantle plumes beneath the following hotspots: Pitcairn, Easter, Galapagos, Crozet, Kerguelen, Caroline and Cape Verde, are most reliably resolved in the present-day tomographic images.

  19. Transient mantle plumes: hot heads and cold stems

    NASA Astrophysics Data System (ADS)

    Touitou, F.; Davaille, A.; Brandeis, G.; Kumagai, I.; Vatteville, J.

    2012-04-01

    Recent petrological studies show evidences for secular cooling in mantle plumes: the source temperature of oceanic plateaus could be 100oC hotter than the source temperature of volcanic island chains. In terms of mantle plumes, it would mean that the temperature of the plume head is hotter than that of the plume stem. This is at odd with a model where a plume head would entrain so much ambient mantle on its journey towards the Earth's surface that it would end up being considerably colder than its narrow stem. So we revisited the problem using laboratory experiments and new visualization techniques to measure in situ simultaneously the temperature, velocity and composition fields. At time t=0, a hot instability is created by heating a patch of a given radius at constant power or constant temperature. The fluids are sugar syrups , with a strongly temperature-dependent viscosity. Rayleigh numbers were varied from 104 to 108, and viscosity ratio between 1.8 and 2000. After a stage where heat transport is by conduction only, the hot fluid gathers in a sphere and begins to rise, followed by a stem anchored on the hot patch. In all cases, temperatures in the head start with higher values than in the subsequent stem. However, the head also cools faster than the stem as they rise, so that they will eventually have the same temperature if the mantle is deep enough. However, our scaling laws predict that Earth's mantle plumes can indeed have hot heads and colder stems. Presence of chemically denser material on the bottom of the mantle would only increase this trend. Moreover, all the material sampled by partial melting in the plume head or stem would be coming from the heated area around the deep source, and very little entrainment from the ambient mantle is predicted.

  20. A mantle plume model for the Equatorial Highlands of Venus

    NASA Technical Reports Server (NTRS)

    Kiefer, Walter S.; Hager, Bradford H.

    1991-01-01

    The possibility that the Equatorial Highlands are the surface expressions of hot upwelling mantle plumes is considered via a series of mantle plume models developed using a cylindrical axisymmetric finite element code and depth-dependent Newtonian rheology. The results are scaled by assuming whole mantle convection and that Venus and the earth have similar mantle heat flows. The best model fits are for Beta and Atla. The common feature of the allowed viscosity models is that they lack a pronounced low-viscosity zone in the upper mantle. The shape of Venus's long-wavelength admittance spectrum and the slope of its geoid spectrum are also consistent with the lack of a low-viscosity zone. It is argued that the lack of an asthenosphere on Venus is due to the mantle of Venus being drier than the earth's mantle. Mantle plumes may also have contributed to the formation of some smaller highland swells, such as the Bell and Eistla regions and the Hathor/Innini/Ushas region.

  1. Seismic Migration Imaging of the Lithosphere beneath the Afar Rift System, East Africa

    NASA Astrophysics Data System (ADS)

    Lee, T. T. Y.; Chen, C. W.; Rychert, C.; Harmon, N.

    2015-12-01

    The Afar Rift system in east Africa is an ideal natural laboratory for investigating the incipient continental rifting, an essential component of plate tectonics. The Afar Rift is situated at the triple junction of three rifts, namely the southern Red Sea Rift, Gulf of Aden Rift and Main Ethiopian Rift (MER). The ongoing continental rifting at Afar transitions to seafloor spreading toward the southern Red Sea. The tectonic evolution of Afar is thought to be influenced by a mantle plume, but how the plume affects and interacts with the Afar lithosphere remains elusive. In this study, we use array seismic data to produce high-resolution migration images of the Afar lithosphere from scattered teleseismic wavefields to shed light on the lithospheric structure and associated tectonic processes. Our preliminary results indicate the presence of lithospheric seismic discontinuities with depth variation across the Afar region. Beneath the MER axis, we detect a pronounced discontinuity at 55 km depth, characterized by downward fast-to-slow velocity contrast, which appears to abruptly deepen to 75 km depth to the northern flank of MER. This discontinuity may be interpreted as the lithosphere-asthenosphere boundary. Beneath the Ethiopian Plateau, on the other hand, a dipping structure with velocity increase is identified at 70-90 km depth. Further synthesis of observations from seismic tomography, receiver functions, and seismic anisotropy in the Afar region will offer better understanding of tectonic significance of the lithospheric discontinuities.

  2. Anatomy of mantle plumes: hot heads and cold stems

    NASA Astrophysics Data System (ADS)

    Davaille, A. B.; Kumagai, I.; Vatteville, J.; Touitou, F.; Brandeis, G.

    2012-12-01

    Recent petrological studies show evidences for secular cooling in mantle plumes: the source temperature of oceanic plateaus could be 100°C hotter than the source temperature of volcanic island chains (Herzberg and Gazel, Nature, 2009). In terms of mantle plumes, it would mean that the temperature of the plume head is hotter than that of the plume stem. This is at odd with a model where a plume head would entrain so much ambient mantle on its journey towards the Earth's surface that it would end up being considerably colder than its narrow stem. So we revisited the problem using laboratory experiments and new visualization techniques to measure in situ simultaneously the temperature, velocity and composition fields. At time t=0, a hot instability is created by heating a patch of a given radius at constant power or constant temperature. The fluids are mixtures of sugar syrups , with a strongly temperature-dependent viscosity, and salt. Rayleigh numbers were varied from 104 to 108, viscosity ratios between 1.8 and 4000, and buoyancy ratios between 0 and 2. After a stage where heat transport is by conduction only, the hot fluid gathers in a sphere and begins to rise, followed by a stem anchored on the hot patch. In all cases, temperatures in the head start with higher values than in the subsequent stem. This is also the case for the thermal instabilities rising from a infinite plate heated uniformly. However, the head also cools faster than the stem as they rise, so that they will eventually have the same temperature if the mantle is deep enough. Moreover, all the material sampled by partial melting in the plume head or stem would be coming from the heated area around the deep source, and very little entrainment from the ambient mantle is predicted. The difference in temperature between head and stem strongly depends on the mantle depth, the viscosity ratio and the buoyancy ratio. Our scaling laws predict that Earth's mantle plumes can indeed have hot heads and colder

  3. A migratory mantle plume on Venus: Implications for Earth?

    USGS Publications Warehouse

    Chapman, M.G.; Kirk, R.L.

    1996-01-01

    A spatially fixed or at least internally rigid hotspot reference frame has been assumed for determining relative plate motions on Earth. Recent 1:5,000,000 scale mapping of Venus, a planet without terrestrial-style plate tectonics and ocean cover, reveals a systematic age and dimensional progression of corona-like arachnoids occurring in an uncinate chain. The nonrandom associations between arachnoids indicate they likely formed from a deep-seated mantle plume in a manner similar to terrestrial hotspot features. However, absence of expected convergent "plate" margin deformation suggests that the arachnoids are the surface expression of a migratory mantle plume beneath a stationary surface. If mantle plumes are not stationary on Venus, what if any are the implications for Earth?

  4. Asymmetric three-dimensional topography over mantle plumes.

    PubMed

    Burov, Evgueni; Gerya, Taras

    2014-09-01

    The role of mantle-lithosphere interactions in shaping surface topography has long been debated. In general, it is supposed that mantle plumes and vertical mantle flows result in axisymmetric, long-wavelength topography, which strongly differs from the generally asymmetric short-wavelength topography created by intraplate tectonic forces. However, identification of mantle-induced topography is difficult, especially in the continents. It can be argued therefore that complex brittle-ductile rheology and stratification of the continental lithosphere result in short-wavelength modulation and localization of deformation induced by mantle flow. This deformation should also be affected by far-field stresses and, hence, interplay with the 'tectonic' topography (for example, in the 'active/passive' rifting scenario). Testing these ideas requires fully coupled three-dimensional numerical modelling of mantle-lithosphere interactions, which so far has not been possible owing to the conceptual and technical limitations of earlier approaches. Here we present new, ultra-high-resolution, three-dimensional numerical experiments on topography over mantle plumes, incorporating a weakly pre-stressed (ultra-slow spreading), rheologically realistic lithosphere. The results show complex surface evolution, which is very different from the smooth, radially symmetric patterns usually assumed as the canonical surface signature of mantle upwellings. In particular, the topography exhibits strongly asymmetric, small-scale, three-dimensional features, which include narrow and wide rifts, flexural flank uplifts and fault structures. This suggests a dominant role for continental rheological structure and intra-plate stresses in controlling dynamic topography, mantle-lithosphere interactions, and continental break-up processes above mantle plumes. PMID:25186903

  5. Tomographic resolution of plume anomalies in the lowermost mantle

    NASA Astrophysics Data System (ADS)

    Xue, Jing; Zhou, Ying; Chen, Yongshun

    2015-05-01

    Mantle plumes as well as `superplumes' have been imaged in the lowermost mantle in tomographic studies. To investigate seismic resolution of deep mantle plume anomalies, we use a spectral element method (SEM) to simulate global seismic wave propagation in 3-D wave speed models and measure frequency-dependent P-, S-, Pdiff- and Sdiff-wave traveltime anomalies caused by plume structures in the lowermost mantle. We compare SEM time delay measurements with calculations based on ray theory and show that an anticorrelation between bulk sound wave speed and S-wave speed could be produced as an artifact. This is caused by different wavefront healing effects between P and S waves in thermal plume models. The differences in wave diffraction between the two types of waves depend on epicentral distance and wave frequency. We show that bulk-sound speed structure can not be recovered in ray-theoretical tomographic inversions when the lateral extent of the anomaly is smaller than the size of the Fresnel zone in the lowermost mantle. In addition, an anticorrelation between bulk sound speed and S-wave speed can be produced in ray-theoretical tomography when the size of the anomaly is less than ˜2000 km; and, the artifacts become more pronounced as the lateral extent of the plume decreases. This indicates a chemical origin of `superplumes' in the lowermost mantle may not be necessary to explain observed seismic traveltimes of core-mantle diffracted waves. The same set of Pdiff and Sdiff measurements are inverted using finite-frequency tomography based on Born sensitivity kernels. We show that wavefront healing effects can be accounted for in finite-frequency tomography to recover the true velocity model.

  6. Mantle plume capture, anchoring and outflow during ridge interaction

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; Richards, M. A.; Geist, D.

    2015-12-01

    Geochemical and geophysical studies have shown that >40% of the world's mantle plumes are currently interacting with the global ridge system and such interactions may continue for up to 180 Myr[1]. At sites of plume-ridge interaction up to 1400 km of the spreading centre is influenced by dispersed plume material but there are few constraints on how and where the ridge-ward transfer of deep-sourced material occurs, and also how it is sustained over long time intervals. Galápagos is an archetypal example of an off-axis plume and sheds important light on these mechanisms. The Galápagos plume stem is located ~200 km south of the spreading axis and its head influences 1000 km of the ridge. Nevertheless, the site of enriched basalts, greatest crustal thickness and elevated topography on the ridge, together with active volcanism in the archipelago, correlate with a narrow zone (~150 km) of low-velocity, high-temperature mantle that connects the plume stem and ridge at depths of ~100 km[2]. The enriched ridge basalts contain a greater amount of partially-dehydrated, recycled oceanic crust than basalts elsewhere on the spreading axis, or indeed basalts erupted in the region between the plume stem and ridge. The presence of these relatively volatile-rich ridge basalts requires flow of plume material below the peridotite solidus (i.e.>80 km). We propose a 2-stage model for the development and sustainment of a confined zone of deep ridge-ward plume flow. This involves initial on-axis capture and establishment of a sub-ridge channel of plume flow. Subsequent anchoring of the plume stem to a contact point on the ridge during axis migration results in confined ridge-ward flow of plume material via a deep network of melt channels embedded in the normal spreading and advection of the plume head[2]. Importantly, sub-ridge flow is maintained. The physical parameters and styles of mantle flow we have defined for Galápagos are less-well known at other sites of plume

  7. Response of mantle transition zone thickness to plume buoyancy flux

    NASA Astrophysics Data System (ADS)

    Das Sharma, S.; Ramesh, D. S.; Li, X.; Yuan, X.; Sreenivas, B.; Kind, R.

    2010-01-01

    The debate concerning thermal plumes in the Earth's mantle, their geophysical detection and depth characterization remains contentious. Available geophysical, petrological and geochemical evidence is at variance regarding the very existence of mantle plumes. Utilizing P-to-S converted seismic waves (P receiver functions) from the 410 and 660 km discontinuities, we investigate disposition of these boundaries beneath a number of prominent hotspot regions. The thickness of the mantle transition zone (MTZ), measured as P660s-P410s differential times (tMTZ), is determined. Our analyses suggest that the MTZ thickness beneath some hotspots correlates with the plume strength. The relationship between tMTZ, in response to the thermal perturbation, and the strength of plumes, as buoyancy flux B, follows a power law. This B-tMTZ behavior provides unprecedented insights into the relation of buoyancy flux and excess temperature at 410-660 km depth below hotspots. We find that the strongest hotspots, which are located in the Pacific, are indeed plumes originating at the MTZ or deeper. According to the detected power law, even the strongest plumes may not shrink the transition zone by significantly more than ~40 km (corresponding to a maximum of 300-400° excess temperature).

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  9. Rollback subduction: the great killer of mantle plumes

    NASA Astrophysics Data System (ADS)

    Druken, K. A.; Kincaid, C. R.; Griffiths, R. W.

    2010-12-01

    Subduction driven mantle flow is shown to stall and decapitate buoyant upwellings, thereby severely limiting vertical heat and mass transport. Ongoing debate tends to focus on the expected surface expression of plumes rising independently of the background circulation, however we present 3-D laboratory results that suggest rollback subduction greatly alters this classic plume model. A Phenolic sheet and temperature dependent glucose fluid, are used to model the subducting plate and upper ~2000 km of the mantle, respectively. Experiments varied style and rate of rollback subduction as well as plume temperature and position. Results show that buoyant upwellings located as far as 1500 km behind the trench fall under two regimes, (I) plate dominated or (II) plume dominated. In either regime, down-dip sinking of the slab initially stalls vertical plume motion and the combination of down-dip sinking and trench rollback redistributes material throughout the system. Plumes with as much as 400°C excess temperature behave as passive features in the subduction-induced 3-D flow (Regime I). Less than 10% of plume material in this regime is capable of reaching zones for melt generation, with rollback subduction trapping or re-subducting the majority of plume material at depth. Only plumes of 600°C excess temperature (or more) are able to overcome the dominant 3-D flow and transport heat and mass to the surface (Regime II). Regardless of plume temperature, conduit velocities (proxy for melt generation) show cycles of high and low hotspot activity also due to distortion from subduction-induced flow. As a result of both the sinking and rollback motions, the temporal hotspot trend is variable and differs from conventional plate-conduit interaction.

  10. Temperature fluctuation of the Iceland mantle plume through time

    NASA Astrophysics Data System (ADS)

    Spice, Holly E.; Fitton, J. Godfrey; Kirstein, Linda A.

    2016-02-01

    The newly developed Al-in-olivine geothermometer was used to find the olivine-Cr-spinel crystallization temperatures of a suite of picrites spanning the spatial and temporal extent of the North Atlantic Igneous Province (NAIP), which is widely considered to be the result of a deep-seated mantle plume. Our data confirm that start-up plumes are associated with a pulse of anomalously hot mantle over a large spatial area before becoming focused into a narrow upwelling. We find that the thermal anomaly on both sides of the province at Baffin Island/West Greenland and the British Isles at ˜61 Ma across an area ˜2000 km in diameter was uniform, with Al-in-olivine temperatures up to ˜300°C above that of average mid-ocean ridge basalt (MORB) primitive magma. Furthermore, by combining our results with geochemical data and existing geophysical and bathymetric observations, we present compelling evidence for long-term (>107 year) fluctuations in the temperature of the Iceland mantle plume. We show that the plume temperature fell from its initial high value during the start-up phase to a minimum at about 35 Ma, and that the mantle temperature beneath Iceland is currently increasing.

  11. Mantle plume helium in submarine basalts from the galapagos platform.

    PubMed

    Graham, D W; Christie, D M; Harpp, K S; Lupton, J E

    1993-12-24

    Helium-3/helium-4 ratios in submarine basalt glasses from the Galapagos Archipelago range up to 23 times the atmospheric ratio in the west and southwest. These results indicate the presence of a relatively undegassed mantle plume at the Galápagos hot spot and place Galápagos alongside Hawaii, Iceland, and Samoa as the only localities known to have such high helium-3/helium-4 ratios. Lower ratios across the rest of the Galápagos Archipelago reflect systematic variations in the degree of dilution of the plume by entrainment of depleted material from the asthenosphere. These spatial variations reveal the dynamics of the underlying mantle plume and its interaction with the nearby Galápagos Spreading Center. PMID:17794969

  12. 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

  13. Experimental, numerical and analytical models of mantle starting plumes

    NASA Astrophysics Data System (ADS)

    Coulliette, D. L.; Loper, D. E.

    1995-12-01

    The results of a combined experimental, numerical and analytical investigation of starting thermal plumes are described, to obtain a better perspective on plumes within the Earth's mantle. Thermal plumes were generated experimentally in a tank of corn syrup by means of an electrical heater. Viscosity ratios of 400, 30 000, and 10 8 were generated by varying the temperature of the tank. Plumes for the smaller ratios had the traditional 'balloon-on-astring' shape, but that at the highest ratio had a novel morphology. The plume heads in the first two cases were observed to rise at roughly a constant speed, in contrast to most previous studies which found the plume heads to accelerate. Loss of buoyancy from the plume head owing to heat loss is believed to be responsible for this difference. Starting plumes were simulated numerically using an axisymmetric, finite-element code to solve the Boussinesq equations at finite Prandtl numbers. The constant rise speed observed experimentally was confirmed by the numerical simulation for the viscosity ratios of 400 and 30 000, but numerical instability prevented simulation of the case with a viscosity ratio of 10 8. There was very good agreement between the experimental and numerical rise speeds. An analytical model was developed which reduces to previous models in limiting cases. This parameterization gives better agreement with the experimental and numerical results than does any previous model.

  14. Capture of the Canary mantle plume head by the Gibraltar arc mantle wedge during slab roll back

    NASA Astrophysics Data System (ADS)

    Duarte, Sílvia; Duarte, João; Mériaux, Catherine; Rosas, Filipe; Mata, João; Schellart, Wouter; Chen, Zhihao; Terrinha, Pedro

    2015-04-01

    Recent evidence suggests that a portion of the Canary plume travelled northeastwards across the Atlas Mountains in North Africa and was captured ~10 Ma ago by the Western Mediterranean Gibraltar subduction system. The capture would have been associated with the retreating slab-induced mantle return flow that would have dragged and trapped a portion of the plume head in the mantle wedge of the Gibraltar subduction zone. Such material eventually contaminated the subduction related volcanism in the Alboran region. In this work we used scaled analogue models of slab-plume head interaction to investigate the plausibility of the plume capture. A 400 km narrow dense plate was drawn into subduction in a viscous upper mantle, while a buoyant plume was initiated at the base of the upper mantle 800 km aside of the plate centreline. First, a transient phase took place during which the plate sunk to the base of the upper mantle and the plume rose up to the surface without interaction, as plate and plume were 2375 km apart. During the second phase of interest, the slab started retreating towards the plume whose head began to grow. The influence of the subducting plate on the spreading plume head was seen with the onset of asymmetry in the plume head in a direction of the trench and parallel to the plate. The asymmetry began as the trench was 862 km away from the plume head centre and the plume head edge was 138 km far from the plate edge. With the passing of the trench at the apex of the plume head centre, capture of the plume head towards the mantle wedge began, and after 6 Myr, 9.5% of the plume head had been captured. Our results support the evidence that mantle plume material may have been sucked towards the mantle wedge of the Gibraltar subduction system during slab rollback induced toroidal mantle flow.

  15. Petrological evidence for secular cooling in mantle plumes.

    PubMed

    Herzberg, Claude; Gazel, Esteban

    2009-04-01

    Geological mapping and geochronological studies have shown much lower eruption rates for ocean island basalts (OIBs) in comparison with those of lavas from large igneous provinces (LIPs) such as oceanic plateaux and continental flood provinces. However, a quantitative petrological comparison has never been made between mantle source temperature and the extent of melting for OIB and LIP sources. Here we show that the MgO and FeO contents of Galapagos-related lavas and their primary magmas have decreased since the Cretaceous period. From petrological modelling, we infer that these changes reflect a cooling of the Galapagos mantle plume from a potential temperature of 1,560-1,620 degrees C in the Cretaceous to 1,500 degrees C at present. Iceland also exhibits secular cooling, in agreement with previous studies. Our work provides quantitative petrological evidence that, in general, mantle plumes for LIPs with Palaeocene-Permian ages were hotter and melted more extensively than plumes of more modern ocean islands. We interpret this to reflect episodic flow from lower-mantle domains that are lithologically and geochemically heterogeneous. PMID:19340079

  16. Seismic tomographic evidence for upwelling mantle plume in NE China

    NASA Astrophysics Data System (ADS)

    He, Chuansong; Santosh, M.

    2016-05-01

    In this study, we collected teleseismic data recorded by permanent and mobile seismic stations and carried out a teleseismic P-wave tomographic study. The results reveal low velocity perturbation regions at the central part of NE China and specifically in the Songliao basin at different depths, which correspond to the location of a proposed upwelling mantle plume identified by receiver function in a recent study. Receiver function data show a predominantly mafic/ultra-mafic lower crust in the Songliao basin, in contrast to the predominantly felsic lower crust in the other regions. The vestige of upwelling mantle plume is well defined at the mantle transition region. Based on the above results, we suggest that the volcanism in NE China and the Songliao basin formation might be related to Mesozoic mantle plume beneath NE China. We also evaluate alternate models on lower crustal delamination contributing to the volcanism in NE China following collision and amalgamation between the Siberia craton and the North China-Mongolian block during late Jurassic and early Cretaceous.

  17. On the origin of noble gases in mantle plumes.

    PubMed

    Coltice, Nicolas; Ricard, Yanick

    2002-11-15

    The chemical differences between deep- and shallow-mantle sources of oceanic basalts provide evidence that several distinct components coexist within the Earth's mantle. Most of these components have been identified as recycled in origin. However, the noble-gas signature is still a matter of debate and questions the preservation of primitive regions in the convective mantle. We show that a model where the noble-gas signature observed in Hawaii and Iceland comes from a pristine homogeneous deep layer would imply a primitive (3)He content and (3)He/(22)Ne ratio that are very unlikely. On the contrary, mass balances show that the partly degassed peridotite of a marble-cake mantle can be the noble-gas end-member with an apparent 'primitive'-like composition. This component is mixed with recycled oceanic crust in different proportions in the plume sources and in the shallow mantle. A recycling model of the mantle, involving gravitational segregation of the oceanic crust at the bottom of the mantle, potentially satisfies trace-element as well as noble-gas constraints. PMID:12460484

  18. Chondritic Xenon in the Earth's mantle: new constrains on a mantle plume below central Europe

    NASA Astrophysics Data System (ADS)

    Caracausi, Antonio; Avice, Guillaume; Bernard, Peter; Furi, Evelin; Marty, Bernard

    2016-04-01

    Due to their inertness, their low abundances, and the presence of several different radiochronometers in their isotope systematics, the noble gases are excellent tracers of mantle dynamics, heterogeneity and differentiation with respect to the atmosphere. Xenon deserves particular attention because its isotope systematic can be related to specific processes during terrestrial accretion (e.g., Marty, 1989; Mukhopadhyay, 2012). The origin of heavy noble gases in the Earth's mantle is still debated, and might not be solar (Holland et al., 2009). Mantle-derived CO2-rich gases are particularly powerful resources for investigating mantle-derived noble gases as large quantities of these elements are available and permit high precision isotope analysis. Here, we report high precision xenon isotopic measurements in gases from a CO2 well in the Eifel volcanic region (Germany), where volcanic activity occurred between 700 ka and 11 ka years ago. Our Xe isotope data (normalized to 130Xe) show deviations at all masses compared to the Xe isotope composition of the modern atmosphere. The improved analytical precision of the present study, and the nature of the sample, constrains the primordial Xe end-member as being "chondritic", and not solar, in the Eifel mantle source. This is consistent with an asteroidal origin for the volatile elements in Earth's mantle and it implies that volatiles in the atmosphere and in the mantle originated from distinct cosmochemical sources. Despite a significant fraction of recycled atmospheric xenon in the mantle, primordial Xe signatures still survive in the mantle. This is also a demonstration of a primordial component in a plume reservoir. Our data also show that the reservoir below the Eifel region contains heavy-radiogenic/fissiogenic xenon isotopes, whose ratios are typical of plume-derived reservoirs. The fissiogenic Pu-Xe contribution is 2.26±0.28 %, the UXe contribution is negligible, the remainder being atmospheric plus primordial. Our

  19. The origin of chemical heterogeneity in the Hawaiian mantle plume

    NASA Astrophysics Data System (ADS)

    Pietruszka, A. J.; Norman, M. D.; Garcia, M. O.; Marske, J. P.; Burns, D. H.

    2012-12-01

    Inter-shield differences in the composition of lavas from Hawaiian volcanoes are generally thought to result from the melting of a heterogeneous mantle source containing variable amounts or types of oceanic crust (sediment, basalt, and/or gabbro) that was recycled into the mantle at ancient subduction zones (e.g., [1-3]). Here we investigate the origin of chemical heterogeneity in the Hawaiian mantle plume by comparing the incompatible trace element abundances of tholeiitic basalts from (1) Kilauea, Mauna Loa, and Loihi Seamount (the three active Hawaiian volcanoes) and (2) the extinct Koolau shield (a compositional end member for Hawaiian volcanoes). Model calculations (based on these incompatible trace element abundances) suggest that the mantle sources of Hawaiian volcanoes contain variable amounts of recycled oceanic crust (ROC), consisting of basalt and gabbro (but little or no marine sediment) that was altered by interaction with seawater or hydrothermal fluids prior to being variably dehydrated in an ancient subduction zone. The estimated fraction of ROC in the Hawaiian plume varies from ~8-16% at Kilauea and Loihi to ~15-21% at Mauna Loa and Koolau (the remainder is assumed to be ambient depleted Hawaiian mantle). The ROC in the mantle source of Kilauea and Loihi lavas is dominated by the uppermost portion of the residual slab (gabbro-free, strongly dehydrated basalt), whereas the ROC in the mantle source of Mauna Loa and Koolau lavas is dominated by the lowermost portion of the residual slab (weakly dehydrated basalt and gabbro). The model results suggest that the large-scale distribution of compositional heterogeneities in the Hawaiian plume at the present time cannot be described by either a radial zonation [1] or a bilateral asymmetry [4,5]. Instead, the Hawaiian plume is heterogeneous on a small scale with a NW-SE oriented spatial gradient in the amount, type (i.e., basalt vs. gabbro) and extent of dehydration of the ancient ROC. [1] Hauri (1996

  20. Diamonds sampled by plumes from the core-mantle boundary.

    PubMed

    Torsvik, Trond H; Burke, Kevin; Steinberger, Bernhard; Webb, Susan J; Ashwal, Lewis D

    2010-07-15

    Diamonds are formed under high pressure more than 150 kilometres deep in the Earth's mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core-mantle boundary. PMID:20631796

  1. An analytic model of axisymmetric mantle plume due to thermal and chemical diffusion

    NASA Technical Reports Server (NTRS)

    Liu, Mian; Chase, Clement G.

    1990-01-01

    An analytic model of axisymmetric mantle plumes driven by either thermal diffusion or combined diffusion of both heat and chemical species from a point source is presented. The governing equations are solved numerically in cylindrical coordinates for a Newtonian fluid with constant viscosity. Instead of starting from an assumed plume source, constraints on the source parameters, such as the depth of the source regions and the total heat input from the plume sources, are deduced using the geophysical characteristics of mantle plumes inferred from modelling of hotspot swells. The Hawaiian hotspot and the Bermuda hotspot are used as examples. Narrow mantle plumes are expected for likely mantle viscosities. The temperature anomaly and the size of thermal plumes underneath the lithosphere can be sensitive indicators of plume depth. The Hawaiian plume is likely to originate at a much greater depth than the Bermuda plume. One suggestive result puts the Hawaiian plume source at a depth near the core-mantle boundary and the source of the Bermuda plume in the upper mantle, close to the 700 km discontinuity. The total thermal energy input by the source region to the Hawaiian plume is about 5 x 10(10) watts. The corresponding diameter of the source region is about 100 to 150 km. Chemical diffusion from the same source does not affect the thermal structure of the plume.

  2. One hundred million years of mantle geochemical history suggest the retiring of mantle plumes is premature

    NASA Astrophysics Data System (ADS)

    Konter, Jasper G.; Hanan, Barry B.; Blichert-Toft, Janne; Koppers, Anthony A. P.; Plank, Terry; Staudigel, Hubert

    2008-11-01

    Linear chains of intraplate volcanoes and their geochemistry provide a record of mantle melting through geological time. The isotopic compositions of their lavas characterize their mantle sources, and their ages help backtrack these volcanoes to their original, eruptive source regions. Such data may shed light on a much-debated issue in Earth Sciences: the origin of intraplate volcanism and its underlying mantle and lithosphere dynamics. We show here that three major Western Pacific Seamount groups, ˜ 50-100 million years in age, display distinct Sr, Nd, Hf, and Pb isotopic signatures that can be traced back in time, both geographically and geochemically, to three separate, recently-active intraplate volcanoes in the South Pacific Cook-Austral Islands. Their unique 100 million year history, which shows a persistent geochemical fingerprint, suggests formation from large volumes of laterally fixed, long-lived source regions. Such longevity is unlikely to be attained in the relatively dynamic upper mantle. Therefore, these sources are likely anchored deep in the mantle, isolated from homogenization by mantle convection, and imply a primary origin from deep mantle plumes rather than resulting from lithosphere extension.

  3. A >100 Ma Mantle Geochemical Record: Retiring Mantle Plumes may be Premature

    NASA Astrophysics Data System (ADS)

    Konter, J. G.; Hanan, B. B.; Blichert-Toft, J.; Koppers, A. A.; Plank, T.; Staudigel, H.

    2006-12-01

    Hotspot volcanism has long been attributed to mantle plumes, but in recent years suggestions have been made that plate tectonic processes, such as extension, can account for all hotspot tracks. This explanation involves a profoundly less dynamic lower mantle, which justifies a critical evaluation before the plume model is dismissed. Such an evaluation has to involve a wide range of geochemical, geological, and geophysical techniques, broadly investigating the products of volcanism as well as the underlying lithosphere and mantle. We argue here that the combined geological record and geochemistry of intraplate volcanoes holds some important clues that help us decide between models of plume-like upwelling versus passive upwelling with lithospheric extension. The best of these integrated datasets can be obtained from the long seamount chains in the Pacific Ocean. A new combined dataset of trace element and isotopic compositions, along with modern 40Ar/39Ar ages from seamounts in the Gilbert Ridge, Tokelau chain, and West Pacific Seamount Province (WPSP) provides a record of current to Cretaceous volcanism in the South Pacific. We have reconstructed the eruptive locations of the seamounts using a range of absolute plate motion models, including some models with hotspot motion and others that use the Indo-Atlantic hotspot reference frame. Our results show that the backtracked locations consistently form clusters (300km radius) around the active ends of the Macdonald, Rurutu and Rarotonga hotspot chains, while closely matching their distinct C-HIMU and C-EM1 signatures. The oldest WPSP seamounts (older than 100 Ma) form the only exception and backtrack, with larger uncertainty, to north of Rarotonga. Therefore, the mantle currently underlying the Cook-Austral islands has produced volcanoes in three geochemically distinct areas for at least 100 m.y. Furthermore, we find the shortest mantle residence time, 0.6 Ga, for a source of mixed recycled DMM and an EM1-like

  4. The interaction of plume heads with compositional discontinuities in the Earth's mantle

    NASA Technical Reports Server (NTRS)

    Manga, Michael; Stone, Howard A.; O'Connell, Richard J.

    1993-01-01

    The effects of compositional discontinuities of density and viscosity in the Earth's mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique. Three specific problems are considered: (1) a plume head rising away from a deformable interface, (2) a plume head passing through an interface, and (3) a plume head approaching the surface of the Earth. For the case of a plume attached to a free-surface, the calculated time-dependent plume shapesare compared with experimental results. Two principle modes of plume head deformation are observed: plume head elingation or the formation of a cavity inside the plume head. The inferred structure of mantle plumes, namely, a large plume head with a long tail, is characteristic of plumes attached to their source region, and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high- to low-viscosity region. As a rising plume head approaches the upper mantle, most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle. If the plume head moves from a high- to low-viscosity region, the plume head becomes significantly elongated and, for the viscosity contrasts thought to exist in the Earth, could extend from the 670 km discontinuity to the surface. Plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry. The dynamic surface topography induced by plume heads is bell-shaped when the top of the plume head is at depths greater than about 0.1 plume head radii. As the plume head approaches the surface and spreads, the dynamic topography becomes plateau-shaped. The largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical, and the surface expression of these stresses is likely to be dominated by radial extension. As the plume spreads, compressional

  5. Linking mantle plumes, large igneous provinces and environmental catastrophes.

    PubMed

    Sobolev, Stephan V; Sobolev, Alexander V; Kuzmin, Dmitry V; Krivolutskaya, Nadezhda A; Petrunin, Alexey G; Arndt, Nicholas T; Radko, Viktor A; Vasiliev, Yuri R

    2011-09-15

    Large igneous provinces (LIPs) are known for their rapid production of enormous volumes of magma (up to several million cubic kilometres in less than a million years), for marked thinning of the lithosphere, often ending with a continental break-up, and for their links to global environmental catastrophes. Despite the importance of LIPs, controversy surrounds even the basic idea that they form through melting in the heads of thermal mantle plumes. The Permo-Triassic Siberian Traps--the type example and the largest continental LIP--is located on thick cratonic lithosphere and was synchronous with the largest known mass-extinction event. However, there is no evidence of pre-magmatic uplift or of a large lithospheric stretching, as predicted above a plume head. Moreover, estimates of magmatic CO(2) degassing from the Siberian Traps are considered insufficient to trigger climatic crises, leading to the hypothesis that the release of thermogenic gases from the sediment pile caused the mass extinction. Here we present petrological evidence for a large amount (15 wt%) of dense recycled oceanic crust in the head of the plume and develop a thermomechanical model that predicts no pre-magmatic uplift and requires no lithospheric extension. The model implies extensive plume melting and heterogeneous erosion of the thick cratonic lithosphere over the course of a few hundred thousand years. The model suggests that massive degassing of CO(2) and HCl, mostly from the recycled crust in the plume head, could alone trigger a mass extinction and predicts it happening before the main volcanic phase, in agreement with stratigraphic and geochronological data for the Siberian Traps and other LIPs. PMID:21921914

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  7. Mantle plume interactions and the spacing of Tharsis and Elysium on Mars

    NASA Astrophysics Data System (ADS)

    Rose, I.; Manga, M.

    2010-12-01

    Tharsis and Elysium are the two largest magmatic provinces on Mars and are characterized by large geoid anomalies and extensive volcanism. Such features on Earth and other terrestrial planets have been explained by mantle plumes. Analytical and numerical studies indicate that the flow excited by one plume can draw neigboring plumes nearer, leading to clustering and merging of mantle plumes. If Tharsis and Elysium, which lie some 98 degrees from each other and have been active for much of Mars' history, are indeed underlain by large plumes, then there might be significant interactions between the two. We investigate the spacing of model mantle plumes by calculating the Stokes flow that one plume excites in the neighborhood of another in order to determine a characteristic interaction distance. We find that plumes beneath Tharsis and Elysium fall outside of the separation at which we would expect them to cluster.

  8. The role of heat source for spatio-temporal variations of mantle plumes

    NASA Astrophysics Data System (ADS)

    Kumagai, I.; Yamagishi, Y.; Davaille, A.

    2014-12-01

    Hot mantle plumes ascending from the core-mantle boundary experience a filtering effect by the endothermic phase change at the 660-km discontinuity. Fluid dynamics predicts that some hot mantle plumes stagnate at the phase boundary and locally heat the bottom of the upper mantle. This generates the secondary plumes in the upper mantle originating hotspots volcanic activities on the Earth's surface. Recently, seismic tomographic images around the upper-lower mantle boundary showed that the horizontal scale of the low velocity regions, which corresponds to that of the thermally buoyant heat sources, is the order of 100-1000 km. Although most of the fluid dynamic theories on the thermal plumes have been developed using an assumption that the heat source effect is negligible, the behaviors of the starting plumes in the upper mantle should depend on the size of heat source, which is generated by the hotter plume from the CMB. In order to understand the effects of heater size on the starting plume generation, we have experimentally investigated the behaviors of thermally buoyant plumes using a localized heat source (circular plate heater). The combination of quantitative visualization techniques of temperature (Thermochromic Liquid Crystals) and velocity (Particle Image Velocimetry) fields reveals the transient nature of the plume evolution: a variety of the spatio-tempotal distribution of plumes. Simple scaling laws for their ascent velocity and spacing of the plumes are experimentally determined. We also estimate the onset time of the secondary plumes in the upper mantle which depends on the local characteristics of the thermal boundary layer developing at the upper-lower mantle boundary.

  9. Identification of an ancient mantle reservoir and young recycled materials in the source region of a young mantle plume: Implications for potential linkages between plume and plate tectonics

    NASA Astrophysics Data System (ADS)

    Wang, Xuan-Ce; Li, Zheng-Xiang; Li, Xian-Hua; Li, Jie; Xu, Yi-Gang; Li, Xiang-Hui

    2013-09-01

    Whether or not mantle plumes and plate subduction are genetically linked is a fundamental geoscience question that impinges on our understanding of how the Earth works. Late Cenozoic basalts in Southeast Asia are globally unique in relation to this question because they occur above a seismically detected thermal plume adjacent to deep subducted slabs. In this study, we present new Pb, Sr, Nd, and Os isotope data for the Hainan flood basalts. Together with a compilation of published results, our work shows that less contaminated basaltic samples from the synchronous basaltic eruptions in Hainan-Leizhou peninsula, the Indochina peninsula and the South China Sea seamounts share the same isotopic and geochemical characteristics. They have FOZO-like Sr, Nd, and Pb isotopic compositions (the dominant lower mantle component). These basalts have primitive Pb isotopic compositions that lie on, or very close to, 4.5- to 4.4-Ga geochrons on 207Pb/204Pb versus 206Pb/204Pb diagram, suggesting a mantle source developed early in Earth's history (4.5-4.4 Ga). Furthermore, our detailed geochemical and Sr, Nd, Pb and Os isotopic analyses suggest the presence of 0.5-0.2 Ga recycled components in the late Cenozoic Hainan plume basalts. This implies a mantle circulation rate of >1 cm/yr, which is similar to that of previous estimates for the Hawaiian mantle plume. The identification of the ancient mantle reservoir and young recycled materials in the source region of these synchronous basalts is consistent with the seismically detected lower mantle-rooted Hainan plume that is adjacent to deep subducted slab-like seismic structures just above the core-mantle boundary. We speculate that the continued deep subduction and the presence of a dense segregated basaltic layer may have triggered the plume to rise from the thermal-chemical pile. This work therefore suggests a dynamic linkage between deep subduction and mantle plume generation.

  10. Plume Generation Zones On The Core Mantle Boundary: their origin and what they tell about how the Earth works - and how it has worked (Arthur Holmes Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Burke, Kevin

    2014-05-01

    It is more than 50 years since Wilson (1963) suggested that a fixed plume of deep origin from the convecting mantle is generating the Hotspots of the Hawaiian chain on the overlying moving rigid lithosphere and nearly 45 years since Morgan (1972) followed by suggesting that the plumes which generate Hotspots rise only from the Core/Mantle Boundary (CMB). During the past ~ 15 years testing has begun of a refinement of Morgan's idea based on the observation that Plumes responsible for Hotspots, Large Igneous Provinces (LIPs) and a significant fraction of other igneous rocks (including kimberlites) originate only in Plume Generation Zones (PGZs) at the edges on the CMB of one or other of TUZO and JASON the 2 antipodal, equatorial, Large Low Shear Wave Velocity Provinces (LLSVPs) of the deep mantle (Garnero et al. 2007) or from similar PGZs at the edges on the CMB of ~8 smaller Low Shear Wave Velocity Provinces. Today I will: (i) demonstrate using dated Hotspot, Large Igneous Province and Kimberlite occurrence history and paleomagnetic rotations (e.g. Torsvik et al. 2010, Burke et al.2008) the stability throughout the past 0.55 Ga of the LLSVPs and LSVPs (ii) show from the history of the Earth and Mars how the LLSVPs and LSVPs are likely to have formed early in Earth history and to have been stable since ~ 4.4 Ga (Burke et al. 2012) (iii) show, following an analogy suggested by Jack Whitehead of similarity to atmospheric fronts, why plumes are generated only from PGZs on the CMB at the margins of LLSVPs and LSVPs. (iv) show from results of recent seismological studies of Iceland, Jan Mayen, Hawaii, Yellowstone, the Afar and Ontong Java, that although plumes rise vertically in the deep mantle from the CMB their fate in the top ~ 1, 000 km of the mantle is proving to be varied and to depend largely, as Wilson suggested, on how they interact with the plates above them. Properties of the Plume Generation Zones (PGZs) on the CMB and of the plumes that rise from them are

  11. Seismically Imaging the Destruction of Continental Lithosphere beneath Afar

    NASA Astrophysics Data System (ADS)

    Rychert, C. A.; Hammond, J. O.; Kendall, J. M.; Harmon, N.; Keir, D.; Ebinger, C. J.; Stuart, G. W.; Belachew, M.

    2011-12-01

    The onset of continental rifting is often accompanied by production of large volumes of molten rock. However, the influence of magmatism on the deforming lithosphere during the breakup process is not well understood. In particular, whether lithosphere is predominantly thinned by mechanical stretching or thermal destruction from melt infiltration, and how this impacts melt production during the breakup process remains unconstrained. Here we use S-to-P (Sp) receiver functions to image the onset of decompression melting beneath Afar, Ethiopia; a region where continental breakup gives way to oceanic spreading. We analyze three broadband datasets using S-to-p (Sp) imaging, which provide high resolution imaging beneath the rift and surrounding regions: the Ethiopia/Kenya Broadband Seismic Experiment (EKBSE), the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE), and a new UK/US led deployment of 46 stations in the Afar depression and surrounding area. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. At ~75 km depth we image a strong, sharp, velocity reduction on the flank of the rift that likely represents the lithosphere-asthenosphere boundary, versus a strong velocity increase with depth beneath the rift. The sharpness of the negative gradient can only be explained by melt ponded at the base of the lithosphere. The depth and magnitude of the positive gradient resemble those expected from numerical estimates for the onset of decompression melting in a mid-ocean ridge environment where ~1% melt is retained in the mantle. This implies that the mantle lithosphere beneath Afar has been destroyed; melt intrusion likely played a key role in the initial destruction of continental lithosphere, but the degree of influence from a thermal plume today in Afar is minimal.

  12. The role of a mantle plume in the formation of Yellowstone volcanism

    NASA Astrophysics Data System (ADS)

    Leonard, Tiffany; Liu, Lijun

    2016-02-01

    The origin of the Yellowstone volcanic province remains debated. Proposed hypotheses involve either a mantle plume or not. Recent tomographic images allow a quantitative evaluation of the plume hypothesis and its interaction with the Farallon slabs. Using 4-D geodynamic models with data assimilation, we find that the slab is always in the way of the initially rising plume and that the plume could reach the surface only through the broken slab hinge at ~15 Ma. For most of the time, the sinking slabs dominate the mantle flow and prohibit upwelling. We find that a plume that satisfies the present mantle image beneath Yellowstone fails to predict both voluminous hot materials at shallow depths beneath the western U.S. and the age migration of the hot spot tracks. We suggest that a plume is likely to have much less influence on the Yellowstone volcanism than previously thought.

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

    NASA Astrophysics Data System (ADS)

    Ballmer, Maxim D.; Ito, Garrett; Wolfe, Cecily J.; Solomon, Sean C.

    2013-08-01

    According to classical plume theory, purely thermal upwellings rise through the mantle, pond in a thin layer beneath the lithosphere, and generate hotspot volcanism. Neglected by this theory, however, are the dynamical effects of compositional heterogeneity carried by mantle plumes even though this heterogeneity has been commonly identified in sources of hotspot magmas. Numerical models predict that a hot, compositionally heterogeneous mantle plume containing a denser eclogite component tends to pool at ∼300-410 km depth before rising to feed a shallower sublithospheric layer. This double-layered structure of a thermochemical plume is more consistent with seismic tomographic images at Hawaii than the classical plume model. The thermochemical structure as well as time dependence of plume material rising from the deeper into the shallower layer can further account for long-term fluctuations in volcanic activity and asymmetry in bathymetry, seismic structure, and magma chemistry across the hotspot track, as are observed.

  14. Crustal Structure of the Gulf of Aden Continental Margins, from Afar to Oman, by Ambient Noise Seismic Tomography

    NASA Astrophysics Data System (ADS)

    Korostelev, F.; Weemstra, C.; Boschi, L.; Leroy, S. D.; Ren, Y.; Stuart, G. W.; Keir, D.; Rolandone, F.; Ahmed, A.; Al Ganad, I.; Khanbari, K. M.; Doubre, C.; Hammond, J. O. S.; Kendall, J. M.

    2014-12-01

    Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Gulf of Aden presents volcanic margins to the west, where they are influenced by the Afar hotspot, and non volcanic margins east of longitude 46° E. We imaged the crustal structure of the Gulf of Aden continental margins from Afar to Oman to evaluate the role of the Afar plume on the evolution of the passive margin and its extent towards the East. We use Ambient Noise Seismic Tomography to better understand the architecture and processes along the Gulf of Aden. This recent method, developed in the last decade, allows us to study the seismic signal propagating between two seismic stations. Ambient Noise Seismic Tomography is thus free from artifacts related to the distribution of earthquakes. We collected continuous records from about 200 permanent or temporary stations since 1999 to compute Rayleigh phase velocity maps over the Gulf of Aden.

  15. Receiver function imaging of the onset of melting, implications for volcanism beneath the Afar Rift in contrast to hotspot environments

    NASA Astrophysics Data System (ADS)

    Rychert, C. A.; Harmon, N.; Hammond, J. O.; Laske, G.; Kendall, J.; Ebinger, C. J.; Shearer, P. M.; Bastow, I. D.; Keir, D.; Ayele, A.; Belachew, M.; Stuart, G. W.

    2012-12-01

    Heating, melting, and stretching destroy continents at volcanic rifts. Mantle plumes are often invoked to thermally weaken the continental lithosphere and accommodate rifting through the influx of magma. However the relative effects of mechanical stretching vs. melt infiltration and weakening are not well quantified during the evolution of rifting. S-to-p (Sp) imaging beneath the Afar Rift and hotspot regions such as Hawaii provides additional constraints. We use data from the Ethiopia/Kenya Broadband Seismic Experiment (EKBSE), the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE), a new UK/US led deployment of 46 stations in the Afar depression and surrounding area, and the PLUME experiment. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. We image a lithosphere-asthenosphere boundary at ~75 km beneath the flank of the Afar Rift vs. its complete absence beneath the rift, where the mantle lithosphere has been totally destroyed. Instead a strong velocity increase with depth at ~75 km depth matches geodynamic model predictions for a drop in melt percentage at the onset of decompression melting. The shallow depth of the onset of melting is consistent with a mantle potential temperature = 1350 - 1400°C, i.e., typical for adiabatic decompression melting. Therefore although a plume initially destroyed the mantle lithosphere, its influence directly beneath Afar today is minimal. Volcanism continues via adiabatic decompression melting assisted by strong melt buoyancy effects. This contrasts with a similar feature at much deeper depth, ~150 km, just west of Hawaii, where a deep thermal plume is hypothesized to impinge on the lithosphere. Improved high resolution imaging of rifting, ridges, and hotspots in a variety of stages and tectonic settings will increase constraints on the

  16. Mantle plume-subduction zone interactions over the past 60 Ma

    NASA Astrophysics Data System (ADS)

    Fletcher, Michael; Wyman, Derek A.

    2015-09-01

    A variety of mantle plume types have been proposed and there is a wide range of ways that these plumes might interact with subduction zone arcs. This study looks at the frequency of interaction between previously catalogued plumes and subduction zones while also assessing the potential role of slab windows to either generate false plume signals or contribute to genuine examples of mantle plume-subduction zone interactions. Of the plumes included in several widely cited catalogues, 29% have moved within 1000 km of a subduction zone and 17% have moved within 500 km of a subduction zone over the past 60 Ma, assuming that the plume life span extended over this period. Of the plumes that moved within 1000 km of a subduction zone, 56% are rated as either a deep or mid-mantle plume by an author of at least one of the catalogues. The 44% of interacting plumes that are not rated as mid-mantle or deep by at least one author are the most likely to be related to "top-down" plate tectonic processes. This study shows that they were never coincident with a slab window, although they have often interacted within distances of 1000 km. The manner of interaction between plumes and slab windows depends on the relative positions of the plume, ridge, and slab window. Of the plumes that interact over a 1000 km circular "Zone of Potential Interaction" (ZPI), 28% are no longer interacting today, but have survived that process. While most plumes interact in the form of a ZPI moving over a trench from either behind or in front of the trench, several plumes do cross the trench, demonstrating that plumes can survive even that interaction. Plume-trench interaction occurs in clusters in the northeast and southwest Pacific with limited events in the northwest and southeast Pacific. The presence of clusters in the northeast and southwest Pacific may be caused by the closer proximity of mid-mantle and deep plumes to a subduction zone in these areas. Whereas some deeper plumes may be modified by

  17. New observational and experimental evidence for a plume-fed asthenosphere boundary layer in mantle convection

    NASA Astrophysics Data System (ADS)

    Morgan, J. P.; Hasenclever, J.; Shi, C.

    2013-03-01

    The textbook view is that the asthenosphere is the place beneath the tectonic plates where competing temperature and pressure effects on mantle rheology result in the lowest viscosity region of Earth's mantle. We think the sub-oceanic asthenosphere exists for a different reason, that instead it is where rising plumes of hot mantle stall and spread out beneath the strong tectonic plates. Below this plume-fed asthenosphere is a thermal and density inversion with cooler underlying average-temperature mantle. Here we show several recent seismic studies that are consistent with a plume-fed asthenosphere. These include the seismic inferences that asthenosphere appears to resist being dragged down at subduction zones, that a sub-oceanic thermal inversion ∼250-350 km deep is needed to explain the seismic velocity gradient there for an isochemical mantle, that a fast 'halo' of shear-wave travel-times surrounds the Hawaiian plume conduit, and that an apparent seismic reflector is found ∼300 km beneath Pacific seafloor near Hawaii. We also present 2D axisymmetric and 3D numerical experiments that demonstrate these effects in internally consistent models with a plume-fed asthenosphere. If confirmed, the existence of a plume-fed asthenosphere will change our understanding of the dynamics of mantle convection and melting, and the links between surface plate motions and mantle convection.

  18. Volatile content and distribution in the Azorean mantle plume

    NASA Astrophysics Data System (ADS)

    Costa, K.; Parman, S. W.; Saal, A. E.; Kelley, K. A.; Shimizu, N.; Nunes, J. C.; Rose-Koga, E. F.

    2012-12-01

    In order to assess pre-eruptive volatile contents of magmas in the central Azores, we have measured major element, trace element, and volatile contents of olivine hosted melt inclusions. Seventy tephra samples were collected from Sao Jorge, Pico and Faial islands. Three samples yielded naturally glassy melt inclusions, while five samples produced crystallized melt inclusions that were rehomogenized with either a one atmosphere furnace or a heating stage. The melt inclusions were analyzed for major elements, volatiles, and trace elements by electron microprobe, secondary ion mass spectrometry (SIMS), and laser ablation ICP-MS, respectively. Olivine host crystals for the melt inclusions are Fo77-88. Melt inclusions compositionally are alkali basalts with Mg #50-68, 40-51wt% SiO2, and 0.82-1.63wt% K2O (corrected for post-entrapment olivine crystallization), which is consistent with existing whole-rock data. They are trace element enriched with 19.3-49.9ppm La and 3.22-4.33 La/Sm. Volatile contents are 270-2509ppm CO2, 0.06-1.52wt% H2O, 120-1465ppm F, 30-2298ppm S, and 28-727ppm Cl. Volatile to trace element ratios are 8.4-46.5 CO2/Nb, 7-220 H2O/Ce, 2.1-42.4 F/Nd, 4-381 S/Dy, and 0.002-0.084 Cl/K. Correlation between Cl and F precludes seawater contamination as a source for the high volatile content. These data suggest that the HIMU component of the Azorean mantle plume is volatile rich, which is consistent with previously published volatile data from other HIMU sources, such as the Austral Islands plume (Lassiter et. al., 2002).

  19. Of Mantle Plumes, Their Existence, and Their Nature: Insights from Whole Mantle SEM-Based Seismic Waveform Tomography

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Many questions remain on the detailed morphology of mantle convection patterns. While high resolution P wave studies show a variety of subducted slab behaviors, some stagnating in the transition zone, others penetrating into the lower mantle (e.g. Fukao & Obayashi, 2013), low velocity structures - the upwelling part of flow - are more difficult to resolve at the same scale. Indeed, depth extent and morphology of the low velocity roots of hotspot volcanoes is still debated, along with the existence of "mantle plumes". Using spectral element waveform tomography, we previously constructed a global, radially anisotropic, upper mantle Vs model (SEMum2, French et al., 2013) and have now extended it to the whole mantle by adding shorter period waveform data (SEMUCB-WM1, French & Romanowicz, GJI, in revision). This model shows long wavelength structure in good agreement with other recent global Vs models derived under stronger approximations (Ritsema et al. 2011; Kustowski, et al. 2008), but exhibits better focused, finer scale structure throughout the mantle. SEMUCB-WM1 confirms the presence in all major ocean basins of the quasi-periodic, upper mantle low velocity anomalies, previously seen in SEMum2. At the same time, lower mantle low velocity structure is dominated by a small number (~15 globally) of quasi-vertical anomalies forming discrete "column"" rooted at the base of the mantle. Most columns are positioned near major hotspots, as defined by buoyancy flux, and are wider (~800-1000 km diameter) than expected from the thermal plume model - suggestive of thermo-chemical plumes, which may be stable for long times compared to purely thermal ones. Some columns reach the upper mantle, while others deflect horizontally near 1000 km - the same depth where many slabs appear to stagnate. As they reach the transition zone, the wide columnar structure can be lost, as these "plumes" appear to meander through the upper mantle, perhaps entrained by more vigorous, lower viscosity

  20. Geodynamically Consistent Interpretation of Seismic Tomography for Thermal and Thermochemical Mantle Plumes

    NASA Astrophysics Data System (ADS)

    Samuel, H.; Bercovici, D.

    2006-05-01

    Recent theoretical developments as well as increased data quality and coverage have allowed seismic tomographic imaging to better resolve narrower structures at both shallow and deep mantle depths. However, despite these improvements, the interpretation of tomographic images remains problematic mainly because of: (1) the trade off between temperature and composition and their different influence on mantle flow; (2) the difficulty in determining the extent and continuity of structures revealed by seismic tomography. We present two geodynamic studies on mantle plumes which illustrate the need to consider both geodynamic and mineral physics for a consistent interpretation of tomographic images in terms of temperature composition and flow. The first study aims to investigate the coupled effect of pressure and composition on thermochemical plumes. Using both high resolution 2D numerical modeling and simple analytical theory we show that the coupled effect of composition and pressure have a first order impact on the dynamics of mantle thermochemical plumes in the lower mantle: (1) For low Si enrichment of the plume relative to a reference pyrolitic mantle, an oscillatory behavior of the plume head is observed; (2) For Si-enriched plume compositions, the chemical density excess of the plume increases with height, leading to stagnation of large plume heads at various depths in the lower mantle. As a consequence, these thermochemical plumes may display broad (~ 1200 km wide and more) negative seismic velocity anomalies at various lower mantle depths, which may not necessarily be associated with upwelling currents. The second study focuses on the identification of thermal mantle plumes by seismic tomography beneath the Hawaiian hot spot: we performed a set of 3D numerical experiments in a spherical shell to model a rising plume beneath a moving plate. The thermal structure obtained is converted into P and S wave seismic velocities using mineral physics considerations. We

  1. Modeling of Mauritius as a Heterogeneous Mantle Plume

    NASA Astrophysics Data System (ADS)

    Moore, J. C.; White, W. M.; Paul, D.; Duncan, R. A.

    2008-12-01

    Mauritius Island (20°20' S, 57°30' E) is located in the western Indian Ocean and is the penultimate volcanic island of the Réunion mantle plume. Mauritius has a well-established history of episodic volcanism and erosional hiatus, traditionally characterized by three chemically and temporally distinct eruptive phases: 1) the voluminous shield-building lavas of the Older Series (8.4-5.5 Ma), 2) the Intermediate Series (3.5-1.9 Ma), and 3) the Younger Series (1.0-0.00 Ma; Duncan, unpub. data). Recent collaboration with the Mauritian Water Resource Unit has permitted the study of a series of newly available drill cores, facilitating an advanced subsurface investigation into the evolution of the island. Radiometric dating of deep lava units from these cores has identified the earliest known sample from Mauritius (B18-1; 8.4 Ma) and demonstrated the existence of Intermediate and Younger Series lavas at previously unanticipated depths, some greater than 150 meters. Calculated volumes for the combined post- erosional lavas exceed 35 km3, closely resembling new results for Hawaiian analogues (20-60 km3; Garcia, pers. comm.). While these two post-erosional series remain temporally distinct (a 0.9 M.y. hiatus remains despite new dates), they are indistinguishable in major, trace, and isotopic composition. The shield building Older Series lavas are enriched in incompatible trace elements relative to the post-erosional lavas, an inverse relationship to that observed at both Hawaii (Maui, Oahu, and Kauai) and Tahaa (Societies). In contrast isotope systematics are consistent, with shield building lavas having more enriched isotopic signatures than post-erosional lavas. The observed differences cannot be explained solely by variations in the extent of partial melting and require distinct and heterogeneous sources for the shield and post-erosional lavas. Two magma generation scenarios for a heterogeneous mantle plume with enriched (eclogitic) and depleted (peridotitic

  2. 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

  3. Tectonic plates, D (double prime) thermal structure, and the nature of mantle plumes

    NASA Technical Reports Server (NTRS)

    Lenardic, A.; Kaula, W. M.

    1994-01-01

    It is proposed that subducting tectonic plates can affect the nature of thermal mantle plumes by determining the temperature drop across a plume source layer. The temperature drop affects source layer stability and the morphology of plumes emitted from it. Numerical models are presented to demonstrate how introduction of platelike behavior in a convecting temperature dependent medium, driven by a combination of internal and basal heating, can increase the temperature drop across the lower boundary layer. The temperature drop increases dramatically following introduction of platelike behavior due to formation of a cold temperature inversion above the lower boundary layer. This thermal inversion, induced by deposition of upper boundary layer material to the system base, decays in time, but the temperature drop across the lower boundary layer always remains considerably higher than in models lacking platelike behavior. On the basis of model-inferred boundary layer temperature drops and previous studies of plume dynamics, we argue that generally accepted notions as to the nature of mantle plumes on Earth may hinge on the presence of plates. The implication for Mars and Venus, planets apparently lacking plate tectonics, is that mantle plumes of these planets may differ morphologically from those of Earth. A corollary model-based argument is that as a result of slab-induced thermal inversions above the core mantle boundary the lower most mantle may be subadiabatic, on average (in space and time), if major plate reorganization timescales are less than those acquired to diffuse newly deposited slab material.

  4. Broad plumes rooted at the base of the Earth's mantle beneath major hotspots.

    PubMed

    French, Scott W; Romanowicz, Barbara

    2015-09-01

    Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique--combining accurate wavefield computations with information contained in whole seismic waveforms--that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection. PMID:26333468

  5. Core Heat Flow and Suppression of Mantle Plumes by Plate-Scale Mantle Flow: Results From Laboratory Experiments

    NASA Astrophysics Data System (ADS)

    Gonnermann, H. M.; Jellinek, A. M.; Richards, M. A.; Manga, M.

    2002-12-01

    Heat flow from the Earth's core to the mantle remains an unresolved quantity. Its value has implications for the core's thermal evolution and growth of the inner core, the geodynamo, and the relative abundance of radioactive elements in the core and mantle. Core heat flow is affected by dynamics of the lowermost mantle in three ways: (1) advection of heat by plume instabilities; (2) conductive heating of subducted material; and (3) suppression of plume instabilities, as well as advection of heat by plate-scale mantle flow. We present results from a boundary-layer analysis and laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high-Rayleigh number (106<=Ra<=109) in a fluid with a strongly temperature-dependent viscosity. The ultimate goal of this work is to better understand the effect of plate-scale mantle flow on heat flux across the CMB and on the dynamics of plume formation at the CMB. Our theoretical analysis is complemented by lab experiments, in which a layer of corn syrup is heated from below and a large-scale flow is induced in the fluid above the hot boundary. We identify 4 convective regions associated with high-Rayleigh number convection in the presence of a large-scale flow: (1) a subcritical TBL region (Domain I), where plume instabilities are suppressed by the advective thinning of the TBL and heat flux is increased relative to convection without large-scale flow; (2) a supercritical TBL region (Domain II), where plume instabilities are no longer suppressed and heat flux is equal to convection without large-scale flow; (3) a flow-dominated region (Domain III), which is free of plumes; and (4) a plume-dominated domain (Domain IV), where the interaction of hot buoyant plumes and imposed large-scale flow results in lateral advection and distortion of rising plumes. In addition, we present a boundary-layer analysis that predicts heat flux, Q, from a hot surface as a function of imposed

  6. Past Plate Motions and The Evolution of Earth's Lower Mantle: Relating LLSVPs and Plume Distribution

    NASA Astrophysics Data System (ADS)

    Bull, A. L.; Torsvik, T. H.; Shephard, G. E.

    2015-12-01

    Seismic tomography elucidates broad, low shear-wave velocity structures in the lower mantle beneath Africa and the central Pacific with uncertain physical and compositional origins. The anomalously slow areas, which cover nearly 50% of the core-mantle boundary, are often referred to as Large Low Shear Velocity Provinces (LLSVPs) due to the reduced velocity of seismic waves passing through them. Several hypotheses have arisen to explain the LLSVPs in the context of large-scale mantle convection. One end-member scenario infers a spatial correlation between LLSVP margins at depth and the reconstructed surface eruption sites of hotspots, kimberlites, and Large Igneous Provinces. Such a correlation has been explained by the preferential triggering of plumes at LLSVP margins by impingement of the subducting lithosphere upon the lower thermal boundary layer at the interface between ambient mantle and the higher density structures. This scenario propounds that Earth's plate motion history plays a controlling role in plume development, and that the location, geometry and morphology of plumes may be influenced by the movement of subducting slabs. Here, we investigate what is necessary to create such a pattern of plume distribution in relation to LLSVPs. We consider what effect past plate motions may have had on the evolution of Earth's lower mantle, and discuss the development of mantle plumes in terms of subduction dynamics. We integrate plate tectonic histories and numerical models of mantle convection to investigate the role that subduction history plays in the development and evolution of plumes in the presence of LLSVPs. To test whether an interaction exists between the surface location of subduction and plume eruption sites, and if so, to what degree over time, we apply varying shifts to the absolute reference frame of the plate reconstruction. With this method, we are able to change the location of subduction at the surface and thus the global flow field. This in turn

  7. 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

  8. The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes.

    PubMed

    Jellinek, A Mark; Manga, Michael

    2002-08-15

    Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of 'ultra-low' seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years. PMID:12181562

  9. Reconstructing the Cenozoic evolution of the mantle: Implications for mantle plume dynamics under the Pacific and Indian plates

    NASA Astrophysics Data System (ADS)

    Glišović, Petar; Forte, Alessandro M.

    2014-03-01

    The lack of knowledge of the initial thermal state of the mantle in the geological past is an outstanding problem in mantle convection. The resolution of this problem also requires the modelling of 3-D mantle evolution that yields maximum consistency with a wide suite of geophysical constraints. Quantifying the robustness of the reconstructed thermal evolution is another major concern. To solve and estimate the robustness of the time-reversed (inverse) problem of mantle convection, we analyse two different numerical techniques: the quasi-reversible (QRV) and the backward advection (BAD) methods. Our investigation extends over the 65 Myr interval encompassing the Cenozoic era using a pseudo-spectral solution for compressible-flow thermal convection in 3-D spherical geometry. We find that the two dominant issues for solving the inverse problem of mantle convection are the choice of horizontally-averaged temperature (i.e., geotherm) and mechanical surface boundary conditions. We find, in particular, that the inclusion of thermal boundary layers that yield Earth-like heat flux at the top and bottom of the mantle has a critical impact on the reconstruction of mantle evolution. We have developed a new regularisation scheme for the QRV method using a time-dependent regularisation function. This revised implementation of the QRV method delivers time-dependent reconstructions of mantle heterogeneity that reveal: (1) the stability of Pacific and African ‘large low shear velocity provinces’ (LLSVP) over the last 65 Myr; (2) strong upward deflections of the CMB topography at 65 Ma beneath: the North Atlantic, the south-central Pacific, the East Pacific Rise (EPR) and the eastern Antarctica; (3) an anchored deep-mantle plume ascending directly under the EPR (Easter and Pitcairn hotspots) throughout the Cenozoic era; and (4) the appearance of the transient Reunion plume head beneath the western edge of the Deccan Plateau at 65 Ma. Our reconstructions of Cenozoic mantle

  10. Oceanic magmatic evolution during ocean opening under influence of mantle plume

    NASA Astrophysics Data System (ADS)

    Sushchevskaya, Nadezhda; Melanholina, Elena; Belyatsky, Boris; Krymsky, Robert; Migdisova, Natalya

    2015-04-01

    Petrology, geochemistry and geophysics as well as numerical simulation of spreading processes in plume impact environments on examples of Atlantic Ocean Iceland and the Central Atlantic plumes and Kerguelen plume in the Indian Ocean reveal: - under interaction of large plume and continental landmass the plume can contribute to splitting off individual lithosphere blocks, and their subsequent movement into the emergent ocean. At the same time enriched plume components often have geochemical characteristics of the intact continental lithosphere by early plume exposure. This is typical for trap magmatism in Antarctica, and for magmatism of North and Central Atlantic margins; - in the course of the geodynamic reconstruction under the whole region of the South Atlantic was formed (not in one step) metasomatized enriched sub-oceanic mantle with pyroxenite mantle geochemical characteristics and isotopic composition of enriched HIMU and EM-2 sources. That is typical for most of the islands in the West Antarctic. This mantle through spreading axes jumping involved in different proportions in the melting under the influence of higher-temperature rising asthenospheric lherzolite mantle; - CAP activity was brief enough (200 ± 2 Ma), but Karoo-Maud plume worked for a longer time and continued from 180 to 170 Ma ago in the main phase. Plume impact within Antarctica distributed to the South and to the East, leading to the formation of extended igneous provinces along the Transantarctic Mountains and along the east coast (Queen Maud Land province and Schirmacher Oasis). Moreover, this plume activity may be continued later on, after about 40 million years cessation, as Kerguelen plume within the newly-formed Indian Ocean, significantly affects the nature of the rift magmatism; - a large extended uplift in the eastern part of the Indian Ocean - Southeastern Indian Ridge (SEIR) was formed on the ancient spreading Wharton ridge near active Kerguelen plume. The strongest plume

  11. Searching for the signal of the Iceland plume: Seismic observations of mantle discontinuities

    NASA Astrophysics Data System (ADS)

    Jenkins, J.; Cottaar, S.; White, R. S.; Deuss, A. F.

    2015-12-01

    The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin slow velocity anomalies representative of a plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Here, we use P to S seismic wave conversions at mantle discontinuities to search for the signal of a mantle plume beneath Iceland. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including more than 100 University of Cambridge run stations. Data are used to create over 6000 receiver functions which are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities. The global transition zone discontinuities at depths of 410 and 660km are thought to be caused by phase changes in the olivine component of mantle rocks. We find that both the 410 and 660 discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The opposite signs of the Clapeyron slopes describing the olivine phase transitions predict anti-correlation of discontinuity topography, thus observations of correlated discontinuities are generally dismissed as an artefact due to under corrected upper mantle velocity variations. We suggest instead that the correlated topography we observe is caused by a garnet (as opposed to olivine) phase transition at 660 described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth. Observations of additional discontinuities in the upper mantle as well as observations of a deep ~1000km discontinuity also have the potential to shed light on the presence of a mantle plume at depth.

  12. Osmium-187 enrichment in some plumes: Evidence for core-mantle interaction?

    USGS Publications Warehouse

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

    1995-01-01

    Calculations with data for asteroidal cores indicate that Earth's outer core may have a rhenium/osmium ratio at least 20 percent greater than that of the chondritic upper mantle, potentially leading to an outer core with an osmium-187/osmium-188 ratio at least 8 percent greater than that of chondrites. Because of the much greater abundance of osmium in the outer core relative to the mantle, even a small addition of metal to a plume ascending from the D??? layer would transfer the enriched isotopic signature to the mixture. Sources of certain plume-derived systems seem to have osmium-187/osmium-188 ratios 5 to 20 percent greater than that for chondrites, consistent with the ascent of a plume from the core-mantle boundary.

  13. The LIP-OIB transitional phase in the Galapagos mantle plume

    NASA Astrophysics Data System (ADS)

    Trela, J.; Gazel, E.; Vidito, C. A.; Class, C.; Jicha, B. R.; Bizimis, M.; Herzberg, C. T.; Alvarado-Induni, G.

    2014-12-01

    Although significant work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available at this point. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages in the Cretaceous. Our petrological models (PRIMELT2) suggest that the Galapagos plume head that formed the Caribbean Large Igneous Province (CLIP) at ~95 Ma melted at hotter temperatures than the ocean island basalt (OIB) equivalents of the modern archipelago. While this work suggests a significant decrease in mantle potential temperatures (Tp) over time, the exact mechanism responsible for secular cooling of the Galapagos plume remains unclear. One viable explanation is that plumes entraining recycled oceanic crust (pyroxenite) will be cooler than purely peridotite plumes, due to the effect of dense pyroxenite on the plume's buoyancy. High-precision electron microprobe analyses on olivine cores from the ~70 Ma Galapagos-related Quepos terrane in Costa Rica indicate a mixed peridotite-pyroxenite source lithology, not evident during the LIP stage. The appearance of this pyroxenitic component correlates with the first record of an EMII isotopic signature (Northern Galapagos Domain), and significant high-field strength enrichments in the Galapagos plume related lavas. This dense pyroxenite component may explain the marked decrease in Tp observed at ~70 Ma due to its effect on the plume's buoyancy. Otherwise, the pyroxenite component may have been diluted during voluminous basalt production of the CLIP by high peridotite melt fractions. Future research will incorporate further petrological modeling, olivine chemistry, and radiogenic isotope work of accreted Galapagos terranes in Central America to test whether or not a decrease in Tp correlates with increasing pyroxenite content in source melts.

  14. The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle.

    PubMed

    Tarduno, John A; Duncan, Robert A; Scholl, David W; Cottrell, Rory D; Steinberger, Bernhard; Thordarson, Thorvaldur; Kerr, Bryan C; Neal, Clive R; Frey, Fred A; Torii, Masayuki; Carvallo, Claire

    2003-08-22

    The Hawaiian-Emperor hotspot track has a prominent bend, which has served as the basis for the theory that the Hawaiian hotspot, fixed in the deep mantle, traced a change in plate motion. However, paleomagnetic and radiometric age data from samples recovered by ocean drilling define an age-progressive paleolatitude history, indicating that the Emperor Seamount trend was principally formed by the rapid motion (over 40 millimeters per year) of the Hawaiian hotspot plume during Late Cretaceous to early-Tertiary times (81 to 47 million years ago). Evidence for motion of the Hawaiian plume affects models of mantle convection and plate tectonics, changing our understanding of terrestrial dynamics. PMID:12881572

  15. Plumes in the mantle. [free air and isostatic gravity anomalies for geophysical interpretation

    NASA Technical Reports Server (NTRS)

    Khan, M. A.

    1973-01-01

    Free air and isostatic gravity anomalies for the purposes of geophysical interpretation are presented. Evidence for the existance of hotspots in the mantle is reviewed. The prosposed locations of these hotspots are not always associated with positive gravity anomalies. Theoretical analysis based on simplified flow models for the plumes indicates that unless the frictional viscosities are several orders of magnitude smaller than the present estimates of mantle viscosity or alternately, the vertical flows are reduced by about two orders of magnitude, the plume flow will generate implausibly high temperatures.

  16. Upper- and mid-mantle interaction between the Samoan plume and the Tonga–Kermadec slabs

    PubMed Central

    Chang, Sung-Joon; Ferreira, Ana M. G.; Faccenda, Manuele

    2016-01-01

    Mantle plumes are thought to play a key role in transferring heat from the core–mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji–Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations. PMID:26924190

  17. Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs.

    PubMed

    Chang, Sung-Joon; Ferreira, Ana M G; Faccenda, Manuele

    2016-01-01

    Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (V(SH)>V(SV)) with thermo-mechanical calculations. PMID:26924190

  18. Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs

    NASA Astrophysics Data System (ADS)

    Chang, Sung-Joon; Ferreira, Ana M. G.; Faccenda, Manuele

    2016-02-01

    Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations.

  19. Dynamics of three-dimensional hydrous thermal-chemical plumes in the mantle wedge

    NASA Astrophysics Data System (ADS)

    Zhu, Guizhi; Gerya, Taras; Kaus, Boris; Tackley, Paul; Honda, Satoru; Yoshida, Takeyoshi; Yuen, David; Connolly, James

    2010-05-01

    As a plate subducts, fluid release from the subducting slab lowers the melting point of the surrounding mantle, which results in the configuration of more dense and viscous dry mantle overlying a thin layer of hydrated mantle with lowered density and viscosity. These processes trigger Rayleigh-Taylor (RT) type instabilities in a low-viscosity wedge with complex three-dimensional (3-D) geometries. RT-type cold plumes atop the slab were previously studied in 2-D. Here we use 3-D petrological-thermomechanical numerical simulations to investigate the dynamics of 3-D hydrous thermal-chemical plumes in the mantle wedge. The simulations were carried out with the I3ELVIS code which is based on a multigrid approach combined with marker-in-cell methods and conservative finite difference schemes. Our numerical simulations show that three types of upwelling plumes occur above the slab-mantle interface: (1) finger-like plumes forming roll/sheet-like structures parallel to the trench; (2) ridge-like structures perpendicular to the trench; and (3) flattened, wave-like instabilities propagating upward along the upper surface of the slab and forming zigzag patterns parallel to the trench. Plume-related melt productivity correlates well with volcanic activity clustering in natural intraoceanic arcs, such as in northeast Japan. Why do the above different plume patterns form atop the slab? Variation in partially molten rock viscosity notably affects plume patterns and lateral dimensions: wave-like plumes are most pronounced at higher (10^19 Pa s) viscosity, which also favors the development of larger plumes compared to models with lower (10^18 Pa s) viscosity. The "effective" density contrast between solid and molten rocks, which is closely related to melt extraction processes, is the key factor in determining plume patterns. A large to moderate density contrast of >200 kg/m3 (i.e. low to moderate degree of melt extraction) promotes the development of three distinct patterns of the

  20. On thermochemical mantle plumes with an intermediate thermal power that erupt on the Earth's surface

    NASA Astrophysics Data System (ADS)

    Kirdyashkin, A. A.; Kirdyashkin, A. G.

    2016-03-01

    The relative plume thermal power Ka = N/ N 1 is used ( N is the thermal power transferred from the plume base to its conduit and N 1 is the thermal power transferred from the plume conduit into the surrounding mantle in the steady-state heat conduction regime). Thermochemical mantle plumes with small (Ka < 1.15) and intermediate (1.15 < Ka < 1.9) thermal powers are formed at the core-mantle boundary beneath cratons in the absence of horizontal free-convection mantle flows beneath them, or in the presence of weak horizontal mantle flows. Thermochemical plumes reach the Earth's surface when their relative thermal power is Ka > 1.15. The thermal and hydrodynamical structure of the plume conduit ascending from the core-mantle interface to the level from which the magmatic melt erupts on the Earth's surface is presented. The model of two-stage eruption of the melt from the plume conduit to the surface is considered. The critical height of the massif above the plume roof, at which the eruption conduit supplying magmatic melt to the surface forms, is determined. The volume of melt erupting through the eruption conduit to the surface is estimated. The dependence of depth Δ x from which the melt is transported to the surface on the plume diameter for a kinematic viscosity of ν = 0.5-2 m2/s is presented. In the case when the value Δ x is larger than the depth starting from which diamond is stable (150 km), the melt from the plume conduit can transport diamonds to the Earth's surface. The melt flow in the eruption conduit is considered as a turbulent flow in a cylindrical duct. The velocity of the melt flow in the eruption conduit and the time for the melt to be transported to the surface from a depth of Δ x = 150 km for a kinematic viscosity of the melt in the eruption conduit ν v = 0.01-1 m2/s are determined. Tangential stress on the eruption conduit sidewall is estimated in cases of melt flow both in smooth and rough conduits.

  1. Seismic anisotropy and shear wave splitting associated with mantle plume-plate interaction

    NASA Astrophysics Data System (ADS)

    Ito, Garrett; Dunn, Robert; Li, Aibing; Wolfe, Cecily J.; Gallego, Alejandro; Fu, Yuanyuan

    2014-06-01

    Geodynamic simulations of the development of lattice preferred orientation in the flowing mantle are used to characterize the seismic anisotropy and shear wave splitting (SWS) patterns expected for the interaction of mantle plumes and lithospheric plates. Models predict that in the deeper part of the plume layer ponding beneath the plate, olivine a axes tend to align perpendicular to the radially directed plume flow, forming a circular pattern reflecting circumferential stretching. In the shallower part of the plume layer, plate shear is more important and the a axes tend toward the direction of plate motion. Predicted SWS over intraplate plumes reflects the asymmetric influence of plate shear with fast S wave polarization directions forming a pattern of nested U shapes that open in the direction opposing both plate motion and the parabolic shape often used to describe the flow lines of the plume. Predictions explain SWS observations around the Eifel hot spot with an eastward, not westward, moving Eurasian plate, consistent with global studies that require relatively slow net (westward) rotation of all of the plates. SWS at the Hawaiian hot spot can be explained by the effects of plume-plate interaction, combined with fossil anisotropy in the Pacific lithosphere. In ridge-centered plume models, the fast polarization directions angle diagonally toward the ridge axis when the plume is simulated as having low viscosity beneath the thermal lithosphere. Such a model better explains SWS observations in northeast Iceland than a model that incorporates a high-viscosity layer due to dehydration of the shallow-most upper mantle.

  2. 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

  3. Fluid dynamics of active heterogeneities in a mantle plume conduit

    NASA Astrophysics Data System (ADS)

    Farnetani, C. G.; Limare, A.; Hofmann, A. W.

    2015-12-01

    Laboratory experiments and numerical simulations indicate that the flow of a purely thermal plume preserves the azimuthal zonation of the source region, thus providing a framework to attribute a deep origin to the isotopic zonation of Hawaiian lavas. However, previous studies were limited to passive heterogeneities not affecting the flow. We go beyond this simplification by considering active heterogeneities which are compositionally denser, or more viscous, and we address the following questions: (1) How do active heterogeneities modify the axially symmetric velocity field of the plume conduit? (2) Under which conditions is the azimuthal zonation of the source region no longer preserved in the plume stem? (3) How do active heterogeneities deform during upwelling and what is their shape once at sublithospheric depths? We conducted both laboratory experiments, using a Particle Image Velocimetry (PIV) to calculate the velocity field, and high resolution three-dimensional simulations where millions of tracers keep track of the heterogeneous fluid. For compositionally denser heterogeneities we cover a range of buoyancy ratios 0plume axis. We find that by increasing λ, the shape of the heterogeneity changes from filament-like to blob-like characterized by internal rotation and little stretching. By increasing B the heterogeneity tends to spread at the base of the plume stem and to rise as a tendril close to the axis, so that the initial zonation may be poorly preserved. We also find that the plume velocity field can be profoundly modified by active heterogeneities, and we explore the relation between strain rates and the evolving shape of the upwelling heterogeneity.

  4. Biotic effects of late Maastrichtian mantle plume volcanism: implications for impacts and mass extinctions

    NASA Astrophysics Data System (ADS)

    Keller, Gerta

    2005-02-01

    During the late Maastrichtian, DSDP Site 216 on Ninetyeast Ridge, Indian Ocean, passed over a mantle plume leading to volcanic eruptions, islands built to sea level, and catastrophic environmental conditions for planktic and benthic foraminifera. The biotic effects were severe, including dwarfing of all benthic and planktic species, a 90% reduction in species diversity, exclusion of all ecological specialists, near-absence of ecological generalists, and dominance of the disaster opportunist Guembelitria alternating with low O 2-tolerant species. These faunal characteristics are identical to those of the K-T boundary mass extinction, except that the fauna recovered after Site 216 passed beyond the influence of mantle plume volcanism about 500 kyr before the K-T boundary. Similar biotic effects have been observed in Madagascar, Israel, and Egypt. The direct correlation between mantle plume volcanism and biotic effects on Ninetyeast Ridge and the similarity to the K-T mass extinction, which is generally attributed to a large impact, reveal that impacts and volcanism can cause similar environmental catastrophes. This raises the inevitable question: Are mass extinctions caused by impacts or mantle plume volcanism? The unequivocal correlation between intense volcanism and high-stress assemblages necessitates a review of current impact and mass extinction theories.

  5. The Kea- and Loa- trends and magma genesis in the Hawaiian mantle plume

    NASA Astrophysics Data System (ADS)

    Ren, Z.; Ingle, S.; Takahashi, E.; Hirano, N.; Hirata, T.; Tatsumi, Y.

    2005-12-01

    The Hawaiian-Emperor volcanic island and seamount chain has been created by a hot mantle plume located beneath the Pacific lithosphere. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear parallel trends, termed _eKea_Eand _eLoa_E(Jackson et al., 1972). Lavas from these two trends are commonly believed to have different geochemical characteristics (Tatsumoto, 1978; Frey et al., 1994; Hauri, 1996; Lassiter et al., 1996; Abouchami et al., 2005). The Kea- and Loa- geochemical trends within the Hawaiian shield volcanoes have been interpreted to reflect melting above a compositionally concentrically zoned (Hauri, 1996; Lassiter et al., 1996; Kurz et al., 1996; DePaolo et al., 2001) or compositionally left-right asymmetrically zoned mantle plume (Abouchami et al., 2005). In order to evaluate the homogeneity of the mantle plume source sampled by the Kea- and Loa- trends, we analyzed major and trace element compositions of olivine-hosted melt inclusions from Hawaiian shield lavas, using EPMA and Laser ICP-MS. We selected lava samples form submarine Hana Ridge, Haleakala volcano (Kea trend) and submarine exposures of the Makapuu stage, Koolau volcano (Loa trend), respectively. We found both Kea- and Loa-like major and trace element compositions from olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes, even within single rock samples. We infer from these data that although one mantle source component may dominate a single lava flow, the two (or more) mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. On the basis of whole rock geochemical characteristics (Ren et al., J. pet., 2004; 2005) combined with the melt inclusion data (Ren et al., 2005, Nature), we propose a Hawaiian mantle plume characterized by more random heterogeneity than would be present in a simple compositionally zoned mantle plume. The geochemical differences in

  6. A new model for the development of the active Afar volcanic margin

    NASA Astrophysics Data System (ADS)

    Pik, Raphaël; Stab, Martin; Bellahsen, Nicolas; Leroy, Sylvie

    2016-04-01

    response to the deformation of the lithosphere, through a petrological and geochemical study of the pre- to syn-rift lavas and concluded that the lithospheric mantle experienced the combined effect of post-plume cooling, but also thinning during the Miocene. This is accompanied by the early channelization of the plume head into narrower zones, which helped focus extension at the future volcanic margins location. The anomalous mantle potential temperature increased during the very last localization phase (< 1 Ma), which leads us to argue in favor of the focussed activity of a plume stem below the volcanic margin, instead of purely passive adiabatic decompression. Our new interpretation of the regional isotopic signatures of lavas depicts a clear framework of the Afar plume and lithospheric mantle relationships to on going extension and segmentation of these margins, and allow us to propose new contrasted models for their development.

  7. Evolution of North Atlantic Passive Margins Controlled by the Iceland Mantle Plume

    NASA Astrophysics Data System (ADS)

    Parnell-Turner, R. E.; White, N. J.; Henstock, T.; Murton, B. J.; Jones, S. M.

    2015-12-01

    Evolution of North Atlantic passive margins has been profoundly influenced by the Iceland mantle plume over the past 60 Ma. Residual depth anomalies of oceanic lithosphere, long wavelength gravity anomalies and seismic tomographic models show that upwelling mantle material extends from Baffin Bay to Western Norway. At fringing passive margins such as Northwest Scotland, there is evidence for present-day dynamic support of the crust. The Iceland plume is bisected by the Reykjanes Ridge ridge, which acts as a tape-recorder of the temporal variability of the plume. We present regional seismic reflection profiles that traverse the oceanic basin between northwest Europe and Greenland. A diachronous pattern of V-shaped ridges and troughs are imaged beneath marine sediments, revealing a complete record of transient periodicity that can be traced continuously back to ~55 Myrs. This periodicity increases from ~3 to ~8 Ma with clear evidence for minor, but systematic, asymmetric crustal accretion. V-shaped ridges grow with time and reflect small (5-30°C) changes in mantle temperature, consistent with episodic generation of hot solitary waves triggered by growth of thermal boundary layer instabilities within the mantle. Our continuous record of convective activity suggests that the otherwise uniform thermal subsidence of sedimentary basins, which fringe the North Atlantic Ocean, has been punctuated by periods of variable dynamic topography. This record can explain a set of diverse observations from the geologic record. Paleogene unconformities in the Faroe-Shetland Basin, the punctuated deposition of contourite drifts and variations in deep-water current strength can all be explained by transient mantle plume behavior. These signals of convective activity should lead to improved insights into the fluid dynamics of the mantle, and into the evolution of volcanic passive margins.

  8. Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume.

    PubMed

    Abouchami, W; Hofmann, A W; Galer, S J G; Frey, F A; Eisele, J; Feigenson, M

    2005-04-14

    The two parallel chains of Hawaiian volcanoes ('Loa' and 'Kea') are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Kea's position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume. PMID:15829954

  9. Chemical heterogeneity in the Hawaiian mantle plume from the alteration and dehydration of recycled oceanic crust

    NASA Astrophysics Data System (ADS)

    Pietruszka, Aaron J.; Norman, Marc D.; Garcia, Michael O.; Marske, Jared P.; Burns, Dale H.

    2013-01-01

    Inter-shield differences in the composition of lavas from Hawaiian volcanoes are generally thought to result from the melting of a heterogeneous mantle source containing variable amounts or types of oceanic crust (sediment, basalt, and/or gabbro) that was recycled into the mantle at an ancient subduction zone. Here we investigate the origin of chemical heterogeneity in the Hawaiian mantle plume by comparing the incompatible trace element abundances of tholeiitic basalts from (1) the three active Hawaiian volcanoes (Kilauea, Mauna Loa, and Loihi) and (2) the extinct Koolau shield (a compositional end member for Hawaiian volcanoes). New model calculations suggest that the mantle sources of Hawaiian volcanoes contain a significant amount of recycled oceanic crust with a factor of ˜2 increase from ˜8-16% at Loihi and Kilauea to ˜15-21% at Mauna Loa and Koolau. We propose that the Hawaiian plume contains a package of recycled oceanic crust (basalt and gabbro, with little or no marine sediment) that was altered by interaction with seawater or hydrothermal fluids prior to being variably dehydrated during subduction. The recycled oceanic crust in the mantle source of Loihi and Kilauea lavas is dominated by the uppermost portion of the residual slab (gabbro-free and strongly dehydrated), whereas the recycled oceanic crust in the mantle source of Mauna Loa and Koolau lavas is dominated by the lowermost portion of the residual slab (gabbro-rich and weakly dehydrated). The present-day distribution of compositional heterogeneities in the Hawaiian plume cannot be described by either a large-scale bilateral asymmetry or radial zonation. Instead, the mantle source of the active Hawaiian volcanoes is probably heterogeneous on a small scale with a NW-SE oriented spatial gradient in the amount, type (i.e., basalt vs. gabbro), and extent of dehydration of the ancient recycled oceanic crust.

  10. Bent Mantle Plumes and Channel Flow Under the 660 Km Discontinuity in Western Pacific

    NASA Astrophysics Data System (ADS)

    Yuen, D. A.; Tosi, N.

    2011-12-01

    Recent high-resolution seismic imaging by multiply-reflected S waves of the transition zone topography beneath the Hawaiian archipelago gives strong evidence for a 1000 to 2000 km wide hot thermal anomaly ponding beneath the 660 km boundary west of Hawaii islands (Cao et al., Science ,2011). This scenario suggests that Hawaiian volcanism may not be caused by a stationary narrow plume rising from the core-mantle boundary but by hot plume material first held back beneath the 660 km discontinuity and then entrained under the transition zone before coming up to the surface. Using a cylindrical convection model with multiple phase transitions, we investigate the particular dynamical conditions needed for obtaining this peculiar plume morphology. Focusing on the role exerted by pressure-dependent thermodynamic and transport parameters, we show that a strong reduction of the coefficient of thermal expansion in the lower mantle and a viscosity hill at a depth of around 1800 km are needed for plumes to have enough focused buoyancy to reach and pass through the 660 km phase boundary. The lateral spreading of plumes near the top of the lower mantle manifests itself as a channel flow whose length is controlled by the viscosity contrast due to temperature variations . For small amounts of viscosity contrast , broad and highly viscous plumes are generated which tend to pass through the transition zone relatively unscathed. For higher values , between 100 and 1000 ,we obtain horizontal channel flows beneath the 660 km boundary as long as 1500 km within a timescale that resembles that of Hawaiian hotspot history. This finding may account for the origin of the broad hot anomaly observed west of Hawaii. For a normal thermal anomaly of 450 K associated with a lower mantle plume, we obtain activation energies of about 400 kJ/mol and 600 kJ/mol for viscosity contrasts of 100 and 1000, respectively, in good agreement with values based on lower mantle mineral physics. If an increase of

  11. The Yellowstone Hotspot and Related Plume: Volcano-Tectonics, Tomography, Kinematics, Dynamics and Mantle Flow

    NASA Astrophysics Data System (ADS)

    Jordan, M.; Smith, R. B.; Puskas, C.; Farrell, J.; Waite, G.

    2005-12-01

    Earth's violent forces have produced the renowned scenery and the world's largest display of geysers at Yellowstone National Park. The energy responsible for these features is related to the Yellowstone hotspot, a coupled crust-mantle magmatic system that has had a profound influence on a much larger area of the western US: the Yellowstone-Snake River Plain-Newberry volcanic field (YSRPN). The volcanic system has produced a 16 Ma track of NE-trending, time progressive, silicic-basaltic volcanism from the Snake River Plain (SNR) to Yellowstone with a mirror image of NW-trending magmatism across the high lava plains to the Newberry caldera, OR. The origin of this magmatic-tectono system has been variously ascribed to plume-plate interaction, lithosphere extension, return mantle flow, decompression melting, etc. We interpret and integrate results from modeling of data from a prototype EarthScope experiment in 1999-2002. These include crust-mantle tomography, geoid and gravity modeling, kinematics from GPS, and geodynamic models. We present a comprehensive model for the mechanism behind YSRPN that is in accordance with our observations and models, e.g. from GPS and seismology. Geodetic data show high rates of deformation at the Yellowstone Plateau, with periods of pronounced uplift and subsidence as well as significant EW extension. Seismic tomography reveals a pronounced mid-crustal P- and S-wave low velocity body of > 8% melt extending from ~6 km to 15 km beneath the caldera. This system is fed by an upper-mantle low velocity plume-like body of up to 1.5% melt in the upper 200 km. The body further extends down to the the base of the transition zone at 650 km depth, notably tilting WNW. At this depth, we estimate the excess temperature between 85 K and 120 K, depending on the water content. Using the inclined plume-geometry and the 650-km source depth we extrapolate the mantle source southwestward as a plume-head in oceanic-type lithosphere beneath the Columbia

  12. 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

  13. Mantle plume influence on the Neogene uplift and extension of the US western Cordillera?

    USGS Publications Warehouse

    Parsons, T.; Thompson, G.A.; Sleep, N.H.

    1994-01-01

    Despite its highly extended and thinned crust, much of the western Cordillera in the United States is elevated more than 1km above sea level. Therefore, this region cannot be thought of as thick crust floating isostatically in a uniform mantle; rather, the lithospheric mantle and/or the upper asthenosphere must vary in thickness or density across the region. Utilizing crustal thickness and density constraints, the residual mass defcicit that must occur in the mantle lithosphere and asthenosphere beneath the western Cordillera was modelled. A major hot spot broke out during a complex series of Cenozoic tectonic events that included lithospheric thickening, back-arc extension, and transition from subduction to a transform plate boundary. It is suggested that many of the characteristics that make the western Cordillera unique among extensional provinces can be attributed to the mantle plume that created the Yellowstone hot spot. -Authors

  14. Midcontinent Rift and Remnants of Initiating Mantle Plume Imaged With Magnetotellurics

    NASA Astrophysics Data System (ADS)

    Bowles-martinez, E.; Schultz, A.

    2015-12-01

    Geologic evidence has long suggested that the Midcontinent Rift (MCR) was initiated by a mantle plume 1.1 Ga in the western Lake Superior region. EarthScope magnetotelluric data has been inverted to create a 3D resistivity model that shows remnants of the plume to depths of at least 150 km. The mantle plume remnants are imaged as a body of highly conductive material in the lithosphere. It is focused below western Lake Superior and northwestern Wisconsin, and elongated in a NW-SE direction, consistent with plate motion vectors. Recent seismic velocity models from EarthScope data also show an anomaly at this location. The presence of a plume after so much time has passed invites many questions regarding the long-term stability of conductive materials, the thickness of the lithosphere, and the stability of sub-craton mantle over long time periods. The resistivity model also shows features defining the length of the MCR as well as the Grenville orogeny. New data being collected this summer is incorporated into the model, extending it southeast across Grenville.

  15. Processes accompanying of mantle plume emplacement into continental lithosphere: Evidence from NW Arabian plate, Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, E. V.

    2015-12-01

    Lower crustal xenoliths occurred in the Middle Cretaceous lamprophyre diatremes in Jabel Ansaria (Western Syria) (Sharkov et al., 1992). They are represented mainly garnet granulites and eclogite-like rocks, which underwent by deformations and retrograde metamorphism, and younger fresh pegmatoid garnet-kaersutite-clinopyroxene (Al-Ti augite) rocks; mantle peridotites are absent in these populations. According to mineralogical geothermobarometers, forming of garnet-granulite suite rocks occurred under pressure 13.5-15.4 kbar (depths 45-54 kn) and temperature 965-1115oC. At the same time, among populations of mantle xenoliths in the Late Cenozoic platobasalts of the region, quite the contrary, lower crustal xenoliths are absent, however, predominated spinel lherzolites (fragments of upper cooled rim of a plume head), derived from the close depths (30-40 km: Sharkov, Bogatikov, 2015). From this follows that ancient continental crust was existed here even in the Middle Cretaceous, but in the Late Cenozoic was removed by extended mantle plume head; at that upper sialic crust was not involved in geomechanic processes, because Precambrian metamorphic rocks survived as a basement for Cambrian to Cenozoic sedimentary cover of Arabian platform. In other words, though cardinal rebuilding of deep-seated structure of the region occurred in the Late Cenozoic but it did not affect on the upper shell of the ancient lithosphere. Because composition of mantle xenolithis in basalts is practically similar worldwide, we suggest that deep-seated processes are analogous also. As emplacement of the mantle plume heads accompanied by powerful basaltic magmatism, very likely that range of lower (mafic) continental crust existence is very convenient for extension of plume heads and their adiabatic melting. If such level, because of whatever reasons, was not reached, melting was limited but appeared excess of volatile matters which led to forming of lamprophyre or even kimberlite.

  16. Bunbury Basalt: Gondwana breakup products or earliest vestiges of the Kerguelen mantle plume?

    NASA Astrophysics Data System (ADS)

    Olierook, Hugo K. H.; Jourdan, Fred; Merle, Renaud E.; Timms, Nicholas E.; Kusznir, Nick; Muhling, Janet R.

    2016-04-01

    In this contribution, we investigate the role of a mantle plume in the genesis of the Bunbury Basalt using high-precision 40Ar/39Ar geochronology and whole-rock geochemistry, and by using crustal basement thickness of the eastern Indian Ocean and the western Australian continent. The Bunbury Basalt is a series of lava flows and deep intrusive rocks in southwestern Australia thought to be the earliest igneous products from the proto-Kerguelen mantle plume. Nine new plateau ages indicate that the Bunbury Basalt erupted in three distinct phases, at 136.96 ± 0.43 Ma, 132.71 ± 0.43 Ma and 130.45 ± 0.82 Ma. All Bunbury Basalt samples are enriched tholeiitic basalts with varying contributions from the continental lithosphere that are similar to other Kerguelen plume-products. Based on plate reconstructions and the present geochronological constraints, the eruption of the oldest Bunbury Basalt preceded the emplacement of the Kerguelen large igneous province by at least 10-20 m.y. Such age differences between a precursor and the main magmatic event are not uncommon but do require additional explanation. Low crustal stretching factors beneath the Bunbury Basalt (β ≈ 1.4) indicate that decompression melting could not have been generated from asthenospheric mantle with a normal chemistry and geotherm. An elevated geotherm from the mantle plume coupled with the geochemical similarity between the Bunbury Basalt and other Kerguelen plume-products suggests a shared origin exists. However, new age constraints of the oldest Bunbury Basalt are synchronous with the breakup of eastern Gondwana and the initial opening of the Indian Ocean at ca. 137-136 Ma, which may mean an alternative explanation is possible. The enriched geochemistry can equally be explained by a patch of shallow mantle beneath the southern Perth Basin. The patch may have been enriched during Gondwana suturing at ca. 550-500 Ma, during early rifting events by magmatic underplating or by intruded melts into the

  17. Magmatic zoning of Late Cenozoic volcanism in Central Mongolia: Relation with the mantle plume

    NASA Astrophysics Data System (ADS)

    Kudryashova, E. A.; Yarmolyuk, V. V.; Kozlovsky, A. M.; Savatenkov, V. M.

    2010-05-01

    The concentric zonal structure of the Late Cenozoic volcanism areal in Central Mongolia which is situated on the territory of the Khangai vault has been educed. The central part of the structure conforms to the axial part of the vault and is presented with volcanic fields of the Watershed graben and newest valley flows. The peripheral zone is presented with volcanic fields located along the vault frame (Taryat graben, Lake Valley graben, and grabens of the Orkhon-Selenga interfluve). The structural zoning of the areal comports with the substantial zoning of volcanism products. The rocks of the central part have isotopic (Sr, Nd, Pb) and geochemical characteristics conforming to the most primitive (like PREMA) compositions of mantle sources of magmatism. Magmatism sources in the peripheral zone of the volcanic areal, besides the PREMA mantle, contained a substance of enriched mantle like EMI. The character of substantial and structural zoning of volcanism is caused by the influence of the mantle plume on the Central Asia lithosphere. According to geophysical and isotopic-geochemical data, this plume had a lower mantle nature.

  18. Modulation of mantle plumes and heat flow at the core mantle boundary by plate-scale flow: results from laboratory experiments

    NASA Astrophysics Data System (ADS)

    Gonnermann, Helge M.; Jellinek, A. Mark; Richards, Mark A.; Manga, Michael

    2004-09-01

    We report results from analog laboratory experiments, in which a large-scale flow is imposed upon natural convection from a hot boundary layer at the base of a large tank of corn syrup. The experiments show that the subdivision of the convective flow into four regions provides a reasonable conceptual framework for interpreting the effects of large-scale flow on plumes. Region I includes the area of the hot thermal boundary layer (TBL) that is thinned by the large-scale flow, thereby suppressing plumes. Region II encompasses the critically unstable boundary layer where plumes form. Region III is the area above the boundary layer that is devoid of plumes. Region IV comprises the area of hot upwelling and plume conduits. Quantitative analysis of our experiments results in a scaling law for heat flux from the hot boundary and for the spatial extent of plume suppression. When applied to the Earth's core-mantle boundary (CMB), our results suggest that large-scale mantle flow, due to sinking lithospheric plates, can locally thin the TBL and suppress plume formation over large fractions of the CMB. Approximately 30% of heat flow from the core may be due to increased heat flux from plate-scale flow. Furthermore, CMB heat flux is non-uniformly distributed along the CMB, with large areas where heat flux is increased on average by a factor of 2. As a consequence, the convective flow pattern in the outer core may be affected by CMB heat-flux heterogeneity and sensitive to changes in plate-scale mantle flow. Because of plume suppression and 'focusing' of hot mantle from the CMB into zones of upwelling flow, plume conduits (hotspots) are expected to be spatially associated with lower-mantle regions of low seismic velocities, inferred as hot upwelling mantle flow.

  19. Cenozoic intraplate volcanism in Mongolia; if not a mantle plume then what?

    NASA Astrophysics Data System (ADS)

    Barry, T. L.; Saunders, A. D.; Kempton, P. D.

    2003-04-01

    Diffuse, small-volume basaltic volcanism has occurred throughout Mongolia for the past 30 My. This region provides an excellent opportunity to study intraplate volcanism because it is clearly on continental crust and far removed from the effects of subduction-related processes. Although magma has been erupted onto 45 km thick crust, there appears to be very little crustal contamination (Barry et al., 2003). The volcanism also provides an important link between the basaltic volcanism to the north around the Baikal rift zone, which has often been related to a mantle plume and Cenozoic basaltic volcanism that infills extensional grabens within NE China. Very clear chemical similarities within all the Baikal-Mongolia-NE China Cenozoic basalts exists (Barry &Kent, 1998) suggesting that the mantle source region beneath this vast area may be the same. Therefore, one general model should be able to explain all the volcanism. Trace element, REE and isotopic modeling of Mongolian basalt compositions indicate that the melts most likely formed within the lowermost lithospheric mantle from recently metasomatised lithosphere. There is no evidence for high heat flow within the mantle (Khutorskoy &Yarmolyuk, 1989), but geophysical studies infer anomalously dense material to be present at the base of the lithospheric mantle (Petit et al., 2002) which is coincident with a low velocity zone at ~200 km depth (Villaseñor et al., 2001). However, there does not appear to be anomalous low velocity material within the asthenospheric mantle. Geochemistry of the basalts give no positive indication for the presence of an underlying mantle plume. Conversely, whilst localized extensional tectonics may have aided the extrusion of basaltic melts, the small amount of extension cannot account for the generation of the basalts (McKenzie &Bickle, 1988). Lacking evidence for a high heat flux mantle plume, we may suggest the presence of a thermal anomaly, i.e. additional heat within the asthenosphere

  20. Daly Lecture: Geochemical Insights into Mantle Geodynamics and Plume Structure (Invited)

    NASA Astrophysics Data System (ADS)

    Weis, D. A.

    2010-12-01

    It is almost 50 years since the first documentation of mantle heterogeneity through the study of ocean island basalts (OIB) [1]. The origin, scale and source of these heterogeneities have been the subject of debate since then. One of the most common approaches in the study of mantle heterogeneities is to analyze the geochemistry of oceanic basalts brought to the surface by mantle plumes. The composition of these ocean island basalts is usually different from those extruded at mid-ocean ridges (MORB), even if some of the post-shield/rejuvenated volcanism of some islands present depleted isotopic signatures. Improved analytical precision for radiogenic isotopes, combined with statistical data treatment, allow for more detailed investigations into the geochemical variations of basalts related to hotspots and mantle plumes and for modeling of the shallow and deep plume structure. Identification of two clear geochemical trends (Loa and Kea) among Hawaiian volcanoes [2, 3] in all isotope systems [4], together with the recurrence of similar isotopic signatures at >350 kyr intervals, have implications for the dynamics and internal structure of the Hawaiian mantle plume conduit [5]. In this lecture, I will present a compilation of recent isotopic data for samples from the shield, post-shield/late shield and rejuvenated stages on Hawaiian volcanoes, focusing specifically on high-precision Pb isotopic data (MC-ICP-MS or DS, TS TIMS) and integrated with Sr, Nd and Hf isotopes. The Hawaiian mantle plume represents >80 Myr of volcanic activity in a pure oceanic setting and corresponds to a high plume flux. All isotopic systems indicate source differences for Loa- and Kea-trend volcanoes that are maintained throughout the ~1 Myr activity of each volcano and that extend back in time on all the Hawaiian Islands (to ~5 Ma). The Loa-trend source is more heterogeneous in all isotopic systems by a factor of ~1.5 than the Kea-trend source. There are also different geochemical trends

  1. Greenland ice reveals imprint of the Early Cenozoic passage of the Iceland mantle plume

    NASA Astrophysics Data System (ADS)

    Rogozhina, I.; Petrunin, A. G.; Vaughan, A. P.; Kaban, M. K.; Mulvaney, R.; Steinberger, B. M.; Koulakov, I.; Thomas, M.; Johnson, J. V.

    2013-12-01

    Modelling and observation of ice sheet basal conditions suggests that elevated values of geothermal heat flow (GHF) result in enhanced basal melting. For the Greenland Ice Sheet (GIS), radar soundings and deep ice core measurements indicate unexpectedly high local values of GHF in areas where thick and stable Early Proterozoic lithosphere suggests they should be low. Rapid basal ice melt and accelerated ice flow, linked to abnormal GHF, indicate that regional heat flow patterns strongly influence the present-day thermodynamic state of the GIS and may affect its evolution in the future. Using a coupled model of climate-driven GIS and lithosphere, constrained by a wide range of interdisciplinary data, we detect a laterally continuous west-to-east area of high GHF in central-northern Greenland. The area of elevated heat flow closely coincides with a west-to-east negative anomaly in seismic velocity, which recent high-resolution tomography models tie to the present-day location of the Iceland mantle plume. Plate paleoreconstructions and analysis of magmatism in eastern and western Greenland suggest passage of the Greenland lithosphere over a mantle plume between around 80 and 35 Ma. Independent evidence under the GIS for magmatism along the putative mantle plume track comes from local gravity anomalies, igneous rock fragments recovered from the bedrock beneath the deep ice core GISP2, and radar sounding evidence of a caldera-like bedrock structure under the central GIS. We argue that long-lived, non-stationary effects of the mantle plume still affect the thermal state of the present-day Greenland lithosphere and are the origin of rapid basal ice melting over vast areas of central and northern Greenland.

  2. Correlating Large Igneous Provinces with Lower Mantle Seismic Structure - Where Is the Plume Generation Zone?

    NASA Astrophysics Data System (ADS)

    Austermann, J.; Kaye, B. T.; Mitrovica, J. X.; Huybers, P. J.

    2014-12-01

    Deep mantle seismic structure is dominated by two large, low shear wave velocity provinces (LLSVPs) below Africa and the Pacific. While different tomography models have come to a consensus over the general geometry of these provinces, the degree of thermal versus chemical heterogeneity that defines them is contentious. The location of plumes that rise from these large structures may provide insight into this question. Large Igneous Provinces (LIPs) are thought to be the surface expression of plumes that formed in the deep mantle and subsequently rose through the mantle and erupted at the surface. When restored to their original location of eruption, these LIPs appear to lie approximately above the margins of LLSVPs. This spatial correlation has been used to argue that plumes are preferentially generated at margins of LLSVPs, a notion that would tend to favor a significant chemical gradient at this margin. We assessed the robustness of this correlation by performing a series of Monte Carlo-based statistical tests (Austermann et al., Geophys. J. Int., 2014). These tests confirm that the reconstructed locations of LIPs are spatially correlated with margins of LLSVPs, but they also show that LIPs are correlated with the full areal extent of LLSVPs (parameterized as regions of slower-than-average shear wave velocity). These two correlations cannot be statistically distinguished, which means the areal extent of LLSVPs is an equally likely zone for plume generation. Therefore, based on current tomography models and reconstructed locations of LIPs, we cannot distinguish whether LIPs originated preferentially at the margins of LLSVPs or whether this correlation is merely an outcome of their origin across the full areal extent of these large scale, deep mantle structures. We will discuss the implications of our findings on the growing debate over the relative contributions of thermal and chemical effects on the net buoyancy of the LLSVPs, a factor that ultimately controls

  3. Lode Gold Deposits and Archean Mantle Plume-Island Arc Interaction, Abitibi Subprovince, Canada.

    PubMed

    Wyman; Kerrich; Groves

    1999-11-01

    In combination with seismic interpretations and geochronological constraints, the association of juvenile arc-type low-Ti tholeiitic basalts with komatiites in the southeastern Abitibi subprovince, Canada, supports a history of subduction step back following Late Archean mantle plume-island arc interaction. The resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of Canada's Precambrian gold resources in a small area of the southern Abitibi subprovince. PMID:10517886

  4. Rapid Mantle Ascent Rates Beneath Brazil: Diamond Bullets from a Smoking Plume?

    NASA Astrophysics Data System (ADS)

    Walter, M. J.; Frost, D. J.

    2010-12-01

    The concept of upwelling plumes of mantle material is, for many, integral to plate tectonics theory. However, proving that plumes exist has been frustrating, and a growing cadre of geoscientists either deny their existence, or remain uncomfortably agnostic. To the uninitiated, seismic tomography can seem a game of now-you-see-it, now-you-don’t, and igneous petrology a malarial fever of now-it's-hot, now-it's-cold. We suggest that diamonds and their mineral inclusions from Juina, Brazil, may provide direct evidence for rapid mantle ascent caused by an upwelling plume. Cretaceous kimberlites in Juina are famous for producing diamonds with inclusions that originated at transition zone and lower mantle depths [1]. Many of these sublithospheric inclusions show evidence of un-mixing of original single-phase minerals into composite inclusions during ascent in the mantle unrelated to kimberlite eruption [2,3]. What is not known is the timeframe or causality of mantle ascent. Diamonds are notoriously hard to date, but Re/Os dates of sulfide inclusions in lithospheric diamonds are generally Early Proterozoic or older, whereas host kimberlites are typically much younger [4]. If the Brazilian diamonds were also ancient, then un-mixing could have been the result of a couple billion years of passive upward migration in the mantle, unrelated to anything so torrid as a mantle plume. Diamond J1 from the Collier4 kimberlite has a composite CaTiO3+CaSiO3 inclusion in a core growth zone (originally perovskite) and a majoritic garnet inclusion in a rim zone. On the basis of excess silica in its formula, the garnet crystallized at 6-7 GPa (about 200 km), consistent with the un-mixing pressure obtained from the perovskite [5]. Experimental phase relations show that the original single-phase perovskite must have formed deeper, between about 300 and 700 km [5]. Thus, diamond J1 exhibits polybaric growth, having ascended some 100 to 500 km during its growth history. Many other mineral

  5. Alignments of volcanic features in the southern hemisphere of Mars produced by migrating mantle plumes

    NASA Astrophysics Data System (ADS)

    Leone, Giovanni

    2016-01-01

    Mars shows alignments of volcanic landforms in its southern hemisphere, starting from the equatorial regions and converging towards the South Pole, and visible at global scale. These composite alignments of volcanoes, calderas, shields, vents, heads of valley networks and massifs between the equatorial regions and the southern polar region define twelve different lines, fitted by rhumb lines (loxodromes), that I propose to be the traces of mantle plumes. The morphology of the volcanic centres changes along some of the alignments suggesting different processes of magma emplacement and eruptive style. The diameters of the volcanic centres and of the volcanic provinces are largest at Tharsis and Elysium, directly proportional to the number of alignments starting from them. A minor presence of unaligned volcanic features is observed on the northern lowlands and on the highlands outside the 12 major alignments. The heads of channels commonly interpreted as fluvial valleys are aligned with the other volcanic centres; unaltered olivine is present along their bed-floors, raising severe doubts as to their aqueous origin. Several hypotheses have tried to explain the formation of Tharsis with the migration of a single mantle plume under the Martian lithosphere, but the discovery of twelve alignments, six starting from Tharsis, favours the hypothesis of several mantle plumes as predicted by the model of the Southern Polar Giant Impact (SPGI) and provides a new view on the formation of the volcanic provinces of Mars.

  6. Seismic evidence for an Iceland thermo-chemical plume in the Earth's lowermost mantle

    NASA Astrophysics Data System (ADS)

    He, Yumei; Wen, Lianxing; Capdeville, Yann; Zhao, Liang

    2015-05-01

    We constrain the geographic extent, geometry and velocity structure of the seismic anomaly near the Earth's core-mantle boundary (CMB) beneath Iceland, based on travel time and three-dimensional waveform modeling of the seismic data sampling the lowermost mantle beneath Iceland. Our analysis suggests a mushroom-shaped low velocity anomaly situated in the lowermost mantle beneath Iceland surrounded by a high velocity province. The best fitting mushroom-shaped model is 600 km high and has a stem with a radius of 350 km in the lowermost 250 km of the mantle and a cap with increasing radii from 550 km at 250 km above the CMB to 650 km at 600 km above the CMB. The shear velocity structure varies from 0% at the top to - 3% at 250 km above the CMB and to - 6% at the CMB. These inferred seismic features, in combination with the previous evidence of existence of ultra-low velocity zones at the base of the mantle beneath the region, suggest that Iceland represents a thermo-chemical plume generated by interaction of downwelling and a localized chemical anomaly at the base of the mantle.

  7. Global P-wave tomography of mantle plumes and subducting slabs

    NASA Astrophysics Data System (ADS)

    Yamamoto, Y.; Zhao, D.

    2008-12-01

    There are many volcanoes on the Earth which can be generally classified into 3 categories: island arc volcanoes, mid-ocean ridge volcanoes, and hotspot volcanoes. Hotspot volcanoes denote intraplate volcanoes like Hawaii, or anomalously large mid-ocean ridge volcanoes like Iceland. So far many researchers have studied the origin of hotspot volcanoes and have used mantle plume hypothesis to explain them. However, we still have little knowledge about mantle plumes yet. In this study, we determined a new model of whole mantle P-wave tomography to understand the origin of hotspot volcanoes. We used the global tomography method of Zhao (2001, 2004). A 3-D grid net was set up in the mantle, and velocity perturbations at every grid nodes were taken as unknown parameters. The iasp91 velocity model (Kennett and Engdahl, 1991) was taken as the 1-D initial model. We selected 9106 earthquakes from the events occurred in the last forty years from the ISC catalog. About 1.6 million arrival-time data of five-type P phases (P, pP, PP, PcP, and Pdiff) were used to conduct the tomographic inversion. In our previous model (Zhao, 2004), the grid interval in the E-W direction is too small in the polar regions. In this study, in order to remedy this problem, we use a flexible-grid approach to make the lateral grid intervals in the polar regions nearly the same as the other portions of the mantle. As a result, the tomographic images in the polar regions are remarkably improved. Our new tomographic model shows huge low-velocity (low-V) zones in the entire mantle under Tahiti and Lake Victoria, which reflect the Pacific and African superplumes, being consistent with the previous studies. A clear low-V zone is revealed under Mt. Erebus volcano in Antarctica. Other major hotspots also exhibit significant low-V zones in the mantle under their surface locations. Beneath Bering Sea, we found that the Pacific slab is subducting from the Aleutian trench and it is stagnant in the mantle transition

  8. Geochemical evidence of mantle reservoir evolution during progressive rifting

    NASA Astrophysics Data System (ADS)

    Rooney, T. O.; Mohr, P.; Dosso, L.; Hall, C. M.

    2010-12-01

    The Afar region in East Africa, which represents the triple junction of three well-exposed Cenozoic rift systems, is a pivotal domain in the study of rift evolution. The western margin of Afar, defined by a wide transitional region from plateau to rift floor, developed in response to the rifting of the Red Sea commencing shortly after the eruption of the ~31-29 Ma Ethiopian-Yemen flood basalts. The Oligocene lava sequence which covers this rift margin was fed from intensive diking. The dikes and the block-faulting and monoclinal warping that followed provide an opportunity to probe the geochemical reservoirs preserved in the magmatic record and the development of the rifting processes. Argon geochronology reveals that dikes along the western Afar margin span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major and trace element, and isotopic results (Sr-Nd-Pb-Hf) from these dikes demonstrate temporal geochemical heterogeneity defined by variable contributions from the Afar plume, depleted mantle and African lithosphere, consistent with studies of Quaternary basalts from the Ethiopian Rift. On a broader scale our results show that as the western Afar margin matures, the initially significant contribution from the Afar plume wanes in favor of shallow asthenospheric and lithospheric reservoirs. The early dikes, which are coincident with the initial weakening of the lithosphere in a magma-assisted rifting model, geochemically resemble the widespread plume-derived flood basalts and shields that constitute the Ethiopian Plateau. Subsequent diking is characterized by a lesser role for the Afar plume and greater contributions from the African lithosphere and depleted mantle. During the terminal stage of dike emplacement, where focused magmatic intrusion accommodated extension, a more significant fraction is derived from the depleted mantle and less of a

  9. Mantle Plume Temperature Variations Immediately Following Continental Breakup of the Northern North Atlantic

    NASA Astrophysics Data System (ADS)

    Parkin, C. J.; White, R. S.; Kusznir, N. J.

    2005-05-01

    The amount of melt generated by mantle decompression beneath an oceanic spreading centre and hence the oceanic crustal thickness is controlled in part by the temperature of the mantle. By measuring the thickness of the oceanic crust formed immediately after breakup of the northern North Atlantic during the early Tertiary, we are able to deduce the maximum elevated mantle temperatures caused by the presence of the Iceland mantle plume. Crustal thickness variations are caused by temporal variations in the mantle plume temperature: at the present Reykjanes Ridge spreading centre the plume temperature pulses on a 3-5 Myr timescale with temperature variations of c.30 K. We show results from two long-offset profiles acquired over oceanic crust; firstly a 170km line perpendicular to the Faroes rifted continetal margin where oceanic spreading developed close to the Iceland mantle plume; and secondly, a 200km line perpendicular to the Hatton rifted continental margin where oceanic spreading developed 800km south of the plume. Each survey recorded long-offset refractions and reflections on OBS (Ocean Bottom Seismometers); 25 instruments, with a spacing of 2-3 km, were used for the Faroes line; and 45 instruments, with a spacing of 4-10 km were used for the Hatton-Rockall line. Accurate information for sediment velocity and thickness was acquired for the Faroes profile using a 12 km long streamer; whilst adequate sediment information was determined for the Hatton-Rockall profile using a 2.4 km streamer. By incorporating sediment structure into a joint reflection and refraction tomographic inversion of the wide-angle OBS data, we have been able to map crustal thickness across the oceanic crust in both regions. Crustal sections across the Faroes and Hatton lines cover the first 14 Myr and 17 Myr respectively, corresponding to the time interval from continental breakup through to mature seafloor spreading. With no apparent decrease in spreading rate observed thinning of the

  10. Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes.

    PubMed

    Lin, Shu-Chuan; van Keken, Peter E

    2005-07-14

    The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted. The Siberian Traps at the Permian-Triassic boundary and the Deccan Traps at the Cretaceous-Tertiary boundary were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces. PMID:16015328

  11. Ridge push, mantle plumes and the speed of the Indian plate

    NASA Astrophysics Data System (ADS)

    Eagles, Graeme; Wibisono, Affelia D.

    2013-08-01

    The buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics. In late Cretaceous and early Paleogene times, the Indian plate moved more rapidly over the mantle than freely subducting slabs sink within it. This signal event has been attributed to arrival of the Deccan-Réunion mantle plume beneath the plate, but it is unknown in which proportions the plume acted to alter the balance of existing plate driving torques and to introduce torques of its own. Our plate kinematic analysis of the Mascarene Basin yields a detailed Indian plate motion history for the period 89-60 Ma. Plate speed initially increases steadily until a pronounced acceleration in the period 68-64 Ma, after which it abruptly returns to values much like those beforehand. This pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate. A simple analysis of the gravitational force related to the Indian plate's thickening away from its boundary with the African plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head. In this instance, torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion.

  12. Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust.

    PubMed

    Cabral, Rita A; Jackson, Matthew G; Rose-Koga, Estelle F; Koga, Kenneth T; Whitehouse, Martin J; Antonelli, Michael A; Farquhar, James; Day, James M D; Hauri, Erik H

    2013-04-25

    Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials. However, the residence time of these subducted materials in the mantle is uncertain and model-dependent, and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust. Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago. Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur--probably derived from hydrothermally altered oceanic crust--was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Δ(33)S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots. PMID:23619695

  13. Mantle Plume Temperature Variations Immediately Following Continental Breakup of the Northern North Atlantic

    NASA Astrophysics Data System (ADS)

    Parkin, C. J.; White, R. S.; Kusznir, N.

    2004-12-01

    The amount of melt generated by mantle decompression beneath an oceanic spreading centre and hence the oceanic crustal thickness is controlled in part by the temperature of the mantle. By measuring the thickness of the oceanic crust formed immediately after breakup of the northern North Atlantic we are able to deduce the maximum elevated mantle temperatures caused by the presence of the Iceland Mantle Plume. Crustal thickness variations are caused by temporal variations in the mantle plume temperature: at the present Reykjanes Ridge spreading centre the plume temperature pulses on a 3-5 Myr timescale with temperature variations of c.30K. We show results from the oceanic end of the Hatton-Rockall iSIMM (integrated Seismic Imaging and Modelling of Margins) project, where for this study, 45 OBS (Ocean Bottom Seismometers) were deployed across the oceanic crust of the Iceland Basin adjacent to the rifted margin. The OBS were deployed perpendicular to the margin extending 200km oceanwards from the continent-ocean boundary at Hatton bank and along a 90km strike line on the oceanic crust. OBS spacing was 10km across the oceanic crust with a denser spacing of 4km adjacent to the margin. The profile spans magnetic anomalies 18-24 (39-56 Ma) with the strike line parallel and coincident with magnetic anomaly 20 (44Ma). The seismic source was a 101.4 Ltr (6340 cu inch), low frequency, broadband airgun source designed to optimise seismic penetration at large offsets. Magnetics, bathymetry and gravity data were acquired as well as seismic reflection data using a 3km multichannel streamer (MCS). Processing of MCS reflection data has allowed accurate determination of the sediment velocity structure covering the igneous oceanic layers. Incorporation of sediment structure into a joint reflection and refraction tomographic inversion of the wide-angle OBS data has enabled us to map crustal thickness across the oceanic crust. Results span the first 17Myr of formation of the northern

  14. Recent off-axis volcanism in the eastern Gulf of Aden: Implications for plume-ridge interaction

    NASA Astrophysics Data System (ADS)

    Leroy, Sylvie; d'Acremont, Elia; Tiberi, Christel; Basuyau, Clémence; Autin, Julia; Lucazeau, Francis; Sloan, Heather

    2010-04-01

    Evidence of anomalous volcanism is readily observed in the Gulf of Aden, although, much of this oceanic basin remains as yet unmapped. In this paper, we investigate the possible connection of the Afar hotspot with a major off-axis volcanic structure and its interpretation as a consequence of a the anomalous presence of melt by integrating several data sets, both published and unpublished, from the Encens-Sheba cruise, the Aden New Century (ANC) cruise and several other onshore and marine surveys. These include bathymetric, gravity, magnetic, magneto-telluric data, and rock samples. Based upon these observations, interpretations were made of seafloor morphology, gravity and magnetic models, seafloor age, geochemical analyses and tectonic setting. We discuss the possible existence of a regional melting anomaly in the Gulf of Aden area and of the probability of its connection to the Afar plume. Several models that might explain the anomalous volcanism are taken into account, such as a local melting anomaly unrelated to the Afar plume, an anomalously large volume of melt associated with seafloor spreading, and interaction of the ridge with the Afar plume. A local melting anomaly and atypical seafloor spreading prove inconsistent with our observations. Two previously proposed models of plume-ridge interactions are examined: the diffuse plume dispersion called pancaked flow and channelized along-axis flow. We conclude that the configuration and structure of this young ocean basin may have the effect of channeling material away from the Afar plume along the Aden and Sheba Ridges to produce the off-axis volcanism observed on the ridge flanks. This interpretation implies that the influence of the Afar hotspot may extend much farther eastwards into the Gulf of Aden than previously believed. The segmentation of the Gulf of Aden and the configuration of the Aden-Sheba system may provide a potential opportunity to study channeled flow of solid plume mantle from the plume along

  15. On the Spatial Variability of the Martian Elastic Lithosphere Thickness: Evidence for Mantle Plumes?

    NASA Astrophysics Data System (ADS)

    Grott, M.; Breuer, D.

    2009-12-01

    The elastic lithosphere thickness at the Martian North Pole has recently been constrained by estimating the flexural response of the lithosphere to loading at the polar caps and a minimum elastic thickness of 300 km has been determined. This is a factor of three to four larger than elastic thickness estimates for other Amazonian surface units like the Tharsis volcanoes, which exhibit elastic thicknesses around 75 to 90 km. Here we investigate the spatial heterogeneity of the Martian elastic lithosphere thickness and present a model which takes the locally varying crustal thickness, the local concentration of heat producing elements as well as variations of strain rate into account. The model predicts D = 225 km at the North Pole today, whereas D = 75-90 km is obtained at the Tharsis volcanoes if a mid Amazonian loading age is assumed. Therefore, although a large degree of spatial heterogeneity can be explained by the presented model, large elastic thicknesses in excess of 300 km cannot be reproduced. In order to fit all elastic thickness values derived from observations the mantle heat flow at the North Pole needs to be smaller than the global average. A local reduction of heat flow by 25% with respect to the chondritic value would be sufficient to explain the large elastic thicknesses observed there. However, a local reduction of heat flow can only be reconciled with a bulk chondritic concentration of heat producing elements in the Martian interior if the excess heat is deposited elsewhere. This could be achieved by mantle plumes, possibly active underneath Tharsis. The size and strength of such a plume is constrained by the elastic thickness at the Tharsis Montes and maximum average heat flows between 8 and 20 mW/m2, corresponding to central peak heat flows of 40 to 100 mW/m2, are consistent with the observations. Such a plume would leave a clear signature in the surface heat flow and should be readily detectable by in-situ heat flow measurements. Gray

  16. Abnormal high surface heat flow caused by the Emeishan mantle plume

    NASA Astrophysics Data System (ADS)

    Jiang, Qiang; Qiu, Nansheng; Zhu, Chuanqing

    2016-04-01

    It is commonly believed that increase of heat flow caused by a mantle plume is small and transient. Seafloor heat flow data near the Hawaiian hotspot and the Iceland are comparable to that for oceanic lithosphere elsewhere. Numerical modeling of the thermal effect of the Parana large igneous province shows that the added heat flow at the surface caused by the magmatic underplating is less than 5mW/m2. However, the thermal effect of Emeishan mantle plume (EMP) may cause the surface hear-flow abnormally high. The Middle-Late Emeishan mantle plume is located in the western Yangtze Craton. The Sichuan basin, to the northeast of the EMP, is a superimposed basin composed of Paleozoic marine carbonate rocks and Mesozoic-Cenozoic terrestrial clastic rocks. The vitrinite reflectance (Ro) data as a paleogeothermal indicator records an apparent change of thermal regime of the Sichuan basin. The Ro profiles from boreholes and outcrops which are close to the center of the basalt province exhibit a 'dog-leg' style at the unconformity between the Middle and Upper Permian, and they show significantly higher gradients in the lower subsection (pre-Middle Permian) than the Upper subsection (Upper Permian to Mesozoic). Thermal history inversion based on these Ro data shows that the lower subsection experienced a heat flow peak much higher than that of the upper subsection. The abnormal heat flow in the Sichuan basin is consistent with the EMP in temporal and spatial distribution. The high-temperature magmas from deep mantle brought heat to the base of the lithosphere, and then large amount of heat was conducted upwards, resulting in the abnormal high surface heat flow.

  17. Seismic Anisotropy near Hawaii - Evidence for plume-related mantle flow

    NASA Astrophysics Data System (ADS)

    Laske, Gabi; Marzen, Rachel

    2016-04-01

    During the Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) deployment, we collected continuous seismic data at ten land stations and nearly 70 ocean bottom sites from 2005 through mid-2007. Both the usage broad-band seismometers as well as the central location of Hawaii with good azimuthal seismicity coverage has allowed us to conduct a comprehensive analysis of surface wave azimuthal anisotropy at periods between 20 and 100 s. We use a sub-array approach to successively fit propagating spherical wave fronts in order to obtain frequency-dependent estimates at a large number of points. We use the standard Smith-and-Dahlen parameterization to express azimuthal variations. A systematic comparison between results obtained for different truncation levels in the trigonometric expansion allows us to assess stability of the results and assign error bars. At short periods, the fast direction aligns coherently with the fossil spreading direction across the entire PLUME network. This result supports the idea that flow-aligned asthenospheric material is "frozen" to the bottom of the cooling plate as it thickens. However, at longer periods, that sense the asthenosphere below the fast direction rotates incoherently, indicating that flow in the asthenosphere is significantly perturbed from the direction of current plate motion. A published shear-wave splitting study (Collins et al., 2012) found no evidence for such an anomalous mantle flow and therefore seems inconsistent with our results. We present initial surface-wave inversion results that suggest that plume-related mantle flow does not reach into the upper lithosphere. We also present forward-modeling results attempting to reconcile both surface-wave and shear-wave splitting observations. Collins, J.A., Wolfe, C.J. and Laske, G., 2012. Shear wave splitting at the Hawaiian hots pot from the PLUME land and ocean bottom seismometer deployments, Geochem. Geophys. Geosys., 13, doi:10.1029/2011gc003881.

  18. Rayleigh Wave Azimuthal Anisotropy beneath the Hawaiian Swell - Evidence for plume-related mantle flow

    NASA Astrophysics Data System (ADS)

    Laske, Gabi; Marzen, Rachel

    2015-04-01

    During the two-stage Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) deployment, we collected continuous seismic data at ten land stations and nearly 70 ocean bottom sites from 2005 through mid-2007. Both the usage broad-band seismometers as well as the central location of Hawaii with good azimuthal seismicity coverage allows us to conduct a comprehensive analysis of surface wave azimuthal anisotropy at periods between 20 and 100 s. Using a triangle method that we developed for earlier studies, we fit propagating spherical wave fronts to the phases at three stations simultaneously to determine the frequency-dependent average phase velocity within these triangles. We use the standard Smith-and-Dahlen parameterization to express azimuthal variations. A systematic comparison between results obtained for different truncation levels in the trigonometric expansion allows us to assess stability of the results and assign error bars. We observe a marked shift in the overall geometry of fast directions. At periods shorter than about 30 s, the fast direction aligns coherently with the fossil spreading direction across the entire PLUME network. This result supports the idea that flow-aligned asthenospheric material is added to the cooling plate as it thickens. This is also consistent with published PLUME shear-wave splitting observations. However, at longer periods, that sense the asthenosphere below the fast direction rotates incoherently, indicating that flow in the asthenosphere is significantly perturbed from the direction of current plate motion. We present results from forward modeling as well as initial inversions that suggest that plume-related mantle flow does not reach into the upper lithosphere, at the scales imposed by both the PLUME station spacing and the surface waves used in this study.

  19. Mixing and entrainment in mantle plumes: A 3D experimental investigation

    NASA Astrophysics Data System (ADS)

    Newsome, William; Cotel, Aline; Lithgow-Bertelloni, Carolina; Hart, Stanley; Whitehead, John

    2011-11-01

    Significant differences exist between isotopic signatures of typical mid-ocean ridge basalts (MORB) and those associated with many ocean islands, with ocean island basalts (OIB) generally exhibiting more variability in trace element concentrations and also a bias towards enrichment in radiogenic isotopes such as Sr, Nd, Hf and Pb. Such observations coupled with other geophysical evidence have been used to suggest that OIB's are surface manifestations of thermal plumes originating in the deep interior near the core-mantle boundary that interact with distinct, heterogeneous reservoirs as material is transported from the Earth's interior to the surface. We experimentally investigate the structure and transport characteristics of isolated thermal plumes in corn syrup. The 3D velocity field is measured using a scanning stereoscopic particle image velocimetry system. Two types of tracer particles are simultaneously utilized, with thermochromic liquid crystals providing an estimate of the temperature field. Lagrangian coherent structures computed from the velocity field identify key material lines and surfaces that provide a taxonomic picture of plumes operating in different regimes. These govern how the plume interacts with the ambient during its ascent.

  20. Constraining temperature and heat flux at the core-mantle boundary with plumes as a probe (Invited)

    NASA Astrophysics Data System (ADS)

    Leng, W.; Zhong, S.

    2010-12-01

    Understanding the temperature and heat flux at the core-mantle boundary (CMB) region is crucial for inferring the thermal evolution history of the Earth’s core and mantle. Plumes, which may originate at the CMB as a result of thermal boundary instabilities and rise to the Earth’s surface, are one of the most important probes which can be employed to study the temperature and heat flux at the CMB region, as Davies [1988] originally proposed. With analytical and numerical models, here we show that the CMB temperature and heat flux can be well deduced from the surface plume-related observations and the thermodynamic parameters of the Earth’s mantle. The results are summarized as following. First, we demonstrate that the total adiabatic cooling effect is exactly balanced out by the total viscous dissipation at any instant in time in compressible mantle convection. Second, we show that the cooling of plumes is dominated by the adiabatic cooling effect, thus the temperature of plumes along depth and the CMB temperature can be derived from surface observed plume temperature. Third, although high CMB heat flux occurs in regions with cold downwellings, the plume heat flux above the CMB respresents reasonably well the total CMB heat flux, supporting the original proposal by Davies [1988] but inconsistent with Labrosse [2002]. However, the plume heat flux decreases significantly from the CMB to the Earth’s surface due to the adiabatic cooling effect for plumes. At last, we obtain that the CMB temperature is ~3750 K and the CMB heat flux is ~11 TW. Our study suggests that to acquire better estimates of CMB temperature and heat flux, improved knowledge about the coefficient of thermal expansion and specific heat in the Earth’s mantle is essential.

  1. The volcanism of the Comores archipelago: mantle plume or lithosperic deformation?

    NASA Astrophysics Data System (ADS)

    Michon, Laurent

    2015-04-01

    The development of the Comores archipelago in the Mozambique channel has been diversely interpreted since the 1970's. The two end-members causes are, on the one hand, a deep mantle plume that developed a hotspot track from the Seychelles Plateau to the Grande Comore, and, on the other hand, a lithospheric deformation that reactivated transform faults and controlled the magma path. The present work first surveys the sparse geological, geophysical and geochronological data available for this archipelago, re-evaluates the age of the magmatic activity and integrates this evolution at a regional scale. Combining realistic magma production rates, the volume of each edifice and the geochronological, it is showed that the magmatic activity started first in Mayotte about 20 Ma and second, almost simultaneously, in Anjouan, Mohéli and Grande Comore about 10 Ma ago. This magmatism, coeval with magmatic periods in areas surrounding the Mozambic channel, the southern East African rift and Madagascar, is organised in three periods since Late Oligocene. Magmatic provinces are now superimposed with seismic zones and graben structures. In consequence, the Comores archipelago is tentatively interpret as part of the East African rift rather than related to a distinct deep mantle plume.

  2. Geoid anomalies and dynamic topography from convection in cylindrical geometry - Applications to mantle plumes on earth and Venus

    NASA Technical Reports Server (NTRS)

    Kiefer, Walter S.; Hager, Bradford H.

    1992-01-01

    A variety of evidence suggests that at least some hotspots are formed by quasi-cylindrical mantle plumes upwelling from deep in the mantle. Such plumes are modeled in cylindrical, axisymmetric geometry with depth-dependent, Newtonian viscosity. Cylindrical and sheet-like, Cartesian upwellings have significantly different geoid and topography signatures. However, Rayleigh number-Nusselt number systematics in the two geometries are quite similar. The geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer, provided that it is at least 1000 times the interior viscosity. Increasing the Rayleigh number or including a low-viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume.

  3. A common mantle plume source beneath the entire East African Rift System revealed by coupled helium-neon systematics

    NASA Astrophysics Data System (ADS)

    Halldórsson, Sæmundur A.; Hilton, David R.; Scarsi, Paolo; Abebe, Tsegaye; Hopp, Jens

    2014-04-01

    We report combined He-Ne-Ar isotope data of mantle-derived xenoliths and/or lavas from all segments of the East Africa Rift System (EARS). Plume-like helium isotope (3He/4He) ratios (i.e., greater than the depleted MORB mantle (DMM) range of 8 ± 1RA) are restricted to the Ethiopia Rift and Rungwe, the southernmost volcanic province of the Western Rift. In contrast, neon isotope trends reveal the presence of an ubiquitous solar (plume-like) Ne component throughout the EARS, with (21Ne/22Ne)EX values (where (21Ne/22Ne)EX is the air-corrected 21Ne/22Ne ratio extrapolated to Ne-B) as low as 0.034, close to that of solar Ne-B (0.031). Coupling (21Ne/22Ne)EX with 4He/3He ratios indicates that all samples can be explained by admixture between a single mantle plume source, common to the entire rift, and either a DMM or subcontinental lithospheric mantle source. Additionally, we show that the entire sample suite is characterized by low 3He/22NeS ratios (mostly < 0.2)—a feature characteristic of oceanic hot spots such as Iceland. We propose that the origin of these unique noble gas signatures is the deeply rooted African Superplume which influences magmatism throughout eastern Africa. We argue that the Ethiopia and Kenya domes represent two different heads of this common mantle plume source.

  4. Mantle plume related dynamic uplift and plate kinematics: The NE Atlantic case with global implications.

    NASA Astrophysics Data System (ADS)

    Skogseid, Jakob; Khabbaz Ghazian, Reza; Lunt, Ian

    2014-05-01

    At present a pronounced residual depth anomaly (RDA), centred on Iceland, is characterizing the bathymetry of the NE Atlantic region. For the oceanic lithosphere this anomaly represents a <500 to >2500 m elevation difference compared to 'normal' oceanic lithosphere. The observed depth anomaly has since Cochran and Talwani (1978) been ascribed to a 200 -300 km thick moderate thermal anomaly beneath the oceanic lithosphere, the existence of which today has been proven by a sizable low velocity zone on seismic tomography data. The sub-lithosphere low velocities are, however, not limited to the oceanic domain, but also underlie the adjacent continental lithosphere, thus causing a similar magnitude anomalous elevation of the continental shelves and landmasses. The thermal anomaly is presumed to relate to the arrival of the Iceland mantle plume demonstrated by excess Paleocene and Early Eocene magmatism and the formation of the North Atlantic Volcanic Province (NAVP), and subsequent volcanic margin formation. The present width of the RDA compares with the size of the regions that experienced excess magmatism during rifting and breakup, which implies that the sub-lithospheric thermally anomalous body was emplaced in Paleocene time, but still resides in the area. This presentation aims to describe the temporal and spatial development of uplift based on combining plate kinematic modeling with models of lithospheric and plume body thickness development through Late Cretaceous-Paleocene extension, and subsequent seafloor spreading. The model prediction of uplift compares well with descriptions of erosional episodes and depositional sequences off Greenland, in the Northern North Sea, off mid-Norway and in the SW Barents Sea, and represents a mechanism that explains the present elevation of East Greenland as well as western Norway. In a global perspective the close correlation between Large Igneous Provinces (LIP's), the arrival of known mantle plumes and formation of volcanic

  5. A mantle plume beneath California? The mid-Miocene Lovejoy Flood Basalt, northern California

    USGS Publications Warehouse

    Garrison, N.J.; Busby, C.J.; Gans, P.B.; Putirka, K.; Wagner, D.L.

    2008-01-01

    The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1-15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal "point source" explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone-Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid

  6. Geochemical characteristics of Antarctic magmatism connected with Karoo-Maud and Kerguelen mantle plumes

    NASA Astrophysics Data System (ADS)

    Sushchevskaya, Nadezhda; Krymsky, Robert; Belyatsky, Boris; Antonov, Anton; Migdisova, Natalya

    2013-04-01

    dykes of the Schirmacher Oasis and basalts and dolerites of the Queen Maud Land (180 Ma) are identical in petrology and geochemistry terms and supposedly could be interpreted as the manifestation of the Karoo-Maud plume activity in Antarctica [Sushchevskaya et al., 2012]. The spatial distribution of the dikes indicates the eastward spreading of the plume material from DML to the Schirmacher Oasis within at least 10 Ma (up to ~35 Ma, taking into account the uncertainty of age determination). On the other hand, the considerable duration and multistage character of plume magmatism related to the activity of the Karoo-Maud plume in Antarctica and Africa [Leat et al., 2007; Luttinen et al., 2002] may indicate that the Mesozoic dikes of the oasis correspond to a single stage of plume magmatism. On the basis of obtained isotopic data it has been determined two magmatic melt evolution trends for basalts from: Queen Maud Land - Kerguelen Archipelago - Afanasy Nikitin Rise (Indian Ocean) and Jetty - Schirmacher oasises which mantle sources are quite different. Thus the Jetty - Schirmacher oasises magmatic melt sources are characterized by prevalence of the matter of moderately enriched or primitive chondritic mantle source and lithospheric mantle of Proterozoic ages but the substances of depleted mantle source similar to MORB-type and ancient mantle are absent. New data obtained on Nd, Sr, Pb isotopic and lithophile elements compositions of the alkaline-ultrabasic rocks from the Jetty oasis and Gaussberg volcano completed imagine of the Kerguelen-plume evolution. It has been confirmed unique character of the alkaline lamproiites of the Gaussberg volcano enrichments. Highly radiogenic Sr and Pb isotope ratios of these lamproiites reflect melting of the ancient sublithospheric depleted mantle which was stored from the Archean till nowadays unaffected by metasomatic-enrichment processes. During modern melting of this mantle part there is input of additional substances (crustal fluid

  7. The geomagnetic field at the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries and lower mantle plumes

    NASA Astrophysics Data System (ADS)

    Pechersky, D. M.

    2007-10-01

    The data on the amplitude of variations in the direction and paleointensity of the geomagnetic field and the frequency of reversals throughout the last 50 Myr near the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries, characterized by peaks of magmatic activity of Siberian and Deccan traps, and data on the amplitude of variations in the geomagnetic field direction relative to contemporary world magnetic anomalies are generalized. The boundaries of geological eras are not fixed in recorded paleointensity, polarity, reversal frequency, and variations in the geomagnetic field direction. Against the background of the “normal” field, nearly the same tendency of an increase in the amplitude of field direction variations is observed toward epicenters of contemporary lower mantle plumes; Greenland, Deccan, and Siberian superplumes; and world magnetic anomalies. This suggests a common origin of lower mantle plumes of various formation times, world magnetic anomalies, and the rise in the amplitude of geomagnetic field variations; i.e., all these phenomena are due to a local excitation in the upper part of the liquid core. Large plumes arise in intervals of the most significant changes in the paleointensity (drops or rises), while no correlation exists between the plume generation and the reversal frequency: times of plume formation correlate with the very diverse patterns of the frequency of reversals, from their total absence to maximum frequencies, implying that world magnetic anomalies, variations in the magnetic field direction and paleointensity, and plumes, on the one hand, and field reversals, on the other, have different sources. The time interval between magmatic activity of a plume at the Earth’s surface and its origination at the core-mantle boundary (the time of the plume rise toward the surface) amounts to 20 50 Myr in all cases considered. Different rise times are apparently associated with different paths of the plume rise, “delays” in the plume

  8. Archaean to Paleozoic mantle plumes in the N-E Baltic shield

    NASA Astrophysics Data System (ADS)

    Bayanova, T.

    2003-04-01

    In Archaean the largest alkaline province occurred in the Keivy terrane. U-Pb dated zircon from alkaline and nepheline syenite (Ponoy, Zapadnokeivsky, Belaya Tundra and Saharjok) gave the age of 2.75-2.61Ga. The alkaline granite of the terrane belongs to A-type granite and has high HFSE and low Y/Nb and Yb/Ta ratios typical of enriched mantle. Nepheline syenite of Saharjok massif corresponds to analogous of the OIB-magma. ɛNd(T) ranges from +0.6 to -9.0 and reflects high metamorphic alterations of the rocks. ɛSr(T) shows a large scatter from -10000 to +100 for alkaline granite and from +100 to +5000 for subalkaline rocks which is interpreted as long-term magmatic differentiation. On ɛSr-ɛNd diagram less-altered alkaline granosyenite and alkaline gabbro lie in EM2 field (Zozulya and Bayanova, 2002). He^3/He^4 ratio for ilmenite from Ponoy massif is 0.6x10-6 suggesting contribution of mantle helium (Vetrin et al., 2000). U-Pb ages for zircons from the alkaline massifs are quite similar to those of 2613±18 and 2611±10 Ma for baddeleyite and zircon from Siilinjarvi's carbonatite. Geochronological results for the Kola alkaline massifs of the Baltic Shield increase well-known world data (Blichert-Toft et al., 1996). Close U-Pb ages are yielded by zircons from anorthosite of Achinsky, Tsaga and Medvezhje-Schuchjeozersky terranes. Duration of magmatism and isotope signatures for the rocks and minerals from alkaline and anorthosite association reflect the Archaean plume existence. In Proterozoic there are distinguished two 350 km-long belts of the PGE-bearing intrusions: Mt. Generalskaya, Monchepluton, Pana massif and Imandra Lopolith - Northern belt and Finnish Russian Southern belt with Penikat, Koiliismaa, Kontilainen and Olanga-group intrusions and Burakovsky massif in Karelia. U-Pb precise zircon-baddeleyite ages for the different parts of the intrusions (gabbronorite, gabbropegmatite and anorthosite) show 100 m.y. (2.5-2.4 Ga) duration of the mantle plume. Rocks

  9. Reconciling the geophysical and geochemical mantles: Plume flows, heterogeneities, and disequilibrium

    NASA Astrophysics Data System (ADS)

    Davies, Geoffrey F.

    2009-10-01

    Geophysical evidence and numerical models of mantle stirring imply the source of mid-ocean ridge basalts (MORBs) comprises most of the mantle, excepting only the D″ region and the "superpile" anomalies deep under Africa and the Pacific. Geophysical evidence is also strong that the mantle is heated substantially from within. Geochemical inferences of a strongly depleted MORB source are inconsistent with this picture because they would require the MORB source to be heated mainly from below and because they cannot accommodate all of the Earth's incompatible elements. Lacking any other large mantle reservoir, the MORB source is required to balance the global uranium budget, which implies a U concentration of about 10 ng/g, more than double recent estimates. The MORB source would then have been depleted only by a factor of two in highly incompatible elements, rather than four or more, relative to is primitive composition. Both geophysical and geochemical evidence support a heterogeneous, multicomponent MORB source. Surprisingly, former plume material may comprise 25% of the MORB source, and this alone could add 50-100% to previous inventories of incompatible elements. Previous geochemical estimates may also be less secure because of a continuing focus on the more common, more depleted MORBs, because of long chains of geochemical inference, and because of a reliance on peridotites that may not have equilibrated with the mean composition of the heterogeneous source. Mean compositions are of most geophysical relevance, rather than putative end-member compositions, but mean compositions will be difficult to estimate accurately because more enriched components are less common and more variable. Nevertheless, a reconciliation of geochemical and geophysical inferences seems possible.

  10. The High Arctic Large Igneous Province Mantle Plume caused uplift of Arctic Canada

    NASA Astrophysics Data System (ADS)

    Galloway, Jennifer; Ernst, Richard; Hadlari, Thomas

    2016-04-01

    The Sverdrup Basin is an east-west-trending extensional sedimentary basin underlying the northern Canadian Arctic Archipelago. The tectonic history of the basin began with Carboniferous-Early Permian rifting followed by thermal subsidence with minor tectonism. Tectonic activity rejuvenated in the Hauterivian-Aptian by renewed rifting and extension. Strata were deformed by diapiric structures that developed during episodic flow of Carboniferous evaporites during the Mesozoic and the basin contains igneous components associated with the High Arctic Large Igneous Province (HALIP). HALIP was a widespread event emplaced in multiple pulses spanning ca. 180 to 80 Ma, with igneous rocks on Svalbard, Franz Josef Island, New Siberian Islands, and also in the Sverdrup Basin on Ellef Ringnes, Axel Heiberg, and Ellesmere islands. Broadly contemporaneous igneous activity across this broad Arctic region along with a reconstructed giant radiating dyke swarm suggests that HALIP is a manifestation of large mantle plume activity probably centred near the Alpha Ridge. Significant surface uplift associated with the rise of a mantle plume is predicted to start ~10-20 my prior to the generation of flood basalt magmatism and to vary in shape and size subsequently throughout the LIP event (1,2,3) Initial uplift is due to dynamical support associated with the top of the ascending plume reaching a depth of about 1000 km, and with continued ascent the uplift topography broadens. Additional effects (erosion of the ductile lithosphere and thermal expansion caused by longer-term heating of the mechanical lithosphere) also affect the shape of the uplift. Topographic uplift can be between 1 to 4 km depending on various factors and may be followed by subsidence as the plume head decays or become permanent due to magmatic underplating. In the High Arctic, field and geochronological data from HALIP relevant to the timing of uplift, deformation, and volcanism are few. Here we present new evidence

  11. Zircon U Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

    NASA Astrophysics Data System (ADS)

    Xu, Yi-Gang; Luo, Zhen-Yu; Huang, Xiao-Long; He, Bin; Xiao, Long; Xie, Lie-Wen; Shi, Yu-Ruo

    2008-07-01

    SHRIMP zircon U-Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U-Pb dates at ˜260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios ( ɛHf( t) = -1.4 to +13.4), with corresponding TDM1 of 400-900 Ma. The highest ɛHf( t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255-251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ɛHf values of -1.3 to -4.4, and of -7.7 to -14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (˜260 Ma) and insignificant mantle contribution to 255-238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth

  12. Shear-wave splitting as a diagnostic tool for resolving plume-related mantle flow around hotspots

    NASA Astrophysics Data System (ADS)

    Bokelmann, G. H.; Walker, K. T.; Klemperer, S. L.

    2003-12-01

    The plate tectonics hypothesis successfully explains most of Earth's geological and geophysical features. However, mantle hotspots, regions often associated with large magmatic provinces, linear age progressions of volcanism, and/or large topographic swells, do not fit into a simple plate-tectonic model. Most hotspots are explained by a simple plume model, i.e. a conduit of hot buoyant upwelling material that originates from a deeper thermal boundary layer, the origin of which is often assumed to be in the lower mantle. Past global seismic tomography has had little success in resolving plume-like structures due to resolution limitations. Recent regional teleseismic imaging above hotspots has had limited success in imaging plume-like conduits down to depths of ˜400 km. Receiver function and SS precursor studies have also had limited success in detecting the thinning of the transition zone beneath hotspots, which is expected on physical grounds due to the penetration of hot plume material from below. It has been proposed recently that some of these features associated with hotspots can be explained by more complicated plate tectonic models, leading to significant debate. We analyze shear-wave splitting of upward propagating shear waves through the seismically anisotropic upper mantle around the Hawaii and Eifel hotspots, and southwest of Yellowstone along the hotspot axis. If plumes exist beneath these hotspots, these data may resolve the geometry and magnitude of upper-mantle flow and anisotropy associated with the interaction between the moving plate and upwelling plume material. Each of the hotspots we investigate is in a unique geophysical setting. For each study region, we observe an approximately parabolic pattern in map view of the splitting fast directions that is predicted by a simple kinematic plume model. The common pattern we observe, along with inferences about the location of anisotropy, suggests that plume conduits exist in at least the upper mantle

  13. Volcanology. The Yellowstone magmatic system from the mantle plume to the upper crust.

    PubMed

    Huang, Hsin-Hua; Lin, Fan-Chi; Schmandt, Brandon; Farrell, Jamie; Smith, Robert B; Tsai, Victor C

    2015-05-15

    The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we revealed a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 cubic kilometers, ~4.5 times that of the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment. PMID:25908659

  14. 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.

  15. Rapid formation of Ontong Java Plateau by Aptian mantle plume volcanism

    USGS Publications Warehouse

    Tarduno, J.A.; Sliter, W.V.; Kroenke, L.; Leckie, M.; Mayer, H.; Mahoney, J.J.; Musgrave, R.; Storey, M.; Winterer, E.L.

    1991-01-01

    The timing of flood basalt volcanism associated with formation of the Ontong Java Plateau (OJP) is estimated from paleomagnetic and paleontologic data. Much of OJP formed rapidly in less than 3 million years during the early Aptian, at the beginning of the Cretaceous Normal Polarity Superchron. Crustal emplacement rates are inferred to have been several times those of the Deccan Traps. These estimates are consistent with an origin of the OJP by impingement at the base of the oceanic lithosphere by the head of a large mantle plume. Formation of the OJP may have led to a rise in sea level that induced global oceanic anoxia. Carbon dioxide emissions likely contributed to the mid-Cretaceous greenhouse climate but did not provoke major biologic extinctions.

  16. Interaction of mantle plume heads with the earth's surface and onset of small-scale convection

    NASA Astrophysics Data System (ADS)

    Griffiths, R. W.; Campbell, I. H.

    1991-10-01

    The interaction of a mantle plume head with the earth's surface was examined by studying the behavior of a spherical blob of a buoyant fluid under the effect of gravity which forces it toward either a rigid horizontal boundary or a free surface. In the experiments, buoyant spheres of diapir fluid having no surface tension and extremely small Reynolds numbers but diameters as large as are practical in the laboratory were injected into wide cylindrical tanks filled with viscous (nu = 149 sq cm/sec) glucose syrup. Experimental results are presented for the thinning and lateral spreading of the bouyant fluid and for the thinning of the squeeze layer for both the case of a rigid, nonslip boundary (a rigid Perspex lid) and that of a free surface. These are compared with similarity scaling laws based on a balance between the buoyancy of the diapir and the viscous stresses in the diapir's surroundings.

  17. Constraints on Mantle Plume Melting Conditions in the Martian Mantle Based on Improved Melting Phase Relationships of Olivine-Phyric Shergottite Yamato 980459

    NASA Technical Reports Server (NTRS)

    Kiefer, Walter S.; Rapp, Jennifer F.; Usui, Tomohiro; Draper, David S.; Filiberto, Justin

    2016-01-01

    Martian meteorite Yamato 980459 (hereafter Y98) is an olivine-phyric shergottite that has been interpreted as closely approximating a martian mantle melt [1-4], making it an important constraint on adiabatic decompression melting models. It has long been recognized that low pressure melting of the Y98 composition occurs at extremely high temperatures relative to martian basalts (1430 degC at 1 bar), which caused great difficulties in a previous attempt to explain Y98 magma generation via a mantle plume model [2]. However, previous studies of the phase diagram were limited to pressures of 2 GPa and less [2, 5], whereas decompression melting in the present-day martian mantle occurs at pressures of 3-7 GPa, with the shallow boundary of the melt production zone occurring just below the base of the thermal lithosphere [6]. Recent experimental work has now extended our knowledge of the Y98 melting phase relationships to 8 GPa. In light of this improved petrological knowledge, we are therefore reassessing the constraints that Y98 imposes on melting conditions in martian mantle plumes. Two recently discovered olivine- phyric shergottites, Northwest Africa (NWA) 5789 and NWA 6234, may also be primary melts from the martian mantle [7, 8]. However, these latter meteorites have not been the subject of detailed experimental petrology studies, so we focus here on Y98.

  18. Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; Thompson, R. N.; Day, J. A.; Humphris, S. E.; Dickin, A. P.

    2005-09-01

    The enriched mantle 1 (EM-1) component in ocean-island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and < 3 Ma basanites and basalts from Tristan da Cunha, Inaccessible and Gough. Additionally, we have analysed Cretaceous mafic-potassic magmas from south-west Africa and eastern South America in order to establish the compositional variation of metasomatised lithospheric mantle that may have been delaminated during Gondwana break-up. The results of our rare-earth-element inversion and Sr-, Nd- and Pb-isotopic mixing models suggest that the 'depleted' mantle plume component resembles FOZO and that the composition of the 'enriched' mantle component in central South Atlantic OIB has varied both spatially and temporally. At least three different enriched mantle end-members are required to explain the compositional range of 100 to 30 Ma OIB magmas. These resemble the source regions of mafic-potassic magmas from: (i) the Congo craton and Damara belt of south-west Africa; (ii) the São Francisco craton and Brasilia belt of south-east Brazil; and (iii) the Rio Apa-Luis Alves craton of southern Brazil and Paraguay. The most isotopically enriched EM-1 basalts ( ɛNd = - 0.8 to - 4.5), generated on the Walvis Ridge and Rio Grande Rise between 89 and 78 Ma, appear to contain a 10% to 15% contribution from a melt source region with low ɛNd, 206Pb / 204Pb and high [La / Nb] n, similar in composition to metasomatised subcratonic lithospheric mantle beneath southern Brazil and Paraguay. Reconstructions of plate motions indicate

  19. Petrological processes in mantle plume heads: Evidence from study of mantle xenoliths in the late Cenozoic alkali Fe-Ti basalts in Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2015-04-01

    It is consensus now that within-plate magmatism is considered with ascending of mantle plumes and adiabatic melting of their head. At the same time composition of the plumes' matter and conditions of its adiabatic melting are unclear yet. The major source of objective information about it can be mantle xenoliths in alkali basalts and basanites which represent fragments of material of the plume heads above magma-generation zone. They are not represent material in melting zone, however, carry important information about material of modern mantle plumes, its phase composition and components, involved in melting. Populations of mantle xenoliths in basalts are characterized by surprising sameness in the world and represented by two major types: (1) dominated rocks of ``green'' series, and (2) more rare rocks of ``black'' series, which formed veins in the ``green'' series matrix. It can evidence about common composition of plume material in global scale. In other words, the both series of xenoliths represent two types of material of thermochemical mantle plumes, ascended from core-mantle boundary (Maruyama, 1994; Dobretsov et al., 2001). The same types of xenoliths are found in basalts and basanites of Western Syria (Sharkov et al., 1996). Rocks of ``green'' series are represented by Sp peridotites with cataclastic and protogranular structures and vary in composition from dominated spinel lherzolites to spinel harzburgites and rare spinel pyroxenites (websterites). It is probably evidence about incomplete homogenizing of the plume head matter, where material, underwent by partial melting, adjoins with more fertile material. Such heterogeneity was survived due to quick cooling of upper rim of the plume head in contact with relatively cold lithosphere. Essential role among xenoliths of the ``black'' series play Al-Ti-augite and water-bearing phases like hornblende (kaersutute) and Ti-phlogopite. Rocks of this series are represented by wehrlite, clinopyroxenite, amphibole

  20. Paleogeothermal record of the Emeishan mantle plume: evidences from borehole Ro data in the Sichuan basin, SW China

    NASA Astrophysics Data System (ADS)

    Hu, S.

    2013-12-01

    The Emeishan basalt province located in the southwest of China is widely accepted to be a result of the eruption of a mantle plume at the time of middle-late Permian. If it was a mantle plume, the ambient sedimentary rocks must be heated up during the development of the mantle plume and this thermal effect must be recorded by some geothermometers in the country rocks. The vitrinite reflectance (Ro) data as a maximum paleotemperature recorder from boreholes in Sichuan basin was employed to expose the thermal regime related to the proposed Emeishan mantle plume. The Ro profiles from boreholes which drilled close to the Emeishan basalts shows a ';dog-leg' (break) style at the unconformity between the middle and the upper Permian, and the Ro profiles in the lower subsection (pre-middle Permian) shows a significantly higher slopes (gradients) than those in the upper subsection. In contrast, those Ro profiles from boreholes far away from the center of the basalt province have no break at the uncomformity. Based on the chemical kinetic model of Ro, the paleo-temperature gradients for the upper and the lower subsections in different boreholes, as well as the erosion at the unconformity between the middle and the upper Permian, were reconstructed to reveal the variations of the temperature gradients and erosion thickness with geological time and space. Both the thermal regime and the erosion thickness together with their spatial variation (structure) provide strong geothermal evidence for the existence of the Emeishan mantle plume in the middle-late Permian.

  1. Indian MORB-source mantle: not just a case of plume contamination or sediment recycling

    NASA Astrophysics Data System (ADS)

    Kempton, P. D.; Pearce, J. A.

    2003-04-01

    Mid-ocean ridge basalts (MORB) from the Indian Ocean have long been known for their distinctive Pb and Sr isotope compositions relative to other MORBs. Most models for their origin involve contamination of a "normal" depleted mantle by a distinctly enriched material, the most favoured being (1) recycled oceanic crust plus pelagic sediments, (2) mantle plumes and/or (3) delaminated sub-continental lithosphere. Based on quantitative mixing models, Rehkämper and Hofmann (1997) showed that recycling of an old, compositionally heterogeneous component could explain the range of Sr, Nd and Pb isotope compositions for Indian MORBs. Their model predicts that the predominant recycled component is ancient (1.5Ga) altered ocean crust, with pelagic sediment comprising less than 10% of the contaminant. However, Hf-Nd isotope systematics are difficult to explain in this way because Indian MORBs have higher eHf values (i.e. greater time-integrated depletion of Lu relative to Hf) for a given eNd than nearly all other MORBs - and considerably higher than any of the enriched materials suggested as contaminants. Essentially, Indian and Pacific MORBs form separate and parallel arrays in Nd-Hf isotope space. What is required is a mechanism that involves not only enrichment of some elements, but also relative depletion of others. Based on new Nd-Hf isotope data for Indian and Pacific MORBs from the Australian-Antarctic Discordance, we propose that the distinctive Indian MORB source composition can be explained by recycling of subduction-modified mantle. This mantle could have been generated within the convergent margin that existed off the east coast of Gondwana throughout most of the Paleozoic and Mesozoic Eras. It was subsequently recycled into the upper mantle beneath Gondwana and became the source of Indian MORBs following the break-up of the Gondwanan supercontinent. Rehkämper, M., and A. W. Hofmann, Recycled ocean crust and sediment in Indian Ocean MORB, Earth and Planetary

  2. Mongolian plateau: Evidence for a late Cenozoic mantle plume under central Asia

    NASA Astrophysics Data System (ADS)

    Windley, Brian F.; Allen, Mark B.

    1993-04-01

    The 2500 x 700 km Mongolian plateau (average elevation 2000 m) is situated between the Altai orogen and the Siberian craton and occupies much of Mongolia and Transbaikalia in Russia. The plateau is characterized by (1) basin and range topography and two major domes(Hentai, 600 x 300 km, and Hangai, 800 x 550 km), where altitudes reach 3905 m; (2) lithosphere that is thinner than adjacent areas (minimum ˜50 km); (3) elevated heat flow (up to 120 mW/m2); (4) dominantly alkaline basaltic volcanism in the form of cones, lava fields, and volcanic plateaus mostly of Miocene-Quaternary age, and (5) rifts, including Baikal (main evolution in the Pliocene-Quaternary), Tunka (Oligocene-early Miocene), and Hobsogol (Pliocene-Quaternary). Existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and/or as a long-distance effect of the India-Asia collision. We propose that the late Cenozoic uplift of the whole Mongolian plateau and associated rifting, magmatism, high heat flow, and lithospherec thinning are not externally driven by the India-Asia collision, but are the expression of the interaction of a mantle plume with overlying lithosphere. Some rifts link and interact with major strike-slip faults, such as the Bolnai. Such faults may be the major expression of the India-Asia collision in this region.

  3. Relative and Absolute Plate Motions, Mantle Plumes and Volcanism in the Arctic region

    NASA Astrophysics Data System (ADS)

    Gaina, C.; Torsvik, T. H.

    2012-04-01

    Seafloor spreading in the North Atlantic ocean from Mesozoic until present day involved relative motion between three major tectonic plates: North America, Greenland and Eurasia and a number of microplates. Relative motions between these tectonic plates and movement of northern Pacific terranes since the Jurassic led to the development of the Arctic region as we know it today. Studying the connection between the two realms involve good knowledge of the development of the North Atlantic and Arctic margins and oceanic basins and ideally, model uncertainties. Here we review the kinematics of North Atlantic and asses the implications of different models for locating the plate boundaries in the Arctic. One set of models implies extension before opening of the Eurasia basin and we postulate that this was accommodated in the proximity of Alpha- Mendeleev Ridge. The origin of (mainly) Cretaceous large igneous activity in the central Arctic (the Alpha Mendeleev Ridge) and in the proximity of rifted margins, the so-called HALIP, is still debated. New models of global plate circuits and the connection with deep mantle are used to re-evaluate a possible link between the Arctic volcanism and mantle plumes.

  4. Sampling the Cape Verde Mantle Plume: Evolution of Santo Antão, Cape Verde Islands

    NASA Astrophysics Data System (ADS)

    Holm, P. M.; Wilson, J. R.; Christensen, B. P.; Hansen, L.; Hansen, S. L.; Hein, K. M.; Mortensen, A. K.; Pedersen, R.; Plesner, S.; Runge, M. K.

    2003-12-01

    The 7.5 - 0.1 Ma old volcanics of the northwesternmost Cape Verde Island of Santo Antão show a change from early incompatible element enriched basanite-phonolite series to more enriched nephelinite/melilite nephelinite-phonolite series volcanics all of HIMU OIB type. Mantle melts were derived by 1-4 % melting and had around 12 wt.% MgO. Olivine Fo88-91 is found in many primitive volcanics. Incompatible element modelling shows that the geochemical change of the composition of the primary magmas requires source enrichment by silicate melts of mainly two compositional types. One of these is MORB. Isotopically the > 2 Ma Old Volcanics group can largely be explained by mixing of two components both with relatively radiogenic Sr and unradiogenic Nd of which one is a young HIMU type source (Δ 8/4 ˜ 0 and Δ 7/4 ˜ -5). The period 2 - 0.7 Ma saw two component mixing of two other end members of which one is a young HIMU source with less radiogenic Sr, more radiogenic Nd, Δ 8/4 ˜ -38 and Δ 7/4 ˜ -5, which isotopically is identical to an end member of carbonatites from the neighbouring island of São Vicente and the southern island Santiago. The youngest volcanics show stronger source enrichment, the most silica undersaturated magmas and an old HIMU-type component (Δ 7/4 > 2). The characteristic EM1-type enrichment of the southern Cape Verde Island is not detected on Santo Antão. We argue that the main components of Santo Antão volcanism are plume derived and reflect vertical variation in composition of rising plume material. The inter island variation of the Cape Verdes may reflect a lateral variation of the plume or lithosphere derived components in the southern island volcanics

  5. Contourite Deposition in the North Atlantic Ocean Moderated By Mantle Plume Activity: Evidence from Seismic Reflection Images

    NASA Astrophysics Data System (ADS)

    Parnell-Turner, R. E.; McCave, I. N. N.; White, N. J.; Henstock, T.; Murton, B. J.; Jones, S. M.

    2014-12-01

    It is generally accepted that the strength of Northern Component Water overflow, the ancient precursor of North Atlantic Deep Water, has varied throughout Neogene times. Variations in dynamic support of the lithosphere, due to transient behavior of the Iceland mantle plume, probably control spatial and temporal water depth variations this region. Pathways and intensities of oceanic bottom currents, together with deposition of contourite drifts, are strongly influenced by changing bathymetry. Here, we combine detailed observations of contourite drift deposits from seismic reflection profiles with a chronology of plume activity, to test the relationships between deep-water circulation, sedimentary drift accumulation and mantle convection. We present multi-channel seismic reflection profiles acquired over Bjorn, Gardar and Hatton Drifts in the Iceland Basin and over the northernmost portion of Eirik Drift, east of Greenland. Depositional hiatuses are easily identified and correlated between these high-quality images and nearby boreholes, which allows us to construct history of sedimentation across the North Atlantic Ocean over the past 5 Ma. We observe kilometer-scale westward-migration of Bjorn Drift, which can be explained by varying current strength and sediment supply, probably moderated by fluctuating dynamic support on overall subsidence. We place these observations into a new continuous 55 Ma record of Iceland mantle plume activity. There is compelling evidence to support the hypothesis that variations in mantle convection deep beneath the plates has profound consequences for deep-water flow and sediment deposition at Earth's surface.

  6. Geographic Variations in Hotspot Geochemistry Caused by 3D Dynamics and Melting of a Heterogeneous Mantle Plume

    NASA Astrophysics Data System (ADS)

    Bianco, T. A.; Ito, G.; van Hunen, J.; Ballmer, M.; Mahoney, J. J.

    2006-12-01

    Spatial variations in magma geochemistry among hotspot volcanoes hold clues to the dynamics and composition of the mantle feeding hotspot volcanism. We use a 3D geodynamic model of plume-lithosphere interaction to explore the causes of spatial patterns of magmatic volumes and compositions at intraplate hotspots. This study focuses on coupling between upper mantle flow, heat transfer, and melting of a heterogeneous (veined) plume. We assume multiple lithologies have different solidi, trace-element, and isotope composition. We use the Cartesian finite-element code, CITCOM, (Zhong and Watts, 2002) to simulate mantle convection with the extended Boussinesq approximation in a volume of upper mantle 400 km in thickness. A parameterized melting model is used to simulate melting of materials with different water contents (Katz et al., 2003). Melt depletion (F) for each lithology is calculated at finite element nodes as a function of temperature, pressure, and water content and is advected using particle tracers. We quantify the response of the geographic pattern of the volume and composition of magmas to different lithospheric thicknesses, and plume temperatures and viscosities, which together control the melting rates and sizes of the melting zones for the different lithologies. In the case of two-lithologies, preliminary results of a sluggishly convecting plume rising beneath thick lithosphere (60-100 km) predict that the melting zone of the least refractory "lithology 1" is wider than that of the more refractory "lithology 2". This leads to the prediction that on the surface, the isotope signature of lithology 1 is most prominent at the leading edge (i.e., upwind edge of plate motion) of the hotspot, whereas the isotope signature of lithology 2 is strongest at the hotspot center. This pattern will likely change for plumes convecting more vigorously or thinner lithosphere.

  7. Geodynamic models of a Yellowstone plume and its interaction with subduction and large-scale mantle circulation

    NASA Astrophysics Data System (ADS)

    Steinberger, B. M.

    2012-12-01

    Yellowstone is a site of intra-plate volcanism, with many traits of a classical "hotspot" (chain of age-progressive volcanics with active volcanism on one end; associated with flood basalt), yet it is atypical, as it is located near an area of Cenozoic subduction zones. Tomographic images show a tilted plume conduit in the upper mantle beneath Yellowstone; a similar tilt is predicted by simple geodynamic models: In these models, an initially (at the time when the corresponding Large Igneous Province erupted, ~15 Myr ago) vertical conduit gets tilted while it is advected in and buoyantly rising through large-scale flow: Generally eastward flow in the upper mantle in these models yields a predicted eastward tilt (i.e., the conduit is coming up from the west). In these models, mantle flow is derived from density anomalies, which are either inferred from seismic tomography or from subduction history. One drawback of these models is, that the initial plume location is chosen "ad hoc" such that the present-day position of Yellowstone is matched. Therefore, in another set of models, we study how subducted slabs (inferred from 300 Myr of subduction history) shape a basal chemically distinct layer into thermo-chemical piles, and create plumes along its margins. Our results show the formation of a Pacific pile. As subduction approaches this pile, the models frequently show part of the pile being separated off, with a plume rising above this part. This could be an analog to the formation and dynamics of the Yellowstone plume, yet there is a mismatch in location of about 30 degrees. It is therefore a goal to devise a model that combines the advantages of both models, i.e. a fully dynamic plume model, that matches the present-day position of Yellowstone. This will probably require "seeding" a plume through a thermal anomaly at the core-mantle boundary and possibly other modifications. Also, for a realistic model, the present-day density anomaly derived from subduction should

  8. Impact of lithosphere rheology on 3D continental rift evolution in presence of mantle plumes: insights from numerical models

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Burov, Evgueni; Gerya, Taras

    2015-04-01

    We implement fully-coupled high resolution 3D thermo-mechanical numerical models to investigate the impact of the laterally heterogeneous structure and rheological stratification of the continental lithosphere on the plume-activated rifting and continental break-up processes in presence of preexisting far-field tectonic stresses. In our experiments, the "plumes" represent short-lived diapiric upwellings that have no continuous feeding from the depth. Such upwellings may be associated with "true" plumes but also with various instabilities in the convective mantle. The models demonstrate that the prerequisite of strongly anisotropic strain localization during plume-lithosphere interaction (linear rift structures instead of axisymmetric radial faulting) refers to simultaneous presence of a mantle upwelling and of (even extremely weak) directional stress field produced by far-field tectonic forces (i.e. ultra-slow far field extension at < 3 mm/y). Although in all experiments the new-formed spreading centers have similar orientations perpendicular to the direction of the main far-field axis, the models with homogeneous lithosphere show that their number and spatial location is different for various extension rates and thermo-rheological structures of the lithosphere: relatively slow extension (3 mm/year) and colder isotherm (600-700°C at Moho depth) at the crustal bottom lead to the development of single rifts, whereas "faster" external velocities (6 mm/year) and "hotter" crustal geotherm (800°C at Moho depth) result in dual (sometimes asymmetric) rift evolution. On the contrary, the models with heterogeneous lithosphere (thick cratonic block with cold and thick depleted mantle embedded into «normal» lithosphere) and the plume centered below the craton, systematically show similar behaviors: two symmetrical and coeval rifting zones embrace the cratonic micro-plate along its long sides. The experiments where the initial plume position has been laterally shifted with

  9. Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge-mantle plume interaction?

    NASA Astrophysics Data System (ADS)

    Breivik, AsbjøRn Johan; Faleide, Jan Inge; Mjelde, Rolf

    2008-02-01

    According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian-Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge-driven convection, or by asthenosphere flow-lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid-ocean ridge basalt-capture model, in which this magmatism can be understood through plume-spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low-degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low-degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

  10. Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau (Invited)

    NASA Astrophysics Data System (ADS)

    Kerr, A. C.; Hastie, A.

    2009-12-01

    The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate due to melting of (possibly) the Galapagos hotspot at ~93 Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high-MgO) Caribbean oceanic plateau lavas from Curaçao, shows that the primary magmas of the plateau contained ~20 wt.% MgO and were derived from 30-32 % partial melting of a fertile peridotite source region which had a potential temperature (Tp) of 1564-1614 °C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle Tp cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in, or close to, a slab window setting, is therefore, highly improbable. Reference Hastie, A.R., Kerr, A.C. 2010. Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau. Earth Science Reviews, in press.

  11. Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau

    NASA Astrophysics Data System (ADS)

    Hastie, Alan R.; Kerr, Andrew C.

    2010-02-01

    The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate from the Galapagos hotspot at ˜ 90 Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high MgO) Caribbean oceanic plateau lavas from Curaçao, shows that the primary magmas of the plateau contained ˜ 20 wt.% MgO and were derived from 30 to 32% partial melting of a fertile peridotite source region which had a potential temperature ( Tp) of 1564-1614 °C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle Tp cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in a slab window setting, is therefore, highly improbable.

  12. Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

    USGS Publications Warehouse

    Nicholson, S.W.; Shirey, S.B.

    1990-01-01

    Between 1091 and 1098 Ma, most of a 15- to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the younget MRS flood basalts, fall into distinctly high- and low-TiO2 types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle and both basalt types are isotopically indistinguishable. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850 ?? 103 km3) of relatively homogeneous magma in a restricted time interval. -from Authors

  13. Hot Spots and Mantle Plumes: A Window Into the Deep Earth and a Lesson on How Science Really Works

    NASA Astrophysics Data System (ADS)

    Caplan-Auerbach, J.

    2010-12-01

    Despite years of discussion, debate and controversy over the causes of ocean island volcanism, most students simply learn that such features form from fixed plumes of hot material rising from the core mantle boundary. Although we know that the Hawaiian plume exhibited substantial southward motion, most introductory geology textbooks still report that hot spots are fixed and that the Hawaiian-Emperor bend reflects a change in plate motion. That mantle plumes are the focus of significant controversy within the scientific community is rarely, if ever, discussed, and alternative models for the formation of intraplate volcanoes are ignored. Students may thus complete their studies without learning about the dynamic debate focused on the existence and formation of mantle plumes. This issue represents an opportunity for students to see how science really works, how new models are constructed, and what distinguishes a hypothesis from a theory. The culminating project in Western Washington University’s Introduction to Geophysics class, a course required for the BS degree in geology, focuses on the hot spot and mantle plume debate. For the first nine weeks of the quarter students learn about general topics in geophysics including plate tectonics, magnetism, seismology, gravity and heat flow. At the end of the course, students break into small research groups with the goal of investigating how geophysics may be used to address three questions: (1) Do ocean island volcanoes form from mantle plumes? (2) Are “hot spots” actually hot? (3) Are hot spots stationary? Each group examines how these questions may be addressed using a specific geophysical tool. In addition to the five topics described above, a sixth group investigates the question of “if not hot spots/mantle plumes, how do ocean island volcanoes form?” Students read the current literature on the topic and present their results to their classmates. Presentations focus on topics such as the use of seismic

  14. The Hawaiian Mantle Plume from Toe to Head along the Northwest Hawaiian Ridge

    NASA Astrophysics Data System (ADS)

    Harrison, L.; Weis, D.; Garcia, M. O.

    2015-12-01

    The Hawaiian-Emperor (HE) chain records ~82 Myr of volcanism1 with two distinct geochemical and geographical trends, Kea and Loa, identified on the archipelago. The Northwest Hawaiian Ridge (NWHR) includes 51 volcanoes, spanning ~42 Myr between the bend in the HE chain and the Hawaiian Islands (47% of the HE chain2), that has no high-precision isotopic data aside from two volcanoes near the bend1. Only Kea compositions have been observed on Emperor seamounts (>50 Ma)1,3, whereas the Hawaiian Islands (<6.5 Ma) have both Kea and Loa lavas3,4. We have analyzed 23 samples of shield stage tholeiitic lavas from 13 NWHR volcanoes for Pb isotopes to test if the Loa trend exhibits a persistent presence along the ridge after Diakakuji seamount1. Age corrected 206Pb/204Pb range from 17.870 at Diakakuji to 18.654 at Midway atoll. The most enriched Loa isotopic compositions are erupted at Diakakuji (comparable to Lanai), and Mokumanamana, West Nihoa, and Nihoa have isotopic compositions similar to Mauna Loa. These observations suggest an ephemeral presence of the Loa geochemical trend along the NWHR. When shield-stage lavas of each Hawaiian volcano is averaged, NWHR volcanoes shows the most and least radiogenic Pb of the entire HE dataset: Diakakuji (0.9703) and Midway (0.9247). The NWHR exhibits the most geochemically extreme lava compositions along a region where many geophysical parameters (volcanic propagation rate, magmatic flux, mantle potential temperature) were changing significantly2,5. At a broader scale, correlation between radiogenic Pb and magmatic flux suggests source composition may control some of these changes, and help explain why the Hawaiian mantle plume seems to be strengthening5 rather than waning like classic plumes and LIPs. 1Regelous et al., 2003, J. Pet., 44, 1, 113-140. 2Garcia et al., 2015, GSA Sp. Pap. 511. 3Tanaka et al., 2008, EPSL, 265, 450-465. 4Weis et al., 2011, Nat. Geosci., 4, 831-838. 5Vidal & Bonneville, 2004, J. Geophy. Res., 109.

  15. Two Plumes Beneath the East African Rift System: a Geochemical Investigation into Possible Interactions in Ethiopia

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Furman, T.; van Keken, P. E.; Lin, S.

    2007-12-01

    East African Rift System magmatism began over 40 my ago and has continued through the present. Numerical models have determined two plumes are necessary to create the spatial and temporal distribution of volcanism. Geochemical data support the presence of two chemically distinct plumes initially located beneath the Afar Depression (NE Ethiopia) and the Turkana Depression (SW Ethiopia/N Kenya). The timing and eruptive of the Afar and Kenya plumes are also distinct. While there is growing evidence to support the existence of two dynamically and chemically distinct plumes beneath the East African Rift System, the interactions between them remain unclear. Our study focuses on the geochemistry of mafic shield lavas from three locations on the eastern flank of the Ethiopian plateau. These lavas are spatially located between the surface manifestation of the Afar and Kenya plumes. The majority of the lava is alkaline and has experienced varying degrees of olivine and pyroxene fractionation. The northernmost lavas (9°10'N) are transitional and display the most fractionation. Primitive mantle melts were generated at depths near the fertile mantle garnet-spinel transition zone and deeper (80-100km) and are free of metasomatic influence. Minor HREE depletions also support derivation of melts from a garnet-bearing source. Lavas with lithospheric influence are generated from shallower depths and show minor amphibole influence. Overall, geochemical data show the lavas in this study closely resemble those from various episodes of Kenya plume magmatism with modifications attributed to lithospheric contamination. This interpretation is consistent with current numerical models suggesting episodic northward movement of Kenya plume magmas along the lithosphere-asthenosphere boundary. The data imply that the Kenya plume has a much larger spatial influence and therefore a larger geodynamic influence in the EARS than previously recognized.

  16. Late Palaeocene Mantle Plume Uplift on The Fugloy Ridge, NE Faroes

    NASA Astrophysics Data System (ADS)

    Hurst, N. W.; Kusznir, N. J.; Roberts, A. M.

    2004-12-01

    The Fugloy ridge is a large (~100 km wide) anticlinal structure situated to the NE of the Faroe Islands separating the Norwegian Ocean basin from the Faroe-Shetland trough. Flexural backstripping and post-breakup thermal subsidence modelling has been performed on a profile crossing the Fugloy Ridge to provide an estimate of mantle plume uplift at the end of the Palaeocene (~55 Ma). The modelling is carried out on a 370 km Q-marine multi-streamer swath reflection profile acquired by the M/V Geco Topaz during the summer of 2002 as part of the iSIMM (integrated Seismic Imaging and Modelling of Margins) project seismic acquisition programme. The profile provides good resolution of post-breakup sediment structure across the margin and also of deeper sub-basaltic structure along the profile. Flexural backstripping and reverse post-breakup thermal subsidence modelling is a 2D (or 3D) technique which is used to restore present day stratigraphic cross sections to earlier post-breakup times. The method removes units of stratigraphy from the top-downwards and calculates isostatic and sediment decompaction responses to this unloading. Thermal subsidence arises from the cooling of stretched continental lithosphere and the recently formed oceanic lithosphere, and may be predicted from the lithosphere beta stretching factor (McKenzie, 1978). Two approaches have been used to determine beta stretching estimates for the profile, the first approach uses beta stretching factors from crustal thinning estimates derived from a gravity anomaly inversion technique (Hurst et al., 2004). The second approach uses palaeo-bathymetric constraints to determine the beta stretching estimates for the profile. Results from the modelling show that the Fugloy Ridge present day stratigraphy flattens out progressively as the 2D cross section is restored to breakup (55 Ma) using beta stretching factor estimates derived from gravity anomaly inversion. The Fugloy Ridge has been proposed as a possible

  17. Mantle support of the East African Rift System

    NASA Astrophysics Data System (ADS)

    Lin, S.; van Keken, P. E.; Brandenburg, J. P.; Furman, T.; Bryce, J.

    2007-12-01

    The African Superplume is a region of slow seismic wave velocities in the lower mantle under southern Africa. The uplift, volcanism and rifting that defines the much of eastern and southern Africa suggest a dynamic link between lower mantle dynamics and near-surface processes affecting the African plate. The dynamic link between the lower mantle and the surface, and the structure and dynamics of the upper mantle below the East African Rift System (EARS) remain unclear. As part of a comprehensive geochemical and numerical investigation of basaltic magmatism in the EARS we have modeled the interaction between putative upper mantle plumes and the rifting continental lithosphere. The modeling provides dynamically tested scenarios that explain the observed episodes of Cenozoic volcanism. Results from recent models that provided an explanation for the present day distribution of volcanism (Lin et al., EPSL, 237, 2005) suggest two plumes below Afar and Tanzania whose uplift is influenced by lithospheric topography. In new 3D modeling we provide improved quantification of the mantle involvement in generating EARS volcanism as constrained by the timing of uplift and regional volcanism. The time scales of episodicity of the volcanism observed at Turkana (related to the Tanzania-Kenya plume) since 45 Ma can be explained by deep- seated time-dependent plume activity. We suggest that this time-dependence is due to thermochemical interactions of dense recycled oceanic crust in the thermally hot regions in the African superplume region (Lin and Van Keken, Nature, 436, 2005).

  18. Influence of the Iceland mantle plume on North Atlantic continental margins

    NASA Astrophysics Data System (ADS)

    White, R. S.; Isimm Team

    2003-04-01

    Early Tertiary breakup of the North Atlantic was accompanied by widespread magmatism. The histories of the Iceland mantle plume, of rifting and of magmatism are intimately related. The magmatism provides a challenge both to imaging structure, and to modelling the subsidence and development of the continental margins. We report new work which integrates state-of-the-art seismic imaging and new acquisition on the Atlantic volcanic margins with new techniques for modelling their evolution. We discuss the distribution of igneous rocks along the North Atlantic margins and discuss the temporal and spatial variations in the Iceland mantle plume in the early Tertiary, which have largely controlled this pattern of magmatism. Igneous rocks are added to the crust on rifted margins as extrusive lavas, as sills intruded into the sub-surface and as lower crustal intrusions or underplate. Each provide different, but tractable problems to seismic imaging. We show that many of these difficulties can be surmounted by using very long offsets (long streamers or two-ship methods) with a broad-band, low-frequency source, and by using fixed ocean bottom receivers. We report results from surveys on the North Atlantic continental margins using these methods. Imaging results are shown from the recent FLARE project and from the iSIMM project, which recorded new seismic data recorded in summer 2002. The iSIMM project acquired two seismic surveys, using 85 4-component ocean bottom seismometers with long streamers for wide-angle data, and vertical arrays for far-field source signature recording. One survey crosses the Faroes Shelf and adjacent continental margin, and a second the Hatton-Rockall Basin, Hatton Bank and adjacent oceanic crust. The Faroes wide-angle profiles were overshot by WesternGeco's Topaz using three single-sensor, Q-Marine streamers, 12km plus two 4km. We designed deep-towed, broad-band low-frequency sources tuned to enhance the bubble pulses, with peak frequencies at 8

  19. Time-dependent convection models of mantle thermal structure constrained by seismic tomography and geodynamics: implications for mantle plume dynamics and CMB heat flux

    NASA Astrophysics Data System (ADS)

    Glišović, P.; Forte, A. M.; Moucha, R.

    2012-08-01

    surface plates and a rigid surface. The thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature (i.e. the geotherm) in the mantle. One important goal of this study is to obtain a steady-state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the Earth's evolution. We obtain surface heat flux in the range of Earth-like values : 37 TW for a rigid surface and 44 TW for a surface with tectonic plates coupled to the mantle flow. Also, our convection simulations deliver CMB heat flux that is on the high end of previously estimated values, namely 13 TW and 20 TW, for rigid and plate-like surface boundary conditions, respectively. We finally employ these two end-member surface boundary conditions to explore the very-long-time scale evolution of convection over billion-year time windows. These billion-year-scale simulations will allow us to determine the extent to which a 'memory' of the starting tomography-based thermal structure is preserved and hence to explore the longevity of the structures in the present-day mantle. The two surface boundary conditions, along with the geodynamically inferred radial viscosity profiles, yield steady-state convective flows that are dominated by long wavelengths throughout the lower mantle. The rigid-surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4, and the plate-like surface condition yields a pattern dominated by degree 1. Our exploration of the time-dependence of the spatial heterogeneity shows that, for both types of surface boundary condition, deep-mantle hot upwellings resolved in the present-day tomography model are durable and stable features. These deeply rooted mantle plumes show remarkable longevity over very long geological time spans, mainly owing to the geodynamically inferred high viscosity in the lower

  20. Recycling of crustal material by the Iceland mantle plume: New evidence from nitrogen elemental and isotope systematics of subglacial basalts

    NASA Astrophysics Data System (ADS)

    Halldórsson, Sæmundur A.; Hilton, David R.; Barry, Peter H.; Füri, Evelyn; Grönvold, Karl

    2016-03-01

    We report new nitrogen (N2) abundance and isotope (δ15N) data for 43 subglacial basaltic glasses from the neovolcanic zones of Iceland, a key locality in studies of mantle plume geochemistry and crust-mantle processes. New helium and argon abundance and isotope data are also reported to supplement previous studies (Füri et al., 2010; Barry et al., 2014), allowing elemental ratios (e.g., N2/40Ar∗ where 40Ar∗ = radiogenic 40Ar) to be calculated. Subglacial basaltic glasses with N2 > 2 μcm3 STP/g show a wide range in δ15N values, from -2.91 to +11.96‰ (vs. Air), with values >6‰ only observed at one locality in the Eastern Rift Zone. Elemental ratios involving N2, i.e., N2/3He, and N2/40Ar∗, span several orders of magnitude from 2.5 × 105 to 9.0 × 107, and 32.8 to 1.46 × 106, respectively. In contrast, argon isotope ratios (40Ar/36Ar) are limited, ranging from air-like (∼298.6) values up to 1330. Glasses exhibit a wide range in helium isotope ratios (8-26 RA), with clear distinctions between individual rift segments. A number of processes have extensively modified original mantle source N isotope and relative abundance compositions - most significantly air interaction, crustal contamination in some instances, and possibly degassing-induced fractionation. Under the assumption that the starting 4He/40Ar∗ production ratio of Iceland mantle is identical to the depleted MORB mantle (DMM), a filtering protocol for the entire N dataset, based upon 40Ar/36Ar and 4He/40Ar∗ ratios, was adopted to identify samples with unmodified δ15N values. Consequently, we identify 22 samples that define the Icelandic mantle N-isotope distribution (δ15N = -2.29 to +5.71‰). Using the filtered dataset, we investigate simple binary mixing scenarios involving N2/3He-N2/40Ar∗-δ15N variations to identify mantle end-member compositions. Mixing scenarios are consistent with a recycled component in the Iceland mantle source, defined by a high and heterogeneous δ15N end

  1. Receiver function imaging of the lithosphere-asthenosphere boundary and melt beneath the Afar Rift in comparison to other systems

    NASA Astrophysics Data System (ADS)

    Rychert, Catherine A.; Harmon, Nicholas

    2015-04-01

    Heating, melting, and stretching destroy continents at volcanic rifts. Mantle plumes are often invoked to thermally weaken the continental lithosphere and accommodate rifting through the influx of magma. However the relative effects of mechanical stretching vs. melt infiltration and weakening are not well quantified during the evolution of rifting. S-to-p (Sp) imaging beneath the Afar Rift provides additional constraints. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. We image a lithosphere-asthenosphere boundary at ~75 km beneath the flank of the Afar Rift vs. its complete absence beneath the rift. Instead, a strong velocity increase with depth at ~75 km depth is imaged. Beneath the rift axis waveform modeling suggests the lack of a mantle lithosphere with a velocity increase at ~75 km depth. Geodynamic models that include high melt retention and suppress thermal convection easily match the required velocity-depth profile, the velocity increase arising from a drop in melt percentage at the onset of decompression melting. Whereas, models with conservative melt retention that include thermal buoyancy effects cannot reproduce the strong velocity increase. The shallow depth of the onset of melting is consistent with a mantle potential temperature = 1350 - 1400°C, i.e., typical for adiabatic decompression melting. Trace element signatures and geochemical modeling have been used to argue for a thick lithosphere beneath the rift and slightly higher mantle potential temperatures ~1450°C, although overall, given modeling assumptions, the results are not in disagreement. Therefore, although a plume initially destroyed the mantle lithosphere, its influence directly beneath Afar today is not strong. Volcanism continues via adiabatic decompression melting assisted by strong melt buoyancy

  2. Platinum-Group Elements in Basalts Derived From the Icelandic Mantle Plume -Past and Present.

    NASA Astrophysics Data System (ADS)

    Momme, P.; Oskarsson, N.; Gronvold, K.; Tegner, C.; Brooks, K.; Keays, R.

    2001-12-01

    Paleogene basalts ( ~55Ma) derived from the ancestral Iceland mantle plume and extruded during continental rifting are exposed along the Blosseville Kyst in central East Greenland. These basalts comprise three intercalated series, viz: a low-Ti, high-Ti and a very high-Ti series. The two Ti-rich series are interpreted to represent continental flood basalts formed by low degrees of partial melting (degree of melting F=3-9%) while the low-Ti series are believed to have formed by higher degrees of partial melting (F:15-25%). All three of the East Greenland basalt series are enriched in the PGE, relative to normal MORB. During differentiation of the low-Ti series, Pd increase from 11 to 24 ppb whereas Pt and Ir decrease from 12 and 0.6 ppb to 3 and <0.05 ppb respectively. The primitive basalts (molar Mg#60) of the dominant high-Ti series contain ~6-10 ppb Pd, ~7-10 ppb Pt and ~0.2 ppb Ir whereas the most evolved basalts (Mg#43) contain 25 ppb Pd, 5 ppb Pt and <0.05 ppb Ir. The PGE-rich nature of these basalts is surprising because low degree partial melts are generally S-saturated and hence strongly depleted in the PGE (cf, Keays, 1995). However, our data indicates that all of the East Greenland magmas were S-undersaturated and as they underwent differentiation, Pd behaved incompatibly while Ir and Pt behaved compatibly. Primitive Holocene Icelandic olivine tholeiites contain 120 ppm Cu, 6 ppb Pd, 4 ppb Pt and 0.2 ppb Ir while their picritic counterparts contain 74 ppm Cu, 17 ppb Pd, 7 ppb Pt and 0.3 ppb Ir. Both the olivine tholeiites and the picrites are believed to have formed by high degrees of partial melting (15-25%) which would have exhausted all of the sulphides in the mantle source region and produced S-undersaturated magmas. In Icelandic samples with 10-14wt% MgO, Cu and the PGEs vary systematically between the primitive picrite and olivine tholeiite compositions given above i.e there is an inverse correlation between Cu and the PGEs. This is best explained

  3. Mantle plumes - A boundary layer approach for Newtonian and non-Newtonian temperature-dependent rheologies. [modeling for island chains and oceanic aseismic ridges

    NASA Technical Reports Server (NTRS)

    Yuen, D. A.; Schubert, G.

    1976-01-01

    Stress is placed on the temperature dependence of both a linear Newtonian rheology and a nonlinear olivine rheology in accounting for narrow mantle flow structures. The boundary-layer theory developed incorporates an arbitrary temperature-dependent power-law rheology for the medium, in order to facilitate the study of mantle plume dynamics under real conditions. Thermal, kinematic, and dynamic structures of mantle plumes are modelled by a two-dimensional natural-convection boundary layer rising in a fluid with a temperature-dependent power-law relationship between shear stress and strain rate. An analytic similarity solution is arrived at for upwelling adjacent to a vertical isothermal stress-free plane. Newtonian creep as a deformation mechanism, thermal anomalies resulting from chemical heterogeneity, the behavior of plumes in non-Newtonian (olivine) mantles, and differences in the dynamics of wet and dry olivine are discussed.

  4. Plume or Mantle Heterogeneity? Numerical Modeling of the Mid-Atlantic Ridge between 2°-14°S

    NASA Astrophysics Data System (ADS)

    Hasenclever, J.; Hort, M. K.; Phipps-Morgan, J.

    2009-12-01

    The ˜900 km long section of the Mid-Atlantic ridge (MAR) between 2°-14°S is a study area of the German priority program SPP 1144, and has been the site of several ship cruises since 2004. Ridge morphology, ridge axis bathymetry, and crustal thickness vary considerably along this part of the MAR, several large- and small-offset fracture zones displace the ridge axis by up to 250 km. The most prominent (and mostly unexplained) geodynamic features are the active volcanic island of Ascension, located 80 km west of the MAR, and a melting anomaly beneath the MAR at 9°30'S causing an elevated ridge axis without axial valley, with crustal thicknesses of up to 11 km, and large seamounts. Possible explanations are enhanced melting of a mantle heterogeneity or the influence of a weak mantle plume located either close to the ridge axis or beneath the Circe seamount (˜450 km east of the MAR). By numerically modeling the different scenarios we test their feasibility in terms of mantle flow and melting. We use a self-developed parallel 3D finite element MATLAB code to numerically solve for viscous flow, advection and diffusion of heat, and melting of a multi-component mantle, discretized on unstructured tetrahedra meshes. The velocity-pressure formulation is based on Maday and Patera [State of the Art Surveys in Computational Mechanics, 1989] and solved using a multigrid-preconditioned conjugate gradient algorithm. Advection of temperature and compositional fields is achieved by a semi-Lagrange Predictor-Corrector scheme combined with bi-cubic interpolation on the unstructured mesh. Diffusion of heat is solved using standard operator-splitting methods and a conjugate gradient algorithm. The melting formulation is based on Phipps Morgan [G3, 2001], to which we added the effect of source water using the parameterization of Katz et al. [G3, 2004]. We include the buoyancy effects of Fe-depletion and melt in pores, and assume a rheology that depends on depth, temperature, water

  5. Transition of Mount Etna lavas from a mantle-plume to an island-arc magmatic source.

    PubMed

    Schiano, P; Clocchiatti, R; Ottolini, L; Busà, T

    2001-08-30

    Mount Etna lies near the boundary between two regions that exhibit significantly different types of volcanism. To the north, volcanism in the Aeolian island arc is thought to be related to subduction of the Ionian lithosphere. On Sicily itself, however, no chemical or seismological evidence of subduction-related volcanism exists, and so it is thought that the volcanism-including that on Mount Etna itself-stems from the upwelling of mantle material, associated with various surface tectonic processes. But the paucity of geological evidence regarding the primary composition of magma from Mount Etna means that its source characteristics remain controversial. Here we characterize the trace-element composition of a series of lavas emitted by Mount Etna over the past 500 kyr and preserved as melt inclusions inside olivine phenocrysts. We show that the compositional change in primary magmas from Mount Etna reflects a progressive transition from a predominantly mantle-plume source to one with a greater contribution from island-arc (subduction-related) basalts. We suggest that this is associated with southward migration of the Ionian slab, which is becoming juxtaposed with a mantle plume beneath Sicily. This implies that the volcanism of Mount Etna has become more calc-alkaline, and hence more explosive, during its evolution. PMID:11528476

  6. The Ninetyeast Ridge and its Relation to the Kerguelen, Amsterdam and St. Paul Mantle Plumes in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Nobre Silva, I. G.; Weis, D.; Scoates, J. S.

    2011-12-01

    The Ninetyeast Ridge, a 5000 km long north-south oriented submarine volcanic ridge in the eastern Indian Ocean, has been interpreted to have formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Samples recovered along the ridge have the characteristic Dupal geochemical signatures of Indian Ocean basalts, but debate concerning the nature and number of components in the mantle source contributing to the formation of these basalts persists. New MC-ICP-MS (Pb, Hf) and TIMS (Sr, Nd) isotopic analyses were determined for tholeiites representative of the ~180 m of basaltic basement recovered from three drill sites (758: 82 Ma; 757: 58 Ma, 756: 43 Ma) along the Ninetyeast Ridge during ODP Leg 121. These analyses provide substantially greater precision compared to earlier studies (ppm vs. % range) after removal of the effects of post-magmatic alteration by thorough acid leaching [1]. No systematic temporal or spatial isotopic variations are observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge [2]. The Pb-Hf-Sr-Nd isotopic compositions of the Ninetyeast Ridge basalts are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam-St. Paul mantle plumes and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The less radiogenic isotopic signatures of some Ninetyeast Ridge basalts (87Sr/86Sr = 0.70381-0.70438) are not consistent with mixing with shallow level Indian MORB and are instead consistent with the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian ocean island basalts (e.g., Crozet, Réunion [3, 4]) with typical EM-1-like

  7. Convection experiments in a centrifuge and the generation of plumes in a very viscous fluid. [for earth mantle models

    NASA Technical Reports Server (NTRS)

    Nataf, H.-C.; Hager, B. H.; Scott, R. F.

    1984-01-01

    In this paper, experiments are described for which inertial effects are negligible. A small aspect-ratio tank filled with a very viscous fluid (Pr = 10 to the 6th) is used to observe the behavior of convection for Rayleigh numbers up to 6.3 x 10 to the 5th. These high values are reached by conducting the experiment in a centrifuge which provides a 130-fold increase in apparent gravity. Rotational effects are small, but cannot be totally dismissed. In this geometry, thermal boundary layer instabilities are indeed observed, and are found to be very similar to their lower Prandtl number counterparts. It is tentatively concluded that once given a certain degree of 'vulnerability' convection can develop 'plume' like instabilities, even when the Prandtl number is infinite. The concept is applied to the earth's mantle and it is speculated that 'plumes' could well be the dominant mode of small-scale convection under the lithospheric plates.

  8. Plume formation in the D-double prime-layer and the roughness of the core-mantle boundary

    NASA Technical Reports Server (NTRS)

    Olson, Peter; Schubert, Gerald; Anderson, Charles

    1987-01-01

    A series of very-high-resolution finite element calculations of plume formation in the D-double prime-layer has been performed for several plausible rheologies and boundary conditions in order to study both the early and later stages of boundary layer development. The results show that plumes are initiated by coalescence of small-scale convective instabilities within the low-viscosity region immediately above the core-mantle boundary (CMB). These instabilities support topographic roughness on the CMB having horizontal scales of 20-50 km and provide a source for scattered P-waves seen as precursors to the phases PKIKP and PKKP. The calculated structure of fully developed plumes emerging from the D-double prime-layer consists of 5-50 cm/yr flow confined to 50-100 km thick vertical conduits. With strongly temperature-dependent viscosity, plumes exhibit time-dependent behavior, including upward propagating solitary conduit waves, which may contribute to episodicity in hotspot volcanism.

  9. Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components?

    USGS Publications Warehouse

    Meibom, A.; Anderson, D.L.; Sleep, N.H.; Frei, R.; Chamberlain, C.P.; Hren, M.T.; Wooden, J.L.

    2003-01-01

    The existence of a primordial, undegassed lower mantle reservoir characterized by high concentration of 3He and high 3He/4He ratios is a cornerstone assumption in modern geochemistry. It has become standard practice to interpret high 3He/4He ratios in oceanic basalts as a signature of deep-rooted plumes. The unfiltered He isotope data set for oceanic spreading centers displays a wide, nearly Gaussian, distribution qualitatively similar to the Os isotope (187Os/188 Os) distribution of mantle-derived Os-rich alloys. We propose that both distributions are produced by shallow mantle processes involving mixing between different proportions of recycled, variably aged radiogenic and unradiogenic domains under varying degrees of partial melting. In the case of the Re-Os isotopic system, radiogenic mid-ocean ridge basalt (MORB)-rich and unradiogenic (depleted mantle residue) endmembers are constantly produced during partial melting events. In the case of the (U+Th)-He isotope system, effective capture of He-rich bubbles during growth of phenocryst olivine in crystallizing magma chambers provides one mechanism for 'freezing in' unradiogenic (i.e. high 3He/4He) He isotope ratios, while the higher than chondritic (U+Th)/He elemental ratio in the evolving and partially degassed MORB melt provides the radiogenic (i.e. low 3He/4He) endmember. If this scenario is correct, the use of He isotopic signatures as a fingerprint of plume components in oceanic basalts is not justified. Published by Elsevier Science B.V.

  10. Polar wandering history of Mars: triggered by giant impact and mantle super-plume?

    NASA Astrophysics Data System (ADS)

    Xiao, L.; Huang, Q.; Wang, D.

    2012-12-01

    Polar wander is not an unusual phenomenon for many terrestrial planets, but the history, processes and triggers are complexity and little is well understood. Mars may be the best planets for studying this process, because most geological history was well preserved. Much evidence indicates that the present spin axis of Mars is not the same as that in its ancient and recent time. Many authors have discussed the polar wander history based on magnetic anomalies (e.g. Arkani-Hamed, 2001; Hood and Zacharian, 2001; Hood et al., 2005; Arkani-Hamed and Boutin, 2004; Boutin and Arkani-Hamed, 2006), topography and sediments (Murray and Malin, 1973; ) , geoid (Sprenke et al., 2005), and giant impact basins (Schultz and Lutz-Garihan, 1982; Arkani-Hamed, 2009) respectively. These studies suggest that the martian spin axis has wandered about 10o-20o in the past 100myr (Murray and Malin, 1973), 15-90o polar motion in the past 4.2Ga due to load of Tharsis bulge (Sprenke et al., 2005), and a combined model suggests forming of Alba Patera and Elysium Rise caused spin axis rotating counterclockwise to equator and subsequent volcanism and giant impacts deduced mass concentrations caused further clockwise rotation to its present position during 4.2-3.9Ga. However, most of these hypotheses are model-dependent and have not well correlated to geological records, especially unique polar deposits and geomorphology. There are many unanswered questions about paleopole deposits, paleo-magnetic poles locations, polar wander, true polar wander, obliquity, and their relation with external and internal force driven events, such as giant impact, mantle plume and it caused volcanic mass loading. By examining possible ancient polar deposits, combining with giant impact history, orientation of magnetic field and compared with previously suggested polar wander models, this study proposed a comprehensive hypothesis that could explain major polar wander events and suggest that giant impacts and volcanism

  11. Geophysical Evidence for a Possible Late Jurassic Mantle Plume in the Gulf of Mexico

    NASA Astrophysics Data System (ADS)

    Bird, D. E.; Hall, S. A.; Casey, J. F.; Burke, K.

    2001-12-01

    Gravity, magnetic and seismic refraction data reveal a prominent basement structure beneath the Keathley Canyon area of the western Gulf of Mexico. Several seismic refraction profiles acquired near and over the structure indicate depths to its crest range from 10.5 to 12 km, rising from basement depths of 14 to 16 km below sea level. Because of the presence of extensive salt features, seismic reflection data are unable to accurately image the structure but several reflection profiles indicate the existence of a basement high in the area. A positive free-air gravity anomaly associated with this basement structure extends 200 km from 93.9o W, 26.4o N along a roughly WNW-ESE directed path to 91.7o W, 25.9o N where it turns northeastward. Bathymetric and seismic reflection data indicate the gravity anomaly is not produced by seafloor topography or shallow sedimentary sources, but can be attributed to the basement relief documented. Its amplitude and wavelength decrease to the ESE, from 70 mGal and 100 km wavelength to 35 mGal and 40 km wavelength. A positive magnetic anomaly with a 130 nT amplitude and 30 km wavelength coincides with the WNW end of the free air gravity anomaly. It extends to the ESE in a similar manner to the gravity anomaly, but its amplitude decays more rapidly. Most models for the formation of the Gulf of Mexico basin culminate in a late Jurassic-early Cretaceous phase of seafloor spreading as the Yucatan Block rotates counterclockwise away from North America. The shape of the free air gravity anomaly over the deep basement structure defines a geometry that is similar to those produced by other hotspot tracks, such as the New England Seamounts, Rio Grande Rise or Vitoria-Trindade seamount chain. The WNW-ESE direction is broadly consistent with motion of North America in the hotspot reference frame at the time of basin formation. Such an interpretation suggests that a minor mantle plume may have been active during spreading and played a significant

  12. Plume Capture by Divergent Plate Motions: Implications for the Distribution of Hotspots, Geochemistry of Mid-Ocean Ridge Basalts, and Heat Flux from the Core-Mantle Boundary

    NASA Astrophysics Data System (ADS)

    Jellinek, A. M.; Richards, M. A.

    2001-12-01

    The coexistence of mantle plumes with plate-scale flow is problematic in geodynamics. Significant problems include the fixity of hotspots with respect to plate motions, the spatial distribution and duration of hotspots, the geophysical and geochemical signatures of plume-ridge interactions, and the relation between mantle plumes and heat flux across the core-mantle boundary. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (up to 109) in a fluid with a strongly temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the velocity ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary. When V<<1, large scale circulation has a negligible effect and convective upwelling occurs as randomly-spaced axisymmetric plumes that interact with one another. When V>10, plume instabilities are suppressed entirely and the heat flux from the hot lower boundary is carried by a central sheet-like upwelling. At intermediate V, ascending plumes are advected along the bottom boundary layer, and the heat flux from the boundary is found to scale (according to a simple boundary layer theory) with V and the ratio of the viscosity of cold fluid above the thermal boundary layer to the viscosity of the hottest fluid in contact with the bottom boundary. For large viscosity ratios (10-100), only about 1/5th or less of the total heat flux from the hot boundary layer is carried by plume instabilities, even for modest imposed horizontal flow velocities (V of order 1). When applied to Earth, our results suggest that plate-scale flow focuses ascending mantle plumes toward mid-ocean ridges, and that plumes may be

  13. Generation of hydrous-carbonated plumes in the mantle transition zone linked to tectonic erosion and subduction

    NASA Astrophysics Data System (ADS)

    Safonova, Inna; Maruyama, Shigenori; Litasov, Konstantin

    2015-11-01

    This paper presents a model for the generation of hydrous-carbonated plumes (HCPs) in the mantle transition zone (MTZ) linking (i) the Pacific-type convergent margins; (ii) melt generation in the MTZ under the influence of volatiles (water, carbon dioxide) and subducted granitic material and oceanic slabs and (iii) the Meso-Cenozoic intra-plate magmatism in Central Asia. The model is based on four groups of evidences obtained from geology, petrology, seismic tomography and numerical simulations. The double-sided subduction at the Pacific-type margins around post-Miocene Asia supplies hydrated-carbonated oceanic crust and continental crust materials down to the deep mantle, which accumulate in the MTZ at 410-660 km. The delivery of crustal material to the MTZ is provided by the direct subduction of intra-oceanic arcs in the Western Pacific and by the tectonic erosion of convergent margin hanging walls. The U-Th-K-enriched continental material accumulated in the MTZ can serve an additional source of heat. Evidence for the subduction of continental crust materials comes from seismic tomography and numerical modelling data. The subducting oceanic slab consisting of serpentinites, hydrated sediments, carbonates and carbonatized basalts can supply water and carbon dioxide to the deep mantle and metasomatize it. The presence of volatiles, which can reduce melting temperature, and the presence of the subducted crustal material, which may serve an additional heater, can synergistically trigger the generation of HCPs. Those HCPs can induce mantle upwelling, melting of the metasomatized mantle and subducted MORB slabs, ascent of melts, surface rifting and formation of mafic and bimodal volcanic series. In addition, they can contribute to the supercontinent cycle. The HCPs generated in the MTZ beneath Central and East Asia resulted in a shift of the tectonic regime from transpression to transtension and in the formation of numerous Meso-Cenozoic intra-plate volcanic fields.

  14. An optical method for measuring temperature in laboratory models of mantle plumes

    NASA Astrophysics Data System (ADS)

    Laudenbach, N.; Christensen, U. R.

    2001-05-01

    We present a method for measuring radial temperature profiles in laboratory thermal plumes using the deflection of a laser beam that passes through the fluid. Plumes are created by injecting hot corn syrup into a column of cold syrup at a well-defined rate. Every second a new radial temperature profile can be taken, which makes the method suitable for monitoring time-dependent phenomena. We compare the thermal structure of stationary plume conduits and of propagating solitary waves with numerical results obtained with a 2-D axisymmetric convection code. The agreement is excellent and shows that accurate high-resolution temperature profiles can be obtained without perturbing the flow.

  15. Co-location of eruption sites of the Siberian Traps and North Atlantic Igneous Province: Implications for the nature of hotspots and mantle plumes

    NASA Astrophysics Data System (ADS)

    Smirnov, Aleksey V.; Tarduno, John A.

    2010-09-01

    One of the striking exceptions to the mantle plume head-tail hypothesis that seeks to explain magmatism of large igneous provinces (LIPs) and hotspot tracks is the ~250 million-year-old Siberian Traps. The lack of a clear hotspot track linked to this LIP has been one motivation to explore non-plume alternative mechanisms. Here, we use a paleomagnetic Euler pole analysis to constrain the location of the Siberian Traps at the time of their eruption. The reconstructed position coincides with the mantle region that also saw eruption of the ~ 61-58 million year-old North Atlantic Igneous Province (NAIP). Together with LIP volume estimates, this reconstruction poses a dilemma for some non-plume models: the partial-melts needed to account for the Siberian Traps should have depleted the enriched upper mantle source that is in turn crucial for the later formation of the NAIP. The observations instead suggest the existence of a long-lived (>250 million-year-long) lower mantle chemical and/or thermal anomaly, and significant temporal changes in mantle plume flux.

  16. On the relationship between tectonic plates and thermal mantle plume morphology

    NASA Technical Reports Server (NTRS)

    Lenardic, A.; Kaula, W. M.

    1993-01-01

    Models incorporating plate-like behavior, i.e., near uniform surface velocity and deformation concentrated at plate boundaries, into a convective system, heated by a mix of internal and basal heating and allowing for temperature dependent viscosity, were constructed and compared to similar models not possessing plate-like behavior. The simplified numerical models are used to explore how plate-like behavior in a convective system can effect the lower boundary layer from which thermal plumes form. A principal conclusion is that plate-like behavior can significantly increase the temperature drop across the lower thermal boundary layer. This temperature drop affects the morphology of plumes by determining the viscosity drop across the boundary layer. Model results suggest that plumes on planets possessing plate-like behavior, e.g., the Earth, may differ in morphologic type from plumes on planets not possessing plate-like behavior, e.g., Venus and Mars.

  17. Impact of a fixed Siberian Traps mantle plume on the tectonics of the Arctic

    NASA Astrophysics Data System (ADS)

    Lawver, L. A.; Norton, I. O.; Gahagan, L.

    2012-12-01

    Eruption of the Siberian Traps at the Permo-Triassic boundary [~250 Ma] produced more than 3 x 106 km3 of rapidly emplaced magma throughout a region ~2.5 x 106 km2 in extent. Dates from the New Siberian Islands of 252 ± 2 Ma (Kuzmichev & Pease, 2007) indicate that Siberian Trap-related magmas are found ~500 km to the east of where they are generally shown to terminate to the west of the Lena River. Cenozoic opening of the Eurasian Basin would account for some of this discrepancy. A Siberian Trap mantle plume in an absolute reference frame fixed to the present day location of the Iceland hot spot, tracks through time across the Taimyr Peninsula region during the Late Triassic period and then to north of the Severnaya Zemlya archipelago by the end of the Middle Jurassic. With the exception of some Middle Triassic dates from the Taimyr Peninsula there is no apparent expression of a hot spot track during the this period. Motion of Laurasia in a paleomagnetically controlled reference frame has the Franz Josef Land archipelago over the fixed hotspot from about 155 Ma to 147 Ma prior to the early phase of the High Arctic Large Igneous Province [HALIP], generally taken to be 130 Ma to 120 Ma. Campsie et al (1988) have one date of 145 Ma from samples collected by Fridthof Nansen in 1895-1896 on Solsberi Island. Dibner et al (1988) have a dozen ages from dolerite samples from various islands spanning the period 175 ±12 Ma to 138 ±10 Ma with five of them between 158 Ma to 144 Ma. During the Late Jurassic into the earliest Cretaceous the track of the fixed hotspot follows the future margin of the Barents Shelf just inboard of a reconstructed Lomonosov Ridge. By the end of the Valanginian, the hotspot tracks curves slightly, mimicking the southern curve of the Lomonosov Ridge off North America. The early phase of the HALIP moves the region of the northern Ellesmere Island over the hotspot while forming the Mendeleev and Alpha ridges. By middle Albian time, the Siberian Traps

  18. Searching for Seismic Signatures of a Plume Source at the Base of the Mantle Below the Galapagos Island Hotspot

    NASA Astrophysics Data System (ADS)

    Vanacore, E.; Niu, F.

    2007-12-01

    , in the Northeast portion of the sampled region bounded to the south and west at approximately \\m-3°S and \\m267° longitude. While the residual differential travel times and the anisotropy measurements do not conclusively show that there is a mantle plume source at the base of the mantle in this region, the data does indicate there the lower mantle beneath the Galapagos Islands has significant structure meriting further study.

  19. Understanding the nature of mantle upwelling beneath East-Africa

    NASA Astrophysics Data System (ADS)

    Civiero, Chiara; Hammond, James; Goes, Saskia; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

    2014-05-01

    The concept of hot upwelling material - otherwise known as mantle plumes - has long been accepted as a possible mechanism to explain hotspots occurring at Earth's surface and it is recognized as a way of removing heat from the deep Earth. Nevertheless, this theory remains controversial since no one has definitively imaged a plume and over the last decades several other potential mechanisms that do not require a deep mantle source have been invoked to explain this phenomenon, for example small-scale convection at rifted margins, meteorite impacts or lithospheric delamination. One of the best locations to study the potential connection between hotspot volcanism at the surface and deep mantle plumes on land is the East African Rift (EAR). We image seismic velocity structure of the mantle below EAR with higher resolution than has been available to date by including seismic data recorded by stations from many regional networks ranging from Saudi Arabia to Tanzania. We use relative travel-time tomography to produce P- velocity models from the surface down into the lower mantle incorporating 9250 ray-paths in our model from 495 events and 402 stations. We add smaller earthquakes (4.5 < mb < 5.5) from poorly sampled regions in order to have a more uniform data coverage. The tomographic results allow us to image structures of ~ 100-km length scales to ~ 1000 km depth beneath the northern East-Africa rift (Ethiopia, Eritrea, Djibouti, Yemen) with good resolution also in the transition zone and uppermost lower mantle. Our observations provide evidence that the shallow mantle slow seismic velocities continue trough the transition zone and into the lower mantle. In particular, the relatively slow velocity anomaly beneath the Afar Depression extends up to depths of at least 1000 km depth while another low-velocity anomaly beneath the Main Ethiopian Rift seems to be present in the upper mantle only. These features in the lower mantle are isolated with a diameter of about 400 km

  20. Implications of 187Os isotopic heterogeneities in a mantle plume: evidence from Gorgona Island and Curaçao

    NASA Astrophysics Data System (ADS)

    Walker, Richard J.; Storey, Michael; Kerr, Andrew C.; Tarney, John; Arndt, Nicholas T.

    1999-03-01

    Recent work has suggested that the mafic-ultramafic volcanism in evidence throughout portions of the Caribbean, Central America, and northern South America, including the islands of Gorgona and Curaçao, was generated as part of a middle-Cretaceous, large igneous province. New Re-Os isochron results for tholeiitic basalts from Gorgona and Curaçao indicate crystallization ages of 89.2 ± 5.2 and 85.6 ± 8.1 Ma, respectively, consistent with reported Ar ages. The Gorgona ultramafic suite shows a large range in initial Os isotopic composition, with γ Os values ranging from -0.5 to +12.4. This large range reflects isotopic heterogeneities in the mantle source similar to those observed for modern ocean island basalts. In contrast to ocean island basalts, however, Os isotopic compositions do not correlate with variations in Nd, Sr, or Pb isotopic compositions, which are within the range of depleted mid-ocean ridge basalts. The processes that produced these rocks evidently resulted in the decoupling of Os isotopes from the Nd, Sr, and Pb isotopic systems. Picrites from Curaçao have very uniform, chondritic initial Os isotopic compositions, with initial γ Os values ranging only from -0.4 to ±1.4. Basalts from Curaçao, however, define an isochron with a 187Os-enriched initial isotopic composition (γ Os = +9.5). In contrast to the 187Os-enriched ultramafic rocks from Gorgona, the enrichment in these basalts could have resulted from lithospheric contamination. If the Gorgona and Curaçao rocks were derived from the same plume, Os results, combined with Sr, Nd, and Pb data indicate a heterogeneous plume, with multiple compositionally and isotopically distinct domains. The Os isotopic results require derivation of Os from a minimum of two distinct reservoirs, one with a composition very similar to the chondritic average and one with long-term enriched Re/Os. Oceanic crustal recycling has been invoked to explain most of the 187Os enrichments that have been observed in

  1. Finite frequency tomography shows a variety of plumes

    NASA Astrophysics Data System (ADS)

    Nolet, G.; Montelli, R.; Masters, G.; Dahlen, F. A.; Hung, S.

    2003-04-01

    The new technique of finite-frequency tomography (see abstract by Montelli et al., this meeting) is very powerful in imaging objects of small dimension in the lower mantle. The first global images of P velocity anomalies obtained by using this technique to invert a small but very accurate data set of long period P arrivals bottoming in the lower mantle show 18 low velocity anomalies in excess of -0.5%, all but two of which are associated with a known hotspot at the surface, and they serve as an unprecented glimpse into the deep mechanisms that give rise to hotspots. The following synopsis is given under the caveat that we have not yet incorporated high frequency waves into the interpretation, nor completed a full resolution analysis at the time of writing of this abstract (both will be presented at the meeting). We observe six or seven hotspots fed by a plume extending to the core-mantle boundary: Cap Verde, Easter Island, Hawaii, Kerguelen, St Helena, Tahiti, and perhaps also Azores. Several hotspots, among which are Bouvet, Bowie, and Mount Erebus, seem to originate at mid-mantle depth, while others (Afar, Ascension, Galapagos, Iceland, la Reunion and others) seem to be mostly confined to the upper mantle. Many renowned hotspots (such as Eifel, Samoa and Yellowstone) have only very weak low velocity anomalies at depth and may be the result of superficial processes confined to the top of the upper mantle. We confirm the existence of the two superplumes which both have Δ V_P < -0.5% extending as high as 2000 km depth. It is clear that no one plume/hotspot model can explain the variety in deep expressions of hotspots in the mantle. If midmantle plume origins represent originally deep plumes in their end stage, while the two unidentified anomalies are either beginning new plumes (Greenland) or plumes cut off in their initial ascent (W. Pacific), the large number of plumes caught in this phase would point to lengthy rise times of the order of tens of millions of

  2. The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

    NASA Astrophysics Data System (ADS)

    Hagos, Miruts; Koeberl, Christian; van Wyk de Vries, Benjamin

    2016-05-01

    The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3-8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036-0.7041, 143Nd/144Nd = 0.51286-0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta'Ale and Alu Shield basalts).

  3. Isotopic variations with distance and time in the volcanic islands of the Cameroon line: evidence for a mantle plume origin

    NASA Astrophysics Data System (ADS)

    Lee, Der-Chuen; Halliday, Alex N.; Fitton, J. Godfrey; Poli, Giampero

    1994-05-01

    probably represents ambient upper mantle, entrained with the plume head during ascent. This entrained component is like 'PREMA', but the Nd and Sr isotopic data indicate that it represents variably mixed depleted and enriched components, such as DMM and EMI. The HIMU component is probably representative of a lower mantle source from which the plume head was derived. The long-lived episodic magmatism on Principe provides evidence that the initial melt migration paths from the upper mantle form a hot zone that can be re-activated after long periods (10 7 yr) of apparent quiescence. The progression to HIMU characteristics within each island probably reflects the gradual flattening of the contaminated plume head within this hot zone, near the base of the lithosphere, and the melting of a stem composed of relatively uncontaminated HIMU mantle.

  4. Geochemistry and petrology of greenstones from the Erdenetsogt formation, central Mongolia: New evidence for a middle Paleozoic mantle plume

    NASA Astrophysics Data System (ADS)

    Ganbat, E.; Ishiwatari, A.; Demberel, O.

    2012-12-01

    implies highly depleted (high melting degree) magma compare with MORB, and more identical to spinel from Hawaiian tholeiitic basalts. From those facts, it is concluded that the Cr-spinel of greenstones may have been derived from a mantle plume source. Furthermore, notable exceptions of the Hangay greenstones are very low ratios of Nb/Zr and Zr/Y (0.05-0.08 and 0.2-0.5, respectively), whereas Hentey basalts show HIMU characteristic (Tsukada, 2006). The greenstones are slightly enriched in LREE and TiO2 (1.6-2.2 wt.%). We suggest that greenstones in Erdenetsogt formation may have been formed as plume-related oceanic island (hotspot or oceanic plateau) within paleo-oceanic plate located between the Siberian and the North China Cratons, and then accreted to the active continental margin of Siberian Craton during middle to late Paleozoic. This setting is analogous to the present southwest Pacific realm. Keywords: Hangay-Hentey accretionary complex, Erdenetsogt Formation, greenstones, clinopyroxene, Cr-spinel, mantle plume

  5. New 40Ar / 39Ar age and geochemical data from seamounts in the Canary and Madeira volcanic provinces: Support for the mantle plume hypothesis

    NASA Astrophysics Data System (ADS)

    Geldmacher, J.; Hoernle, K.; Bogaard, P. v. d.; Duggen, S.; Werner, R.

    2005-08-01

    The role of mantle plumes in the formation of intraplate volcanic islands and seamount chains is being increasingly questioned. Particular examples are the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. New 40Ar / 39Ar ages and Sr-Nd-Pb isotope geochemistry of volcanic rocks from seamounts northeast of the Madeira Islands (Seine and Unicorn) and northeast of the Canary Islands (Dacia and Anika), however, provide support for the plume hypothesis. The oldest ages of shield stage volcanism from Canary and Madeira volcanic provinces confirm progressions of increasing age to the northeast. Average volcanic age progression of ∼1.2 cm/a is consistent with rotation of the African plate at an angular velocity of ∼0.20° ± 0.05 /Ma around a common Euler pole at approximately 56° N, 45° W computed for the period of 0-35 Ma. A Euler pole at 35° N, 45° W is calculated for the time interval of 35-64 Ma. The isotope geochemistry further confirms that the Madeira and Canary provinces are derived from different sources, consistent with distinct plumes having formed each volcanic group. Conventional hotspot models, however, cannot easily explain the up to 40 m.y. long volcanic history at single volcanic centers, long gaps in volcanic activity, and the irregular distribution of islands and seamounts in the Canary province. A possible explanation could involve interaction of the Canary mantle plume with small-scale upper mantle processes such as edge-driven convection. Juxtaposition of plume and non-plume volcanism could also account for observed inconsistencies of the classical hotspot concept in other volcanic areas.

  6. Petrogenesis of nephelinites from the Tarim Large Igneous Province, NW China: Implications for mantle source characteristics and plume-lithosphere interaction

    NASA Astrophysics Data System (ADS)

    Cheng, Zhiguo; Zhang, Zhaochong; Hou, Tong; Santosh, M.; Zhang, Dongyang; Ke, Shan

    2015-04-01

    The nephelinite exposed in the Wajilitage area in the northwestern margin of the Tarim large igneous province (TLIP), Xinjiang, NW China display porphyritic textures with clinopyroxene, nepheline and olivine as the major phenocryst phases, together with minor apatite, sodalite and alkali feldspar. The groundmass typically has cryptocrystalline texture and is composed of crystallites of clinopyroxene, nepheline, Fe-Ti oxides, sodalite, apatite, rutile, biotite, amphibole and alkali feldspar. We report rutile SIMS U-Pb age of 268 ± 30 Ma suggesting that the nephelinite may represent the last phase of the TLIP magmatism, which is also confirmed by the field relation. The nephelinite shows depleted Sr-Nd isotopic compositions with age-corrected 87Sr/86Sr and εNd(t) values of 0.70348-0.70371 and + 3.28 to + 3.88 respectively indicating asthenospheric mantle source. Based on the reconstructed primary melt composition, the depth of magma generation is estimated as 115-140 km and the temperatures of mantle melting as 1540-1575 °C. The hotter than normal asthenospheric mantle temperature suggests the involvement of mantle thermal plume. The Mg isotope values display a limited range of δ26Mg from - 0.35 to - 0.55‰, which are lower than the mantle values (- 0.25‰). The Mg isotopic compositions, combined with the Sr-Nd isotopes and major and trace element data suggest that the Wajilitage nephelinite was most likely generated by low-degree partial melting of the hybridized carbonated peridotite/eclogite source, which we correlate with metasomatism by subducted carbonates within the early-middle Paleozoic convergent regime. A plume-lithosphere model is proposed with slight thinning of the lithosphere and variable depth and degree of melting of the carbonated mantle during the plume-lithosphere interaction. This model also accounts for the variation in lithology of the TLIP.

  7. Double Mantle Plume Upwelling—A Possible Formation Mechanism of Beta Plateau and Devana Chasma,Venus

    NASA Astrophysics Data System (ADS)

    Ding, N.

    2009-12-01

    Ning Ding,Zuoxun Zeng,China University of Geosciences,Wuhan,430074,China NingDing.eagle@gmail.com Introduction:Venus represents a‘one plate planet’[1],and the uplift,fractures and volcanism in Beta Regio on Venus are considered to be formed by lithosphere uplift driven by a hot plume[2]. Based on the double peaking saddle landform,we suggest the tectonic pattern of double mantle plume upwelling to interpret the formation mechanism of Beta Plateau and Devana Chasma.We take a physical modeling to validate this possibility. Model:There is no ductile shear in Venus[3],so we use quartz sands to simulate the crust of Venus.We use two wood stickes 1.5cm in diameter rising from the rubber canvas slowly and straight till about half of the model,then falling down slowly and straight.The base is a hard rubber plate,in the center of which,there are two holes 3cm in diameter,and the distance between them is 5cm.The holes are covered by rubber canvas.We use the quartz sands in colours of white, red and black with particle size of 70 mess as the model materials. Result:Fig.1:At the beginning of the wood stickes upwelling,only fine radial cracks are formed above the upwelling from central to outside.With the upwelling continue,surface energy of the fine radial cracks increase and make the cracks unstable,finally,the fine radial cracks connect each other and form a fracture zone.And then the two mantle plume downwelling,the fracture zone is developed to form a chasma at the end. Fig.2:The four profiles all form reverse faults outside and normal faults inside.But the difference is the faults in the middle of the chasma goes deeper than others.It is the pattern of Beta Plateau where the tectonic rising is cut by Devana Chasma zone in the topographic features. Fig.3:From the tow fig., we can see two points similar:a.the elevation is high and distribution area is large around the area of two upwelling and it is high around the area of chasma,but the distribution area is small

  8. Petrology and Geochronology of Kaula Volcano lavas: An off-axis window into the Hawaiian Mantle Plume

    NASA Astrophysics Data System (ADS)

    Garcia, M. O.; Weis, D.; Jicha, B. R.; Tree, J. P.; Bizimis, M.

    2014-12-01

    The Hawaiian Islands extend NW for 625 km from Lō'ihi to Ka'ula island. One anomalous feature cross-cutting the Hawaiian Islands is the Kaua'i Ridge, a 165 km-long bathymetric high with three well-defined gravity highs. These gravity highs are centered under or near the islands of Ka'ula, Ni'ihau and Kaua'i, and represent the cores of three shield volcanoes whose volumes decrease dramatically with distance from the axis of the Hawaiian Chain (Kaua'i, 58 x 103 km3, Ni'ihau x 103 km, Ka'ula 10 x 103 km; Robinson and Eakins 2006). Ka'ula Volcano, on the SW end of the Kaua'i Ridge, is centered 100 km off the axis of the Hawaiian mantle plume. The volcano is capped by a small island, which is a remnant of a nephelinitic tuff cone. The cone contains abundant accidental bombs of lava (tholeiite, phonolite and basanite), peridotite and pyroxenite, and unexploded ordnance from US military bombing. Two JASON dives on the flanks of Ka'ula recovered only alkalic lavas. Three stage of Ka'ula volcanism have been identified from sampling the volcanic bombs and flanks of the volcano. These rocks were dated using 40Ar/39Ar methods for the basalts and K-Ar for the phonolites. A tholeiitic shield basalt yielded an age of 6.2 Ma, the oldest reliable age for any Hawaiian Island tholeiite. Post-shield phonolites gave ages of 4.0 to 4.2 Ma (Garcia et al., 1986) and rejuvenation stage alkalic basalts yielded ages of 1.9 to 0.5 Ma. These ages are nearly identical to those for the same stages for adjacent Ni'ihau volcano but slightly older than on Kauai, 100 km to the NE (Sherrod et al. 2007). Thus, volcanism was nearly simultaneous along Kaua'i Ridge. The new age results extend to 420 km the distance within the Hawaiian Islands that experienced coeval rejuvenated volcanism. Geochemically, the rejuvenated and tholeiitic lavas from the Kaua'i Ridge are very similar with mixed source signatures of Loa and Kea trend compositions. Mixed Loa-Kea sources have been found for many other Hawaiian

  9. A Geochemical Transect Across the Lau and North Fiji Basins: New Evidence for the Distribution of Multiple Mantle Plume Components

    NASA Astrophysics Data System (ADS)

    Price, A. A.; Jackson, M. G.; Blichert-Toft, J.; Arculus, R. J.; Conatser, C. S.; Konter, J. G.; Koppers, A. A. P.; Blusztajn, J.

    2014-12-01

    The Lau and North Fiji backarc basins are located in a tectonically complex region of the South Pacific, where the upper mantle may have been modified by up to five hotspots (Samoa, Rurutu, Rarotonga, Macdonald, and Louisville), each with distinct geochemical fingerprints. We present new Hf, Nd, Sr, and Pb isotopic data for basaltic samples dredged from seven areas along an east-west transect spanning the Lau and North Fiji basins to determine the possible influence and distribution of these various hotspot sources. We find that the isotope ratios of nearly all samples can be explained by mixing a depleted mantle component, which is ubiquitous in the Lau Basin, with a component similar to that found in Samoan shield (EMII) and/or rejuvenated (EMI) lavas. Lavas as far southwest as the Fiji Triple Junction (North Fiji Basin) show enriched geochemical signatures (87Sr/86Sr and 206Pb/204Pb up to 0.7037 and 18.635 respectively, and 143Nd/144Nd and 176Hf/177Hf down to 0.51285 and 0.283023, respectively) trending toward Samoa. This observation extends the range of Samoan influence into the North Fiji Basin 400 km south of its previous observed extent at South Pandora Ridge. The few samples that cannot be explained solely by incorporation of Samoan material are from the northeastern Lau Basin (Falloon et al., 2007) and host a dilute HIMU component that may relate to the incorporation of material from the Rurutu hotspot. This component is not observed further to the west in the Lau and North Fiji basins. A ubiquitous EMI signature in the region may be linked to the Rarotonga hotspot. New dredges from the northeast Lau Basin may give clearer signals that will reveal the identity of the enriched plume component.

  10. Radial Anisotropy beneath the Main Ethiopian Rift and Afar Depression

    NASA Astrophysics Data System (ADS)

    Accardo, N. J.; Gaherty, J. B.; Jin, G.; Shillington, D. J.

    2014-12-01

    The Main Ethiopian Rift (MER) and Afar uniquely capture the final stages of transition from continental rifting in the broader East African Rift System to incipient seafloor spreading above a mantle hotspot. Studies of the region increasingly point to magmatism as a controlling factor on continental extension. However, the character and depth extent of these melt products remain contentious. Radial anisotropy derived from surface waves provides a unique diagnostic constraint on the presence of oriented melt pockets versus broader oriented anisotropic fabrics. This study investigates the thermal and radially anisotropic structure beneath the broader MER and Afar to resolve the magmatic character of the region and ultimately to understand the role of magmatism in present day rift development. We utilize 104 stations from 4 collocated arrays in the MER/Afar region to constrain radial anisotropy within the upper mantle via the inversion of Love- and Rayleigh-wave observations between 25 and 100 s period. We employ a multi-channel cross-correlation algorithm to obtain inter-station phase and amplitude information. The multi-channel phase observations are inverted for dynamic phase velocity across the array, which are then corrected for focusing and multipathing using the amplitude observations via Helmholtz tomography. We jointly invert Love- and Rayleigh-wave structural phase velocity measurements employing crustal constraints from co-located active source experiments to obtain estimates of Vsv and Vsh between 50 - 170 km depth. Preliminary results readily reveal the distinct shear velocity structure beneath the MER and Afar. Within the MER, shear velocity structure suggests pronounced low velocities accompanied by strong anisotropy between 80 - 140 km depth beneath the western Ethiopian plateau and rift valley. Within Afar, shear velocity structure is more varied with the slowest velocities found at shallow depths (less than 70 km depth), accompanied by weak

  11. Major influence of plume-ridge interaction, lithosphere thickness variations, and global mantle flow on hotspot volcanism—The example of Tristan

    NASA Astrophysics Data System (ADS)

    Gassmöller, Rene; Dannberg, Juliane; Bredow, Eva; Steinberger, Bernhard; Torsvik, Trond H.

    2016-04-01

    Hotspot tracks are thought to originate when mantle plumes impinge moving plates. However, many observed cases close to mid-ocean ridges do not form a single age-progressive line, but vary in width, are separated into several volcanic chains, or are distributed over different plates. Here we study plume-ridge interaction at the example of the Tristan plume, which features all of these complexities. Additionally, the South Atlantic formed close to where plume volcanism began, opening from the south and progressing northward with a notable decrease in magmatism across the Florianopolis Fracture Zone. We study the full evolution of the Tristan plume in a series of three-dimensional regional models created with the convection code ASPECT. We then compute crustal thickness maps and compare them to seismic profiles and the topography of the South Atlantic. We find that the separation of volcanism into the Tristan and Gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field. Plume material below the off-ridge track can flow toward the ridge and regions of thinner lithosphere, where decompression melting leads to the development of a second volcanic chain resembling the Tristan and Gough hotspot tracks. Agreement with the observations is best for a small plume buoyancy flux of 500 kg/s or a low excess temperature of 150 K. The model explains the distribution of syn-rift magmatism by hot plume material that flows into the rift and increases melt generation.

  12. Mantle source of the 2.44-2.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions

    NASA Astrophysics Data System (ADS)

    Yang, Sheng-Hong; Hanski, Eero; Li, Chao; Maier, Wolfgang D.; Huhma, Hannu; Mokrushin, Artem V.; Latypov, Rais; Lahaye, Yann; O'Brien, Hugh; Qu, Wen-Jun

    2016-08-01

    Significant PGE and Cr mineralization occurs in a number of 2.44-2.50-Ga mafic layered intrusions located across the Karelian and Kola cratons. The intrusions have been interpreted to be related to mantle plume activity. Most of the intrusions have negative ɛNd values of about -1 to -2 and slightly radiogenic initial Sr isotope compositions of about 0.702 to 0.703. One potential explanation is crustal contamination of a magma derived from a mantle plume, but another possibility is that the magma was derived from metasomatized sub-continental lithospheric mantle. Samples from the upper chromitite layers of the Kemi intrusion and most samples from the previously studied Koitelainen and Akanvaara intrusions have supra-chondritic γOs values indicating some crustal contamination, which may have contributed to the formation of chromitites in these intrusions. Chromite separates from the main ore zone of the Kemi and Monchepluton intrusions show nearly chondritic γOs, similar to the coeval Vetreny belt komatiites. We suggest that the Os isotope composition of the primitive magma was not significantly changed by crustal contamination due to a high Os content of the magma and a low Os content of the contaminant. Modeling suggests that the Os and Nd isotope compositions of the Monchepluton and Kemi intrusions cannot be explained by assuming a magma source in the sub-continental lithospheric mantle with sub-chondritic γOs. A better match for the isotope data would be a plume mantle source with chondritic Re/Os and Os isotope composition, followed by crustal contamination.

  13. Are the Equatorial Highlands on Venus formed by mantle plume diapirs?

    NASA Technical Reports Server (NTRS)

    Olson, Peter

    1991-01-01

    Several origins have been proposed for the Equatorial Highlands on Venus, including spreading centers and plume-related uplift. Recently, the spreading center hypothesis has been shown to be incompatible with the measured geoid and topography variations over the highlands. It is also difficult to reconcile the range of geoid anomalies over the highlands with a steady-state plum model. There is a large variation in admittance values (geoid/topography ratios) among highland regions. This variation suggests that different uplifted regions represent distinct stages in a time dependent process. It has been proposed that the Beta Regio, Thetis Regio, Ovda Regio, and Artemis Plateau Equatorial Highland Regions are formed by large mantel diapirs. According to this model, topography and geoid height decrease with increasing age of the highland, as the diapir spreads out beneath the lithosphere. In order to determine if the diapir model is compatible with the sequence of tectonic and volcanic events recorded in the surface geology of the highlands, a series of finite difference calculations were made of the ascent and partial melting of a spherical thermal diapir in an incompressible, infinite Prandtl number, isoviscous fluid.

  14. Pb, Nd, and Sr isotopic evidence for a multicomponent source for rocks of Cook-Austral Islands and heterogeneities of mantle plumes

    USGS Publications Warehouse

    Nakamura, Y.; Tatsumoto, M.

    1988-01-01

    Sr, Nd, and Pb isotopic compositions were measured in alkaline volcanic rocks (alkali basalt, ankaramite, nephelinite, phonolite, and trachyte) from the South Cook Islands (Aitutaki, Mauke, Rarotonga, Atiu, and Mangaia) and the Austral Islands (Rimatara and Rurutu). The results show that the Cook-Austral rocks have an extremely wide range in isotopic compositions of Pb: 206Pb 204Pb from 18.25 to 21.76, 207pb 204pb from 15.48 to 15.83, and sol208pb 204Pb from 38.37 to 40.62, whereas isotopic compositions of Sr and Nd are less variable. Isotopically, Mangaia, Rimatara, and Rurutu form one group (Mangaia group), which shows extremely radiogenic Pb isotopic compositions but near-MORB (mid-oceanic ridge basalts) values for Sr and Nd isotopic ratios. In contrast, samples from Aitutaki, Rarotonga, Mauke, and Atiu (Aitutaki group) have high 207Pb 204Pb and 208Pb 204Pb and moderately high 87Sr 86Sr (Dupal anomaly). The Aitutaki group could have been derived from heterogeneous mantle plumes, which rose from the enriched deep mantle (the almost primitive lower mantle or recycled continental and oceanic slabs). On the other hand, the Mangaia component could have been derived from the depleted upper mantle which may have been metasomatized with a CO2-rich fluid, as indicated by the near-MORB values of Sr and Nd isotopes. Although Pb isotopic data of the two groups cannot be distinguished from each other statistically, the end components of the Pb-Pb system do not match with those of the Nd-Sr system. Thus, the data must be explained by a multi-, at least three, component mixing model: the mantle plumes (Dupal component and a recycled oceanic slab), metasomatized upper mantle, and lithosphere. The K-Ar ages and isotopic characteristics of the Cook-Austral rocks indicate that if one mantle plume rises from the deep mantle in this region, it has separated into at least two segments on the way to the surface. ?? 1988.

  15. Anisotropic structure of the African upper mantle from Rayleigh and Love wave tomography

    NASA Astrophysics Data System (ADS)

    Sebai, Amal; Stutzmann, Eléonore; Montagner, Jean-Paul; Sicilia, Déborah; Beucler, Eric

    2006-04-01

    The geodynamics of the mantle below Africa is not well understood and anisotropy tomography can provide new insight into the coupling between the African plate and the underlying mantle convection. In order to study the anisotropic structure of the upper mantle beneath Africa, we have measured phase velocities of 2900 Rayleigh and 1050 Love waves using the roller-coaster algorithm [Beucler, E., Stutzmann, E., Montagner, J.-P., 2003. Surface-wave higher mode phase velocity measurments, using a roller-coaster type algorithm. Geophys. J. Int. 155 (1), 289-307]. These phase velocities have been inverted to obtain a new tomographic model that gives access to isotropic S V-wave velocity perturbations, azimuthal and radial anisotropies. Isotropic S V-wave velocity maps have a lateral resolution of 500 km. Anisotropy parameters have a lateral resolution of 1000 km which is uniform over Africa for azimuthal anisotropy but decreases at the West and South of Africa for radial anisotropy. At shallow depth, azimuthal anisotropy varies over horizontal distances much smaller than the continent scale. At 280 km depth, azimuthal anisotropy is roughly N-S, except in the Afar area, which might indicate differential motion between the African plate and the underlying mantle. The three cratons of West Africa, Congo and Kalahari are associated with fast velocities and transverse anisotropy that decrease very gradually down to 300 km depth. On the other hand, we observe a significant change in the direction and amplitude of azimuthal anisotropy at about 180 km depth, which could be the signature of the root of these cratons. The Tanzania craton is a shallower structure than the other African cratons and the slow velocities (-2%) observed on the maps at 180 and 280 km depth could be the signature of hot material such as a plume head below the craton. This slow velocity anomaly extends toward the Afar and azimuthal anisotropy fast directions are N-S at 180 km depth, indicating a possible

  16. Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge: A Nd-Sr-Pb isotopic study

    NASA Astrophysics Data System (ADS)

    Schilling, J.-G.; Hanan, B. B.; McCully, B.; Kingsley, R. H.; Fontignie, D.

    1994-06-01

    We report on a Pb-Nd-Sr isotope and rare earth study of Mid-Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600-km-long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3-2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peter's Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb-rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400-km-long MAR segment between the Charcot and Ascension fracture zones is also

  17. Cocos Plate Seamounts offshore NW Costa Rica and SW Nicaragua: Implications for large-scale distribution of Galápagos plume material in the upper mantle

    NASA Astrophysics Data System (ADS)

    Herbrich, Antje; Hoernle, Kaj; Werner, Reinhard; Hauff, Folkmar; Bogaard, Paul v. d.; Garbe-Schönberg, Dieter

    2015-01-01

    The origin of intraplate volcanism not directly part of a hotspot track, such as diffuse seamount provinces, and the extent of mantle plume influence on the upper mantle remain enigmatic. Here we present new 40Ar/39Ar age data and geochemical (major and trace-element and Sr-Nd-Pb isotopic) data from seamounts on the Cocos Plate presently located offshore of NW Costa Rica and SW Nicaragua. The seamounts (~ 7-24 Ma) require mixing of an enriched ocean island basalt composition, similar to that of the Northern Galápagos Domain, with two depleted components. One of the depleted components is similar to East Pacific Rise normal mid-ocean ridge basalt and the other has more depleted incompatible elements, either reflecting secondary melting of MORB or a depleted Galápagos plume component. Seamounts with ages significantly younger than the ocean crust formed in an intraplate setting and can be explained by northward transport of Galápagos plume material along the base of the Cocos Plate up to 900 km away from the hotspot and 250-500 km north of the Galápagos hotspot track. We propose that melting occurs due to decompression as the mantle upwells to shallower depth as it flows northwards, either due to changes in lithospheric thickness or as a result of upwelling at the edge of a viscous plug of accumulated plume material at the base of the lithosphere. The tholeiitic to alkaline basalt compositions of the Cocos Plate Seamounts compared to the more silica under-saturated compositions of Hawaiian rejuvenated and arch (alkali basalts to nephelinites) lavas are likely to reflect the significant difference in age (< 25 vs ~ 90 Ma) and thus thickness of the lithosphere on which the lavas were erupted.

  18. Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China

    NASA Astrophysics Data System (ADS)

    Wei, Xun; Xu, Yi-Gang; Luo, Zhen-Yu; Zhao, Jian-Xin; Feng, Yue-Xing

    2015-08-01

    Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (0.26-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenocrysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The 87Sr/86Sri (0.7035-0.7037) and εNdi (4.5-4.8) of the clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the wehrlites (Mg# = 75-84, 87Sr/86Sri = 0.7038-0.7041, εNdi = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts.

  19. Is the track of the Yellowstone hotspot driven by a deep mantle plume? - Review of volcanism, faulting, and uplift in light of new data

    USGS Publications Warehouse

    Pierce, K.L.; Morgan, L.A.

    2009-01-01

    Geophysical imaging of a tilted mantle plume extending at least 500??km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17??Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle-plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River-Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14??Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely "scraped off" the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250??km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62??km/m.y. from 17 to 10??Ma, compared to the rate of ~ 25??km/m.y. from 10 to 2??Ma. A plume head-to-tail transition occurred in the 14-to-10-Ma interval in the central Snake River Plain and was characterized by frequent (every 200-300??ka for about 2??m.y. from 12.7 to 10

  20. Is the track of the Yellowstone hotspot driven by a deep mantle plume? — Review of volcanism, faulting, and uplift in light of new data

    NASA Astrophysics Data System (ADS)

    Pierce, Kenneth L.; Morgan, Lisa A.

    2009-11-01

    Geophysical imaging of a tilted mantle plume extending at least 500 km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17 Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle-plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River-Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14 Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely "scraped off" the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250 km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62 km/m.y. from 17 to 10 Ma, compared to the rate of ~ 25 km/m.y. from 10 to 2 Ma. A plume head-to-tail transition occurred in the 14-to-10-Ma interval in the central Snake River Plain and was characterized by frequent (every 200-300 ka for about 2 m.y. from 12.7 to 10.5 Ma

  1. Tectono-magmatic interaction of mantle plumes with thin and warm Venus lithosphere: consequences for the origin of novae and coronae structures

    NASA Astrophysics Data System (ADS)

    Gerya, Taras

    2014-05-01

    Recent global mapping of crustal and lithospheric thickness on Venus suggest that 47% of the planet has an estimated very low elastic thickness value of 0-20 km (Anderson and Smrekar, 2006), possibly indicating thin and warm lithosphere (Diament and Burov, 1992). These finding suggest that some of the prominent Venus surface structures such as coronae and novae may actually result from mantle plumes interaction with the thin and warm Venus lithosphere that may allow penetration of mantle upwellings to the bottom of the crust. Here we present new 3D high-resolution thermomechanical model of thermal mantle plume impingement into warm and thin lithosphere with Venus-like surface temperature. Numerical results suggests that nova-like and corona-like structures can result from magma-assisted convection of weak ductile crust, induced by decompression melting of the hot rising mantle plume. During the initial stage, nova forms by stellate fracturing of a topographic rise forming atop the growing crustal convection cell. Few million years later, nova can convert to coronae by inward dipping concentric fracturing of the nova rise margins and subsequent outward thrusting of partially molten crustal rocks over the surface. An outer annulus of concentric normal faults forms in the outer rise region of the downbending brittle upper crust. Whereas an inner annulus of concentric thrust faults forms in front of the outward thrusting crustal wedge. A trench-like depression forms between these two annuli. Resembling retreating subduction, the rudimentary concentric upper-crustal slab warms up rapidly and recycles into the convection cell. The convection cell remains active for up to 15 million years, fueled by heat and magma from the plume. Predicted surface topography and fracturing patterns agree with some small to moderate size novae and coronae on Venus. References: Anderson, F.S., Smrekar, S.E. (2006) Global mapping of crustal and lithospheric thickness on Venus. J. Geophys

  2. Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

    NASA Astrophysics Data System (ADS)

    Tolstikhin, I. N.; Kamensky, I. L.; Marty, B.; Nivin, V. A.; Vetrin, V. R.; Balaganskaya, E. G.; Ikorsky, S. V.; Gannibal, M. A.; Weiss, D.; Verhulst, A.; Demaiffe, D.

    2002-03-01

    During the Devonian magmatism (370 Ma ago) ˜20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ˜300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ˜70 samples. 4He/ 3He ratios in He released by fusion vary from pure radiogenic values ˜10 8 down to 6 × 10 4. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of 3He, up to 4 × 10 -9 cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low 4He/ 3He = 3 × 10 4, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 10 4, indicating the contribution of 3He-rich plume component. Magnetites are principal carriers of this component. Trapped 3He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic 4He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of 4He sites degraded by U, Th decay. Similar 4He/ 3He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source. The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional 20Ne/ 22Ne versus 21Ne/ 22Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). 20Ne/ 22Ne ratios (up to 12.1) correlate well with 40

  3. Temporal variations in the mantle potential temperatures along the Northwest Hawaiian Ridge using olivine-liquid equilibria: Implications for Hawaiian plume melt flux variations

    NASA Astrophysics Data System (ADS)

    Tree, J. P.; Garcia, M. O.; Putirka, K. D.

    2013-12-01

    samples from Midway, Hancock, and Gardner Pinnacles will be analyzed in order to obtain additional estimates of temporal variations in mantle potential temperature variations along the Ridge. Other parameters may also be affecting the magmatic productivity such as source heterogeneities (ie: pyroxenite vs peridotite) or melting pressure. These will be investigated using the computational methods of PRIMELT2 to understand significance of temperature, pressure, and compositional variations on the melt flux history of Hawaiian mantle plume.

  4. Are Mantle Plumes Periodic?

    NASA Astrophysics Data System (ADS)

    Rampino, Michael R.; Prokoph, Andreas

    2013-03-01

    In the past few years, researchers have uncovered evidence that several kinds of geological and biological events seem to show regular cycles of similar lengths. For example, Rohde and Muller [2005] looked at the record of diversity of marine organisms over the past 540 million years and found evidence for two cycles in the data—a roughly 62-million-year cycle and a longer cycle of about 140 million years. This was followed by reports of an approximately 56-million-year cycle in long-term stratigraphic sequences in sedimentary basins [Meyers and Peters, 2011] and a 59-million-year period in the marine strontium-isotope record [Melott et al., 2012]. A similar period may even exist in atmospheric carbon dioxide over the past 542 million years of the Phanerozoic [Franks et al., 2012]. A cycle of about 140 million years was reported by Veizer et al. [2000] and Mayhew et al. [2008] in long-term fluctuations in global climate.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  6. The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

    NASA Astrophysics Data System (ADS)

    Hagos, Miruts; Koeberl, Christian; van Wyk de Vries, Benjamin

    2016-05-01

    The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3-8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036-0.7041, 143Nd/144Nd = 0.51286-0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta'Ale and Alu Shield basalts).

  7. Uppermost mantle velocity from Pn tomography in the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Corbeau, Jordane; Rolandone, Frédérique; Leroy, Sylvie; Al-Lazki, Ali; Keir, Derek; Stuart, Graham; Stork, Anna

    2013-04-01

    We present an analysis of Pn traveltimes to determine lateral variations of velocity in the uppermost mantle and crustal thickness beneath the Gulf of Aden and its margins. No detailed tomographic image of the entire Gulf of Aden was available. Previous tomographic studies covered the eastern Gulf of Aden and were thus incomplete or at a large scale with a too low resolution to see the lithospheric structures. From 1990 to 2010, 49206 Pn arrivals were selected from the International Seismological Center catalogue. We also used temporary networks : YOCMAL (Young Conjugate Margins Laboratory) networks with broadband stations located in Oman, Yemen and Socotra from 2003 to 2011, and Djibouti network from 2009 to 2011. From these networks we picked Pn arrivals and selected 4110 rays. Using a least-squares tomographic code (Hearn, 1996), these data were analyzed to solve for velocity variations in the mantle lithosphere. We perform different inversions for shorter and longer ray path data sets in order to separate the shallow and deep structure within the mantle lid. In the upper lid, zones of low velocity (7.7 km/s) around Sanaa, Aden, Afar, and along the Gulf of Aden are related to active volcanism. Off-axis volcanism and a regional melting anomaly in the Gulf of Aden area may be connected to the Afar plume, and explained by the model of channeling material away from the Afar plume along ridge-axis. Our study validates the channeling model and shows that the influence of the Afar hotspot may extend much farther eastwards along the Aden and Sheba ridges into the Gulf of Aden than previously believed. Still in the upper lid, high Pn velocities (>8,2 km/s) are observed in Yemen and may be related to the presence of a magmatic underplating under the volcanic margin of Aden and under the Red Sea margins. In the lower lid, zones of low velocities are spatially located differently than in the upper lid. On the Oman margin, a low velocity zone (7.6 km/s) suggests deep partial

  8. Interaction of extended mantle plume head with ancient lithosphere: evidence from deep-seated xenoliths in basalts and lamprophyre diatremes in Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2016-04-01

    The Middle Cretaceous lamprophyric diatremes of the Jabel Ansaria Ridge contain xenoliths of ancient lower crustal rocks mainly represented by the suite of partly altered garnet granulite and eclogite-like rocks, which were formed at the expense of ferrogabbros and ferroclinopyroxenites most likely in the course of underplating of Fe-Ti basalt. Garnet (Alm26Grs11Py63) megacrysts and coarse-granular garnet-clinopyroxene intergrowths are most likely the varieties of rocks of this series. Garnet megacrysts are represented by large (up to 10 cm in diameter) round "nodules," often molten from the surface. Garnet is usually fractured, and the kelyphite material similar to that in rocks of the eclogite-granulite series occurs in fractures. In addition, we found several intergrowths of garnet with large (up to 3-5 cm in length) crystals of high-Al augite with the low of Ti and Na contents like in rocks of the eclogite-granulite suite. Coarse-grained garnet-clinopyroxene-hornblende rocks with spinel, as well as megacrysts of Al-Ti augite with kaersutite, form the second group in prevalence. This group is close to mantle xenoliths of the "black series" in alkali Fe-Ti basalt worldwide. Kaersutite in these rocks contains gaseous cavities, which provides evidence for the origin of rocks at the expense of a strongly fluidized melt/fluid. In contrast to rocks of the eclogite-granulite series, these rocks did not undergo alteration. Garnet Alm19-26Grs12-13.5Py59-67.5 usually associates with dark opaque spinel. In contrast, the Late Cenozoic plateaubasalts of the region practically do not contain lower crustal xenoliths, whereas xenoliths of mantle spinel lherzolite (fragments of the upper cooled rim of the plume head) are widely abundant. According to data of mineralogical thermobarometry, rocks of the eclogite-granulite suite were formed at 13.5-15.4 kbar (depths of 45-54 km) and 965-1115°C. Rocks of this suite are typical representatives of the continental lower crust

  9. Sub-km HIMU-type Enriched Mantle at a Mid-ocean Ridge Far From a Plume: Endeavour, JdFR

    NASA Astrophysics Data System (ADS)

    Gill, J. B.; Michael, P. J.; Dreyer, B. M.; Clague, D. A.; Ramos, F. C.

    2015-12-01

    The Endeavour segment of the Juan de Fuca Ridge is characterized by abundant enriched (E) MORB since the currently inflated axial ridge formed <105 years ago, and by the full range of depleted (D) to E-MORB during the last 2300 years in the km-wide axial graben. Two different styles of enrichment of moderately incompatible elements are present. The first characterized basalts across the ~5 km-wide ridge from >10,000 to ~4000 years ago, whereas the second characterizes more recent basalts erupted in the axial graben. We attribute the first to a higher proportion of pyroxenite to enriched peridotite in the mantle source during ridge inflation. The more recent style reflects the reduced role of pyroxenite after the axial graben formed. The enriched component for both styles is a HIMU-type because it has low 87Sr/86Sr and 176Hf/177Hf relative to 143Nd/144Nd, lower 3He/4He (~8.1 RA) than in the more depleted basalts, shallow slopes on Pb isotope diagrams, and high Nb/LREE ratios. It is regionally widespread and shared with the West Valley and Explorer segments to the north. At least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, indicating that the scale of mantle heterogeneity is <1 km in this setting that is far from a plume.

  10. Three-dimensional inversion of large-scale EarthScope magnetotelluric data based on the integral equation method: Geoelectrical imaging of the Yellowstone conductive mantle plume

    NASA Astrophysics Data System (ADS)

    Zhdanov, Michael S.; Smith, Robert B.; Gribenko, Alexander; Cuma, Martin; Green, Marie

    2011-04-01

    Interpretation of the EarthScope MT (magnetotelluric) data requires the development of a large-scale inversion method which can address two common problems of 3D MT inversion: computational time and memory requirements. We have developed an efficient method of 3D MT inversion based on an IE (integral equation) formulation of the MT forward modeling problem and a receiver footprint approach, implemented as a massively parallel algorithm. This method is applied to the MT data collected in the western United States as a part of the EarthScope project. As a result, we present one of the first 3D geoelectrical images of the upper mantle beneath Yellowstone revealed by this large-scale 3D inversion of the EarthScope MT data. These images show a highly conductive body associated with the tomographically imaged mantle plume-like layer of hot material rising from the upper mantle toward the Yellowstone volcano. The conductive body identified in these images is west-dipping in a similar way to a P-wave low-velocity body.

  11. New attempts to identify core-mantle interactions in plume-derived materials using ultra-high precision tungsten isotope measurements

    NASA Astrophysics Data System (ADS)

    Touboul, M.; Puchtel, I. S.; Walker, R. J.

    2010-12-01

    Some mantle plume derived materials show coupled 187,186Os enrichments relative to upper-mantle materials that have been interpreted by some to reflect core-mantle interaction (Brandon et al., 1999, 2003, Puchtel et al., 2005). In addition to osmium, tungsten is another element whose isotopic composition can potentially be used to trace core-mantle interactions. Tungsten has one radiogenic isotope, 182W, a decay product of 182Hf, with a half-life of ~9 Myr. Like Os, W is siderophile, under reducing conditions, and, hence, is preferentially incorporated into Earth’s core, whereas Hf is lithophile and is retained in the mantle. Fractionation of Hf from W during core formation is predicted to have led to large differences in 182W/184W between the core and mantle. The use of W isotopes as tracers of core-mantle interaction has been hampered by limitations in the ability to measure W isotopic ratios at the level of ± 10 ppm or better. Within analytical uncertainty, MC-ICP-MS measurements of terrestrial rocks investigated so far show no resolvable 182W anomalies (Scherstén et al., 2004). Over the past year, we have developed a new ultra-high precision 182W/184W measurement protocol using a Triton thermal ionization mass spectrometer, allowing us to resolve 182W anomalies at a ± 6 ppm level (2σ, n=40). All W isotope measurements are performed in a negative ionization mode (WO3-) using a dynamic acquisition scheme. This precision improvement allows us to more rigorously interrogate the W isotopic compositions of materials with potentially deep mantle origins. A major problem in this application of W isotopes is the acquisition of mantle-derived materials that have not been contaminated with crustal W. Here we present W abundances, measured using isotope dilution, and corresponding ultra-high precision W isotopic composition measurements of Archean komatiites from the Kostomuksha greenstone belt (Baltic Shield), for which coupled 186Os-187Os enrichment has been

  12. Mapping Mantle Mixing and the Extent of Superplume Influence Using He-Ne-Ar-CO2-N2 Isotopes: The Case of the East Africa Rift System

    NASA Astrophysics Data System (ADS)

    Hilton, D. R.; Halldorsson, S. A.; Scarsi, P.; Castillo, P.; Abebe, T.; Kulongoski, J. T.

    2014-12-01

    Earth's mantle possesses distinct and variable volatile characteristics as sampled by magmatic activity in different tectonic environments. In general, trace element depleted mid-ocean ridge basalts, with low Sr and Pb isotope values (but high ɛNd and ɛHf), release mantle-derived noble gases characterised by 3He/4He ~8 ± 1RA, (21Ne/22Ne)ex ~0.06 and 40Ar/36Ar ≥ 10,000 with CO2 and N2 having δ13C~-5‰ and δ15N ~-5‰, respectively. In contrast, enriched intraplate lavas possess higher 3He/4He (up to 50RA), lower (21Ne/22Ne)ex ~0.035 and 40Ar/36Ar ≤ 10,000 with generally higher but variable δ13C and δ15N. These isotopic attributes of mantle-derived volatiles can be exploited to map the extent, and mixing characteristics, of enriched (plume) mantle with depleted asthenospheric mantle ± the effects of over-riding lithosphere and/or crust. The East African Rift System (EARS) is superimposed upon two massive plateaux - the Ethiopia and Kenya domes - regarded as geophysical manifestations of a superplume source, a huge thermochemical anomaly originated at the core-mantle boundary and providing dynamic support for the plateaux. We present new volatile isotopic and relative abundance data (on the same samples) for geothermal fluids (He-CO2-N2), lavas (He-Ne-Ar) and xenoliths (He-Ne-Ar-CO2-N2) which provide an unprecedented overview of the distribution of mantle volatiles of the Ethiopia Dome, from the Red Sea via the Afar region and Main Ethiopian Rift (MER) to the Turkana Depression. Notably, peaks in geothermal fluid 3He/4He (16RA) and δ15N (+6.5‰) are coincident within the MER but the maximum δ13C (-0.78‰) lies ~100 km to the south. Highs in 3He/4He (14RA), δ13C (~-1‰) and δ15N (+3.4‰) for mafic crystals occur in the Afar region ~ 500km to the north. We assess the significance of the off-set in these volatile isotope signals, for sampling volatile heterogeneity in the plume source and/or the relative sensitivity of different volatiles to

  13. Linking the Earth's surface with the deep-mantle plume beneath a region from Iceland to the city of Perm

    NASA Astrophysics Data System (ADS)

    Glišović, Petar; Forte, Alessandro; Simmons, Nathan; Grand, Stephen

    2014-05-01

    Current tomography models consistently reveal three large-scale regions of strongly reduced seismic velocity in the lowermost mantle under the Pacific, Africa and a region that extends from below Iceland to the city of Perm (the Perm Anomaly). We have carried out mantle dynamic simulations (Glišović et al., GJI 2012; Glišović & Forte, EPSL 2014) of the evolution of these large-scale structures that directly incorporate: 1) robust constraints provided by joint seismic-geodynamic inversions of mantle density structure with constraints provided by mineral physics data (Simmons et al., GJI 2009); and 2) constraints on mantle viscosity inferred by inversion of a suite of convection-related and glacial isostatic adjustment data sets (Mitrovica & Forte, EPSL 2004) characterised by Earth-like Rayleigh numbers. The convection simulations provide a detailed insight into the very-long-time evolution of the buoyancy of these lower-mantle anomalies. We find, in particular, that the buoyancy associated with the Perm Anomaly generates a very long-lived superplume that is connected to the paleomagnetic location of the Siberian Traps at the time of their eruption (Smirnov & Tarduno, EPSL 2010) and also to location of North Atlantic Igneous Provinces (i.e., the opening of North Atlantic Ocean).

  14. Multiple mantle upwellings beneath the Northern East-African Rift System from relative P- and S-wave traveltime tomography

    NASA Astrophysics Data System (ADS)

    Civiero, Chiara; Hammond, James; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

    2015-04-01

    Mantle plumes have been invoked as the likely cause of East African Rift volcanism and extension. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume, the African Superplume, connected to the LLSVP beneath Southern Africa, to one or more distinct lower-mantle sources along the rift. We present a new relative travel-time tomography model that images detailed P- and S- wave velocities from P,S and SKS phases below the northern East-African, Red Sea and Gulf of Aden rift. Data comes from stations that cover the area from Tanzania to Saudi Arabia. The aperture of the integrated dataset allows us to image for the first time structures of ~100 km length scale down to depths of 900 km beneath this region. Our images provide evidence of at least two low-velocity structures with a diameter of ~200 km that continue through the transition zone and into the lower mantle: the first extends to at least 900 km beneath Afar, and a second reaching at least 750 km depth just west of the Main Ethiopian Rift, a region with off-rift volcanism. Taking into account seismic sensitivity to temperature and thermally controlled phase boundary topography, we interpret these features as multiple focused upwellings from below the transition zone with excess temperatures of 100±50 K. The scale of the upwellings is smaller than any of the previously proposed lower mantle plume sources. This suggests the ponding or flow of deep-plume material below the transition zone may be spawning smaller upper-mantle upwellings.

  15. Melting during late-stage rifting in Afar is hot and deep.

    PubMed

    Ferguson, D J; Maclennan, J; Bastow, I D; Pyle, D M; Jones, S M; Keir, D; Blundy, J D; Plank, T; Yirgu, G

    2013-07-01

    Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin. PMID:23823795

  16. Full seismic waveform inversion of the African crust and Mantle - Initial Results

    NASA Astrophysics Data System (ADS)

    Afanasiev, Michael; Ermert, Laura; Staring, Myrna; Trampert, Jeannot; Fichtner, Andreas

    2016-04-01

    We report on the progress of a continental-scale full-waveform inversion (FWI) of Africa. From a geodynamic perspective, Africa presents an especially interesting case. This interest stems from the presence of several anomalous features such as a triple junction in the Afar region, a broad region of high topography to the south, and several smaller surface expressions such as the Cameroon Volcanic Line and Congo Basin. The mechanisms behind these anomalies are not fully clear, and debate on their origin spans causative mechanisms from isostatic forcing, to the influence of localized asthenospheric upwelling, to the presence of deep mantle plumes. As well, the connection of these features to the African LLSVP is uncertain. Tomographic images of Africa present unique challenges due to uneven station coverage: while tectonically active areas such as the Afar rift are well sampled, much of the continent exhibits a severe dearth of seismic stations. As well, while mostly surrounded by tectonically active spreading plate boundaries (a fact which contributes to the difficulties in explaining the South's high topography), sizeable seismic events (M > 5) in the continent's interior are relatively rare. To deal with these issues, we present a combined earthquake and ambient noise full-waveform inversion of Africa. The noise component serves to boost near-surface sensitivity, and aids in mitigating issues related to the sparse source / station coverage. The earthquake component, which includes local and teleseismic sources, aims to better resolve deeper structure. This component also has the added benefit of being especially useful in the search for mantle plumes: synthetic tests have shown that the subtle scattering of elastic waves off mantle plumes makes the plumes an ideal target for FWI [1]. We hope that this new model presents a fresh high-resolution image of sub-African geodynamic structure, and helps advance the debate regarding the causative mechanisms of its surface

  17. The effect of mantle plume heads on the motion between the African and Antarctic plates in the Late Cretaceous and Early Cenozoic

    NASA Astrophysics Data System (ADS)

    Cande, S. C.; Patriat, P.

    2012-12-01

    Indo-Atlantic plate kinematics during the Late Cretaceous and Early Cenozoic were dominated by a period of roughly 25 million years during which the motions of India and Africa appear to have been coupled: a rapid speedup of India's absolute motion starting around 68 Ma was accompanied by a dramatic slowdown of Africa's absolute motion and the subsequent slowdown of India between 52 and 45 Ma was accompanied by a speedup of Africa. Cande and Stegman (2011) proposed that the coupled nature of these plate motions was caused by the arrival of the Reunion plume head at the Earth's surface: the speedup of India (slowdown of Africa) was due to the onset of the plume head, while the slowdown of India (speedup of Africa) was due to the waning of the plume head. This hypothesis is controversial since the slowdown of India has long been attributed to the initial collision of India with Eurasia and it is not clear how mantle plume heads affect plate motions. In order to better understand the cause of the coupled motions of India and Africa we have re-examined the motion of Africa relative to Antarctica as constrained by magnetic anomalies and fracture zones on the Southwest Indian Ridge (SWIR). The bends of the SWIR fracture zones contain a particularly important record of plate motion changes: a gradual ccw bend starting at Chron 32 is followed by a sharp cw bend at Chron 24. We present here a set of 13 revised rotations for the SWIR for the time interval from Chron 34 to Chron 18. These rotations quantify in more detail than in previous studies the changes recorded by the SWIR fracture zones. The onset of the ccw change in spreading direction and start of a rapid decrease in spreading rate on the SWIR occurs around Chron 32 (71 Ma). From Chron 32 to Chron 24 the motion between Africa and Antarctica is characterized by a continuous and apparently smooth migration of the Africa-Antarctic stage pole. The most dramatic change in motion along the SWIR is the sudden cw bend of

  18. Constraints on a plume in the mid-mantle beneath the Iceland region from seismic array data

    USGS Publications Warehouse

    Pritchard, M.J.; Foulger, G.R.; Julian, B.R.; Fyen, J.

    2000-01-01

    Teleseismic P waves passing through low-wave-speed bodies in the mantle are refracted, causing anomalies in their propagation directions that can be measured by seismometer arrays. Waves from earthquakes in the eastern Pacific and western North America arriving at the NORSAR array in Norway and at seismic stations in Scotland pass beneath the Iceland region at depths of ~ 1000-2000 km. Waves arriving at NORSAR have anomalous arrival azimuths consistent with a low-wave-speed body at a depth of ~ 1500 km beneath the Iceland-Faeroe ridge with a maximum diameter of ~ 250 km and a maximum wave-speed contrast of ~ 1.5 per cent. This agrees well with whole-mantle tomography results, which image a low-wave-speed body at this location with a diameter of ~ 500 km and a wave-speed anomaly of ~ 0.5 per cent, bearing in mind that whole-mantle tomography, because of its limited resolution, broadens and weakens small anomalies. The observations cannot resolve the location of the body, and the anomaly could be caused in whole or in part by larger bodies farther away, for example by a body imaged beneath Greenland by whole-mantle tomography.

  19. Mantle Anisotropy Beneath the Hawaiian Islands from Measurements of Shear-wave Splitting: Results from the PLUME Ocean-Bottom and Land Seismograph Deployments

    NASA Astrophysics Data System (ADS)

    Collins, J. A.; Wolfe, C. J.; Laske, G.; Solomon, S. C.; Detrick, R. S.; Orcutt, J. A.; Bercovici, D. A.; Hauri, E. H.

    2008-12-01

    The fieldwork component of the Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) project consisted of two consecutive one-year deployments of ocean-bottom seismometer (OBS) and land stations, respectively offshore and on the Hawaiian Islands. Thirty-five OBSs were deployed in the first year in a relatively dense array around the modern locus of the Hawaiian hotspot; in the second year, 38 OBSs were deployed over an area extending from west of Kauai to east of Hawaii. Ten portable land stations were operated for a period spanning both OBS deployments. We have analyzed SKS phases recorded by both OBS and land stations for anisotropy-induced shear-wave splitting. Splitting measurements were typically made in the frequency band 0.05-0.1 Hz in order to minimize tilt-generated noise at the low-frequency end and microseismic noise at the high end. Only events with Mw ≥ 6 yielded measurements with adequate precision. Data quality is such that there are about 5 events per station that yield good splitting measurements. Splitting parameters were measured using the stacking technique of Wolfe and Silver [1998]. The geographical distribution of fast-polarization azimuths does not show an obvious signature of a localized center of mantle upwelling and divergence. Fast azimuths are predominantly parallel to the fossil spreading direction (~75°), with a smaller number parallel to the present-day direction of absolute plate motion (-58°). Measured delay times are typically about 1 s or less, although some stations display larger splitting times of 1-2 s. The variability in the delay times across the different stations may indicate differences in either the degree of anisotropy or thickness of the anisotropic lithosphere. Some well- constrained null measurements may provide constraints on the amount of heating and deformation of the lithosphere due to interaction with upwelling mantle.

  20. Long-term cycles in Large Igneous Province (LIP) eruptions and fossil diversity may be related to periodic mantle plume activity

    NASA Astrophysics Data System (ADS)

    Rampino, M. R.; Prokoph, A.

    2013-12-01

    Wavelet analysis shows evidence for an approximately 62 Myr cycle in the eruptions of Large Igneous Provinces (LIPs) over the last 540 Myr. This agrees with a cycle of 62×3 Myr seen in fossil biodiversity over the same interval. A similar cycle (about 56-62 Myr) has been reported in data related to continental-scale fluctuations of sedimentation, most likely resulting from changes in climate, sea level and tectonics. A longer about 140 Myr cycle is also detected in the LIP eruption data, matching a similar cycle seen in fossil diversity and in global climate. Both the LIP and fossil diversity data sets show a shorter approximately 28-35 Myr period, especially during the last 135 Myr. Cross-spectral wavelet analysis of the LIP occurrences against the marine diversity record shows that almost all cross-variability in the two data sets is concentrated in the 28 to 35 Myr, 62 to 65 Myr and around 140 Myr wavelength bands, showing a sharp change from dominant 62 Myr to 32 Myr cyclicity since the Early Cretaceous. The phase differences for the cycles indicate an inverse LIP-diversity relationship at these wavebands. The LIP eruptions commonly mark the initiation of hotspots, presumably created by upwelling mantle plumes. The agreement among these periodicities suggests that long-term global cycles of biodiversity, sea level, climate and sedimentation are partly driven by periodic or quasi-periodic fluctuations in mantle plume activity. This conclusion is bolstered by the close temporal correlation of several LIPs with times of mass extinction and climatic crises indicated by ocean anoxic events.

  1. Upper mantle seismic structure beneath the Pacific Northwest: A plume-triggered delamination origin for the Columbia River flood basalt eruptions

    NASA Astrophysics Data System (ADS)

    Darold, Amberlee; Humphreys, Eugene

    2013-03-01

    We invert teleseismic P and S body waves constrained by an ambient-noise surface wave model and Moho depth inferred from receiver function analysis (Gao et al., 2011) to image mantle structures continuously from the surface to the base of the upper mantle. The major structures coincide with prominent geological features. We focus on a NE-Oregon structure, termed here the Wallowa anomaly, which coincides with the source area for the ˜16 Ma Columbia River flood basalt eruptions and a circular area of topographic relief created during and after these eruptions. Resolution tests indicate that the curtain-like structure previously interpreted as Farallon lithosphere connects with the Wallowa anomaly above 150 km along the northeast margin of the Wallowa anomaly. This connection, along with the pre-flood basalt magmatic and tectonic history of the Pacific Northwest, lead us to conclude that arrival of the Yellowstone plume to south-central Oregon initiated delamination of remnant Farallon lithosphere from the base of NE Oregon, exposing ocean crust to Yellowstone asthenosphere. This hypothesis accounts for the propagation of flood basalt volcanism far north of the Yellowstone hotspot track, and for the high-silica composition of most of the flood basalt magmas.

  2. Isotope-geochemical Nd-Sr evidence of Palaeoproterozoic plume magmatism in Fennoscandia and mantle-crust interaction on stages of layered intrusions formation

    NASA Astrophysics Data System (ADS)

    Serov, Pavel; Bayanova, Tamara; Kunakkuzin, Evgeniy; Steshenko, Ekaterina

    2016-04-01

    characteristic feature is that in most cases, the proportion of mantle component decreases from the central parts of intrusions to their boundary zones. This may indicate a slight degree of contamination of the magma intrusion by crustal material near the contacts with the frame- rocks. Thus, our investigations show that Palaeoproterozoic layered PGE-bearing intrusions in the N-E Fennoscandian Shield were derived from intraplate magmatism. The same Palaeoproterozoic layered intrusions are known on the Fennoscandian Shield, Superior and Wyoming provinces of the world, and according to [Heaman, 1997; Ernst et.al., 2008] they were derived from the mantle plumes which caused the breakup of the oldest Kenorland supercontinent. These studies were supported by the RFBR 15-35-20501.

  3. He-Ne-Ar isotope studies of mafic volcanic rocks and mantle xenoliths from the East African Rift System - contrasting isotope signals in different rift branches

    NASA Astrophysics Data System (ADS)

    Halldorsson, S. A.; Hilton, D. R.; Scarsi, P.; Abebe, T.; Massi, K. M.; Barry, P. H.; Fischer, T. P.; de Moor, J.; Rudnick, R. L.

    2010-12-01

    Helium isotope studies of the East African Rift System (EARS) suggest the involvement of a deep mantle plume(s) beneath the northern (Ethiopian) segment [1-3]. The highest 3He/4He (RA) signatures found to date show a close association with the greatest magma volumes erupted since the Early Cenozoic in the region. While the helium isotope characteristics are well established in the Ethiopia-Afar region, Ne and Ar systematics remain poorly constrained. Using a combined He-Ne-Ar isotope approach, our aim is to determine the regional extent of the influence of the Afar plume and to distinguish between subcontinental lithospheric mantle (SCLM) and/or a possible second mantle plume sources located to the south of the Turkana Depression. Xenoliths and mafic lavas from N-Tanzania display a limited range in He isotopes (5-7 RA) with exceptions at Arusha (7.8RA) and Labait (8.7RA), through 7.1-8.7 RA in N-Kenya and S-Ethiopia, to 14.3 RA in the Main Ethiopian Rift and Afar, spanning nearly the entire range of previously reported values. The mean 3He/4He ratio from of lavas and xenoliths from N-Tanzania is remarkably close to the global average of 6.1±0.9 (RA) for continental xenoliths and basalts, thought to represent the SCLM [4]. Thus far, only MORB-like values of 7.3-8.3 RA have been found in volcanics of the Western rift. Initial Ne isotope data reveal the presence of a solar-like Ne component in xenoliths from the Ethiopia-Afar region, with extrapolated 21Ne/22Neex ratios of 0.0365 (assuming Ne-B = 12.5). This trend overlaps that of the Loihi-Kilauea line (L-K). Interestingly, a xenolith from N-Tanzania has a 21Ne/22Neex ratio of 0.0415, falling on a trajectory intermediate between MORB and L-K. The highest 40Ar/36Ar ratio obtained on phenocrysts/xenoliths to date is 1510. The generally low 3He/4He ratios of N-Tanzania likely result from different mixing proportions of asthenospheric sources with lithospheric material, the latter having developed lower 3He/4He ratios

  4. A potential link between magmatic volatiles and mantle source lithology in the Hawaiian Plume: a view from olivine-hosted melt inclusions and osmium isotopes

    NASA Astrophysics Data System (ADS)

    Marske, J. P.; Hauri, E. H.; Garcia, M. O.; Pietruszka, A. J.

    2013-12-01

    Variations in radiogenic isotope ratios and magmatic volatile abundances (e.g., CO2 or H2O) in lavas from Hawaiian volcanoes reveal important magmatic processes (e.g., melting of a heterogeneous source and magma degassing). Based on variations in ratios of highly incompatible trace elements (e.g., Nb/La) and radiogenic isotopes (e.g., 206Pb/204Pb), shield-stage Hawaiian lavas likely originate from a plume source containing peridotite and ancient recycled oceanic crust (pyroxenite). The source region may also be heterogeneous with respect to volatile concentrations. However, shallow magma degassing makes it difficult to determine if there is a link between mantle source composition and the volatile budget. We analyzed osmium isotopic ratios and volatile contents in olivines and glasses for 34 samples from Koolau, Mauna Kea, Mauna Loa, Hualalai, Kilauea, and Loihi to determine if volatiles in magmas correlate with geochemical tracers of source lithology. For a given volcano, most 187Os/188Os values of olivines (0.127-0.134) are similar to the whole-rock values, yet some Mauna Loa and Loihi olivines display the lower ratios (0.116-0.118) that may reflect partial melts of ancient recycled mantle lithosphere. SIMS analyses of Hawaiian glasses reveal a range in abundances of CO2 (10-250 ppm), H2O (0.2-1.2 wt.%), S (38-2960 ppm), and Cl (39-2960 ppm). However, most samples have low CO2 contents (<100 ppm) indicating that the lavas are degassed. Olivine-hosted melt inclusions from the same Hawaiian samples display a wider range of volatile abundances (i.e. 10-760 ppm CO2) than matrix glasses that may reflect mixing of undegassed to moderately degassed magmas. The average CO2 and H2O/CO2 contents in the least degassed olivine-hosted melt inclusions (with >200 ppm CO2) display a broad correlation with the osmium isotopic compositions of the olivines. This indicates a potential link between pre-eruptive volatile budgets and mantle sources lithology may exist within the

  5. Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

    NASA Technical Reports Server (NTRS)

    Erickson, Stephanie Gwen; Nelson, Wendy R.; Peslier, Anne H.; Snow, Jonathan E.

    2014-01-01

    The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 kmof continental flood basalts approx.30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle.

  6. Horizontal and vertical zoning of heterogeneities in the Hawaiian mantle plume from the geochemistry of consecutive postshield volcano pairs: Kohala-Mahukona and Mauna Kea-Hualalai

    NASA Astrophysics Data System (ADS)

    Hanano, Diane; Weis, Dominique; Scoates, James S.; Aciego, Sarah; Depaolo, Donald J.

    2010-01-01

    Sr-Nd-Pb-Hf isotopic compositions of postshield lavas from two pairs of Hawaiian volcanoes, Mauna Kea and Kohala (Kea trend) and Hualalai and Mahukona (Loa trend), allow for identification of small-scale (tens of kilometers) heterogeneities in the Hawaiian mantle plume and provide constraints on their distribution. The postshield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g., LaN/YbN = 6.0-16.2). These lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.70343-0.70365, 143Nd/144Nd = 0.51292-0.51301, and 176Hf/177Hf = 0.28311-0.28314) and have distinct Pb isotopic compositions (206Pb/204Pb = 17.89-18.44, 207Pb/204Pb = 15.44-15.49, and 208Pb/204Pb = 37.68-38.01) that correspond to their respective Kea or Loa side of the Pb-Pb isotopic boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to postshield stage and they trend to low 87Sr/86Sr toward, but not as extreme as, compositions characteristic of rejuvenated stage lavas. Hualalai postshield lavas lie distinctly above the Hf-Nd Hawaiian array and have much lower Pb isotopic ratios than shield lavas, including some of the least radiogenic values (e.g., 206Pb/204Pb = 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona volcano shows that these older postshield lavas become more radiogenic in Pb during the late stages of volcanism. The isotope systematics of the postshield lavas cannot be explained by mixing between Hawaiian plume end-members (e.g., Kea, Koolau, and Loihi) or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust (±sediments) in their source. More than one depleted component is sampled by the postshield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions

  7. the role of magmatism and segmentation in the structural evolution of the Afar Rift

    NASA Astrophysics Data System (ADS)

    Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël; Quidelleur, Xavier; Ayalew, Dereje; Leroy, Sylvie

    2015-04-01

    -rift magmatic supply. The difference in tectono-magmatic style between Central Afar (distributed extension and thick crust) and Northern Afar Erta Ale segment (narrow graben, thin crust) may be explained by the difference of magma volume (extruded & underplated) brought to the crust during extension. Magma supply in Central Afar thus allows the crust to be stretched without extreme thinning despite high degree of divergence. Thus, break-up may occur in both Central and Northern Afar, not depending on the apparent thickness of the crust but rather on the ability of the system to localize deformation. - There appears to be a link between early-rift transform zones and distribution of magmatic activity that affects in turn the structural style. We suggest that the closest feature from the SDR at mature VPM is the Stratoid series. The difference of volume between the Stratoid and the enormous volume of SDR imaged in seismic studies (e.g South Atlantic) is probably best explained by an initial low mantle potential temperature in Afar. Contrasted structural styles in Afar are the product of magma supply and segmentation, controlling thinning and extension distribution in the rift.

  8. A kinematic model for the development of the Afar Depression and its paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Redfield, T. F.; Wheeler, W. H.; Often, M.

    2003-11-01

    The Afar Depression is a highly extended region of continental to transitional oceanic crust lying at the junction of the Red Sea, the Gulf of Aden and the Ethiopian rifts. We analyze the evolution of the Afar crust using plate kinematics and published crustal models to constrain the temporal and volumetric evolution of the rift basin. Our reconstruction constrains the regional-scale initial 3D geometry and subsequent extension and is well calibrated at the onset of rifting (˜20 Ma) and from the time of earliest documented sea-floor spreading anomalies (˜6 Ma Red Sea; ˜10 Ma Gulf of Aden). It also suggests the Danakil block is a highly extended body, having undergone between ˜200% and ˜400% stretch. Syn-rift sedimentary and magmatic additions to the crust are taken from the literature. Our analysis reveals a discrepancy: either the base of the crust has not been properly imaged, or a (plume-related?) process has somehow caused bulk removal of crustal material since extension began. Inferring subsidence history from thermal modeling and flexural considerations, we conclude subsidence in Afar was virtually complete by Mid Pliocene time. Our analysis contradicts interpretations of late (post 3 Ma) large (˜2 km) subsidence of the Hadar area near the Ethiopian Plateau, suggesting paleoclimatic data record regional, not local, climate change. Tectonic reconstruction (supported by paleontologic and isotopic data) suggests that a land bridge connected Africa and Arabia, via Danakil, up to the Early to Middle Pliocene. The temporal constraints on land bridge and escarpment morphology constrain Afar paleogeography, climate, and faunal migration routes. These constraints (particularly the development of geographic isolation) are fundamentally important for models evaluating and interpreting biologic evolution in the Afar, including speciation and human origins.

  9. Shear-wave splitting in Ethiopia: Precambrian mantle anisotropy locally modified by Neogene rifting

    NASA Astrophysics Data System (ADS)

    Gashawbeza, Ewenet M.; Klemperer, Simon L.; Nyblade, Andrew A.; Walker, Kristoffer T.; Keranen, Katie M.

    2004-09-01

    Twenty-six broadband seismic stations in an areal array spanning 500 × 500 km across Ethiopia were used for shear-wave splitting studies. Our results show small-to-moderate delay times (0.5-1.7s) with fast-polarization azimuths sub-parallel to the orientation of the East African Rift (NNE-SSW) and also to the Proterozoic tectonic fabric across the entire studied area. Our results imply Ethiopian upper-mantle anisotropy is controlled largely by the Proterozoic accretion of the Mozambique belt, with possible minor effects within the rift due to aligned cracks or melt pockets parallel to the rift axis. Our observations are not consistent with anisotropy created by asthenospheric flow parallel either to the Cenozoic extension direction (NW-SE) or to the modern absolute plate motion direction (NNW-SSE), or to asthenospheric radial flow from the ``Afar'' plume.

  10. A melt inclusion study in primitive olivines from the adventive cones of the Piton de la Fournaise volcano, La Réunion Island : Implications for the nature of the Réunion mantle plume

    NASA Astrophysics Data System (ADS)

    Valer, Marina; Schiano, Pierre; Bachèlery, Patrick

    2016-04-01

    According to Courtillot et al. (2003), the mantle plume that forms the Réunion hot spot originates from the deepest part of the lower mantle. Based on the isotopic compositions of the lavas, this long-lived plume appears relatively homogeneous during the last 65 My (e.g Fisk et al., 1988), and is believed to correspond to an ubiquitous mantle component common to ocean island basalts (e.g Bosch et al., 2008). Here, we give additional information on the nature of the Réunion mantle plume by studying the chemical composition of silicate melt inclusions trapped within early-formed, primitive olivine crystals (Fo>85%) from the adventive cones of the Piton de la Fournaise volcano. These cones have emitted distinct magmas from the historical lavas. In particular, we focus on very incompatible trace element ratios, which reflect the long-term characteristics of the basalt sources and do not depend on the age of the source. The results indicate that the trapped melts have primitive compositions (up to 10.55 wt.% MgO) relative to the lavas. They also suggest that the magmas found in the adventive cones originate from a common chemical source, corresponding to either (1) a homogeneous mixed source between different mantle components (HIMU, EM 1, EM 2 and DMM), or (2) a near-primitive less-differentiated mantle source. Some very incompatible trace element ratios (e.g Th/La, Nb/La) display values similar to the primitive mantle ones, giving thus further support for hypothesis (2), as also inferred by Vlastélic et al. (2006) and Schiano et al. (2012). If based on Ce/Pb and Nb/U systematics, Hofmann et al. (1986) argued that the sources of all oceanic basalts (MORB and OIB) have undergone continental crust extraction, we propose an intermediate origin for the Réunion plume, between a primitive-like mantle domain and a depleted one, almost not affected by the recycling processes.

  11. A Melt Inclusion Study in Primitive Olivines from the Adventive Cones of the Piton de la Fournaise Volcano, La Réunion Island : Implications for the Nature of the Réunion Mantle Plume

    NASA Astrophysics Data System (ADS)

    Valer, M.; Schiano, P.; Bachelery, P.

    2015-12-01

    According to Courtillot et al. (2003), the mantle plume that forms the Réunion hot spot originates from the deepest part of the lower mantle. Based on the isotopic compositions of the lavas, this long-lived plume appears relatively homogeneous during the last 65 My (e.g Fisk et al., 1988), and is believed to correspond to an ubiquitous mantle component common to ocean island basalts (e.g Bosch et al., 2008). Here, we give additional information on the nature of the Réunion mantle plume by studying the chemical composition of silicate melt inclusions trapped within early-formed, primitive olivine crystals (Fo>85%) from the adventive cones of the Piton de la Fournaise Volcano. These cones have emitted distinct magmas from the historical lavas. In particular, we focus on very incompatible trace element ratios, which reflect the long-term characteristics of the basalt sources and do not depend on the age of the source. The results indicate that the trapped melts have very primitive compositions (up to 11.93 wt% MgO) relative to the lavas. They also suggest that the magmas found in the adventive cones originate from a common chemical source, corresponding to either (1) a homogeneous mixed source between different mantle components (HIMU, EM 1, EM 2 and DMM), or (2) a near-primitive less-differentiated mantle source. Some very incompatible trace element ratios (e.g Th/La, Nb/La) display values similar to the primitive mantle ones, giving thus further support for hypothesis (2), as also inferred by Vlastélic et al. (2006) and Schiano et al. (2012). If based on Ce/Pb and Nb/U systematics, Hofmann et al. (1986) argued that the sources of all oceanic basalts (MORB and OIB) have undergone continental crust extraction, we propose an intermediate origin for the Réunion plume, between a primitive-like mantle domain and a depleted one, almost not affected by the recycling processes.

  12. Midcontinent rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin

    SciTech Connect

    Nicholson, S.W. Univ. of Minnesota, MN ); Shirey, S.B. )

    1990-07-10

    Between 1091 and 1098 Ma, most of a 15- to 20-km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North American. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high- and low-TiO{sub 2} types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3-5.3; Th/Ta = 2.12-2.16; Zr/Y = 4.3-4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have {sup 87}Sr/{sup 86}Sr{sub i} {approx}0.7038, {epsilon}{sub Nd(1095 Ma)} {approx}0 {plus minus} 2, and {mu}{sub 1} {approx}8.2. Model ages with respect to a depleted mantle source (T{sub DM}) average about 1950-2100 Ma. Portage Lake rhyolits fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics ({epsilon}{sub Nd(1095 Ma)} {approx}0 to {minus}4.7; {mu}{sub 1} {approx}8.2-7.8) consistent with contamination of tholeiitic rocks by 5-10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions ({epsilon}{sub Nd(1095 Ma)} {approx}{minus}13 to {minus}16; {mu}{sub 1} {approx}7.6-7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma.

  13. The role of mantle plumes in the evolution of the African segment of Pangea and the formation of the Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Peyve, A. A.

    2015-09-01

    In this paper, we discuss a broad range of issues related to the formation of large igneous provinces in the African segment of Pangea on the basis of modern seismic tomography data. The formation of older igneous provinces (Central American and Karoo) is attributed to a prolonged phase of upwelling of hot mantle material or fluids in separate jets within a much larger area than the supposed plume head. Owing to its huge size and the thick, dense continental crust, Pangea acted as a shield promoting the accumulation and lateral channeling of heat energy beneath the lithosphere. The changes in global Earth dynamics and the generation of extensional stresses alone may have led to the breakup of Pangea, triggering the eruption of large volumes of magma over short period of time. The same factors led to the opening of the Atlantic Ocean. We provide arguments that the African superplume represents a Cenozoic structure not associated with the emplacement of the Karoo province. At the same time, the hot material brought under the lithosphere by this superplume synchronously with the start of magmatism in east Africa then spread out to the northwest to form local melting areas in Central and Northwestern Africa. We suggest that magmatic activity within the same region may have lasted, with interruptions, over tens of millions of years. Because of plate motion, these lowvelocity zones acting as heat sources appear to have lost their deep-seated roots, so that mantle reservoirs surviving at the base of the lithosphere may have fed magmatism and drifted together with the lithosphere.

  14. 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.

  15. Plume capture by divergent plate motions: implications for the distribution of hotspots, geochemistry of mid-ocean ridge basalts, and estimates of the heat flux at the core-mantle boundary

    NASA Astrophysics Data System (ADS)

    Jellinek, A. Mark; Gonnermann, Helge M.; Richards, Mark A.

    2003-01-01

    The coexistence of stationary mantle plumes with plate-scale flow is problematic in geodynamics. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (10 6≤Ra≤10 9) in a fluid with a temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary, and on the ratio λ of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary. When V≪1 and λ≥1, large-scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes. When V>10 and λ>100, plume formation is suppressed entirely, and the heat flux is carried by a sheet-like upwelling located in the center of the tank. At intermediate V, and depending on λ, established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling. Heat transfer across the layer occurs through a combination of ascending plumes and large-scale flow. Scaling analyses show that the bottom boundary layer thickness and, in turn, the basal heat flux q depend on the Peclet number, Pe, and λ. When λ>10, q∝Pe 1/2 and when λ→1, q∝(Pe λ) 1/3, consistent with classical scalings. When applied to the Earth, our results suggest that plate-driven mantle flow focuses ascending plumes towards upwellings in the central Pacific and Africa as well as into mid-ocean ridges. Furthermore, plumes may be captured by strong upwelling flow beneath fast-spreading ridges. This behavior may explain why hotspots are more abundant near slow

  16. 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

  17. Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography

    NASA Astrophysics Data System (ADS)

    Civiero, Chiara; Hammond, James O. S.; Goes, Saskia; Fishwick, Stewart; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, J.-Michael; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rümpker, Georg; Stuart, Graham W.

    2015-09-01

    Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ˜100 km length-scale down to depths of 700-800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings. This article was corrected on 28 SEP 2015. See the end of the full text for details.

  18. Petrology and geochronology of lavas from Ka'ula Volcano: Implications for rejuvenated volcanism of the Hawaiian mantle plume

    NASA Astrophysics Data System (ADS)

    Garcia, Michael O.; Weis, Dominique; Jicha, Brian R.; Ito, Garrett; Hanano, Diane

    2016-07-01

    Marine surveying and submersible sampling of Ka'ula Volcano, located 100 km off the axis of the Hawaiian chain, revealed widespread areas of young volcanism. New 40Ar/39Ar and geochemical analyses of the olivine-phyric submarine and subaerial volcanic rocks show that Ka'ula is shrouded with 1.9-0.5 Ma alkalic basalts. The ages and chemistry of these rocks overlap with rejuvenated lavas on nearby, northern Hawaiian Island shields (Ni'ihau, Kaua'i and South Kaua'i Swell). Collectively, these rejuvenated lavas cover a vast area (∼7000 km2), much more extensive than any other area of rejuvenated volcanism worldwide. Ka'ula rejuvenated lavas range widely in alkalinity and incompatible element abundances (e.g., up to 10× P2O5 at a given MgO value) and ratios indicating variable degrees of melting of a heterogeneous source. Heavy REE elements in Ka'ula lavas are pinned at a mantle normalized Yb value of 10 ± 1, reflecting the presence of garnet in the source. Trace element ratios indicate the source also contained phlogopite and an Fe-Ti oxide. The new Ka'ula ages show that rejuvenated volcanism was nearly coeval from ∼0.3 to 0.6 Ma along a 450 km segment of the Hawaiian Islands (from West Maui to north of Ka'ula). The ages and volumes for rejuvenated volcanism are inconsistent with all but one geodynamic melting model proposed to date. This model advocates a significant contribution of pyroxenite to rejuvenated magmas. Analyses of olivine phenocryst compositions suggest a major (33-69%) pyroxenite component in Ka'ula rejuvenated lavas, which correlates positively with radiogenic Pb isotope ratios for Ka'ula. This correlation is also observed in lavas from nearby South Kaua'i lavas, as was reported for Atlantic oceanic islands. The presence of pyroxenite in the source may have extended the duration and volume of Hawaiian rejuvenated volcanism.

  19. Anticlockwise P-T evolution at ∼280 Ma recorded from ultrahigh-temperature metapelitic granulite in the Chinese Altai orogenic belt, a possible link with the Tarim mantle plume?

    NASA Astrophysics Data System (ADS)

    Tong, Laixi; Xu, Yi-Gang; Cawood, Peter A.; Zhou, Xin; Chen, Yibing; Liu, Zhao

    2014-11-01

    An ultrahigh-temperature (UHT) metapelitic granulite assemblage consisting of garnet(g)-spinel(sp)-orthopyroxene(opx)-sillimanite(sil)-cordierite(cd)-ilmenite(ilm)-biotite(bi)-plagioclase(pl)-quartz(q) occurs within migmatitic paragneiss near Kalasu in the Chinese Altai, NW China. Textural relations, mineral compositions and P-T estimates, indicate three stages of mineral assemblages: (1) pre-peak prograde stage (M1) consisting of a sp-sil-bearing or sp-opx-bearing inclusion assemblage, with low-Al2O3 contents (4-5 wt.%) in orthopyroxene and P-T conditions of ∼7 kbar and ∼890 °C, (2) peak UHT stage (M2) comprising a g-opx-cd-bearing coarse-grained assemblage, with high-Al2O3 contents (8-9 wt.%) in orthopyroxene and peak conditions of ∼8 kbar and ∼970 °C, and (3) post-peak HT stage (M3) containing an oriented opx-bi-sil-bearing assemblage in matrix, with moderate amounts of Al2O3 (6-7 wt.%) in orthopyroxene and P-T conditions of 8-9 kbar and ∼870 °C. The three discrete stages define an anticlockwise P-T path involving initial prograde heating and post-peak near isobaric cooling. Such a near isobaric cooling anticlockwise P-T path suggests that UHT metamorphism likely occurred in an overall extensional tectonic setting with associated underplating of mantle-derived mafic magma. A SHRIMP zircon U-Pb age of 278 ± 2 Ma obtained from the metapelitic granulite indicates UHT metamorphism in the Altai orogen occurred during the Permian, coeval with spacially associated mantle-derived mafic intrusions (∼280 Ma) and the Tarim mantle plume (∼275 Ma). Thus, the Permian UHT metamorphism of the Chinese Altai is likely associated with underplating and heating of mantle-derived mafic magma as a result of the Tarim mantle plume.

  20. Impact of lithosphere rheology and pre-existing tectonic stress field on surface topography, crustal and mantle deformation during plume-lithosphere interactions in continents: insights from 3D numerical experiments

    NASA Astrophysics Data System (ADS)

    Koptev, Alexander; Burov, Evgueni; Gerya, Taras

    2014-05-01

    We implement high-resolution 3D thermo-mechanical numerical models to elucidate the impact of realistically implemented rheological structure of continental lithosphere and of far-field tectonic stress/strain field on the localization and style of deformation during the emplacement of a mantle plume at the bottom of continental lithosphere. Numerical models demonstrate strong dependence of crustal strain distributions and surface topography on the rheological composition of the lower crust and the initial thermal structure of the lithosphere. In contrast to the usual inferences from passive rifting models, distributed wide rifting takes place in case of cold (500° C at Moho depth) initial isotherm and mafic composition of the lower crust, whereas hotter geotherms and weaker (wet quartzite) lower crustal rheology lead to strong localization of rifting. Moreover, it appears that the prerequisite of strongly anisotropic strain localization (linear rift structures) refers to simultaneous presence of an active mantle plume and of some, even very weak, slow (< 3 mm/y) passive horizontal extension produced by far-field tectonic forces. Higher (than 1.5-3 mm/y) velocities of supplementary far-field extension expectedly lead to enlargement of the active fault zone for the same lapse of time. Yet, simultaneous rise of the lithospheric geotherm associated with active rifting has an opposite effect leading to the narrowing of the rift zone. Consequently, interplays between active and passive rifting result in highly varying rifts styles hence breaking common rift-style classifications. The importance of the rheological properties of the continental crust for deformation regime is demonstrated not only by considerable difference in surface morphology and crustal strain patterns between the models with different lower crustal rheology, but also by a noticeable distinction in deep distribution of the plume head material, with consequent effect for magmatic processes and mantle

  1. Magnetotelluric imaging of upper crustal partial melt at Tendaho graben in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Didana, Yohannes Lemma; Thiel, Stephan; Heinson, Graham

    2014-05-01

    We report on a recent magnetotelluric (MT) survey across the Manda Hararo magmatic segment (MHMS) within the Tendaho graben in the Afar Depression in northeastern Ethiopia. Twenty-two broadband MT sites with ˜1 km station spacing were deployed along a profile with the recorded data covering a period range from 0.003 s to 1000 s. A two-dimensional (2-D) resistivity model reveals an upper crustal fracture zone (fault) and partial melt with resistivity of 1-10Ωm at a depth of >1 km. The partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the Afar Stratoid Series basalts. We estimate a melt fraction of about 13% based on geochemical and borehole data, and bulk resistivity from the 2-D MT inversion model. The interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat. Within the MHMS and Tendaho graben, a magma intrusion is a plausible explanation for the upper crustal conductor. The inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also makes the Tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy.

  2. Along-rift Variations in Deformation and Magmatism in the Ethiopian and Afar Rift Systems

    NASA Astrophysics Data System (ADS)

    Keir, D.; Bastow, I. D.; Corti, G.; Mazzarini, F.; Rooney, T. O.

    2015-12-01

    The geological record at rifts and margins worldwide often reveals along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate, and timing of extension; alternatively, preexisting plate architecture and/or the evolving kinematics of extension during breakup may exert first-order control on magmatism. The Ethiopian and Afar Rift systems provide an excellent opportunity to address this since it exposes, along strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of distribution and style of volcanism and faulting along strike in the MER and Afar. We also incorporate synthesis of geophysical, geochemical, and petrological constraints on magma generation and emplacement in order to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rift. Along-rift changes in extension by magma intrusion and plate stretching, and the three-dimensional focusing of melt where the rift dramatically narrows each influence igneous intrusion, volcanism and subsidence history. In addition, rift obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

  3. Receiver function constraints on crustal seismic velocities and partial melting in the Red Sea Rift, Central Afar

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Almadani, S.; Gao, S. S.; Elsheikh, A. A.; Cherie, S.; Thurmond, A. K.; Liu, K. H.

    2013-12-01

    The Afar Depression is currently a unique locale for the investigation of crustal and mantle processes involved in the transition from continental to oceanic rifting. To provide high-quality images of the crust and upper mantle beneath the Red Sea Rift in Central Afar, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Here we report receiver function stacking results to investigate crustal properties of this presumably incipient oceanic rift. Stacking of approximately 2200 radial receiver functions along the ~200 km long array reveals an average crustal thickness of 22 km, ranging from nearly 18 km within the Red Sea Rift axis to approximately 30 km within the overlap zone between the Red Sea and Gulf of Aden rift axes. The resulting anomalously high Vp/Vs ratios systematically decrease toward the northeast, ranging from 2.40 southwest of the Tendaho Graben to 1.85 within the overlap zone. We utilize theoretical Vp and melt fraction relationships to constrain a highly reduced average crustal P-velocity of 5.0 km/s within the rift axis, which is characterized by a melt percentage of ~11% confined primarily to the lower crust while the overlap zone contains relatively minor quantities of partial melt. An observed asymmetric distribution of high Vp/Vs values within the Tendaho Graben, as well as regionally maximum values on the southwestern rift flank, suggest crustal magmas either delivered from off-axis subcrustal magma chambers or as material present as residuum from the Red Sea Rift axis migration. Comparisons of these crustal properties beneath the Red Sea Rift and those found beneath mature mid-ocean ridges suggest the locus of extensional strain within the Central Afar is currently diffuse and in the process of localizing toward the Tendaho Graben accompanying the northeastward migration of the Afar Triple Junction.

  4. Exposed guyot from the afar rift, ethiopia.

    PubMed

    Bonatti, E; Tazieff, H

    1970-05-29

    A series of originally submarine volcanoes has been found in the Afar Depression. Some of the volcanic structures are morphologically similar to oceanic guyots. One of them consists of strata of finely fragmented and pulverized basaltic glass. The fragmentation of the lava is probably the result of stream explosions taking place during the submarine eruption. The flat top of this guyot is considered to be a constructional feature; by analogy, it is suggested that not all oceanic guyots are necessarily the result of wave truncation of former volcanic islands. PMID:17833451

  5. Combined mantle plume-island arc model for the formation of the 2.9 Ga sumozero-kenozero greenstone belt, SE baltic shield: Isotope and trace element constraints

    NASA Astrophysics Data System (ADS)

    Puchtel, I. S.; Hofmann, A. W.; Amelin, Yu. V.; Garbe-Schönberg, C.-D.; Samsonov, A. V.; Shchipansky, A. A.

    1999-11-01

    The Sumozero-Kenozero greenstone belt in the SE Baltic Shield is ˜400 km long and up to 50 km wide and includes two main units with a total thickness of ˜5 km. The lower unit consists of oceanic plateau-type submarine mafic-ultramafic lavas. The upper unit is made up of island arc-type volcanic BADR (basalt-andesite-dacite-rhyolite)-series rocks and adakite-series subvolcanic rhyolites. Both units are separated by major thrust zones from the 3.2 Ga TTG-gneisses of the Vodla Block microcontinent. Komatiites of the lower unit were derived from a liquid containing ˜30% MgO that erupted at a temperature of ˜1570°C. The komatiite liquid began melting at depths of 300-400 km in a mantle plume. The plume was 250°C hotter than the ambient mantle and had the thermal potential to produce oceanic crust with an average thickness of ˜35 km, which was at least in part unsubductable. The lower unit mafic-ultramafic lavas have high ɛNd(T) of +2.7 ± 0.3, relatively unradiogenic Pb isotope compositions (μ 1 = 8.73 ± 0.20), are depleted in highly incompatible elements, and show Nb-maxima (Nb/Nb∗ = 1.2 ± 0.2, Nb/U = 43 ± 6). These parameters are similar to those found in a number of early Precambrian uncontaminated greenstones and in recent Pacific oceanic flood basalts (OFB). They are regarded as plume source characteristics. The BADR-series mafic-intermediate to felsic volcanic rocks and subvolcanic adakitic-series rhyolites from the upper unit have island arc geochemical signatures (enrichment in highly incompatible elements, large negative HFSE-anomalies, Nb/Nb∗ = 0.32 ± 0.10, Nb/U = 8.8 ± 2.5), but are characterized by high positive ɛNd(T) values of +2.4 ± 1.2, indistinguishable from the lower unit mafic-ultramafic sequences. They represent mantle wedge-derived and slab-derived melts, respectively, erupted in the inner and frontal parts of an intraoceanic island arc. U-Pb zircon age of 2875 ± 2 Ma for the upper unit BADR-series felsic volcanic rocks, and Pb

  6. The basaltic volcanism of the Dumisseau Formation in the Sierra de Bahoruco, SW Dominican Republic: A record of the mantle plume-related magmatism of the Caribbean Large Igneous Province

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Joubert, Marc; Abad, Manuel; Pérez-Valera, Fernando; Gabites, Janet

    2016-06-01

    The basaltic volcanism of the Dumisseau Fm in the Sierra de Bahoruco, SW Dominican Republic, offers the opportunity to study, on land, the volcanism of the Caribbean Large Igneous Province (CLIP). It consists of an at least 1.5 km-thick sequence of submarine basaltic flows and pyroclastic deposits, intruded by doleritic dykes and sills. Three geochemical groups have been identified: low-Ti tholeiites (group I); high-Ti transitional basalts (group II); and high-Ti and LREE-enriched alkaline basalts (group III). These geochemical signatures indicate a plume source for all groups of basalts, which are compositionally similar to the volcanic rocks that make up various CLIP fragments in the northern region of the Caribbean Plate. Trace element modelling indicates that group I magmas are products of 8-20% melting of spinel lherzolite, group II magmas result 4-10% melting of a mixture of spinel and garnet lherzolite, and group III basalts are derived by low degrees (0.05-4%) of melting of garnet lherzolite. Dynamic melting models suggest that basalts represent aggregate melts produced by progressive decompression melting in a mantle plume. There is no compositional evidence for the involvement of a Caribbean supra-subduction zone mantle or crust in the generation of the basalts. Two 40Ar/39Ar whole-rock ages reflect the crystallisation of group II magmas at least in the late Campanian (~ 74 Ma) and the lower Eocene (~ 53 Ma). All data suggest that the Dumisseau Fm is an emerged fragment of the CLIP, which continues southward through the Beata Ridge

  7. Regional kinematic models for the development of the Afar depression

    NASA Astrophysics Data System (ADS)

    Redfield, T. F.; Wheeler, W. H.; Often, M.

    2003-04-01

    Few reconstructions of the Afar rift combine plate kinematics with analyses of the rift basin evolution. The Afar rift is a highly-extended region of continental to transitional oceanic crust lying at the junction of the Red Sea, Gulf of Aden and Ethiopian rifts. Here, we present a new Afar reconstruction taking into account plate kinematics, crustal thinning and magmatic construction. We use a regional plate reconstruction incorporating Nubia, Arabia, Somalia and Danakil to constrain the regional-scale extension and subsidence of the rift and relative movement of Danakil. The plate model is temporally and spatially well constrained at the onset of rifting (ca. 20 Ma) and from sea-floor spreading anomalies in the Red Sea (ca. 6 Ma-present) and Gulf of Aden (ca. 10 Ma-present). The Red Sea pre-rift fit is constrained by piercing points along the Red Sea margins (Sultan et al. 1993). We model the Late Oligocene to present-day evolution of the Afar crust by volume balance using a crustal model based on published topographic and depth-to-Moho interpretations as well as volume estimates of extrusive and sedimentary rocks. Errors stemming from plate boundary uncertainties are small in relation to the reconstructed volume. We partition Afar magmatism into pre-extensional and syn-extensional volumes. From thermal modeling and flexural considerations we infer that the regional-scale subsidence of the Afar depression was virtually complete by Mid Pliocene time. Our model supports the interpretation that the escarpments bounding the Afar Depression achieved nearly their present height (ca. 3 km) by the Late Miocene. Erosional considerations suggest the Late Miocene escarpments were steeper than they are today. Our model does not support the interpretation found in the paleo-anthropological literature that Late Miocene and Pliocene vertical movements were sufficiently large (ca. 2 km) to cause small fault blocks such as Hadar to migrate through climatic temperature zones

  8. Magmatism on rift flanks: Insights from ambient noise phase velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Keir, Derek; Ren, Yong; Molinari, Irene; Ahmed, Abdulhakim; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. M.; Doubre, Cécile; Ganad, Ismail Al; Goitom, Berhe; Ayele, Atalay

    2015-04-01

    During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

  9. Continental aggregation, subduction initiation, and plume generation

    NASA Astrophysics Data System (ADS)

    Heron, P. J.; Lowman, J. P.

    2013-12-01

    Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures through the generation of mantle plumes. Paleogeographic plate reconstructions have indicated that sub-continental mantle upwellings appear below large continents that are extensively ringed by subduction zones. Moreover, several numerical simulations of supercontinent formation and dispersal attribute the genesis of sub-continental plumes to the generation of subduction zones on the edges of the supercontinent, rather than resulting from continental insulation. However, the role of the location of downwellings in producing a return-flow upwelling, and on increasing sub-continental mantle temperatures, is not fully understood. In this mantle convection study, we examine the evolution of mantle dynamics after supercontinent accretion over a subduction zone (analogous to the formation of Pangea) for a range of continental coverage. We present 2D and 3D Cartesian geometry mantle convection simulations, featuring geotherm- and pressure-dependent viscosity with thermally and mechanically distinct oceanic and continental plates. Through changing the size of the continent we are able to analyze the factors involved in the generation of mantle plumes in purely thermal convection. Furthermore, we change the upper and lower mantle viscosity to determine their relation to plume formation in vigorous mantle convection simulations. Elevated sub-continental temperatures are analyzed in relation to continental coverage to further understand the influence of continental tectonics on the thermal evolution of the mantle.

  10. Recent rift-related volcanism in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Ferguson, David J.; Barnie, Talfan D.; Pyle, David M.; Oppenheimer, Clive; Yirgu, Gezahegn; Lewi, Elias; Kidane, Tesfaye; Carn, Simon; Hamling, Ian

    2010-04-01

    Rift zones are the most common magmatic environment on Earth. However opportunities to observe active rifting are rare, and consequently the volcanological characteristics of rift systems are not well understood. An ongoing phase of magmatic rifting along a section of the Red Sea system in Afar, Ethiopia, presents an exceptional opportunity to constrain relationships between volcanism and crustal growth. Here, by integrating analyses of satellite images (i.e. MODIS, OMI, ASTER, and ALI) with field observations, we characterise two recent (August 2007 and June 2009) basaltic fissure eruptions in Afar and evaluate the role and significance of volcanism in the rifting process. Both events were brief (36-72 h) and erupted 4.4-18 × 10 6 m 3 of lava from a fissure system 4-6.5 km in length. Data from the spaceborne Ozone Monitoring Instrument (OMI) suggests total SO 2 emissions for each eruption of 26 ± 5 kt (2007) and 34 ± 7 kt (2009), consistent with complete degassing of the erupted magma volumes. Using geodetic models for the intrusive activity in Afar we estimate the partitioning of magma between intrusive and extrusive components, up to July 2009, to be ˜ 180:1. Comparing the first-order volcanic characteristics and the intrusive-extrusive volume balance for the Afar volcanism with data from the 1975-1984 Krafla rifting cycle (Iceland) suggests that the volcanic flux in Afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking, and subsequent dykes are able to propagate more easily to the surface. As a consequence, basaltic fissure eruptions in this section of the Afar rift will become of increasing large magnitude as the rifting event matures over the next 5-10 yr. Using available models of magmatic rifting we forecast the likely size and location of future eruptions in Afar.

  11. Correlated Os-Pb-Nd-Sr isotopes in the Austral-Cook chain basalts: the nature of mantle components in plume sources

    NASA Astrophysics Data System (ADS)

    Schiano, P.; Burton, K. W.; Dupré, B.; Birck, J.-L.; Guille, G.; Allègre, C. J.

    2001-04-01

    Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pb) isotopes have been measured on a suite of aphyric basalts from 12 islands of the Austral-Cook island archipelago, an area which exhibits a range in Pb isotope compositions that encompasses almost the entire range displayed by ocean island basalts (OIB). Although the samples have Os concentrations (1.69-34.80 ppt) at the lower end of the range measured for OIB, they display a range of initial 187Os/ 188Os ratios (between 0.1279 and 0.1594) similar to that defined by olivine-phyric, Os-rich OIB. Positive Os-Nd, Os-Pb and negative Os-Sr isotope correlations are documented, indicating that the isotopic arrays represent various proportions of mixing between a HIMU-type end-member represented by Mangaia and Tubuai islands and characterized by radiogenic Os and Pb isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, Nd and Pb isotopic compositions. The HIMU signature of the mantle component involved in Tubuai-Mangaia mantle sources requires long-term enrichments of U and Th relative to Pb and Re relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the 'bulk silicate earth' (BSE), which cannot be obtained by mixing the four mantle components (DMM, HIMU and EM1 and 2) generally used to circumscribe the Sr-Nd-Pb isotopic data of OIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence of undepleted material from a lower segment of the mantle in the source of the Austral-Cook island basalts (and more specifically Rarotonga basalts); however, such a hypothesis is challenged by both the absence of a primordial 3He signature and the non-primitive Ce/Pb and Nb/U values for the Austral-Cook island basalts. Alternatively, assuming

  12. Relationship between plume and plate tectonics

    NASA Astrophysics Data System (ADS)

    Puchkov, V. N.

    2016-07-01

    The relationship between plate- and plume-tectonics is considered in view of the growth and breakdown of supercontinents, active rifting, the formation of passive volcanic-type continental margins, and the origin of time-progressive volcanic chains on oceanic and continental plates. The mantle wind phenomenon is described, as well as its effect on plume morphology and anisotropy of the ambient mantle. The interaction of plumes and mid-ocean ridges is discussed. The principles and problems of plume activity analysis in subduction- and collision-related foldbelts are considered and illustrated with examples.

  13. Small scale inhomogeneity in the mantle source of the Cape Verde hotspot is probably related to plume complexity: implications from Sr, Nd and high precision Pb isotopes and geochemistry

    NASA Astrophysics Data System (ADS)

    Holm, P. M.; Sørensen, R. V.

    2009-04-01

    The volcanic rocks of one of the major islands of the Cape Verde hotspot have been investigated in order to test mantle plume models. From the centre of the Cape Verde Rise an array of islands trend west, the northern HIMU-type Cape Verde Islands. Of these, São Nicolau (SN) is the easternmost and Santo Antão the westernmost. Sixty samples of primitive (MgO = 9-14 wt%) basanitic composition from SN that represent the four volcanic stages of the 9 - 0.1 Ma evolution of the island have been analysed for Sr, Nd and high precision Pb isotopic composition. Pb ranges to a less radiogenic composition than on SA [1] and has lower 8/4 than the rocks of the southern EM1-type Cape Verde islands. Most SN lavas have a young HIMU character with negative 7/4. The most radiogenic Pb at SN is less thorogenic than Pb at SA. Temporal variation is also evident: An intermediate age group of samples have particularly low La/Nb = 0.4 - 0.5 and the least LREE-enrichment for SN. The youngest group of rocks has the lowest Zr/Nb = 2.5 - 3.0 and the most unradiogenic Sr and radiogenic Nd in the archipelago. At least four of the mantle source components for the SN magmas are different from any found in the SA magmas. High precision Pb data allow identification of parallel trends for northern SN and the southern island Santiago, which therefore must have unrelated source components. For the northern Cape Verde islands source compositions vary from E to W as well as with time. This cannot be explained by stationary enriched lithosphere components. The derivation of melts from a complex plume source is modelled. [1] Holm P.M., Wilson J.R., Christensen B.P., Hansen S.L., Hein K.M., Mortensen A.K., Pedersen R., Plesner S., and Runge M.K. (2006) JPetrol 47, 145-189.

  14. Zoned mantle convection.

    PubMed

    Albarède, Francis; Van Der Hilst, Rob D

    2002-11-15

    We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced

  15. Carbon isotope and abundance systematics of Icelandic geothermal gases, fluids and subglacial basalts with implications for mantle plume-related CO2 fluxes

    NASA Astrophysics Data System (ADS)

    Barry, P. H.; Hilton, D. R.; Füri, E.; Halldórsson, S. A.; Grönvold, K.

    2014-06-01

    We report new carbon dioxide (CO2) abundance and isotope data for 71 geothermal gases and fluids from both high-temperature (HT > 150 °C at 1 km depth) and low-temperature (LT < 150 °C at 1 km depth) geothermal systems located within neovolcanic zones and older segments of the Icelandic crust, respectively. These data are supplemented by CO2 data obtained by stepped heating of 47 subglacial basaltic glasses collected from the neovolcanic zones. The sample suite has been characterized previously for He-Ne (geothermal) and He-Ne-Ar (basalt) systematics (Füri et al., 2010), allowing elemental ratios to be calculated for individual samples. Geothermal fluids are characterized by a wide range in carbon isotope ratios (δ13C), from -18.8‰ to +4.6‰ (vs. VPDB), and CO2/3He values that span eight orders of magnitude, from 1 × 104 to 2 × 1012. Extreme geothermal values suggest that original source compositions have been extensively modified by hydrothermal processes such as degassing and/or calcite precipitation. Basaltic glasses are also characterized by a wide range in δ13C values, from -27.2‰ to -3.6‰, whereas CO2/3He values span a narrower range, from 1 × 108 to 1 × 1012. The combination of both low δ13C values and low CO2 contents in basalts indicates that magmas are extensively and variably degassed. Using an equilibrium degassing model, we estimate that pre-eruptive basaltic melts beneath Iceland contain ∼531 ± 64 ppm CO2 with δ13C values of -2.5 ± 1.1‰, in good agreement with estimates from olivine-hosted melt inclusions (Metrich et al., 1991) and depleted MORB mantle (DMM) CO2 source estimates (Marty, 2012). In addition, pre-eruptive CO2 compositions are estimated for individual segments of the Icelandic axial rift zones, and show a marked decrease from north to south (Northern Rift Zone = 550 ± 66 ppm; Eastern Rift Zone = 371 ± 45 ppm; Western Rift Zone = 206 ± 24 ppm). Notably, these results are model dependent, and selection of a lower

  16. Plume-lithosphere interaction beneath a fast moving plate

    NASA Astrophysics Data System (ADS)

    Thoraval, Catherine; Tommasi, Andréa; Doin, Marie-Pierre

    2006-01-01

    Two-dimensional numerical simulations of mantle convection with temperature and pressure dependent viscosity are used to study plume-lithosphere interaction beneath a fast moving plate. Plumes behavior and, hence, their erosional and melting potential, depend on the Rayleigh number and plume buoyancy flux. Analysis of the balance between large-scale and plume-induced flow and of the ability of the plume to melt allows to put bounds on the upper mantle viscosity (1020, 1021 Pas), on the plumes diameter (<200 km) and temperature anomaly (200-400°C). Within this range of parameters, strongly time-dependent small-scale instabilities form in the plume-lithosphere boundary layer. They lead to thermal rejuvenation of the lithosphere downstream from the plume. The 1200°C isotherm is raised by up to 30km, but the 800°C isotherm is hardly moved, leading to a steep transient geotherm at the base of the plate.

  17. Evolution of continental-scale drainage in response to mantle dynamics and surface processes: An example from the Ethiopian Highlands

    NASA Astrophysics Data System (ADS)

    Sembroni, Andrea; Molin, Paola; Pazzaglia, Frank J.; Faccenna, Claudio; Abebe, Bekele

    2016-05-01

    Ethiopia offers an excellent opportunity to study the effects and linkage between mantle dynamics and surface processes on landscape evolution. The Ethiopian Highlands (NW Ethiopia), characterized by a huge basaltic plateau, is part of the African Superswell, a wide region of dynamically-supported anomalously high topography related to the rising of the Afar plume. The initiation and steadiness of dynamic support beneath Ethiopia has been explored in several studies. However the presence, role, and timing of dynamic support beneath Ethiopia and its relationship with continental flood basalts volcanism and surface processes are poorly defined. Here, we present a geomorphological analysis of the Ethiopian Highlands supplying new constraints on the evolution of river network. We investigated the general topographic features (filtered topography, swath profiles, local relief) and the river network (river longitudinal profiles) of the study area. We also apply a knickpoint celerity model in order to provide a chronological framework to the evolution of the river network. The results trace the long-term progressive capture of the Ethiopian Highlands drainage system and confirm the long-term dynamic support of the area, documenting its impact on the contrasting development of the Blue Nile and Tekeze basins.

  18. Evolution of continental-scale drainage in response to mantle dynamics and surface processes: an example from the Ethiopian Highlands.

    NASA Astrophysics Data System (ADS)

    Sembroni, Andrea; Molin, Paola; Pazzaglia, Frank J.; Faccenna, Claudio; Abebe, Bekele

    2016-04-01

    Ethiopia offers an excellent opportunity to study the effects and linkage between mantle dynamics and surface processes on landscape evolution. The Ethiopian Highlands (NW Ethiopia), characterized by a huge basaltic plateau, is part of the African Superswell, a wide region of dynamically-supported anomalously high topography related to the rising of the Afar plume. The initiation and steadiness of dynamic support beneath Ethiopia has been explored in several studies. However the presence, role, and timing of dynamic support beneath Ethiopia and its relationship with continental flood basalts volcanism and surface processes are poorly defined. Here, we present a geomorphological analysis of the Ethiopian Highlands supplying new constrains on the evolution of river network. We investigated the general topographic features (filtered topography, swath profiles, local relief) and the river network (river longitudinal profiles) of the study area. We also apply a knickpoint celerity model in order to provide a chronological framework to the evolution of the river network. The results trace the long-term progressive capture of the Ethiopian Highlands drainage system and confirm the long-term dynamic support of the area, documenting its impact on the contrasting development of the Blue Nile and Tekeze basins.

  19. A two-way interaction between the Hainan plume and the Manila subduction zone

    NASA Astrophysics Data System (ADS)

    Mériaux, Catherine A.; Duarte, João. C.; Schellart, Wouter P.; Mériaux, Anne-Sophie

    2015-07-01

    The interaction between mantle plumes and subducting slabs is well accepted, but the influence of slabs on plumes has more often been portrayed than the reverse. Here we present three-dimensional upper mantle laboratory models in which a compositional plume rises underneath a subducting plate. Slab/plume buoyancy flux ratios ranged between 7 and 18. The models exhibit a two-way interaction. While the plume conduit increasingly tilts away from the trench as a result of slab rollback-induced toroidal mantle flow, the slab subduction rate decreases as a function of the amount of plume buoyancy opposing that of the slab, which gets subducted beneath the slab. We propose that our models apply to the Hainan/Manila system and explain the recently imaged tilt of the Hainan plume by the Manila slab-induced mantle return flow. The Hainan plume could lessen the Manila subduction rate from 8 Ma into the future.

  20. Tracking Iceland Plume Motion Using Trace Element Geochemistry

    NASA Astrophysics Data System (ADS)

    Fitton, J. G.; Walters, R. L.; Jones, S. M.

    2011-12-01

    The Greenland-Scotland Ridge (GSR) is a hotspot track built by interaction between the Mid Atlantic Ridge (MAR) and the Iceland mantle plume. Unlike most other hotspot tracks built by ridge-plume interaction, the GSR is 2 to 3 times wider than the plume conduit in the upper mantle. (This unusual wide morphology arises because Icelandic crust changes significantly in thickness within a few million years of accretion, probably mainly by viscous flow in the hot lower crust). The upshot is that the GSR cannot be compared directly with theoretical plume tracks from hotspot reference frame models. However, it is possible to track the position of the Iceland plume conduit using the trace element geochemistry of basaltic lavas. Away from the plume conduit, plate spreading drives upwelling of mantle through the melting region. Above the plume conduit, plume-driven flow forces mantle through the lower part of the melting region faster than the plate-driven upwelling rate. The average depth of melting is therefore greater directly above the plume conduit than away from the plume conduit, and this difference in average melting depth means that melts generated directly above the plume conduit are relatively enriched in incompatible trace elements. Joint modelling of trace element compositions and crustal thickness can also be used to establish location of melting relative to the plume conduit. To date, these concepts have been used only to explain compositional variations in modern (post-glacial) Icelandic lavas; in this study we show that the same concepts can be applied to map the location of the plume conduit throughout the onshore Icelandic geological record (since the middle Miocene, c. 16 Ma). The plume track thus determined is in reasonable agreement with theoretical tracks calculated under the assumption that the Iceland Plume has remained fixed relative to other Indo-Atlantic hotspots. This result also supports the idea that episodic relocations of the onshore part of

  1. Magmatism on rift flanks: insights from Ambient-Noise Phase-velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Ren, Yong; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. Michael; Doubre, Cécile; Ganad, Ismail Al

    2015-04-01

    During the breakup of continent in the presence of magma, strain is commonly assumed to initially occur by border faulting, and progressively migrate in space and time towards axial magma intrusion. Magmatic processes near the rift flanks are commonly ignored. We present phase-velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only upper crust beneath the axial volcanic systems, but also both upper and lower crust beneath rift flanks where ongoing volcanism and hydrothermal activity occurs at the surface. The results show that magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process, and may persist through to early seafloor spreading. Since ongoing flank magmatism during breakup impacts the thermal evolution of the lithosphere, it has implications for the subsidence history of the rift.

  2. Seismic Anisotropy and Mantle Flow Beneath Northeastern Africa and Arabia

    NASA Astrophysics Data System (ADS)

    Elsheikh, A.; Gao, S. S.; Liu, K. H.; Abdelsalam, M. G.

    2011-12-01

    The Afar Depression in NE Africa hosts the Earth's only RRR triple junction exposed on land, and is an ideal place to study processes involved in the transition from continental rifting to seafloor spreading. In order to image the crust and upper mantle structure under Afar with an unprecedented spatial resolution, eighteen broadband seismic stations were deployed between Dec. 2009 and May 2011 along an approximately 250 km long profile across the Tendaho Graben in central Afar as part of the Afar Lithosphere Imaging Experiment (ALIE). Here we present results from teleseismic shear-wave (XKS) splitting analysis using data from ALIE and all the available broadband seismic data archived at the IRIS Data Center. Several previous studies of shear wave splitting in the region suggested that the observed anisotropy could be attributed to preexisting Precambrian fabrics, while others argue that vertical magmatic dikes or rift parallel mantle flow are responsible for the observed anisotropy. All of those studies utilized limited quality and quantity of XKS measurements which resulted in the inconsistency of conclusions. This work presents about 200 XKS splitting measurements obtained using data from ALIE and over 400 measurements from other stations in Afar, the Main Ethiopian Rift (MER), Ethiopian Plateau, and the Arabian plate. A robust process to reliably assess and objectively rank XKS splitting parameters was used to produce the results. Manual screening was applied to the results of the automatic calculations to ensure that no high quality events were ignored and no low quality results were selected. This study found that the ~450 measurements in the Ethiopian Plateau and in the MER have insignificant azimuthal variations with MER dominated by a rift parallel fast direction suggestive of a single layer of anisotropy. In contrast, measurements in the Afar Depression show a systematic azimuthal dependence of splitting parameters with a 90-degree periodicity, which

  3. Geodetic measurements and numerical models of the Afar rifting sequence 2005-2010

    NASA Astrophysics Data System (ADS)

    Ali, T.; Feigl, K.; Calais, E.; Hamling, I. J.; Wright, T. J.

    2012-12-01

    Rifting episodes are characterized by magma migration and dike intrusions that perturb the stress field within the surrounding lithosphere, inducing viscous flow in the lower crust and upper mantle that leads to observable, transient surface deformation. The Manda Hararo-Dabbahu rifting episode that occurred in the Afar depression between 2005 and 2010 is the first such episode to unfold fully in the era of satellite geodesy, thus providing a unique opportunity to probe the rheology of lithosphere at a divergent plate boundary. GPS and SAR measurements over the region since 2005 show accelerated surface deformation rates during post-diking intervals [Wright et al., Nature Geosci., 2012]. Using these observations in combination with a numerical model, we estimate model parameters that best explain the deformation signal. Our model accounts for three distinct processes: (i) secular plate spreading between Nubian and Arabian plates, (ii) time dependent post-rifting viscoelastic relaxation following the 14 dike intrusions that occurred between 2005 and 2010, including the 60 km long mega dike intrusion of September 2005, and (iii) magma accumulation within crustal reservoirs that feed the dikes. To model the time dependent deformation field, we use the open-source unstructured finite element code, Defmod [Ali, 2011, http://defmod.googlecode.com/]. Using a gradient-based iterative scheme [Ali and Feigl, Geochem. Geophys. Geosyst., 2012], we optimize the fit between observed and modeled deformation to estimate parameters in the model, including the locking depth of the rift zone, geometry and depth of magma reservoirs and rheological properties of lower crust and upper mantle, along with their formal uncertainties.

  4. Plume radiation

    NASA Astrophysics Data System (ADS)

    Dirscherl, R.

    1993-06-01

    The electromagnetic radiation originating from the exhaust plume of tactical missile motors is of outstanding importance for military system designers. Both missile- and countermeasure engineer rely on the knowledge of plume radiation properties, be it for guidance/interference control or for passive detection of adversary missiles. To allow access to plume radiation properties, they are characterized with respect to the radiation producing mechanisms like afterburning, its chemical constituents, and reactions as well as particle radiation. A classification of plume spectral emissivity regions is given due to the constraints imposed by available sensor technology and atmospheric propagation windows. Additionally assessment methods are presented that allow a common and general grouping of rocket motor properties into various categories. These methods describe state of the art experimental evaluation techniques as well as calculation codes that are most commonly used by developers of NATO countries. Dominant aspects influencing plume radiation are discussed and a standardized test technique is proposed for the assessment of plume radiation properties that include prediction procedures. These recommendations on terminology and assessment methods should be common to all employers of plume radiation. Special emphasis is put on the omnipresent need for self-protection by the passive detection of plume radiation in the ultraviolet (UV) and infrared (IR) spectral band.

  5. Viscosity jump in Earth's mid-mantle.

    PubMed

    Rudolph, Maxwell L; Lekić, Vedran; Lithgow-Bertelloni, Carolina

    2015-12-11

    The viscosity structure of Earth's deep mantle affects the thermal evolution of Earth, the ascent of mantle plumes, settling of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Based on a reanalysis of the long-wavelength nonhydrostatic geoid, we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth. We detect an increase in viscosity at 800- to 1200-kilometers depth, far greater than the depth of the mineral phase transformations that define the mantle transition zone. The viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation, plume deflection, and changes in large-scale structure and offers a simple explanation of these phenomena. PMID:26659053

  6. South Pacific hotspot swells dynamically supported by mantle flows

    NASA Astrophysics Data System (ADS)

    Adam, Claudia; Yoshida, Masaki; Isse, Takehi; Suetsugu, Daisuke; Fukao, Yoshio; Barruol, Guilhem

    2010-05-01

    The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. According to the plume theory, originally proposed by Morgan, the hotspot volcanoes are created by jets of hot material (plumes) rising from the deep mantle. With later studies, troubling inconsistencies began to emerge and other phenomena are invoked to explain intraplate volcanism, thus tending to nail the plume coffin. However, the problems encountered may simply be "the maturing of a valid theory to deal with the complexity of the real planet". This alternative is tested here by studying the dynamics of the South Pacific plumes through a new numerical model of mantle flow based on a highly-resolved seismic tomography model. We show here, for the first time, that a direct link exists between the surface observations and the mantle flow. We find indeed outstanding correlations between the observed and the modelled swells and between the modelled flow pattern and the active volcanism. This shows that at a first order, the morphology of the volcanic chains and their associated swells is controlled by the mantle flows. The excellent correlation we find between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology has even broader implications. It implies indeed that we can accurately evaluate the heat transported by mantle plumes from a careful estimation of the swell morphology. We show that the heat transported by the South Pacific plumes accounts for 13% of the total plume heat flux.

  7. Determinants of delivery practices among Afar pastoralists of Ethiopia

    PubMed Central

    Mekonnen, Medhanit Getachew; Yalew, Kassahun Negash; Umer, Jemal Yesouf; Melese, Muluken

    2012-01-01

    Introduction In a previous qualitative study in Afar, it was reported that most mothers deliver at home, assisted by Traditional Birth Attendants (TBAs). However, determinant factors of delivery practices in this region remain scarcely explored. The objective of this study was to elucidate these factors among the Afar community. Methods This cross sectional study was conducted in April 2011 in zone 3 of Afar regional state, Ethiopia. Quantitative data were collected from 478 women who had given birth during the preceding one year. Results Out of 478 interviewed mothers in urban/rural areas, 398 (83.3%) gave birth to the youngest child while at home; 370 (92.5%) were assisted by TBAs. Only 3.2% of them were assisted by Health Extension Workers/nurses in health posts or at home during delivery. We found an association between health facility delivery and ante-natal care (ANC) attendance (p<0.001), educational status (p<0.001) and occupation of the husband/wife (mother) and gravidity (p=0.003); but there was no association with the number of wives the husband had (p=0.566). In the adjusted model, ANC attendance and education status of mother were significantly associated with health facility delivery (p=0.036; p<0.001). Conclusion Most deliveries in the study community took place at home. Educated mothers and ANC attending mothers have high tendency for health facility delivery. Programmes need to strengthen the capacities of mothers to attend ANC services, as well as build the capacity of Health Extension Workers (HEWs) and nurses working in health posts, in order to win the confidence of the community. PMID:23467618

  8. Petrological constraints on evolution of continental lithospheric mantle beneath the northwestern Ethiopian plateau: Insight from mantle xenoliths from the Gundeweyn area, East Gojam, Ethiopia

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Zhu, Bin; Fentie, Birhanu; Abraham, Samuel; Haji, Muhammed

    2016-01-01

    Detailed petrographical observations and in-situ major- and trace-element data for minerals from ten spinel peridotite xenoliths from a new locality in Gundeweyn area, East Gojam, have been examined in order to understand the composition, equilibrium temperature and pressure conditions as well as depletion and enrichment processes of continental lithospheric mantle beneath the Ethiopian plateau. The peridotite samples are very fresh and, with the exception of one spinel harzburgite, are all spinel lherzolites. Texturally, the xenoliths can be divided into two groups as primary and secondary textures. Primary textures are protogranular and porphyroclastic while secondary ones include reaction, spongy and lamellae textures. The Fo content of olivine and Cr# of spinel ranges from 86.5 to 90.5 and 7.7 to 14.1 in the lherzolites, respectively and are 89.8 and 49.8, respectively, in the harzburgite. All of the lherzolites fall into the lower Cr# and Fo region in the olivine-spinel mantle array than the harzburgite, which indicates that they are fertile peridotites that experienced low degrees of partial melting and melt extraction. Orthopyroxene and clinopyroxene show variable Cr2O3 and Al2O3 contents regardless of their lithology. The Mg# of orthopyroxene and clinopyroxene are 87.3 to 90.1 and 85.8 to 90.5 for lherzolite and 90.4 and 91.2 for harzburgite, respectively. The peridotites have been equilibrated at a temperature and pressure ranging from 850 to 1100 °C and 10.2 to 30 kbar, respectively, with the highest pressure record from the harzburgite. They record high mantle heat flow between 60 and 150 mW/m2, which is not typical for continental environments (40 mW/m2). Such a high geotherm in continental area shows the presence of active mantle upwelling beneath the Ethiopian plateau, which is consistent with the tectonic setting of nearby area of the Afar plume. Clinopyroxene of five lherzolites and one harzburgite samples have a LREE enriched pattern and the rest

  9. Mapping the Hawaiian plume conduit with converted seismic waves

    PubMed

    Li; Kind; Priestley; Sobolev; Tilmann; Yuan; Weber

    2000-06-22

    The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s(-1)) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of approximately 300 degrees C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100 degrees C higher temperature for the Hawaiian plume. PMID:10879532

  10. Mantle metasomatism

    SciTech Connect

    Menzies, M.; Hawkesworth, C.

    1986-01-01

    The concept of metasomatism and its role in the geochemical enrichment and depletion processes in upper mantle rocks remains contentious. This volume makes a comprehensive contribution to the study of metasomatic and enrichment processes: origin and importance in determining trace element and isotopic heterogeneity in the lithospheric mantle. It begins with a theoretical thermodynamic and experimental justification for metasomatism and proceeds to present evidence for this process from the study of mantle xenoliths. Finally the importance of metasomatism in relation to basaltic volcanism is assessed. The contents are as follows: Dynamics of Translithospheric Migration of Metasomatic Fluid and Alkaline Magma. Solubility of Major and Trace Elements in Mantle Metasomatic Fluids: Experimental Constraints. Mineralogic and Geochemical Evidence for Differing Styles of Metasomatism in Spinel Lherzolite Xenoliths: Enriched Mantle Source Regions of Basalts. Characterization of Mantle Metasomatic Fluids in Spinel Lherzolites and Alkali Clinophyroyxenites from the West Eifel and South-West Uganda. Metasomatised Harzburgites in Kimberlite and Alkaline Magmas: Enriched Resites and ''Flushed'' Lherzolites. Metasomatic and Enrichment Phenomena in Garnet-Peridotite Facies Mantle Xenoliths from the Matsoku Kimberlite Pipe Lesotho. Evidence for Mantle Metasomatism in Periodite Nodules from the Kimberley Pipes South Africa. Metasomatic and Enrichment Processes in Lithospheric Peridotites, an Effective of Asthenosphere-Lithosphere Interaction. Isotope Variations in Recent Volcanics: A Trace Element Perspective. Source Regions of Mid-Ocean Ridge Basalts: Evidence for Enrichment Processes. The Mantle Source for the Hawaiian Islands: Constraints from the Lavas and Ultramafic Inclusions.

  11. The planet beyond the plume hypothesis

    NASA Astrophysics Data System (ADS)

    Smith, Alan D.; Lewis, Charles

    1999-12-01

    Acceptance of the theory of plate tectonics was accompanied by the rise of the mantle plume/hotspot concept which has come to dominate geodynamics from its use both as an explanation for the origin of intraplate volcanism and as a reference frame for plate motions. However, even with a large degree of flexibility permitted in plume composition, temperature, size, and depth of origin, adoption of any limited number of hotspots means the plume model cannot account for all occurrences of the type of volcanism it was devised to explain. While scientific protocol would normally demand that an alternative explanation be sought, there have been few challenges to "plume theory" on account of a series of intricate controls set up by the plume model which makes plumes seem to be an essential feature of the Earth. The hotspot frame acts not only as a reference but also controls plate tectonics. Accommodating plumes relegates mantle convection to a weak, sluggish effect such that basal drag appears as a minor, resisting force, with plates having to move themselves by boundary forces and continents having to be rifted by plumes. Correspondingly, the geochemical evolution of the mantle is controlled by the requirement to isolate subducted crust into plume sources which limits potential buffers on the composition of the MORB-source to plume- or lower mantle material. Crustal growth and Precambrian tectonics are controlled by interpretations of greenstone belts as oceanic plateaus generated by plumes. Challenges to any aspect of the plume model are thus liable to be dismissed unless a counter explanation is offered across the geodynamic spectrum influenced by "plume theory". Nonetheless, an alternative synthesis can be made based on longstanding petrological evidence for derivation of intraplate volcanism from volatile-bearing sources (wetspots) in conjunction with concepts dismissed for being incompatible or superfluous to "plume theory". In the alternative Earth, the sources for

  12. InSAR observations of post-rifting deformation around the Dabbahu rift segment, Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Hamling, Ian J.; Wright, Tim J.; Calais, Eric; Lewi, Elias; Fukahata, Yukitoshi

    2014-04-01

    Increased displacement rates have been observed following manylarge earthquakes and magmatic events. Although an order of magnitude smaller than the displacements associated with the main event, the post-seismic or post-rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion. Due to the rare occurrence of subaerial rifting events, there are very few observations to constrain models of post-rifting deformation. In 2005 September, a 60-km-long dyke was intruded along the Dabbahu segment of the Nubia-Arabia Plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion. Using multiple satellite passes, we are able to separate the rift perpendicular and vertical displacement fields around the Dabbahu segment. Rift perpendicular and vertical rates of up to 180 and 240 mm yr-1, respectively. Here, we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment. Our models suggest upper mantle viscosities of 1018-19 Pa s overlain by an elastic crust of between 15 and 30 km. To fit the observations, inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of ˜0.13 and -0.08 km3 yr-1.

  13. Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode.

    PubMed

    Wright, Tim J; Ebinger, Cindy; Biggs, Juliet; Ayele, Atalay; Yirgu, Gezahegn; Keir, Derek; Stork, Anna

    2006-07-20

    Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the approximately 60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of approximately 2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975-84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia-Arabia plate boundary. PMID:16855588

  14. A long-lived ancient subduction-induced mantle boundary within the Pacific mantle

    NASA Astrophysics Data System (ADS)

    Zhang, G.; Smith-Duque, C. E.; Tang, S.; Li, S.; Alvarez Zarikian, C. A.; D'Hondt, S.; Inagaki, F.

    2012-12-01

    A large-scale mantle discontinuity has been identified along the East Pacific Rise (EPR) and the Pacific-Antarctic Ridge (PAR) with an inferred transition zone between the EPR 23°S-31°S. Because of strong interactions of the EPR with the Easter mantle plume, the nature and genesis of this geochemical transition zone remain unclear. IODP sites U1367 and U1368 drilled into the basement that was accreted from the mantle of the Pacific-Farallon/Nazca ridge at ~33.5 Ma and ~13.5 Ma, respectively, at latitudes of 28°S to 29°S on the EPR. Lavas from sites U1367 and U1368 are used here to track this mantle discontinuity away from the EPR. The Sr-Nd-Pb isotope data reported here show strong discrepancies between the two sites unrelated to the plume-ridge interaction. which suggests the persistence of a mantle boundary near latitudes of the Easter island since at least 33.5 Ma. Comparison of our data with those along the EPR-PAR defines an isotopic anomaly in the Pacific mantle with a mantle boundary near the EPR 29°S and a gentle transition near the PAR 57°S. This isotopic anomaly is coupled with a low-velocity zone near the core-mantle boundary in the south Pacific, low 3He/4He ratios of lavas, and shallow axial depth1 south of the EPR 29°S along the EPR-PAR. Interpretation of this mantle discontinuity involves an ancient subduction zone across the EPR 28°S-29°S that allowed long-lasting introduction of recycled oceanic crust and depleted mantle wedge into the south Pacific mantle. Lavas at sites U1367 and U1368 might have sampled mantle that once was part of this ancient subduction zone that remained largely intact and not stirred by mantle convection.

  15. North Atlantic magmatism controlled by temperature, mantle composition and buoyancy

    NASA Astrophysics Data System (ADS)

    Brown, Eric L.; Lesher, Charles E.

    2014-11-01

    Large igneous provinces are characterized by anomalously high rates of magma production. Such voluminous magmatism is commonly attributed to partial melting of hot, buoyantly upwelling mantle plume material. However, compositional heterogeneity in the mantle, caused by the subduction of oceanic crust, can also enhance magma production, diminishing the need for elevated temperatures associated with upwelling plumes. A plume origin for the North Atlantic large igneous province has been questioned because lava compositions correlate with crustal thickness, implying a link between magma productivity and mantle source composition. Here we use a numerical model that simulates upwelling and melting of compositionally heterogeneous mantle material to constrain the conditions that gave rise to magmatism in the North Atlantic. Using observations of lava compositions and volumes from the North Atlantic, we show that subducted crustal material represented less than 10% of the mantle source. We further show that mantle temperatures have remained elevated by 85-210 °C and increased mantle upwelling up to 14 times the rate of plate separation has occurred over the past 56 Myr. The enhanced temperatures and upwelling rates extended along more than 1,000 km of the Palaeogene rift, but are substantially more restricted along the modern Mid-Atlantic Ridge. These findings reflect the long-term manifestation of a mantle plume.

  16. Evolution of helium isotopes in the Earth's mantle.

    PubMed

    Class, Cornelia; Goldstein, Steven L

    2005-08-25

    Degassing of the Earth's mantle through magmatism results in the irreversible loss of helium to space, and high (3)He/(4)He ratios observed in oceanic basalts have been considered the main evidence for a 'primordial' undegassed deep mantle reservoir. Here we present a new global data compilation of ocean island basalts, representing upwelling 'plumes' from the deep mantle, and show that island groups with the highest primordial signal (high (3)He/(4)He ratios) have striking chemical and isotopic similarities to mid-ocean-ridge basalts. We interpret this as indicating a common history of mantle trace element depletion through magmatism. The high (3)He/(4)He in plumes may thus reflect incomplete degassing of the deep Earth during continent and ocean crust formation. We infer that differences between plumes and the upper-mantle source of ocean-ridge basalts reflect isolation of plume sources from the convecting mantle for approximately 1-2 Gyr. An undegassed, primordial reservoir in the mantle would therefore not be required, thus reconciling a long-standing contradiction in mantle dynamics. PMID:16121171

  17. Hotspots and the evolution of the mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    Trace element patterns show that continental and ocean island basalts are complementary to mid-ocean ridge basalts (MORB). The relative sizes of the two source regions can be estimated from enrichment/depletion patterns. Their combined volume, computed from estimates of whole mantle abundances, occupies the entire upper mantle. The source regions appear to be the result of an early differentiation of the mantle. The MORB source evolved from the melt fraction which lost its late stage enriched fluids to the overlying plume source. The MORB source is primarily garnet and clinopyroxene, consistent with it being an eclogite cumulate.

  18. Dynamical geochemistry of the mantle

    NASA Astrophysics Data System (ADS)

    Davies, G. F.

    2011-09-01

    the hybrid pyroxenite there would be less degassed. Plumes would sample both the degassed, enriched old oceanic crust and the gassy, less enriched hybrid pyroxenite and deliver them to OIBs. These findings can account quantitatively for the main He, Ne and Ar isotopic observations. It has been commonly inferred that the MORB source is strongly depleted of incompatible elements. However it has recently been argued that conventional estimates of the MORB source composition fail to take full account of mantle heterogeneity, and in particular focus on an ill-defined "depleted" mantle component while neglecting less common enriched components. Previous estimates have also been tied to the composition of peridotites, but these probably do not reflect the full complement of incompatible elements in the heterogeneous mantle. New estimates that account for enriched mantle components suggest the MORB source complement of incompatibles could be as much as 50-100 % larger than previous estimates. A major difficulty has been the inference that mass balances of incompatible trace elements could only be satisfied if there is a deep enriched layer in the mantle, but the Earth's topography precludes such a layer. The difficulty might be resolved if either the Earth is depleted relative to chondritic or the MORB source is less depleted than previous estimates. Together these factors can certainly resolve the mass balance difficulties.

  19. Two-way interaction between plume and slab: The Hainan-Manila example

    NASA Astrophysics Data System (ADS)

    Mériaux, Catherine; Duarte, João; Schellart, Wouter

    2015-04-01

    Three-dimensional upper-mantle laboratory models consisting of a compositional plume that is initiated underneath an entirely dynamically driven dense plate fixed at the surface along its trailing edge exhibit a two-way interaction between plume and slab. The slab influence on the plume is driven by the induced mantle flow generated by the plate motion, which includes a sinking and a retreating phase. Slab/Plume buoyancy flux ratios ranged between 7 and 18. In all models, the plume is being swept away from the slab during its rise, and once it has reached the surface, its head spreads towards the trench as a gravity current while its conduit keeps being deflected away. The plume influence on the slab is seen later, when the slab in its retreat gets closer to the plume. The plume buoyancy spreading under the slab then weakens the subduction rate. The degree to which the subduction rate is lessened is conditioned by the level of asymmetry, which the slab may develop along its free edge during its impact at the bottom surface. A lasting symmetric plate causes maximum disturbance of the plume to the slab retreat rate, while plate asymmetry alleviates the plume influence as the plume buoyancy is no longer trapped underneath the plate in its centreline but can escape sideways. Our laboratory model configuration applies to the Hainan plume and Manila subduction system. The geophysical and seismic observations showing the existence of a NW-SE tilting plume-like mantle low-velocity structure in the crust and in the mantle beneath the north Hainan Island-Leizhou Peninsula basalt province are explained by slab rollback induced toroidal mantle flow from the Manila subduction zone. On the basis of our models, it can be foreseen that the Hainan plume is to spread out under the Manila slab towards the mantle wedge in the future, which could lessen the Manila subduction rate.

  20. Mantle Flow beneath Arabia Offset from the Opening Red Sea

    NASA Astrophysics Data System (ADS)

    Stein, S. A.; Chang, S.; Merino, M.; van der Lee, S.; Stein, C. A.

    2010-12-01

    The rifting of continents involves a complex and poorly understood sequence of lithospheric stretching, volcanism, and mantle flow that eventually gives rise to seafloor spreading that forms a new ocean basin. The Red Sea, forming as the Arabian plate diverges from Africa, is a classic area for studying this process. Here, we present new insight from joint inversion of seismic wave travel times and waveforms to map velocity structure beneath Arabia and its surroundings. We find the low velocities expected for hot upwelling mantle material centered beneath the southern Red Sea and Gulf of Aden, consistent with the active spreading there. However, this hot material extends not below the northern Red Sea, but is instead offset to the east beneath Arabia, showing northward upper mantle flow from the Afar hotspot. The location of this low velocity channel beneath volcanic rocks erupted since rifting began 30 million years ago indicates that although the flow originates from the hotspot that is essentially fixed in the upper mantle, the channel moves with the Arabian plate. We thus propose that the absence of seafloor spreading in the northern Red Sea reflects the offset mantle flow. Because this offset has existed for millions of years, it is unclear whether it will evolve into seafloor spreading, rifting of Arabia above the channel, or both. This situation has aspects of the end-member models of rifting initiated by either mantle flow or lithospheric extension, and thus shows that the two can occur somewhat independently in different places before coalescing to seafloor spreading.

  1. P and S wave delays caused by thermal plumes

    NASA Astrophysics Data System (ADS)

    Maguire, Ross; Ritsema, Jeroen; van Keken, Peter E.; Fichtner, Andreas; Goes, Saskia

    2016-05-01

    Many studies have sought to seismically image plumes rising from the deep mantle in order to settle the debate about their presence and role in mantle dynamics, yet the predicted seismic signature of realistic plumes remains poorly understood. By combining numerical simulations of flow, mineral-physics constraints on the relationships between thermal anomalies and wave speeds, and spectral-element method based computations of seismograms, we estimate the delay times of teleseismic S and P waves caused by thermal plumes. Wavefront healing is incomplete for seismic periods ranging from 10 s (relevant in traveltime tomography) to 40 s (relevant in waveform tomography). We estimate P wave delays to be immeasurably small (< 0.3 s). S wave delays are larger than 0.4 s even for S waves crossing the conduits of the thinnest thermal plumes in our geodynamic models. At longer periods (> 20 s), measurements of instantaneous phase misfit may be more useful in resolving narrow plume conduits. To detect S wave delays of 0.4-0.8 s and the diagnostic frequency dependence imparted by plumes, it is key to minimize the influence of the heterogeneous crust and upper mantle. We argue that seismic imaging of plumes will advance significantly if data from wide-aperture ocean-bottom networks were available since, compared to continents, the oceanic crust and upper mantle is relatively simple.

  2. P- and S-wave delays caused by thermal plumes

    NASA Astrophysics Data System (ADS)

    Maguire, Ross; Ritsema, Jeroen; van Keken, Peter E.; Fichtner, Andreas; Goes, Saskia

    2016-08-01

    Many studies have sought to seismically image plumes rising from the deep mantle in order to settle the debate about their presence and role in mantle dynamics, yet the predicted seismic signature of realistic plumes remains poorly understood. By combining numerical simulations of flow, mineral-physics constraints on the relationships between thermal anomalies and wave speeds, and spectral-element method based computations of seismograms, we estimate the delay times of teleseismic S and P waves caused by thermal plumes. Wave front healing is incomplete for seismic periods ranging from 10 s (relevant in traveltime tomography) to 40 s (relevant in waveform tomography). We estimate P-wave delays to be immeasurably small (<0.3 s). S-wave delays are larger than 0.4 s even for S waves crossing the conduits of the thinnest thermal plumes in our geodynamic models. At longer periods (>20 s), measurements of instantaneous phase misfit may be more useful in resolving narrow plume conduits. To detect S-wave delays of 0.4-0.8 s and the diagnostic frequency dependence imparted by plumes, it is key to minimize the influence of the heterogeneous crust and upper mantle. We argue that seismic imaging of plumes will advance significantly if data from wide-aperture ocean-bottom networks were available since, compared to continents, the oceanic crust and upper mantle are relatively simple.

  3. Chondritic xenon in the Earth's mantle.

    PubMed

    Caracausi, Antonio; Avice, Guillaume; Burnard, Peter G; Füri, Evelyn; Marty, Bernard

    2016-05-01

    Noble gas isotopes are powerful tracers of the origins of planetary volatiles, and the accretion and evolution of the Earth. The compositions of magmatic gases provide insights into the evolution of the Earth's mantle and atmosphere. Despite recent analytical progress in the study of planetary materials and mantle-derived gases, the possible dual origin of the planetary gases in the mantle and the atmosphere remains unconstrained. Evidence relating to the relationship between the volatiles within our planet and the potential cosmochemical end-members is scarce. Here we show, using high-precision analysis of magmatic gas from the Eifel volcanic area (in Germany), that the light xenon isotopes identify a chondritic primordial component that differs from the precursor of atmospheric xenon. This is consistent with an asteroidal origin for the volatiles in the Earth's mantle, and indicates that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. Furthermore, our data are consistent with the origin of Eifel magmatism being a deep mantle plume. The corresponding mantle source has been isolated from the convective mantle since about 4.45 billion years ago, in agreement with models that predict the early isolation of mantle domains. Xenon isotope systematics support a clear distinction between mid-ocean-ridge and continental or oceanic plume sources, with chemical heterogeneities dating back to the Earth's accretion. The deep reservoir now sampled by the Eifel gas had a lower volatile/refractory (iodine/plutonium) composition than the shallower mantle sampled by mid-ocean-ridge volcanism, highlighting the increasing contribution of volatile-rich material during the first tens of millions of years of terrestrial accretion. PMID:27111512

  4. East Asia: Seismotectonics, magmatism and mantle dynamics

    NASA Astrophysics Data System (ADS)

    Zhao, Dapeng; Yu, Sheng; Ohtani, Eiji

    2011-02-01

    In this article, we review the significant recent results of geophysical studies and discuss their implications on seismotectonics, magmatism, and mantle dynamics in East Asia. High-resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes, which may reflect magma and fluids that affected the rupture nucleation of large earthquakes. In subduction zone regions, the crustal fluids originate from the dehydration of the subducting slab. Magmatism in arc and back-arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab. The intraplate magmatism has different origins. The continental volcanoes in Northeast Asia (such as Changbai and Wudalianchi) seem to be caused by the corner flow in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well. The Tengchong volcano in Southwest China is possibly caused by a similar process in BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China seems to be a hotspot fed by a lower-mantle plume associated with the Pacific and Philippine Sea slabs' deep subduction in the east and the Indian slab's deep subduction in the west down to the lower mantle. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab-plume interactions. Some of these issues, such as the origin of intraplate magmatism, are still controversial, and so further detailed studies are needed from now.

  5. Numerical and laboratory experiments on the dynamics of plume-ridge interaction. Progress report

    SciTech Connect

    Kincaid, C.; Gable, C.W.

    1995-09-01

    Mantle plumes and passive upwelling beneath ridges are the two dominant modes of mantle transport and thermal/chemical fluxing between the Earth`s deep interior and surface. While plumes and ridges independently contribute to crustal accretion, they also interact and the dispersion of plumes within the upper mantle is strongly modulated by mid-ocean ridges. The simplest mode of interaction, with the plume centered on the ridge, has been well documented and modeled. The remaining question is how plumes and ridges interact when the plume is located off-axis; it has been suggested that a pipeline-like flow from the off-axis plume to the ridge axis at the base of the rigid lithosphere may develop. Mid-ocean ridges migrating away from hot mantle plumes can be affected by plume discharges over long times and ridge migration distances. Salient feature of this model is that off-axis plumes communicate with the ridge through a channel resulting from the refraction and dispersion of an axi-symmetric plume conduit along the base of the sloping lithosphere. To test the dynamics of this model, a series of numerical and laboratory dynamic experiments on the problem of a fixed ridge and an off-axis buoyant upwelling were conducted. Results are discussed.

  6. Mantle flow beneath Arabia offset from the opening Red Sea

    NASA Astrophysics Data System (ADS)

    Chang, Sung-Joon; Merino, Miguel; Van der Lee, Suzan; Stein, Seth; Stein, Carol A.

    2011-02-01

    Continental rifting involves a poorly understood sequence of lithospheric stretching, volcanism, and mantle flow that evolves to seafloor spreading. We present new insight from inversion of seismic traveltimes and waveforms beneath Arabia and surroundings. Low velocities occur beneath the southern Red Sea and Gulf of Aden, consistent with active spreading. However, hot material extends not below the northern Red Sea, but is offset eastward beneath Arabia, showing mantle flow from the Afar hotspot. The location of this channel beneath volcanic rocks erupted since rifting began 30 million years ago indicates that flow moves with Arabia. We propose that the absence of seafloor spreading in the northern Red Sea reflects the offset flow. This geometry may evolve to spreading in the Northern Red Sea, rifting of Arabia, or both. This situation has aspects of both active and passive rifting, showing that both can occur before coalescing to seafloor spreading.

  7. The volcano seismic crisis in Afar, Ethiopia, starting September 2005

    NASA Astrophysics Data System (ADS)

    Ayele, Atalay; Jacques, Eric; Kassim, Mohammed; Kidane, Tesfaye; Omar, Ahmed; Tait, Stephen; Nercessian, Alexandre; de Chabalier, Jean-Bernard; King, Geoffrey

    2007-03-01

    We describe the seismo-volcanic crisis that occurred in northern Afar in late 2005, which involved 15 earthquakes greater than M5 and a small explosive silicic eruption from a vent called Da'Ure (at 12.651°N., 40.519°N) close to Dabbahu volcano. The purpose is to pull together the different sources of information into a coherent preliminary interpretation of what happened. The main geophysical data are the locations of the largest earthquakes, and a radar interferogram of unusually high quality that reveals injection of a 60 km long dyke with surface deformation expressed as normal faulting. Subsidence occurred around the Dabbahu volcanic edifice. Most of the dyke is likely to have been basaltic rather than silicic although the eruption was silicic. The volume of the subsidence represents at most 25% of the magma injected into the dyke. The silicic eruption was possibly triggered by interaction of basaltic magma with a shallow silicic reservoir. At about the same time as the eruption and dyking episode, some activity appears also to have taken place at the lava lake at Erta Ale volcano, some 150 km to the north of the eruption site. We evaluate the possibility that there may be some link by calculating stresses associated with opening of the fissure and looking at the activity of the lava lake as revealed by the thermal anomaly seen by weather satellites.

  8. Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Acocella, Valerio

    2006-10-01

    Erta Ale volcano lies along the on-shore Red Sea Rift (northern Afar, Ethiopia), separating the Nubia and Danakil plates. Erta Ale has a NNW-SSE elongated caldera, with a subvertical rim scarp, hosting a lava lake. Structural field work was aimed at defining the deformation pattern around the caldera. The caldera consists of along-rim and across-rim structures, resulting from local and regional (maximum extension ˜NE-SW) stress fields, respectively. These structures cross-cut each other at high angles, suggesting that the two stress fields remain distinct, each prevailing during rifting or caldera collapse. The local along-rim extensional fractures are gravity-driven structures that formed due to the retreat of the caldera wall after collapse, and are confined to the region of caldera subsidence. The across-rim structures are mainly located to the N and S of the caldera, where they form rift zones each accommodating a similar amount of extension (˜6.3 m), but displaying different trends and extension directions. Analogue models of interacting fractures are consistent with the Southern Rift being representative of the regional fault kinematics, while the Northern Rift is a local perturbation, resulting from the interaction between two right-stepping rift segments along the Erta Ale Range.

  9. Spectral analysis of dike-induced earthquakes in Afar, Ethiopia

    NASA Astrophysics Data System (ADS)

    Tepp, Gabrielle; Ebinger, Cynthia J.; Yun, Sang-Ho

    2016-04-01

    Shallow dike intrusions may be accompanied by fault slip above the dikes, a superposition which complicates seismic and geodetic data analyses. The diverse volcano-tectonic and low-frequency local earthquakes accompanying the 2005-2010 large-volume dike intrusions in the Dabbahu-Manda Hararo rift (Afar), some with fault displacements of up to 3 m at the surface, provide an opportunity to examine the relations among the earthquakes, dike intrusions, and surface ruptures. We apply the frequency index (FI) method to characterize the spectra of swarm earthquakes from six of the dikes. These earthquakes often have broad spectra with multiple peaks, making the usual peak frequency classification method unreliable. Our results show a general bimodal character with high FI earthquakes associated with deeper dikes (top > 3 km subsurface) and low FI earthquakes associated with shallow dikes, indicating that shallow dikes result in earthquakes with more low-frequency content and larger-amplitude surface waves. Low FI earthquakes are more common during dike emplacement, suggesting that interactions between the dike and faults may lead to lower FI. Taken together, likely source processes for low FI earthquakes are shallow hypocenters (<3 km) possibly with surface rupture, slow rupture velocities, and interactions with dike fluids. Strong site effects also heavily influence the earthquake spectral content. Additionally, our results suggest a continuum of spectral responses, implying either that impulsive volcano-tectonic earthquakes and the unusual, emergent earthquakes have similar source processes or that simple spectral analyses, such as FI, cannot distinguish different source processes.

  10. South Pacific hotspot swells dynamically supported by mantle flows

    NASA Astrophysics Data System (ADS)

    Yoshida, M.; Adam, C.; Isse, T.; Suetsugu, D.; Fukao, Y.; Barruol, G.

    2009-12-01

    The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. According to the plume theory, originally proposed by Morgan, the hotspot volcanoes are created by jets of hot material (plumes) rising from the deep mantle. With later studies, troubling inconsistencies began to emerge and other phenomena are invoked to explain intraplate volcanism, thus tending to nail the plume coffin. However, the problems encountered may simply be “the maturing of a valid theory to deal with the complexity of the real planet”. This alternative is tested here by studying the dynamics of the South Pacific plumes through a new numerical model of mantle flow based on a highly-resolved seismic tomography model. We show here, for the first time, that a direct link exists between the surface observations and the mantle flow. We find indeed outstanding correlations between the observed and the modelled swells and between the modelled flow pattern and the active volcanism. This shows that at a first order, the morphology of the volcanic chains and their associated swells is controlled by the mantle flows. The excellent correlation we find between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology has even broader implications. It implies indeed that we can accurately evaluate the heat transported by mantle plumes from a careful estimation of the swell morphology. We show that the heat transported by the South Pacific plumes accounts for 13% of the total plume heat flux.

  11. Stress field during early magmatism in the Ali Sabieh Dome, Djibouti, SE Afar rift

    NASA Astrophysics Data System (ADS)

    Sue, Christian; Le Gall, Bernard; Daoud, Ahmed Mohamed

    2014-09-01

    The so-called Ali Sabieh range, SE Afar rift, exhibits an atypical antiform structure occurring in the overall extensional tectonic context of the Afar triple junction. We dynamically analyzed the brittle deformation of this specific structural high using four different methods in order to better constrain the tectonic evolution of this key-area in the Afar depression. Paleostress inversions appear highly consistent using the four methods, which a posteriori validates this approach. Computed paleostress fields document two major signals: an early E-W extensional field, and a later transcurrent field, kinematically consistent with the previous one. The Ali Sabieh range may have evolved continuously during Oligo-Miocene times from large-scale extensional to transcurrent tectonism, as the result of probable local stress permutation between σ1 and σ2 stress axes.

  12. Mantle cryptology

    SciTech Connect

    Zindler, A.; Jagoutz, E.

    1988-02-01

    A group of anhydrous peridotites from Peridot Mesa, Arizona, document isotopic and trace element heterogeneity in the source mantle. LREE enrichments in two spinel periodotites may have occurred immediately prior to entrainment through interaction with a melt similar to the hose basanite. Detailed characterization of inclusion-free peridotite phases, and washed and unwahsed whole-rock samples, verifies the presence of a ubiquitous secondary contaminant which derives from interaction of the peridotites with local ground waters and host magma. Once the veil of this contamination is removed, coexisting phases are found to be in isotopic equilibrium. Further, a comparison of washed whole rocks and calculated clean-bulk compositions documents the occurrence of an important intragranular fluid-hosted trace element component. For the very incompatible elements (K, Rb, Cs, and Ba, and probably U, Th, Pb and gaseous components as well) this component dominates the nodule budget for two of the three samples studied in detail. Production of basaltic magmas from fertile but incompatible-element-depleted peridotite requires the action of melting processes such as those recently proposed by McKenzie (1985) and O'Hara (1985). The distinctive feature of these models is that they call on effectively larger source volumes for more incompatible elements. In this context, depletions of incompatible trace elements in MORB source mantle will be more extreme than has heretofore been suspected. This would essentially preclude the long-term total isolation of a MORB source mantle above the 670 km seismic discontinuity.

  13. Afar-wide Crustal Strain Field from Multiple InSAR Tracks

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Wright, T. J.; Wang, H.; Calais, E.; Bennati Rassion, L. S.; Ebinger, C. J.; Lewi, E.

    2010-12-01

    Onset of a rifting episode in the Dabbahu volcanic segment, Afar (Ethiopia), in 2005 renewed interest in crustal deformation studies in the area. As a consequence, an extensive geodetic data set, including InSAR and GPS measurements have been acquired over Afar and hold great potential towards improving our understanding of the extensional processes that operate during the final stages of continental rupture. The current geodetic observational and modelling strategy has focused on detailed, localised studies of dyke intrusions and eruptions mainly in the Dabbahu segment. However, an eruption in the Erta ‘Ale volcanic segment in 2008, and cluster of earthquakes observed in the Tat Ale segment, are testament to activity elsewhere in Afar. Here we make use of the vast geodetic dataset available to obtain strain information over the whole Afar depression. A systematic analysis of all the volcanic segments, including Dabbahu, Manda-Hararo, Alayta, Tat ‘Ale Erta Ale and the Djibouti deformation zone, is undertaken. We use InSAR data from multiple tracks together with available GPS measurements to obtain a velocity field model for Afar. We use over 300 radar images acquired by the Envisat satellite in both descending and ascending orbits, from 12 distinct tracks in image and wide swath modes, spanning the time period from October 2005 to present time. We obtain the line-of-sight deformation rates from each InSAR track using a network approach and then combine the InSAR velocities with the GPS observations, as suggested by Wright and Wang (2010) following the method of England and Molnar (1997). A mesh is constructed over the Afar area and then we solve for the horizontal and vertical velocities on each node. The resultant full 3D Afar-wide velocity field shows where current strains are being accumulated within the various volcanic segments of Afar, the width of the plate boundary deformation zone and possible connections between distinct volcanic segments on a

  14. Helium and lead isotopes reveal the geochemical geometry of the Samoan plume.

    PubMed

    Jackson, M G; Hart, S R; Konter, J G; Kurz, M D; Blusztajn, J; Farley, K A

    2014-10-16

    Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume. Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle endmembers with low (3)He/(4)He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high µ = (238)U/(204)Pb) and DM (depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high (3)He/(4)He. This observation is consistent with several low-(3)He/(4)He components in the plume mixing with a common high-(3)He/(4)He component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high-(3)He/(4)He plume matrix that hosts, and mixes with, several distinct low-(3)He/(4)He components. PMID:25318524

  15. Dust Plumes off Libya

    NASA Technical Reports Server (NTRS)

    2007-01-01

    Two-toned dust plumes blew northward off the coast of Libya on October 26, 2007, as the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite took this picture. While plumes in the west are beige, reminiscent of the Sahara's sands, the plumes in the east are distinctly darker. The differences in color can be traced to the plumes's varied origins.

  16. Degassing of carbon dioxide from basaltic magma at spreading centers: I. Afar transitional basalts

    NASA Astrophysics Data System (ADS)

    Gerlach, Terrence M.

    1989-11-01

    This study investigates the hypothesis that a significant fraction of the CO 2 in basalt supplied to axial volcanic ranges of spreading centers in the Afar depression, escapes by degassing during residence in crustal magma reservoirs. The investigation employs volcanic gas data to test the degassing hypothesis. Volcanic gases emitted from source vents at Erta'Ale lava lake are used to represent volatiles present in the basalt supplied to magma reservoirs underlying Afar spreading centers. For comparison, volcanic gases from a large fissure eruption at Ardoukoba are used to represent volatiles in basalt after a period of storage in the Afar magma reservoirs. The results confirm the hypothesis. Gases from the lava lake and fissure eruption are the same except for CO 2. They lie along a common CO 2 control line. The fissure eruption gases are six-fold depleted in CO 2 compared to gases from the continuously supplied lava lake. This difference corresponds to a loss of approximately 85% of the initial CO 2. Moreover, gases from the fissure eruption are nearly identical to those emitted by Kilauea basalts that have lost CO 2 by magma reservoir degassing. Mass balance modeling indicates an initial CO 2 content for Afar basalt of 0.12 wt.% compared to CO 2 concentrations as low as 0.02 wt.% after degassing at depth.

  17. Ethnobotanical study of plants used in management of livestock health problems by Afar people of Ada’ar District, Afar Regional State, Ethiopia

    PubMed Central

    2013-01-01

    Background The great majority of the Afar people of Ethiopia are pastoralists, highly dependent on livestock and livestock products. Livestock productivity is, however, frequently affected by different diseases. Although many districts in the Region have veterinary clinics, they lack basic facilities. As a result, the Afar people are still dependent on local materials, mainly plants, and traditional knowledge to manage livestock health problems. However, there is a serious threat to such local resources mainly due to recurrent drought and influence of modernization. Hence there is a need for proper documentation and evaluation of the existing ethnoveterinary knowledge in the Region. This study was aimed at documenting and analysing ethnoveterinary knowledge of people in Ada’ar District of the Afar Region associated with the use of plants. Methods The study involved interviewing selected knowledgeable Afar people in Ada’ar District on the use of plants to manage livestock ailments. Fidelity Level (FL) values were calculated for the reported medicinal plant to estimate their healing potentials. Specimens of reported medicinal plant were collected, identified and deposited at the National Herbarium, Addis Ababa University. Results The study revealed 49 medicinal plants as being used by the Afar people of Ada’ar District for the treatment of various livestock ailments, the majority of which (67.3%) were shrubs. Highest number of medicinal plants was used to treat blackleg, contagious caprine pleuropneumonia (CCPP), sudden sickness and pneumonia. Leaf was the most frequently sought plant part, accounting for 47% of the reported plants. All the medicnal plants used in the District were uncultivated ones growing in semi-disturbed and disturbed habitats as remnant plants and weeds. Cissus quadrangularis and Solanum incanum were the plants scoring the highest fidelity level values for their use to treat blackleg and respiratory tract problems, respectively. Conclusion

  18. Two views of Hawaiian plume structure

    NASA Astrophysics Data System (ADS)

    Hofmann, Albrecht W.; Farnetani, Cinzia G.

    2013-12-01

    Fundamentally contradictory interpretations of the isotopic compositions of Hawaiian basalts persist, even among authors who agree that the Hawaiian hotspot is caused by a deep-mantle plume. One view holds that the regional isotopic pattern of the volcanoes reflects large-scale heterogeneities in the basal thermal boundary layer of the mantle. These are drawn into the rising plume conduit, where they are vertically stretched and ultimately sampled by volcanoes. The alternative view is that the plume resembles a "uniformly heterogeneous plum pudding," with fertile plums of pyroxenite and/or enriched peridotite scattered in a matrix of more refractory peridotite. In a rising plume, the plums melt before the matrix, and the final melt composition is controlled significantly by the bulk melt fraction. Here we show that the uniformly heterogeneous plum pudding model is inconsistent with several geochemical observations: (1) the relative melt fractions inferred from La/Yb ratios in shield-stage basalts of the two parallel (Kea- and Loa-) volcanic chains, (2) the systematic Pb-isotopic differences between the chains, and the absence of such differences between shield and postshield phases, (3) the systematic shift to uniformly depleted Nd-isotopic compositions during rejuvenated volcanism. We extend our previous numerical simulation to the low melt production rates calculated far downstream (200-400 km) from shield volcanism. Part of these melts, feeding rejuvenated volcanism, are formed at pressures of ˜5 GPa in the previously unmelted underside of the plume, from material that originally constituted the uppermost part of the thermal boundary layer at the base of the mantle.

  19. Modes of rifting in magma-rich settings: Tectono-magmatic evolution of Central Afar

    NASA Astrophysics Data System (ADS)

    Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël.; Quidelleur, Xavier; Ayalew, Dereje; Leroy, Sylvie

    2016-01-01

    Recent research in Afar (northern Ethiopia) has largely focused on the formation of the present-day ocean-continent transition at active segments (e.g., Manda Hararo). However, the Oligo-Miocene history of extension, from the onset of rifting at ~25 Ma to the eruption of the massive Stratoïd flood basalts at ~4 Ma, remains poorly constrained. Here we present new structural data and radiometric dating from Central Afar, obtained along a zone stretching from the undeformed Oligocene Ethiopian plateau to the Manda Hararo and Tat'Ale active volcanic segments. Basaltic and rhyolitic formations were mapped in two key areas corresponding to the proximal and distal parts of a half-rift. We present a balanced composite cross section of Central Afar, reconstructed using our new data and previously published geophysical data on the crustal structure. Our main findings are as follows: (1) Extension during the Mio-Pliocene corresponds to a "wide rift" style of rifting. (2) The lower crust has been underplated/intruded and rethickened during rifting by magmatic injection. (3) Our restoration points to the existence of midcrustal shear zones that have helped to distribute extension in the upper crust and to localize extension at depth in a necking zone. Moreover, we suggest that there is a close relationship between the location of a shear zone and the underplated/intruded material. In magma-rich environments such as Central Afar, breakup should be achieved once the initial continental crust has been completely replaced by the newly, magmatically accreted crust. Consequently, and particularly in Afar, crustal thickness is not necessarily indicative of breakup but instead reflects differences in tectono-magmatic regimes.

  20. A young source for the Hawaiian plume.

    PubMed

    Sobolev, Alexander V; Hofmann, Albrecht W; Jochum, Klaus Peter; Kuzmin, Dmitry V; Stoll, Brigitte

    2011-08-25

    Recycling of oceanic crust through subduction, mantle upwelling, and remelting in mantle plumes is a widely accepted mechanism to explain ocean island volcanism. The timescale of this recycling is important to our understanding of mantle circulation rates. Correlations of uranogenic lead isotopes in lavas from ocean islands such as Hawaii or Iceland, when interpreted as model isochrons, have yielded source differentiation ages between 1 and 2.5 billion years (Gyr). However, if such correlations are produced by mixing of unrelated mantle components they will have no direct age significance. Re-Os decay model ages take into account the mixing of sources with different histories, but they depend on the assumed initial Re/Os ratio of the subducted crust, which is poorly constrained because of the high mobility of rhenium during subduction. Here we report the first data on (87)Sr/(86)Sr ratios for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200-650-Myr-old sea water. We infer that such sea water must have contaminated the Mauna Loa source rock, before subduction, imparting a unique 'time stamp' on this source. Small amounts of seawater-derived strontium in plume sources may be common but can be identified clearly only in ultra-depleted melts originating from generally highly (incompatible-element) depleted source components. The presence of 200-650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 (±1) cm yr(-1), much faster than previously thought. PMID:21832996

  1. Not so hot "hot spots" in the oceanic mantle.

    PubMed

    Bonath, E

    1990-10-01

    Excess volcanism and crustal swelling associated with hot spots are generally attributed to thermal plumes upwelling from the mantle. This concept has been tested in the portion of the Mid-Atlantic Ridge between 34 degrees and 45 degrees (Azores hot spot). Peridotite and basalt data indicate that the upper mantle in the hot spot has undergone a high degree of melting relative to the mantle elsewhere in the North Atlantic. However, application of various geothermometers suggests that the temperature of equilibration of peridotites in the mantle was lower, or at least not higher, in the hot spot than elsewhere. The presence of H(2)O-rich metasomatized mantle domains, inferred from peridotite and basalt data, would lower the melting temperature of the hot spot mantle and thereby reconcile its high degree ofmelting with the lack of a mantle temperature anomaly. Thus, some so-called hot spots might be melting anomalies unrelated to abnormally high mantle temperature or thermal plumes. PMID:17808242

  2. On possible plume-guided seismic waves

    USGS Publications Warehouse

    Julian, B.R.; Evans, J.R.

    2010-01-01

    Hypothetical thermal plumes in the Earth's mantle are expected to have low seismic-wave speeds and thus would support the propagation of guided elastic waves analogous to fault-zone guided seismic waves, fiber-optic waves, and acoustic waves in the oceanic SOund Fixing And Ranging channel. Plume-guided waves would be insensitive to geometric complexities in the wave guide, and their dispersion would make them distinctive on seismograms and would provide information about wave-guide structure that would complement seismic tomography. Detecting such waves would constitute strong evidence of a new kind for the existence of plumes. A cylindrical channel embedded in an infinite medium supports two classes of axially symmetric elastic-wave modes, torsional and longitudinal-radial. Torsional modes have rectilinear particle motion tangent to the cylinder surface. Longitudinal-radial modes have elliptical particle motion in planes that include the cylinder axis, with retrograde motion near the axis. The direction of elliptical particle motion reverses with distance from the axis: once for the fundamental mode, twice for the first overtone, and so on. Each mode exists only above its cut-off frequency, where the phase and group speeds equal the shear-wave speed in the infinite medium. At high frequencies, both speeds approach the shear-wave speed in the channel. All modes have minima in their group speeds, which produce Airy phases on seismograms. For shear wave-speed contrasts of a few percent, thought to be realistic for thermal plumes in the Earth, the largest signals are inversely dispersed and have dominant frequencies of about 0.1-1 Hz and durations of 15-30 sec. There are at least two possible sources of observable plume waves: (1) the intersection of mantle plumes with high-amplitude core-phase caustics in the deep mantle; and (2) ScS-like reflection at the core-mantle boundary of downward-propagating guided waves. The widespread recent deployment of broadband

  3. Optical-radar-DEM remote sensing data integration for geological mapping in the Afar Depression, Ethiopia

    NASA Astrophysics Data System (ADS)

    Thurmond, Allison K.; Abdelsalam, Mohamed G.; Thurmond, John B.

    2006-02-01

    The advantages of integrating optical (Landsat Enhanced Thematic Mapper Plus (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)) and radar (Shuttle Imaging Radar (SIR) - C, X-band Synthetic Aperture Radar (SAR) and RADARSAT-1) remote sensing data, and digital elevation models (DEMs) (Shuttle Radar Topography Mission (SRTM)) for geological mapping in arid regions such as the Afar Depression in Ethiopia are demonstrated. The Afar Depression in NE Africa is a natural laboratory for studying processes of sea-floor spreading and the transition from rifting to true sea-floor spreading. It is ideal for geological remote sensing because of its vastness, remoteness and inaccessibility together with almost continuous exposure, and lack of vegetation and soil cover. Optical-radar-DEM remote sensing data integration is used for: (1) Distinguishing spatial and temporal distribution of individual lava flows in the Quaternary Erta 'Ale Volcanic Range in the northern part of the Afar Depression, by integrating band-ratios of ASTER thermal infrared (TIR) data with Landsat ETM+ visible and near infrared (VNIR) and SIR-C/X-SAR L-band ( λ = 24 cm) data with horizontally transmitted and horizontally received (HH) polarization. (2) Visualizing and interpreting extensional imbrication fans that constitute part of the Dobe Graben in the central part of the Afar Depression by integrating Landsat ETM+ VNIR data with RADARSAT C-band ( λ = 6 cm) data with HH polarization and SRTM DEMs. These imbrication fans were developed as layer-parallel gravitational slip of the border fault hanging-wall towards the graben center. (3) Mapping morphologically defined structures in rhyolite flows exposed on the flanks of the Tendaho Rift by merging ASTER VNIR and short wave infrared (SWIR) with RADARSAT C-band data with HH polarization. The Tendaho Rift constitutes part of the Tendaho-Gobaad Discontinuity that separates the southern and the central eastern parts of the Afar

  4. Anisotropy of thermal diffusivity in the upper mantle.

    PubMed

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

    2001-06-14

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

  5. Survival of the primitive mantle reservoir?

    NASA Astrophysics Data System (ADS)

    Huang, S.; Jacobsen, S. B.; Mukhopadhyay, S.

    2010-12-01

    The high-3He lavas are thought to originate from a deep primitive mantle source that has not been much modified since the formation of Earth’s core. Comparison of 4He/3He in MORBs and plume lavas indicate that the plume sources must be a lower mantle feature, in agreement with most geophysical inferences. However, the lithophile element isotope systems of plume lavas are not primitive. The idea that the high-3He source is significantly less processed and more primitive than MORB source is clearly supported by mixing trends in plots of 4He/3He versus Sr, Nd and Pb isotope ratios, which have been extrapolated to an inferred 4He/3He of ~17,000 (~43x the atmospheric ratio), a mantle reservoir named PHEM (Primitive HElium Mantle). Slightly lower 4He/3He, ~15,000, were reported for Baffin Island picrites. Recently, Jackson et al. (2010) claimed that some Baffin Island and Greenland picrites with single-stage Pb model ages of ~4.5 Ga have low 4He/3He, and argued that “their source is the most ancient accessible reservoir in the Earth’s mantle, and it may be parental to all mantle reservoirs”. However, the available data are insufficient to make such a claim, and we suggest an alternative interpretation. Specially: 1. Four out of ten Baffin Island and Greenland picrites used by Jackson et al. (2010) have 4He/3He higher than average MORB value and all are far removed from the lowest measured value of 15,000. 2. Five Greenland picrites which cluster around the 4.50 Gyr geochron (Jackson et al., 2010) form a curved 207Pb*/206Pb*-4He/3He trend. This trend is best explained as a mixing line, implying that the single-stage Pb ages of these lavas are meaningless. 3. In a 207Pb*/206Pb*-4He/3He plot, Koolau lavas from Hawaii overlap with Baffin Island and Greenland picrites. If Baffin Island and Greenland picrites represent melts from the primitive mantle based on their Pb and He isotopes (Jackson et al., 2010), a similar argument can be applied to Koolau lavas. However, it

  6. Experiments and numerical investigations of wave propagation in thermal plumes

    NASA Astrophysics Data System (ADS)

    Laudenbach, N.

    2001-12-01

    In laboratory experiments thermal plumes are created by injecting hot corn syrup into a column of cold syrup. The viscosity contrast is up to a factor of 1000. Solitary waves, that propagate upwards in the plume conduit, are generated by enhancing the injection rate for a few seconds. For the measurement of the thermal structure of the plume we have implemented a method based on the deflection of a laser beam passing through the plume. Continuous scanning provides a new radial temperature profile each second, which allows detailed studies of the thermal structure of solitary waves. A PIV - (particle image volecimetry) method provides the velocity structure of the thermal plume. Measurements were taken for plume heads, conduits and propagating waves. Comparison between experimental results and numerical 2-D axisymmetric simulations shows a good agreement of the temperature profiles and velocity fields in the plume conduit and waves. Because of thermal diffusion, the conduit widens with height, while its central temperature decreases. The solitary waves start with the same temperature as the unperturbed conduit, however, we find that the temperature in the waves decreases less rapide with rising height. This can be explained by the faster upward propagation and the trapping of fluid within the soliton. If solitary waves exists in mantle plumes, this would imply that they arrive at the bottom of the lithosphere with a larger excess temperature than what the plumes normally exhibits, which could explain strong variations of melt generation with time.

  7. Experiments and Numerical Investigations of Wave Propagation In Thermal Plumes.

    NASA Astrophysics Data System (ADS)

    Laudenbach, N.; Christensen, U. R.

    In laboratory experiments thermal plumes are created by injecting hot corn syrup into a column of cold syrup. The viscosity contrast is up to a factor of 1000. Solitary waves, that propagate upwards in the plume conduit, are generated by enhancing the injection rate for a few seconds. For the measurement of the thermal structure of the plume we have implemented a method based on the deflection of a laser beam passing through the plume. Continuous scanning provides a new radial temperature profile each second, which allows detailed studies of the thermal structure of solitary waves. A PIV - (particle image volecimetry) method provides the velocity structure of the thermal plume. Measurements were taken for plume heads, conduits and propagating waves. Comparison between experimental results and numerical 2-D axisymmetric simulations shows a good agreement of the temperature profiles and velocity fields in the plume conduit and waves. Because of thermal diffusion, the conduit widens with height, while its central temperature decreases. The solitary waves start with the same temperature as the unperturbed conduit, however, we find that the temperature in the waves decreases less rapide with rising height. This can be explained by the faster upward propagation and the trapping of fluid within the soliton. If solitary waves exists in mantle plumes, this would imply that they arrive at the bottom of the lithosphere with a larger excess temperature than what the plumes normally exhibits. Especially for weak hotspots solitary waves could have strong influence on the variation of melt generation with time.

  8. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression.

    PubMed

    Keir, Derek; Belachew, M; Ebinger, C J; Kendall, J-M; Hammond, J O S; Stuart, G W; Ayele, A; Rowland, J V

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  9. Mapping the evolving strain field during continental breakup from crustal anisotropy in the Afar Depression

    PubMed Central

    Keir, Derek; Belachew, M.; Ebinger, C.J.; Kendall, J.-M.; Hammond, J.O.S.; Stuart, G.W.; Ayele, A.; Rowland, J.V.

    2011-01-01

    Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strain remain controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar Triple Junction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strain in a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extension dominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in this zone of incipient plate rupture. PMID:21505441

  10. Imaging the mantle beneath Iceland using integrated seismological techniques

    USGS Publications Warehouse

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

    2002-01-01

    Using a combination of body wave and surface wave data sets to reveal the mantle plume and plume head, this study presents a tomographic image of the mantle structure beneath Iceland to 400 km depth. Data comes primarily from the PASSCAL-HOTSPOT deployment of 30 broadband instruments over a period of 2 years, and is supplemented by data from the SIL and ICEMELT networks. Three sets of relative teleseismic body wave arrival times are generated through cross correlation: S and SKS arrivals at 0.03-0.1 Hz, and P and PKIKP arrivals at 0.03-0.1 and 0.8-2.0 Hz. Prior to inversion the crustal portion of the travel time anomalies is removed using the crustal model ICECRTb. This step has a significant effect on the mantle velocity variations imaged down to a depth of ???250 km. Inversion of relative arrival times only provides information on lateral velocity variations. Surface waves are therefore used to provide absolute velocity information for the uppermost mantle beneath Iceland. The average wave number for the Love wave fundamental mode at 0.020 and 0.024 Hz is measured and used to invert for the average S velocity. Combination of the body wave and surface wave information reveals a predominantly horizontal low-velocity anomaly extending from the Moho down to ???250 km depth, interpreted as a plume head. Below the plume head a near-cylindrical low-velocity anomaly with a radius of ???100 km and peak VP and VS anomalies of -2% and -4%, respectively, extends down to the maximum depth of resolution at 400 km. Within the plume head, in the uppermost mantle above the core of the plume, there is a relatively high velocity with a maximum VP and VS anomaly of +2%. This high-velocity anomaly may be the result of the extreme degree of melt extraction necessary to generate the thick (46 km) crust in central Iceland. Comparison of the plume volumetric flux implied by our images, the crustal generation rate, and the degree of melting suggested by rare earth element inversions

  11. Subducting Slabs: Jellyfishes in the Earth's Mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, C.; Braun, J.; Husson, L.; Le Carlier de Veslud, C.; Thieulot, C.; Yamato, P.; Grujic, D.

    2010-12-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  12. Subducting slabs: Jellyfishes in the Earth's mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, Christelle; Braun, Jean; Husson, Laurent; Le Carlier de Veslud, Christian; Thieulot, Cedric; Yamato, Philippe; Grujic, Djordje

    2010-08-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  13. Comparing the nature of the western and eastern Azores mantle

    NASA Astrophysics Data System (ADS)

    Genske, Felix S.; Beier, Christoph; Stracke, Andreas; Turner, Simon P.; Pearson, Norman J.; Hauff, Folkmar; Schaefer, Bruce F.; Haase, Karsten M.

    2016-01-01

    The Azores islands in the central North-Atlantic originate from a regional melting anomaly, probably created by melting hot, unusually hydrous and geochemically enriched mantle. Here, we present Hf, Pb and Os isotopic data in geochemically well-characterised primitive lavas from the islands Flores and Corvo that are located west of the Mid-Atlantic Ridge (MAR), as well as submarine samples from a subsided island west of Flores and from Deep Sea Drilling Project (DSDP) holes drilled in the western part of the Azores platform and beyond. These are compared to existing data from the Azores islands east of the MAR. The geodynamic origin of the two islands west of the ridge axis and furthest from the inferred plume centre in the central part of the plateau is enigmatic. The new data constrain the source compositions of the Flores and Corvo lavas and show that the western and eastern Azores mantle is isotopically similar, with the exception of an enriched component found exclusively on eastern São Miguel. Trace element ratios involving high field strength elements (HFSE) are distinctly different in the western islands (e.g. twofold higher Nb/Zr) compared to any of the islands east of the MAR. A similar signature is observed in MAR basalts to the south of the Azores platform and inferred to originate from (auto-) metasomatic enrichment of the sub-ridge mantle (Gale et al., 2011, 2013). In a similar fashion, low degree melts from an enriched source component may metasomatise the ambient plume mantle underneath the western Azores islands. Melting such a modified plume mantle can explain the chemical differences between lavas from the western and eastern Azores islands without the need for additional plume components. Recent re-enrichment and intra melting column modification of the upwelling mantle can cause local to regional scale geochemical differences in mantle-derived melts.

  14. The 2004 dyke-fault interaction at Dallol, northern Afar (Ethiopia)

    NASA Astrophysics Data System (ADS)

    Nobile, A.; Pagli, C.; Keir, D.; Wright, T.; Ruch, J.; Acocella, V.

    2012-04-01

    The 2005 Dabbahu intrusive event in Afar highlighted the importance of magma in controlling rifting along divergent plate boundaries. Here we show that a further intrusive event occurred in late 2004 at Dallol, along the axis of the Red Sea Rift in northern Afar. Dallol consists of an hydrothermal area lacking significant volcanic activity at the northern end of the Erta Ale volcanic segment. We use InSAR and seismicity data to constrain the geometric and kinematic features of the intrusion, as well as its possible effects. The dyke intruded a 6 km portion of the rift, from Dallol volcano, which subsided, to the south. The intrusion was accompanied by a seismic sequence, including a Mw 5.5 earthquake on 22 October. The larger events were registered by several seismic stations worldwide and located by the International Seismological Center (ISC). InSAR data from Envisat allow us to measure the surface deformation associated with these events. An inversion for a simple elastic model suggests that a ~3m wide and ~46km3 volume dyke was accompanied by fault slip. Seismicity observations suggest that the Mw 5.5 earthquake was triggered by the dyke intrusion. These results at Dallol provide the northernmost evidence of dyke-induced extension in Afar, occurring shortly before the 2005 event at Dabbahu.

  15. Large magnitude silicic volcanism in north Afar: the Nabro Volcanic Range and Ma'alalta volcano

    NASA Astrophysics Data System (ADS)

    Wiart, Pierre; Oppenheimer, Clive

    2005-02-01

    Much of the volcanological work carried out in north Afar (Ethiopia and Eritrea) has focused on the nature of Quaternary basaltic volcanic ranges, which have been interpreted by some as incipient oceanic ridges. However, we show here that comparable volumes of silicic magmas have been erupted in the region. In particular, the virtually undocumented Nabro Volcanic Range, which runs NNE for more than 100 km from the margin of the Danakil Depression to the Red Sea coast, has a subaerial volume of the order of 550 km3, comparable to the volume of the much better known Erta’Ale axial volcanic range. Nabro volcano itself forms part of an enigmatic double caldera structure with a neighbouring volcano, Mallahle. The twin caldera may have formed simultaneously with the eruption of between 20 and 100 km3 of ignimbrite, which is readily identified in Landsat Thematic Mapper imagery. This may have been the largest explosive eruption in north Afar, and is certain to have deposited a regionally distributed tephra layer which could in the future be located in distal sections as a stratigraphic marker. An integrated analysis of optical and synthetic aperture radar imagery, digital topographic data, field observations and limited geochemical measurements, permits here descriptions and first order inferences about the structure, stratigraphy and compositions of several major volcanoes of the Afar Triangle, and a reappraisal of their regional significance.

  16. 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).

  17. Turbulent Plumes in Nature

    NASA Astrophysics Data System (ADS)

    Woods, Andrew W.

    2010-01-01

    This review describes a range of natural processes leading to the formation of turbulent buoyant plumes, largely relating to volcanic processes, in which there are localized, intense releases of energy. Phenomena include volcanic eruption columns, bubble plumes in lakes, hydrothermal plumes, and plumes beneath the ice in polar oceans. We assess how the dynamics is affected by heat transfer, particle fallout and recycling, and Earth's rotation, as well as explore some of the mixing of the ambient fluid produced by plumes in a confined geometry.

  18. Dust Plume off Mauritania

    NASA Technical Reports Server (NTRS)

    2007-01-01

    A thick plume of dust blew off the coast of Mauritania in western Africa on October 2, 2007. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite observed the dust plume as it headed toward the southwest over the Atlantic Ocean. In this image, the dust varies in color from nearly white to medium tan. The dust plume is easier to see over the dark background of the ocean, but the plume stretches across the land surface to the east, as well. The dust plume's structure is clearest along the coastline, where relatively clear air pockets separate distinct puffs of dust. West of that, individual pillows of dust push together to form a more homogeneous plume. Near its southwest tip, the plume takes on yet another shape, with stripes of pale dust fanning out toward the northwest. Occasional tiny white clouds dot the sky overhead, but skies are otherwise clear.

  19. Modeling Europa's dust plumes

    NASA Astrophysics Data System (ADS)

    Southworth, B. S.; Kempf, S.; Schmidt, J.

    2015-12-01

    The discovery of Jupiter's moon Europa maintaining a probably sporadic water vapor plume constitutes a huge scientific opportunity for NASA's upcoming mission to this Galilean moon. Measuring properties of material emerging from interior sources offers a unique chance to understand conditions at Europa's subsurface ocean. Exploiting results obtained for the Enceladus plume, we simulate possible Europa plume configurations, analyze particle number density and surface deposition results, and estimate the expected flux of ice grains on a spacecraft. Due to Europa's high escape speed, observing an active plume will require low-altitude flybys, preferably at altitudes of 5-100 km. At higher altitudes a plume may escape detection. Our simulations provide an extensive library documenting the possible structure of Europa dust plumes, which can be quickly refined as more data on Europa dust plumes are collected.

  20. Subduction: The Gatekeeper for Mantle Melting.

    NASA Astrophysics Data System (ADS)

    Kincaid, C. R.; Druken, K. A.; Griffiths, R. W.

    2011-12-01

    Geodynamic models are used to show the importance of subduction in controlling vertical thermal and chemical fluxes from Earth's interior to surface. In our models subduction-induced circulation produces conditions favorable to both steady-state and episodic melt production and also plays the role of gatekeeper in thwarting large scale melt production from rising plumes. We use laboratory experiments to characterize three-dimensional (3D) flow fields in convergent margins in response to a range of subduction and back-arc deformation styles, and how these flows interact with upwellings. Models utilize a glucose working fluid with a temperature dependent viscosity to represent the upper 2000 km of the mantle. Subducting lithosphere is modeled with a descending Phenolic plate and back-arc extension is produced by moving Mylar sheets. Thermal plumes are generated from a pressurized, temperature controlled source. Our results show that naturally occurring transitions from downdip- to rollback-dominated subduction produce conditions that favor both widespread decompression melting in the mantle wedge and short-lived pulses of extensive slab melting. For cases of plume-subduction interaction, 3D slab-induced flow quickly converts the active upwelling to a passive thermal anomaly that bears little to no resemblance to traditional models for plume surface expressions. Instead of rising to make LIPs with age-progressive chains, the bulk of the original plume material is trapped below depths of melt production before being re-subducted by the slab. A limited volume of this passive, former plume material is capable of surfacing. Interestingly, this is seen to occur through a range of morphologies that are consistently offset from the original rise location (e.g., conduit). Surface expressions include anything from small circular patches to long, linear features with complex age trends (e.g., progressive or regressive) resulting from the competition between plate motions and

  1. Chondritic xenon in the Earth’s mantle

    NASA Astrophysics Data System (ADS)

    Caracausi, Antonio; Avice, Guillaume; Burnard, Peter G.; Füri, Evelyn; Marty, Bernard

    2016-05-01

    Noble gas isotopes are powerful tracers of the origins of planetary volatiles, and the accretion and evolution of the Earth. The compositions of magmatic gases provide insights into the evolution of the Earth’s mantle and atmosphere. Despite recent analytical progress in the study of planetary materials and mantle-derived gases, the possible dual origin of the planetary gases in the mantle and the atmosphere remains unconstrained. Evidence relating to the relationship between the volatiles within our planet and the potential cosmochemical end-members is scarce. Here we show, using high-precision analysis of magmatic gas from the Eifel volcanic area (in Germany), that the light xenon isotopes identify a chondritic primordial component that differs from the precursor of atmospheric xenon. This is consistent with an asteroidal origin for the volatiles in the Earth’s mantle, and indicates that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. Furthermore, our data are consistent with the origin of Eifel magmatism being a deep mantle plume. The corresponding mantle source has been isolated from the convective mantle since about 4.45 billion years ago, in agreement with models that predict the early isolation of mantle domains. Xenon isotope systematics support a clear distinction between mid-ocean-ridge and continental or oceanic plume sources, with chemical heterogeneities dating back to the Earth’s accretion. The deep reservoir now sampled by the Eifel gas had a lower volatile/refractory (iodine/plutonium) composition than the shallower mantle sampled by mid-ocean-ridge volcanism, highlighting the increasing contribution of volatile-rich material during the first tens of millions of years of terrestrial accretion.

  2. New geodetic measurements in central Afar constraining the Arabia-Somalia-Nubia triple junction kinematics

    NASA Astrophysics Data System (ADS)

    Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Lewi, E.; Grandin, R.; Calais, E.; Wright, T. J.; Bendick, R. O.; Pagli, C.; Peltzer, G.; de Chabalier, J. B.; Ibrahim Ahmed, S.

    2014-12-01

    The Afar Depression is an extraordinary submerged laboratory where the crustal mechanisms involved in the active rifting process can be studied. But the crustal movements at the regional scale are complicated by being the locus of the meeting of three divergent plate boundaries: the oceanic spreading ridges of the Red Sea and the Aden Ridge and the intra-continental East-African Rift (EAR). We present here the first GPS measurements conducted in a new network in Central Afar, complementing existing networks in Eritrea, around the Manda-Harraro 2005-2010 active segment, in the Northern part of the EAR and in Djibouti. Even if InSAR data were appropriate for mapping the deformation field, the results are difficult to interpret for analyzing the regional kinematics because of the atmospheric conditions, the lack of complete data catalogue, the acquisition configuration and the small velocity variations. Therefore, our measurements in the new sites are crucial to obtain an accurate velocity field over the whole depression, and focus specifically on the spatial organization of the deformation to characterize the tripe junction. These first results show that a small part of the motion of the Somalia plate with respect to the Nubia plate or the Arabia plate (2-3 mm/yr) occurs south of the Tadjura Gulf and East of the Adda-do segment in Southern Afar. The complex kinematic pattern involves a clockwise rotation of this Southeastern part of the Afar rift and can be related to the significant seismic activity regularly recorded in the region of Jigjiga (northern Somalia-Ethiopia border). The western continuation of the Aden Ridge into Afar extends West of the Asal rift segment and does not reach the young active segment of Manda-Inakir (MI). A slow gradient of velocity is observed across the Dobi Graben and across the large systems of faults between Lake Abhe and the MI rift segment. A striking change of the velocity direction occurs in the region of Assaïta, west of Lake

  3. Block rotation and continental extension in Afar: A comparison to oceanic microplate systems

    NASA Astrophysics Data System (ADS)

    Acton, Gary D.; Stein, Seth; Engeln, Joseph F.

    1991-06-01

    The reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation. To see whether such microplate behavior has implications for continental rifting, we investigate the application of a microplate model to the Afar region at the Nubia-Somalia-Arabia triple junction. Studies of marine magnetic anomalies, volcanic ages, bathymetry, and seismicity suggest that the westward propagating Gulf of Aden spreading center has propagated into eastern Afar within the past 2 m.y., causing rifting and extension within the continent. We derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that Pliocene to Pleistocene age rocks have undergone a clockwise rotation of ˜11°. We suggest that the history of rifting, the rotation, and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for Afar. In this scenario, motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift. While motion is transferred, the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates. The rifting history and rotations in eastern Afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves. Provided the constraints we use are appropriate, our model better describes the regional kinematics than alternative block models including one based on "bookshelf" faulting. If the tectonics of Afar are typical for continental breakup, they have interesting implications for the geometry of passive margins. In particular, asymmetric rifted margins can be produced if the final location of the rift axis is not

  4. Plume and plate controlled hotspot trails in the South Atlantic

    NASA Astrophysics Data System (ADS)

    O'Connor, John; Jokat, Wilfried; le Roex, Anton; Class, Cornelia; Wijbrans, Jan; Keßling, Stefanie; Kuiper, Klaudia; Nebel, Oliver

    2013-04-01

    Discovering if hotspots observed on the Earth's surface are explained by underlying plumes rising from the deep mantle or by shallow plate-driven processes continues to be an essential goal in Earth Science. Key evidence underpinning the mantle plume concept is the existence of age-progressive volcanic trails recording past plate motion relative to surface hotspots and their causal plumes. Using the icebreaker RV Polarstern, we sampled scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the Earth's two largest and deepest regions of slower-than-average seismic wave speed - the Africa Low Shear Wave Velocity Province - caused by a massive thermo-chemical 'pile' on the core-mantle boundary. We showed recently using Ar/Ar isotopic ages - and crustal structure and seafloor ages - that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O'Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. If the distribution of hotspot trails reflects where plume melts could or could not penetrate the continental or oceanic lithosphere then plumes could have been active for significantly longer than indicated by their volcanic chains. This provides a mechanism for extended late stage interplay between deep mantle

  5. He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components

    NASA Astrophysics Data System (ADS)

    Poreda, R. J.; Craig, H.

    1992-11-01

    Helium isotope ratios in Lau Basin back-arc basalts range from 7 to 22 times the atmospheric value ( R A), i.e. from ratios typical of MORB (Depleted Mantle) helium (R/R A = 8 ± 1) to ratios similar to 'high- 3He' hotspots as observed in the Hawaiian, Icelandic, and nearby Samoan plume ( R/R A = 24 ). Along the Central Lau Basin spreading axis and its northward extension in the region around Niuafo'ou Volcano, 3He/ 4He ratios have typical MORB values (range = 7.5-8.6), but on Rochambeau Bank, the southern flank of a large seamount, ratios up to 22 R A occur. These high 3He/ 4He ratios are extrema of linear arrays (11-22 R A) of He vs. Sr, Nd and Pb isotope ratios, between a Depleted Mantle (MORB) end-member and a Primitive Helium Mantle component (PHEM). PHEM is the Enriched Mantle end-member for the 'depleted' array formed with the DM component, and at the same time the Depleted end-member for the 'enriched' array formed with 'EM' the EM2-type end-member for Masefau Bay, Samoan basalts, as these two binary arrays intersect at its composition. Sr and Nd isotopic arrays vs. each other and vs. 3He are consistent with these binary 'mirror arrays' for Lau and Masefau basalts. The 3He data show unequivocally that deep-mantle plume material is present at Rochambeau Bank, and to some extent in the leaky transform/spreading axis along Peggy Ridge. We suppose that the Samoan plume component regards itself as an 'off-ridge' hotspot relative to the nearby Lau spreading axis, and that some of its material is channeled toward Peggy Ridge in a manner similar to the channeling we observe at the Galapagos and Pascua (Easter Island) hotspots.

  6. Temperature and velocity measurements of a rising thermal plume

    NASA Astrophysics Data System (ADS)

    Cagney, Neil; Newsome, William H.; Lithgow-Bertelloni, Carolina; Cotel, Aline; Hart, Stanley R.; Whitehead, John A.

    2015-03-01

    The three-dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature-dependent viscosity are measured using Particle-Image Velocimetry and thermochromatic liquid crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualize the plume, they do not reveal the finer details of its structure. However, by computing the Finite-Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterize the behavior of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small-scale isotopic variability observed in ocean-island basalt regions. Lastly, the Finite-Time Lyapunov Exponent fields are used to calculate the steady state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature-dependence of the fluid viscosity in the current work.

  7. Geophysical inferences of thermal-chemical structures in the lower mantle

    NASA Technical Reports Server (NTRS)

    Yuen, D. A.; Cadek, O.; Chopelas, A.; Matyska, C.

    1993-01-01

    Lateral variations of the temperature field in the lower mantle have been reconstructed using new results in mineral physics and seismic tomographic data. We show that, with the application of high-pressure experimental values of thermal expansivity and of sound velocities, the slow seismic anomalies in the lower mantle under the Pacific and Africa can be converted into realistic-looking plume structures with large dimensions of 0(1000 km). The outer fringes of the plumes have an excess temperature of around 400 K. In the core of the plumes are found tonguelike structures with extremely high thermal anomalies. These values can exceed 1200 K and are too high to be explained on the basis of thermal anomalies alone. We suggest that these major plumes in the deep mantle may be driven by both thermal and chemical buoyancies or that enhanced conductive heat-transfer may be important there.

  8. An olivine-free mantle source of Hawaiian shield basalts.

    PubMed

    Sobolev, Alexander V; Hofmann, Albrecht W; Sobolev, Stephan V; Nikogosian, Igor K

    2005-03-31

    More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations. PMID:15800614

  9. Modeling Europa's Dust Plumes

    NASA Astrophysics Data System (ADS)

    Southworth, B.; Kempf, S.; Schmidt, J.

    2015-12-01

    The discovery of Europa maintaining a probably sporadic water vapor plume constitutes a huge scientific opportunity for NASA's upcoming mission to this Galilean moon. Measuring the properties of material emerging from interior sources offers a unique chance to understand conditions at Europa's subsurface ocean. Exploiting results obtained for the Enceladus plume, we adjust the ejection model by Schmidt et al. [2008] to the conditions at Europa. In this way, we estimate properties of a possible, yet unobserved dust component of the Europa plume. For a size-dependent speed distribution of emerging ice particles we use the model from Kempf et al. [2010] for grain dynamics, modified to run simulations of plumes on Europa. Specifically, we model emission from the two plume locations determined from observations by Roth et al. [2014] and also from other locations chosen at the closest approach of low-altitude flybys investigated in the Europa Clipper study. This allows us to estimate expected fluxes of ice grains on the spacecraft. We then explore the parameter space of Europa dust plumes with regard to particle speed distribution parameters, plume location, and spacecraft flyby elevation. Each parameter set results in a 3-dimensional particle density structure through which we simulate flybys, and a map of particle fallback ('snowfall') on the surface of Europa. Due to the moon's high escape speed, a Europa plume will eject few to no particles that can escape its gravity, which has several further consequences: (i) For given ejection velocity a Europa plume will have a smaller scale height, with a higher particle number densities than the plume on Enceladus, (ii) plume particles will not feed the diffuse Galilean dust ring, (iii) the snowfall pattern on the surface will be more localized about the plume location, and will not induce a global m = 2 pattern as seen on Enceladus, and (iv) safely observing an active plume will require low altitude flybys, preferably at 50

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

    PubMed

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

    2011-10-01

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

  11. Os-He Isotope Systematics of Iceland Picrites: Evidence for a Deep Origin of the Iceland Plume

    NASA Technical Reports Server (NTRS)

    Brandon, Alan D.; Graham, David W.; Waight, Tod; Gautason, Bjarni

    2007-01-01

    Recent work on the origin of the Iceland hotspot suggests that it may result from upwelling upper mantle material rather than a deep plume. To constrain the depths of origins of Iceland mantle sources, Os and He isotope systematics were obtained on a suite picrites that span the compositional range observed within the neovolcanic zones.

  12. Characteristics of the Bab al Mandab-Northern Afar area of the southern Red Sea

    SciTech Connect

    Allen, R.B. ); Sikander, A.H. ); Abouzakhm, A.G.

    1991-08-01

    The southern Red Sea and adjacent Afar area represent an enigmatic portion of the Red Sea/Gulf of Aden basin system. Although the topographic rift shoulders of the Red Sea and Gulf of Aden can be traced through this elbow, and appear to suggest that a similar width for the zone of extension is maintained across the region, the character of the floor of the rift zone changes. The distinctive character of the southern Red Sea-Afar area results in part from a topographically elevated region, possibly associated with the Red Sea-Gulf of Aden-East African rift triple junction. In addition, however, seismic data from offshore Ethiopia suggests that the distinctive character is due a complex pattern of rifting. The central axial trough of the Red Sea decreases in depth to the south toward the strait of Bab al Mandab, suggesting that rifting is dying out southward. Farther to the west at the same latitude, a major but narrow half-graben can be seen on seismic in the Gulf of Zula, bounded to the east by a large west-dipping normal fault. This structure continues south into the Danakil Depression of the Afar area. Between these two en echelon rift trends, the Danakil Alps form a long-lived high. Seismic data from the southern Red Sea of Ethiopia show southward thinning and pinch-out of the Miocene syn-rift evaporite sequence onto the northern Danakil block. Thus, it appears that the Danakil block has largely escaped Red Sea extension and subsidence. Instead, it forms a large unextended terrain located between overlapping en echelon rift trends, and may represent an accommodation zone structure associated with offset in the rift axis of the southern Red Sea.

  13. The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

    NASA Astrophysics Data System (ADS)

    Ayele, Atalay; Stuart, Graham; Bastow, Ian; Keir, Derek

    2007-06-01

    In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5-7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle-ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.

  14. First Evidence of Epithermal Gold Occurrences in the SE Afar Rift, Republic of Djibouti

    NASA Astrophysics Data System (ADS)

    Moussa, Nima; Fouquet, Yves; Caminiti, Antoine Marie; Le Gall, Bernard; Rolet, Joel; Bohn, Marcel; Etoubleau, Joel; Delacourt, Christophe; Jalludin, Mohamed

    2010-05-01

    The Republic of Djibouti, located at the SE part of the Afar volcanic Triangle, is characterized by intense tectonic and bimodal volcanic activity, and is emplaced over an earlier magmatic rift system, as old as 25-30 Ma. Each magmatic event is accompanied by hydrothermal activity. Few works have been so far published on hydrothermal mineralization in the Afar area. Mineralization generally occur as veins and are mainly associated with acidic volcanic intrusions along the fractures at the edges of grabens established during the last 4 Ma. Eighty samples from hydrothermal quartz ± carbonate veins and breccias were studied on 9 different sites representative of 4 main volcanic events ranging in age from early Miocene up to Present. Gold was found in excess of 200 ppb in 30% of the samples. Mineralogical analyses based on optical reflected light microscopy, X-Ray diffractometry, X-Ray fluorescence, inductively coupled plasma mass spectroscopy and electron microprobe, led us to identify two types of gold mineralization (i) native gold, electrum, hessite and sulfides (chalcopyrite, pyrite, bornite, ± sphalerite, and galena) in massive quartz breccias and banded chalcedony, (ii) gold, electrum, hematite, magnetite, trace minerals (argentite) and adularia in banded chalcedony. Another group without gold is characterized by quartz, pyrite ± goethite. Secondary minerals are characterized by goethite, native silver and native copper. Arsenic is enriched in pyrite in samples with a high gold content. The bimodal volcanism, the occurrence of adularia, the native gold and electrum in banded silica veins, are classically observed in neutral epithermal systems. The discovery of this type of mineralization in a recent-active continental rift system supplies new insights about hydrothermal processes associated with volcanic activity in a spreading context. Keywords: Republic of Djibouti, Afar Triangle, Hydrothermal, Epithermal system, Gold

  15. COOLING TOWER PLUME MODEL

    EPA Science Inventory

    A review of recently reported cooling tower plume models yields none that is universally accepted. The entrainment and drag mechanisms and the effect of moisture on the plume trajectory are phenomena which are treated differently by various investigators. In order to better under...

  16. Evidence for melt channelization in Galapagos plume-ridge interaction

    NASA Astrophysics Data System (ADS)

    Mittal, T.; Richards, M. A.

    2015-12-01

    Many present-day hot spots are located within ~ 1000 km of a mid-ocean ridge, either currently or in the geologic past, leading to frequent interaction between these two magmatic regimes. The consequent plume-ridge interactions provide a unique opportunity to test models for asthenosphere-lithosphere dynamics, with the plume acting as a tracer fluid in the problem, and excess magmatism reflecting otherwise unsampled sub-surface phenomena. Galapagos is an off-ridge hotspot with the mantle plume located ~150-250 km south of the plate boundary. Plume-ridge interaction in Galapagos is expressed by the formation of volcanic lineaments of islands and seamounts - e.g., the Wolf-Darwin lineament (WDL) - providing a direct probe of the plume-ridge interaction process, especially in regards to geochemical data. Although several models have been proposed to explain plume-ridge interaction in Galapagos, none adequately explain the observed characteristics, especially the WDL. In particular, predicted lithospheric fault orientations and melt density considerations appear at odds with observations, suggesting that lithospheric extension is not the primary process for formation of these islands. Other off-ridge hotspots interacting with nearby spreading ridges, such as Reunion and Louisville, also exhibit volcanic lineaments linking the plume and the ridge. Thus these lineament-type features are a common outcome of plume-ridge interaction that are indicative of the underlying physics. We propose that the lineaments are surface expressions of narrow sub-lithospheric melt channels focused towards the spreading ridge. These channels should form naturally due to the reactive infiltration instability in a two-phase flow of magma and solid mantle as demonstrated in two-phase flow simulations (e.g., Katz & Weatherley 2012). For Galapagos, we show that melt channels can persist thermodynamically over sufficient length-scales to link the plume and nearby ridge segments. We also show that

  17. Evolution of Northeast Atlantic Magmatic Continental Margins from an Ethiopian-Afar Perspective

    NASA Astrophysics Data System (ADS)

    England, R. W.; Cornwell, D. G.; Ramsden, A. M.

    2014-12-01

    One of the major problems interpreting the evolution of magmatic continental margins is that the structure which should record the pre-magmatic evolution of the rift and which potentially influences the character of the rifting process is partially or completely obscured by thick basalt lava flows and sills. A limited number of deep reflection seismic profiles acquired with tuned seismic sources have penetrated the basalts and provide an image of the pre-magmatic structure, otherwise the principle data are lower resolution wide-angle/refraction profiles and potential field models which have greater uncertainties associated with them. In order to sidestep the imaging constraints we have examined the Ethiopian - Afar rift system to try to understand the rifting process. The Main Ethiopian rift contains an embryonic magmatic passive margin dominated by faulting at the margins of the rift and en-echelon magmatic zones at the centre. Further north toward Afar the rift becomes in-filled with extensive lava flows fed from fissure systems in the widening rift zone. This rift system provides, along its length, a series of 'snapshots' into the possible tectonic evolution of a magmatic continental margin. Deep seismic profiles crossing the NE Atlantic margins reveal ocean dipping reflector sequences (ODRS) overlying extended crust and lower crustal sill complexes of intruded igneous rock, which extend back beneath the continental margin. The ODRS frequently occur in fault bounded rift structures along the margins. We suggest, by analogy to the observations that can be made in the Ethiopia-Afar rift that these fault bounded basins largely form at the embryonic rift stage and are then partially or completely filled with lavas fed from fissures which are now observed as the ODRS. Also in the seismic profiles we identify volcanic constructs on the ODRS which we interpret as the equivalent of the present day fissure eruptions seen in Afar. The ocean ward dip on the ODRS is

  18. Lithology and temperature: How key mantle variables control rift volcanism

    NASA Astrophysics Data System (ADS)

    Shorttle, O.; Hoggard, M.; Matthews, S.; Maclennan, J.

    2015-12-01

    Continental rifting is often associated with extensive magmatic activity, emplacing millions of cubic kilometres of basalt and triggering environmental change. The lasting geological record of this volcanic catastrophism are the large igneous provinces found at the margins of many continents and abrupt extinctions in the fossil record, most strikingly that found at the Permo-Triassic boundary. Rather than being considered purely a passive plate tectonic phenomenon, these episodes are frequently explained by the involvement of mantle plumes, upwellings of mantle rock made buoyant by their high temperatures. However, there has been debate over the relative role of the mantle's temperature and composition in generating the large volumes of magma involved in rift and intra-plate volcanism, and even when the mantle is inferred to be hot, this has been variously attributed to mantle plumes or continental insulation effects. To help resolve these uncertainties we have combined geochemical, geophysical and modelling results in a two stage approach: Firstly, we have investigated how mantle composition and temperature contribute to melting beneath Iceland, the present day manifestation of the mantle plume implicated in the 54Ma break up of the North Atlantic. By considering both the igneous crustal production on Iceland and the chemistry of its basalts we have been able to place stringent constraints on the viable temperature and lithology of the Icelandic mantle. Although a >100°C excess temperature is required to generate Iceland's thick igneous crust, geochemistry also indicates that pyroxenite comprises 10% of its source. Therefore, the dynamics of rifting on Iceland are modulated both by thermal and compositional mantle anomalies. Secondly, we have performed a global assessment of the mantle's post break-up thermal history to determine the amplitude and longevity of continental insulation in driving excess volcanism. Using seismically constrained igneous crustal

  19. Recycled crust in the Galápagos Plume source at 70 Ma: Implications for plume evolution

    NASA Astrophysics Data System (ADS)

    Trela, Jarek; Vidito, Christopher; Gazel, Esteban; Herzberg, Claude; Class, Cornelia; Whalen, William; Jicha, Brian; Bizimis, Michael; Alvarado, Guillermo E.

    2015-09-01

    Galápagos plume-related lavas in the accreted terranes of the Caribbean and along the west coast of Costa Rica and Panama provide evidence on the evolution of the Galápagos mantle plume, specifically its mantle temperature, size and composition of heterogeneities, and dynamics. Here we provide new 40Ar/39Ar ages, major and trace element data, Sr-Nd-Pb isotopic compositions, and high-precision olivine analyses for samples from the Quepos terrane (Costa Rica) to closely examine the transitional phase of the Galápagos Plume from Large Igneous Province (LIP) to ocean island basalt (OIB) forming stages. The new ages indicate that the record of Quepos volcanism began at 70 Ma and persisted for 10 Ma. Petrological evidence suggests that the maximum mantle potential temperature (Tp) of the plume changed from ∼1650° to ∼1550 °C between 90-70 Ma. This change correlates with a dominant pyroxenite component in the Galapagos source as indicated by high Ni and Fe/Mn and low Ca olivines relative to those that crystallized in normal peridotite derived melts. The decrease in Tp also correlates with an increase in high-field strength element enrichments, e.g., Nb/Nb*, of the erupted lavas. Radiogenic isotope ratios (Nd-Pb) suggest that the Quepos terrane samples have intermediate (Central Domain) radiogenic signatures. The Galápagos plume at 70 Ma represents elevated pyroxenite melt productivity relative to peridotite in a cooling lithologically heterogeneous mantle.

  20. Stealth Plumes on Io

    NASA Technical Reports Server (NTRS)

    Johnson, T. V.; Matson, Dennis L.; Blaney, Diana L.; Veeder, Glenn J.; Davies, Ashley

    1995-01-01

    We suggest that Io's eruptive activity may include a class of previously undetected SO2 geysers. The thermodynamic models for the eruptive plumes discovered by Voyager 'involve low to moderate entropy SO2 eruptions. The resulting plumes are a mixture of solid and gas which emerge from the vent and follow essentially ballistic trajectories. We show that intrusion of silicate magma into buried SO2 deposits can create the required conditions for high entropy eruptions which proceed entirely in the vapor phase. These purely gaseous plumes would have been invisible to Voyager's instruments. Hence, we call them "stealth" plumes. Such eruptions could explain the "patchy" SO2 atmosphere inferred from recent UV and micro-wave spectral observations. The magma intrusion rate required to support the required gas production for these plumes is a negligible fraction of estimated global magma intrusion rates.

  1. Model for Transition Zone Formation from Upwelling Thermo-Chemical Plumes of Intermediate Rheology

    NASA Astrophysics Data System (ADS)

    Nguyen, C. T.; Weeraratne, D. S.

    2008-12-01

    The mantle transition zone has been limited to a layer of approximately 250 km at the base of the upper mantle, identified by studies in seismology and mineral physics. However, there are many uncertainties as to the nature of formation of this mid-mantle layer, its evolution over geological time, physical properties, and its role facilitating or inhibiting whole mantle flow. Here, we conduct laboratory fluid experiments using high viscosity corn syrup fluids and liquid gallium to study mantle convection processes in the early Earth. Specifically, we consider early core formation events involving metal-silicate plumes which sink following impact events and entrain magma ocean material from the surface during descent. Preliminary studies indicate that low viscosity, buoyant material, that makes up the model magma ocean near the surface is entrained in conduits that form behind quickly descending liquid metal plumes to the base of the lower mantle. This low density material brought to the base of the model lower mantle becomes buoyant and subsequently rises back up to the top of the fluid box forming a new intermediate material that has experienced both chemical and thermal diffusion along its mantle pathway and empties at the top of the lower mantle or base of a magma ocean. Two-component fluid experiments are considered in the presence of a hot lower thermal boundary layer at Rayleigh numbers of 103 to 105 and low Reynolds number flow, and indicate that upwelling thermo-chemical plumes may form following core formation events. This new third fluid layer of intermediate rheology is considered as a model for the mantle transition zone. Shadow graph images indicate a sharp density contrast with surrounding fluids that persists for long times, consistent with seismic discontinuities observed for the Earth's transition zone. We will present quantitative estimates of material rheology, density, and flow properties for scaling with a silicate mantle, geophysical, and

  2. Laminar plume formation by high pressure CO2

    NASA Astrophysics Data System (ADS)

    Nadal, Francois; Meunier, Patrice; Pouligny, Bernard; Laurichesse, Eric

    2012-11-01

    Convection flows have often revealed the presence of plumes, especially in the earth's mantle where the Schmidt number is large. There has thus been a large number of studies on plumes created by a point source. However, there are very few results on plumes generated by an extended source. Here, we present experimental, numerical and theoretical results on the flow created by high pressure CO2 dissolved into distilled water. The thin layer of dense fluid created at the surface destabilizes through the Rayleigh-Taylor instability and leads to a laminar and parallel stationary plume. The plume width and amplitude are measured by Particle Image Velocimetry for various aspect ratios, Bond and Rayleigh numbers. They are in good agreement with the numerical result if a no-slip boundary condition is assumed at the free surface. Finally, the theory for a plume generated by a point source is adapted for an extended source, which leads to different scaling exponents (with a logarithmic dependence), in excellent agreement with the experimental and numerical results. This study thus provides a simple and accurate description of axisymmetric plumes generated by an extended source.

  3. Three-dimensional instabilities of mantle convection with multiple phase transitions

    NASA Technical Reports Server (NTRS)

    Honda, S.; Yuen, D. A.; Balachandar, S.; Reuteler, D.

    1993-01-01

    The effects of multiple phase transitions on mantle convection are investigated by numerical simulations that are based on three-dimensional models. These simulations show that cold sheets of mantle material collide at junctions, merge, and form a strong downflow that is stopped temporarily by the transition zone. The accumulated cold material gives rise to a strong gravitational instability that causes the cold mass to sink rapidly into the lower mantle. This process promotes a massive exchange between the lower and upper mantles and triggers a global instability in the adjacent plume system. This mechanism may be cyclic in nature and may be linked to the generation of superplumes.

  4. Did mantle plume magmatism help trigger the Great Oxidation Event?

    NASA Astrophysics Data System (ADS)

    Ciborowski, T. Jake. R.; Kerr, Andrew C.

    2016-03-01

    The Great Oxidation Event (GOE) represents the first sustained appearance of free oxygen in Earth's atmosphere. This fundamental event in Earth's history has been dated to approximately 2450 million years ago (Ma), that is, hundreds of millions of years after the appearance of photosynthetic cyanobacteria in the fossil record. A variety of mechanisms have been suggested to explain this time lag between the onset of photosynthesis and atmospheric oxygenation, including orogenesis, changes in the areal extent and distribution of continental shelves, the secular release of hydrogen to space, and methanogenic bacterial stress. Recently, it has been proposed that subaerial volcanism during the early Proterozoic could have provided a large pulse of sulphate to the ancient oceans, the reduction of which liberated the oxygen to drive the GOE. Here we show that the Matachewan Large Igneous Province (LIP), which is partially preserved in Scandinavia and North America, is both exactly coincident with the onset of the GOE, and of sufficient magnitude to be the source of this sulphate release. We therefore propose that the volcanism associated with the emplacement of the Matachewan LIP was a principal driver of the oxygenation of our planet.

  5. Primordial metallic melt in the deep mantle

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    PubMed

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

    2016-05-12

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

  7. 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.

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

    NASA Astrophysics Data System (ADS)

    Schmandt, B.

    2013-12-01

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

  9. Lithosphere erosion and breakup due to the interaction between extension and plume upwelling

    NASA Astrophysics Data System (ADS)

    Lavecchia, Alessio; Thieulot, Cedric; Beekman, Fred; Cloetingh, Sierd; Clark, Stuart

    2016-04-01

    We have built up 2D numerical models of coupled crust - lithospheric mantle - upper mantle systems. The reconstructed sections are subjected to external velocity fields and mantle plume impingement beneath the lithosphere, both acting simultaneously. The models are designed to simulate the interaction between plumes and lithosphere in an extensional setting with the main purpose to contribute to address the following questions: 1) Are plumes capable of weakening certain lithospheric regions? Where and when are the main effects observed? 2a) Can a plume really cause a plate break-off and drifting with no external contribution; 2b) if yes, are there any particularly favorable conditions required? In our models a novel aspect is melt generation due to plume, upper mantle and lithospheric mantle partial melting. Produced melts are capable to ascend across the reconstructed sections due to buoyancy. Furthermore, heat transport related to melt movement is taken into account and leads to a significant heating of host rocks at the melt neutral buoyancy depth. In absence of external stress or velocity fields, the effects of plume impingement beneath the lithosphere are negligible at surface. Here the main observed feature is the production of doming at various length scales, depending on the adopted rheology for the crust. At depth, the main effect is a thermo-mechanical erosion of the lithospheric mantle with production of melts and subsequent underplating of the crust. The heat flux due to plume impingement and crust underplating determines a weakening of crust and lithosphere. However, the strength drop is not followed by an appreciable deformation. When external stress or velocity fields are applied, the coupled effects with plume presence and melt production lead to great modifications of the lithospheric structure. Topography profiles are characterized by the presence of a horst and graben structure, and extensive erosion of the lithosphere always occurs. The presence

  10. Hot mantle upwelling across the 660 beneath Yellowstone

    NASA Astrophysics Data System (ADS)

    Schmandt, Brandon; Dueker, Kenneth; Humphreys, Eugene; Hansen, Steven

    2012-05-01

    P-to-s receiver functions mapped to depth through P and S body-wave tomography models image continuous 410 and 660 km discontinuities beneath the area covered by USArray prior to the year 2011. Mean depths to the 410 and 660 km discontinuities of 410 and 656 km imply a mantle transition zone that is about 4 km thicker than the global average and hence has a slightly cooler mean temperature and/or enhanced water content. Compared to the mean 660 depth beneath this ~ 2000 km wide area, the 660 beneath the Yellowstone hotspot is deflected upward by 12-18 km over an area about 200 km wide. This is the most anomalous shallowing of the 660 imaged and its horizontal extent is similar to the area where P and S tomography image low-velocity mantle extending from the top of the transition zone to about 900 km depth. Together, these results indicate a high-temperature, plume-like upwelling extending across the 660. The depth of 410 km discontinuity beneath the Yellowstone region is within 5 km of the mean depth implying that the plume is vertically heterogeneous and possibly discontinuous. Tomography indicates a similar vertically heterogeneous thermal plume. The irregular plume structure may be intrinsic to the dynamics of upwelling through the transition zone, or distortion may be caused by subduction-induced mantle flow. Topography of the 410 and 660 confirms that subducted slabs beneath the western U.S. are highly segmented, as inferred from recent tomography studies. We find no evidence of regionally pervasive velocity discontinuities between 750 and 1400 km depth. The plume's depth of origin within the lower mantle remains uncertain.

  11. Hotspot activity and plume pulses recorded by geometry of spreading axes

    NASA Astrophysics Data System (ADS)

    Abelson, Meir; Agnon, Amotz

    2001-06-01

    Anomalous plan view geometry (planform) of spreading axes is shown to be a faithful indicator of hotspot influence, possibly capable of detecting pulses of hotspot discharge. A planform anomaly (PA) occurs when the orientation of second-order ridge segments is prominently oblique to the spreading direction. PA is found in the vicinity of hotspots at shallow ridges (<1.5 km), suggesting hotspot influence. In places the PA and shallow bathymetry are accompanied by geochemical anomalies, corroborating hotspot influence. This linkage is best expressed in the western Gulf of Aden, where the extent of the PA from the Afar hotspot coincides with the extent of La/Sm and Sr isotopic anomalies. Using fracture mechanics we predict PA to reflect overpressurized melt that dominates the stresses in the crust, consistent with hotspot regime. Accordingly, the temporal variations of the planform previously inferred from magnetic anomalies around the Kolbeinsey Ridge (KR), north of Iceland, record episodes of interaction with the hotspot and major pulses of the plume. This suggestion is corroborated by temporal correlation of episodes showing PA north of Iceland with plume pulses previously inferred by the V-shaped ridges around the Reykjanes Ridge (RR), south of Iceland. In contrast to the RR, the temporal correlation suggests simultaneous incidence of the plume pulses at Iceland and KR, hundreds of kilometers to the north. A deep northward branch of the Iceland plume active during pulse-periods may explain these observations.

  12. Mantle dynamics and geodesy

    NASA Technical Reports Server (NTRS)

    Albee, Arden

    1990-01-01

    Both completed work and work that is still in progress are presented. The completed work presented includes: (1) core-mantle boundary topography; (2) absolute value for mantle viscosity; (3) code development; (4) lateral heterogeneity of subduction zone rheology; and (5) planning for the Coolfront meeting. The work presented that is still in progress includes: (1) geoid anomalies for a chemically stratified mantle; and (2) geoid anomalies with lateral variations in viscosity.

  13. Prometheus: Io's wandering plume.

    PubMed

    Kieffer, S W; Lopes-Gautier, R; McEwen, A; Smythe, W; Keszthelyi, L; Carlson, R

    2000-05-19

    Unlike any volcanic behavior ever observed on Earth, the plume from Prometheus on Io has wandered 75 to 95 kilometers west over the last 20 years since it was first discovered by Voyager and more recently observed by Galileo. Despite the source motion, the geometric and optical properties of the plume have remained constant. We propose that this can be explained by vaporization of a sulfur dioxide and/or sulfur "snowfield" over which a lava flow is moving. Eruption of a boundary-layer slurry through a rootless conduit with sonic conditions at the intake of the melted snow can account for the constancy of plume properties. PMID:10817989

  14. 4 Gy of Mantle Recycling: Evolution of Species in the Mantle Zoo

    NASA Astrophysics Data System (ADS)

    Hart, S. R.

    2008-12-01

    (2007) for Samoan lavas that show the undeniable presence of an ancient sediment component. The parentage of EM1 and HIMU are less certain. While typically ascribed to recycled sediment and ocean crust, respectively, adequate models for their derivation and evolution still invoke believable but ad hoc chemical processing during subduction. Recycled metasomatized lower oceanic mantle lithosphere is an alternative model for EM1, with perhaps a similar believability quotient. FOZO was discovered hiding in the mantle zoo post-Hofmann and White 1982, and is perhaps the least understood of the mantle critters. Numerous models and variants exist for FOZO; many involve recycled components and most require ancient processing ages. FOZO unequivocally is not 'primordial' mantle. My current world view: DMM - depleted upper mantle, perhaps containing small meter-scale mafic lithologies. FOZO - very ancient ubiquitous meso- scale (50-100 km) lower mantle association of recycled lithospheric peridotites and highly refractory mafic crust. HIMU - similar, but containing mafic crust that is less processed during subduction. EM1 - old metasomatized lower oceanic lithosphere. EM2 - peridotite with recycled continental crustal components. In any of these species, the mafic or metasomatic components need not exist as separate lithologies, but may be subsumed into a single peridotitic lithology. HIMU, EM1 and EM2 exist in their most pure-bred forms in the lower mantle piles demarcated seismically as the Pacific and African Superplumes. A mantle regime change occurred ~ 1.8 Gy ago, leading to increased plume access to these piles.

  15. ON THE VIGOR OF MANTLE CONVECTION IN SUPER-EARTHS

    SciTech Connect

    Miyagoshi, Takehiro; Tachinami, Chihiro; Kameyama, Masanori; Ogawa, Masaki E-mail: ctchnm.geo@gmail.com E-mail: cmaogawa@mail.ecc.u-tokyo.ac.jp

    2014-01-01

    Numerical models are presented to clarify how adiabatic compression affects thermal convection in the mantle of super-Earths ten times the Earth's mass. The viscosity strongly depends on temperature, and the Rayleigh number is much higher than that of the Earth's mantle. The strong effect of adiabatic compression reduces the activity of mantle convection; hot plumes ascending from the bottom of the mantle lose their thermal buoyancy in the middle of the mantle owing to adiabatic decompression, and do not reach the surface. A thick lithosphere, as thick as 0.1 times the depth of the mantle, develops along the surface boundary, and the efficiency of convective heat transport measured by the Nusselt number is reduced by a factor of about four compared with the Nusselt number for thermal convection of incompressible fluid. The strong effect of adiabatic decompression is likely to inhibit hot spot volcanism on the surface and is also likely to affect the thermal history of the mantle, and hence, the generation of magnetic field in super-Earths.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  17. CHLORINATED SOLVENT PLUME CONTROL

    EPA Science Inventory

    This lecture will cover recent success in controlling and assessing the treatment of shallow ground water plumes of chlorinated solvents, other halogenated organic compounds, and methyl tert-butyl ether (MTBE).

  18. Methane Plumes on Mars

    NASA Video Gallery

    Spectrometer instruments attached to several telescopes detect plumes of methane emitted from Mars during its summer and spring seasons. High levels of methane are indicated by warmer colors. The m...

  19. 3-D numerical modeling of plume-induced subduction initiation

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Gerya, taras; Sobolev, Stephan

    2016-04-01

    Investigation of mechanisms involved in formation of a new subduction zone can help us to better understand plate tectonics. Despite numerous previous studies, it is still unclear how and where an old oceanic plate starts to subduct beneath the other plate. One of the proposed scenarios for nucleation of subduction is plume-induced subduction initiation, which was investigated in detail, using 2-D models, by Ueda et al. (2008). Recently. Gerya et al. (2015), using 3D numerical models, proposed that plume-lithosphere interaction in the Archean led to the subduction initiation and onset of plate tectonic. In this study, we aim to pursue work of Ueda et al. (2008) by incorporation of 3-D thermo-mechanical models to investigate conditions leading to oceanic subduction initiation as a result of thermal-chemical mantle plume-lithosphere interaction in the modern earth. Results of our experiments show four different deformation regimes in response to plume-lithosphere interaction, that are a) self-sustaining subduction initiation where subduction becomes self-sustained, b) freezing subduction initiation where subduction stops at shallow depths, c) slab break-off where subducting circular slab breaks off soon after formation and d) plume underplating where plume does not pass through the lithosphere but spreads beneath it (failed subduction initiation). These different regimes depend on several parameters such as plume's size, composition and temperature, lithospheric brittle/plastic strength, age of the oceanic lithosphere and presence/absence of lithospheric heterogeneities. Results show that subduction initiates and becomes self-sustained when lithosphere is older than 10 Myr and non-dimensional ratio of the plume buoyancy force and lithospheric strength above the plume is higher than 2.

  20. Sulfur plumes off Namibia

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Sulfur plumes rising up from the bottom of the ocean floor produce colorful swirls in the waters off the coast of Namibia in southern Africa. The plumes come from the breakdown of marine plant matter by anaerobic bacteria that do not need oxygen to live. This image was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite on April 24, 2002 Credit: Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC

  1. The Earth's Mantle.

    ERIC Educational Resources Information Center

    McKenzie, D. P.

    1983-01-01

    The nature and dynamics of the earth's mantle is discussed. Research indicates that the silicate mantle is heated by the decay of radioactive isotopes and that the heat energizes massive convention currents in the upper 700 kilometers of the ductile rock. These currents and their consequences are considered. (JN)

  2. Electrification of volcanic plumes

    NASA Astrophysics Data System (ADS)

    Mather, T. A.; Harrison, R. G.

    2006-07-01

    Volcanic lightning, perhaps the most spectacular consequence of the electrification of volcanic plumes, has been implicated in the origin of life on Earth, and may also exist in other planetary atmospheres. Recent years have seen volcanic lightning detection used as part of a portfolio of developing techniques to monitor volcanic eruptions. Remote sensing measurement techniques have been used to monitor volcanic lightning, but surface observations of the atmospheric electric Potential Gradient (PG) and the charge carried on volcanic ash also show that many volcanic plumes, whilst not sufficiently electrified to produce lightning, have detectable electrification exceeding that of their surrounding environment. Electrification has only been observed associated with ash-rich explosive plumes, but there is little evidence that the composition of the ash is critical to its occurrence. Different conceptual theories for charge generation and separation in volcanic plumes have been developed to explain the disparate observations obtained, but the ash fragmentation mechanism appears to be a key parameter. It is unclear which mechanisms or combinations of electrification mechanisms dominate in different circumstances. Electrostatic forces play an important role in modulating the dry fall-out of ash from a volcanic plume. Beyond the local electrification of plumes, the higher stratospheric particle concentrations following a large explosive eruption may affect the global atmospheric electrical circuit. It is possible that this might present another, if minor, way by which large volcanic eruptions affect global climate. The direct hazard of volcanic lightning to communities is generally low compared to other aspects of volcanic activity.

  3. Variability of Water in the Convecting Mantle

    NASA Astrophysics Data System (ADS)

    Hauri, E. H.; Saal, A. E.

    2014-12-01

    Estimation of the abundance of water in mantle sources first requires careful consideration of the shallow-level effects of degassing and contamination by seawater-derived components. Use of submarine glasses erupted at >500m water depth, and critically-evaluated use of melt inclusion volatile contents, can be used to identify and eliminate degassing as an important mitigating factor; widespread evidence for seawater-derived components are evident in halogen contents, but these effects do not typically correlate with water though there may be subtle effects in long-lived magmatic systems at mid-ocean ridges. H2O/Ce ratios show large differences between mid-ocean ridges, hotspots, back-arc basins and arc-front volcanoes that testify to the large input of water at subduction zones; however, at arcs most of the Ce (like most of the water) is derived from the subducting slab, and at hotspots isotopic tracers of recycled components indicate the presence of materials that can, in sufficient quantity, dominate the Ce budget of mantle plumes. Thus H2O/Ce ratios, while useful, are problematic when the goal is to determine the absolute abundances of water in mantle sources because the abundance of Ce cannot normally be assumed with confidence, except perhaps at mid-ocean ridges. A more complete understanding of the abundance of water in mantle sources can be obtained when data for radiogenic isotopes are used as tracers of mantle composition, and when major and trace element data illuminate the process of mantle melting. In areas far from hotspots, normal mid-ocean ridges reveal a remarkably narrow range of H2O/Ce ratios yet display large-scale regional differences between ocean basins [1]. Isotopically enriched signatures at hotspots suggest low absolute H2O abundances [2], yet there is so much water delivered to the sources of arc volcanoes that even >95% dehydration of slabs results in delivery of water to the deep mantle in excess of that observed in MORB sources. The

  4. Rogue Mantle Helium and Neon

    NASA Astrophysics Data System (ADS)

    Albarede, F.

    2007-12-01

    The canonical view of He isotope geochemistry holds that high 3He/4He ratios in basalts fingerprints undegassed mantle sources. Hawaiian basalts with unradiogenic He with 3He/4He up to 30 RA are therefore seen as originating from parts of the mantle that is still primordial, at least much more so than MORB mantle (3He/4He ~ 8 RA). This view was strongly reinforced by the discovery of solar and even planetary Ne components in oceanic basalts and gas wells. The canonical view, however, conflicts with multiple observations on ocean islands, notably Hawaiian basalts: the correlation of {187}Os/{186}Os with δ 18O combined with the presence of unusually radiogenic Hf isotope compositions for a given Nd isotope composition and the correlation between Hf and Pb isotopes are all features strongly reminiscent of ancient subducted oceanic crust and pelagic sediments in the source of the Hawaiian plume. These conflicting observations beg the question of how Hawaiian basalts, which carry the embodiment of a primordial gas signature, at the same time can provide such strong evidence of surface material recycling. I here suggest and alternative model that uses the marble cake paradigm and Shuster et al.'s data on olivine. A solution to this conundrum lies in an analogy with oil genesis: 3He and Ne do not reside in the low-melting point peridotites in which they were originally hosted but rather migrated since early in Earth history into refractory 'reservoir' rocks. Since there can be no free gas phase percolating at pressures in excess of olivine carbonation at ~3 GPa, He must be largely redistributed by diffusion. The time scale of diffusion is the defining parameter: although over billions of years 3He diffuses across large distances, melting events are too short to efficiently strip residual refractory rocks from their high-3He/4He component. Assuming that melts begin forming over the uppermost 100 km with an upwelling rate of 10 m y-1 in plume conduits and 10 cm y-1 under

  5. Pb - Isotopes and Pulses of the Deccan Plume

    NASA Astrophysics Data System (ADS)

    Basu, A. R.; Yannopoulos, A. S.

    2015-12-01

    Mantle plumes are generally implicated for flood basalt generation in both continental and oceanic environments by impact of large plume heads beneath or within the lithosphere. The Deccan and Siberian flood basalt eruptions, synchronous with the Cretaceous-Paleogene and end-Permian extinctions, respectively, continue to fascinate geoscientists in search for the "kill-mechanisms" by impacts, volcanisms or both. Recently, Richards et al. (2015) proposed that bulk of the Deccan eruption was triggered by the Chicxulub impact. We showed (Basu et al., 1993) that early (68.5 Ma) and late (65 Ma) alkalic pulses of the Deccan were before and after the impact event at 66 Ma. Here, we focus on an extensive volcano-stratigraphic study of Pb isotopic systematics of 69 basaltic samples from 3 subgroups and 12 formations of the Deccan, each sampled from bottom to top along the stratigraphic section, covering the 3km thick 12 Deccan formations. Pb is sensitive to crustal contamination of mantle plume-derived magmas as both the upper and lower mantle are low in Pb (0.02 - 0.15 ppm) compared to ~ 4 ppm in continental crust. The lower Deccan formations of Kalsubai and Lonavala have initial 206Pb/204Pb with a widely varying range (16.543 - 22.823) indicating continental crustal contamination. In contrast, the upper formations of the Wai subgroup show a narrow range of 16.883 to 18.956, reflecting the plume signature. In addition, the 206Pb/204Pb and 207Pb/204Pb data of the Kalsubai subgroup lavas give an isochron age of 2603±140 Ma (single-stage, µ = 8). The Wai subgroup shows a narrow and restricted Pb isotopic range plotting closer to the Geochron. We interpret these data to infer that the basement rocks of the Deccan, the Archean Indian craton, were assimilated by the upwelling melt, ultimately clearing the conduit passages for the lavas sourced from direct melting of the plume head.

  6. The size of plume heterogeneities constrained by Marquesas isotopic stripes

    NASA Astrophysics Data System (ADS)

    Chauvel, Catherine; Maury, René C.; Blais, Sylvain; Lewin, Eric; Guillou, Hervé; Guille, GéRard; Rossi, Philippe; Gutscher, Marc-André

    2012-07-01

    The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher 87Sr/86Sr and lower 206Pb/204Pb ratios than the southern Fatu Hiva group at any given 143Nd/144Nd value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or "plumelets" that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other "weak" Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Réunion Island, and the higher degrees of melting blur the isotopic variability of the plume source.

  7. Melt migration through Io's convecting mantle

    NASA Astrophysics Data System (ADS)

    Elder, C. M.; Showman, A. P.

    2013-12-01

    The extensive volcanism occurring on the surface of Io suggests that its interior must contain at least some partial melt. Unlike Earth, Io cannot lose its internal heat through convection alone [1]. Instead, melt moving through the solid mantle helps remove heat from Io's interior by carrying its latent heat towards the surface as it buoyantly ascends through the mantle. We investigate this process by considering melt migration in a column of rock rising through the mantle between downwelling plumes. Convective scaling laws provide the upwelling velocity and the temperature of the rising mantle. Properties of melt migration in this rising mantle are calculated using porous flow equations and an equation for the conservation of energy which includes latent heat consumption, heat advection and heat conduction [2]. This combination of convective scaling laws and porous flow laws allows us to self-consistently determine the radial melt fraction profile in Io's interior, the average melt fraction in Io's interior and the heat flux due to advection of melt. The average melt fraction can be compared to the melt fraction constraints calculated by [3] from Galileo magnetometer measurements. The surface heat flux calculations can be compared to the value of Io's observed surface heat flux which ranges with observation from 1.5-4 W m-2 [4]. [1] Moore W. B. (2003) J. Geophys. Res., 108, E8, 15-1. [2] Hewitt I. J. and Fowler A. C. (2008) Proc. R. Soc. A., 464, 2467-2491. [3] Khurana K. K. et al. (2011) Science, 332, 1186-1189. [4] Moore, W. B. et al. (2007) In: Io After Galileo, Springer-Praxis, 89-108.

  8. The heavy noble gas composition of the depleted MORB mantle (DMM) and its implications for the preservation of heterogeneities in the mantle

    NASA Astrophysics Data System (ADS)

    Tucker, Jonathan M.; Mukhopadhyay, Sujoy; Schilling, Jean-Guy

    2012-11-01

    To characterize the heavy noble gas composition of MORBs we present new He, Ne, Ar, and Xe abundances and isotopic compositions from the equatorial Mid-Atlantic Ridge. Both depleted MORBs nominally devoid of plume influence and more enriched MORBs thought to represent the influence of a HIMU mantle plume are present in close geographical proximity in this region. Ne-Ar-Xe isotopic compositions in individual step-crushes are correlated, which, along with significant radiogenic excesses, allows correction for shallow-level air contamination. Based on the relationship between the noble gases and the lithophile isotopes (Sr, Nd and Pb), the depleted MORB mantle has a 21Ne/22Ne between 0.0617 and 0.0646, 40Ar/36Ar ratio of 41,500±9000 and 129Xe/130Xe ratio of 7.77±0.06. On the other hand, the HIMU-type MORBs are characterized by far less radiogenic Ne, Ar and Xe isotopic compositions with mantle source 21Ne/22Ne between 0.0544 and 0.0610, and 40Ar/36Ar and 129Xe/130Xe ratios of 18,100±600 and 7.21±0.06, respectively. The observation of less nucleogenic 21Ne/22Ne in HIMU-type MORBs is similar to observations from HIMU ocean islands and requires the HIMU plume to be comprised of both recycled and primitive material. Within the depleted MORBs we observe He and Ne to be negatively correlated. The observation suggests that along the equatorial Atlantic the most depleted MORBs are related to normal MORBs through the addition of a small proportion of a HIMU plume component. Our new Xe isotopic measurements demonstrate distinct 129Xe/136Xe ratios in the mantle sources of depleted MORBs, HIMU-type MORBs and the Iceland plume. While substantial injection of atmospheric Xe into these mantle sources is implied, the differences in Xe isotopic composition cannot result solely from recycling of air. Rather, they require that mantle plumes sample a reservoir less degassed than the depleted MORB mantle. This conclusion is consistent with a higher proportion of Pu- to U

  9. Seismic anisotropy and mantle flow beneath Africa and Arabia

    NASA Astrophysics Data System (ADS)

    Elsheikh, Ahmed Abdalla

    In spite of numerous studies, the mechanisms for the rifting, uplifting, and volcanism on the African plate remain enigmatic. The most popular hypotheses proposed for explaining these tectonic phenomena involve edge-driven small-scale mantle convection and the thermal or dynamic effects of one or more mantle plumes. In this study we use continental scale shear-wave splitting (SWS) measurements to provide additional constraints on the various models of rifting, uplifting, and volcanism of the Cameroon Volcanic Line (CVL) and the Arabian plate. The splitting of P-to-S converted phases at the core-mantle boundary on the receiver side (XKS including PKS, SKKS, and SKS) is one of the most effective approaches to constrain convective mantle flow patterns. A robust procedure involving automatic and manual batch processing to reliably assess and objectively rank shear-wave splitting parameters were used. The resulting 1532 pairs of splitting parameters show a NNE dominated fast direction. Spatial distribution of the splitting parameters in the CVL and Arabia is not consistent with the edge-driven small-scale mantle convection hypothesis, the mantle plume hypothesis, fossil fabrics formed by past tectonic events, or the fabric-forming process due to the absolute plate motion relative to the deep mantle. The research suggests that the progressive thinning of the lithosphere through basal erosion by the flow leads to decompression melting is responsible for the formation of the CVL, and olivine lattice preferred orientation in the upper asthenosphere associated with the northward motion of the African plate since 150 Ma, most likely causes the observed anisotropy across the Red Sea.

  10. Geochemical Diversity of the Mantle: 50 Years of Acronyms

    NASA Astrophysics Data System (ADS)

    Hart, S. R.

    2014-12-01

    50 years ago, Gast, Tilton and Hedge demonstrated that the oceanic mantle is isotopically heterogeneous. 28 years ago, Zindler and Hart formalized the concept of geochemical mantle components, with an attendant, to some, odious, acronym soup. Work on a marriage of mantle geochemistry and dynamics continues unabated. We know unequivocally that the mantle is chemically heterogeneous; we do not know the scale lengths of these heterogeneities. We know unequivocally that these heterogeneities have persisted for eons (Gy); we do not know where they were formed or where they are stored. Through the kind auspices of the Plume Model, we plausibly have access to the whole mantle. The most accessible and well understood mantle reservoir is the upper depleted MORB mantle (DMM). Classically, this mantle was depleted by extraction of oceanic and continental crust from a "chondritic" bulk silicate Earth. In this post-Boyet and Carlson world, the complementary enriched reservoir may instead be hidden in the deepest mantle. In this case, DMM will become an endangered acronym. Hofmann and White (1982) argued that radiogenic Pb mantle (HIMU) is re-cycled ocean crust, and this is a comfortably viable model. It does require some ad hoc chemical manipulations during subduction. Given 2 Gy of aggregate mantle strains, the mafic component in HIMU may be of small length scale (< 50 m), possibly subsumed into the dominant peridotitic lithology. This mantle species is globally widespread. Enriched mantles (EM1 and EM2) almost certainly reflect recycling of enriched continental material. This was splendidly verified by Jackson et al (2007), with 87Sr/86Sr in Samoan EM2 lavas up to 0.721. The lithology and length scale of EM1 and EM2 is unconstrained. EM1 is globally present; EM2 is confined to the SW Pacific hotspots. FOZO is a work in progress; many would like to see it become extinct! The trace element signatures of HIMU and FOZO mantles have been constrained using melting models; in both

  11. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii.

    PubMed

    Cao, Q; van der Hilst, R D; de Hoop, M V; Shim, S-H

    2011-05-27

    The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. To investigate pertinent thermal anomalies, we imaged with inverse scattering of SS waves the depths to seismic discontinuities below the Central Pacific, which we explain with olivine and garnet transitions in a pyrolitic mantle. The presence of an 800- to 2000-kilometer-wide thermal anomaly (ΔT(max) ~300 to 400 kelvin) deep in the transition zone west of Hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward Hawaii and, perhaps, other islands. This implies that geochemical trends in Hawaiian lavas cannot constrain lower mantle domains directly. PMID:21617072

  12. Seismic Structure and Origin of the Hainan Plume

    NASA Astrophysics Data System (ADS)

    Huang, J.

    2012-12-01

    tomography and receiver function, we consider that the Hainan plume exist in the northeast of Hainan island and may originate from lower mantle. Our result provided new seismological evidences for the scope and origin of Hainan mantle plume.

  13. Electrification of volcanic plumes

    NASA Astrophysics Data System (ADS)

    Mather, T. A.; Harrison, R. G.

    We present a review of our current understanding of the electrification of volcanic plumes on Earth and discuss the possible implications both in terms of the volcanic monitoring, early Earth evolution and planetary exploration. Volcanic lightning is perhaps the most spectacular consequence of the electrification of volcanic plumes. Recent years have seen volcanic lightning detection used as part of a portfolio of developing techniques to monitor volcanic eruptions. Remote sensing measurement techniques have been used to monitor volcanic lightning, but surface observations of the atmospheric electric Potential Gradient (PG) and the charge carried on volcanic ash also show that many volcanic plumes, whilst not sufficiently electrified to produce lightning, have detectable electrification exceeding that of their surrounding environment. Electrification has only been observed associated with ash-rich explosive plumes, but there is little evidence that the composition of the ash is critical to its occurrence. Different conceptual theories for charge generation and separation in volcanic plumes have been developed to explain the disparate observations obtained, but the ash fragmentation mechanism appears to be a key parameter. It is unclear which mechanisms or combinations of electrification mechanisms dominate in different circumstances. Electrostatic forces play an important role in modulating the dry fall-out of ash from a volcanic plume. Beyond the local electrification of plumes, the higher stratospheric particle concentrations following a large explosive eruption may affect the global atmospheric electrical circuit. It is possible that this might present another, if minor, way by which large volcanic eruptions affect global climate. Volcanic lightning has been implicated in a number of ways in the origin of life on Earth, and may also exist in other planetary atmospheres where measurements of its occurrence might give clues about the nature of volcanism on other

  14. Upper-mantle seismic structure in a region of incipient continental breakup: northern Ethiopian rift

    NASA Astrophysics Data System (ADS)

    Bastow, Ian D.; Stuart, Graham W.; Kendall, J.-Michael; Ebinger, Cynthia J.

    2005-08-01

    The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction, and marks the transition from continental rifting in the East African rift to incipient oceanic spreading in Afar. We determine the P- and S-wave velocity structure beneath the northern Ethiopian rift using independent tomographic inversion of P- and S-wave relative arrival-time residuals from teleseismic earthquakes recorded by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) passive experiment using the regularised non-linear least-squares inversion method of VanDecar. Our 79 broad-band instruments covered an area 250 × 350 km centred on the Boset magmatic segment ~70 km SE of Addis Ababa in the centre of the northern Ethiopian rift. The study area encompasses several rift segments showing increasing degrees of extension and magmatic intrusion moving from south to north into the Afar depression. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the southeastern Somalian Plate faster (~0.65 s for the P waves; ~2 s for the S waves) than the northwestern Nubian Plate. Our tomographic inversions image a 75 km wide tabular low-velocity zone (δVP~-1.5 per cent, δVS~-4 per cent) beneath the less-evolved southern part of the rift in the uppermost 200-250 km of the mantle. At depths of >100 km, north of 8.5°N, this low-velocity anomaly broadens laterally and appears to be connected to deeper low-velocity structures under the Afar depression. An off-rift low-velocity structure extending perpendicular to the rift axis correlates with the eastern limit of the E-W trending reactivated Precambrian Ambo-Guder fault zone that is delineated by Quaternary eruptive centres. Along axis, the low-velocity upwelling beneath the rift is segmented, with low-velocity material in the uppermost 100 km often offset to the side of the rift with the highest rift flank topography. Our observations from this magmatic

  15. Mode of rifting in magmatic-rich setting: Tectono-magmatic evolution of the Central Afar rift system

    NASA Astrophysics Data System (ADS)

    Stab, Martin; Bellahsen, Nicolas; Pik, Raphaël; Leroy, Sylvie; Ayalew, Dereje

    2014-05-01

    Observation of deep structures related to break-up processes at volcanic passive margins (VPM) is often a troublesome exercise: thick pre- to syn-breakup seaward-dipping reflectors (SDR) usually mask the continent-ocean boundary and hide the syn-rift tectonic structures that accommodate crustal stretching and thinning. Some of the current challenges are about clarifying 1) if tectonic stretching fits the observed thinning and 2) what is the effect of continuous magma supply and re-thickening of the crust during extension from a rheological point of view? The Afar region in Ethiopia is an ideal natural laboratory to address those questions, as it is a highly magmatic rift that is probably close enough to breakup to present some characteristics of VPM. Moreover, the structures related to rifting since Oligocene are out-cropping, onshore and well preserved. In this contribution, we present new structural field data and lavas (U-Th/He) datings along a cross-section from the Ethiopian Plateau, through the marginal graben down to the Manda-Hararo active rift axis. We mapped continent-ward normal fault array affecting highly tilted trapp series unconformably overlain by tilted Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. It is itself overlain by flat lying Pliocene series, including the Stratoid. Balanced cross-sections of those areas allow us to constrain a surface stretching factor of about 2.1-2.9. Those findings have the following implications: - High beta factor constrained from field observations is at odd with thinning factor of ~1.3 predicted by seismic and gravimetric studies. We propose that the continental crust in Central Afar has been re-thickened by the emplacement of underplated magma and SDR. - The deformation in Central Afar appears to be largely distributed through space and time. It has been accommodated in a 200-300 km wide strip being a diffuse incipient

  16. Lower mantle thermal structure deduced from seismic tomography, mineral physics and numerical modelling

    NASA Technical Reports Server (NTRS)

    Cadek, O.; Yuen, D. A.; Steinbach, V.; Chopelas, A.; Matyska, C.

    1994-01-01

    The long-wavelength thermal anomalies in the lower mantle have been mapped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high-pressure mineral physics experiments. These parameters are the depth variations of thermal expansivity and of the proportionality factor between changes in density and seismic velocity. The giant plume-like structures in the lower mantle under the Pacific Ocean and Africa have outer fringes with thermal anomalies around 300-400 K, but very high temperatures are found in the center of the plumes near the base of the core-mantle boundary. These extreme values can exceed +1500 K and may reflect large hot thermal anomalies in the lower mantle, which are supported by recent measurements of high melting temperatures of perovskite and iron. Extremely cold anomalies, around -1500 K, are found for anomalies in the deep mantle around the Pacific rim and under South America. Numerical simulations show that large negative thermal anomalies in the mid-lower mantle have modest magnitudes of around -500 K. correlation pattern exists between the present-day locations of cold masses in the lower mantle and the sites of past subduction since the Cretaceous. Results from correlation analysis show that the slab mass-flux in the lower mantle did not conform to a steady-state nature but exhibited time-dependent behavior.

  17. Three-dimensional mantle dynamics with an endothermic phase transition

    NASA Technical Reports Server (NTRS)

    Honda, S.; Balachandar, S.; Yuen, D. A.; Reuteler, D.

    1993-01-01

    3D convection for the spinel to perovskite phase change has been simulated numerically. Results for Rayleigh (Ra) numbers of 0(10 exp 6) show intermittent layering with a strong robust plume rising through the phase boundary. Many descending instabilities are deflected but merging cold sheets come together at a junction. A pool of cold material accumulates underneath in the phase-transition zone. A strong gravitational instability results, which precipitates a rapid and massive discharge of upper-mantle material.

  18. A Brilliant Plume

    NASA Technical Reports Server (NTRS)

    2007-01-01

    The Long Range Reconnaissance Imager (LORRI) on New Horizons captured another dramatic picture of Jupiter's moon Io and its volcanic plumes, 19 hours after the spacecraft's closest approach to Jupiter on Feb. 28, 2007. LORRI took this 75 millisecond exposure at 0035 Universal Time on March 1, 2007, when Io was 2.3 million kilometers (1.4 million miles) from the spacecraft.

    Io's dayside is deliberately overexposed to bring out faint details in the plumes and on the moon's night side. The continuing eruption of the volcano Tvashtar, at the 1 o'clock position, produces an enormous plume roughly 330 kilometers (200 miles) high, which is illuminated both by sunlight and 'Jupiter light.'

    The shadow of Io, cast by the Sun, slices across the plume. The plume is quite asymmetrical and has a complicated wispy texture, for reasons that are still mysterious. At the heart of the eruption incandescent lava, seen here as a brilliant point of light, is reminding scientists of the fire fountains spotted by the Galileo Jupiter orbiter at Tvashtar in 1999.

    The sunlit plume faintly illuminates the surface underneath. 'New Horizons and Io continue to astonish us with these unprecedented views of the solar system's most geologically active body' says John Spencer, deputy leader of the New Horizons Jupiter Encounter Science Team and an Io expert from Southwest Research Institute.

    Because this image shows the side of Io that faces away from Jupiter, the large planet does not illuminate the moon's night side except for an extremely thin crescent outlining the edge of the disk at lower right. Another plume, likely from the volcano Masubi, is illuminated by Jupiter just above this lower right edge. A third and much fainter plume, barely visible at the 2 o'clock position, could be the first plume seen from the volcano Zal Patera.

    As in other New Horizons images of Io, mountains catch the setting Sun just beyond the terminator (the line dividing day and night

  19. Late Miocene hominin teeth from the Gona Paleoanthropological Research Project area, Afar, Ethiopia.

    PubMed

    Simpson, Scott W; Kleinsasser, Lynnette; Quade, Jay; Levin, Naomi E; McIntosh, William C; Dunbar, Nelia; Semaw, Sileshi; Rogers, Michael J

    2015-04-01

    Since 2000, significant collections of Latest Miocene hominin fossils have been recovered from Chad, Kenya, and Ethiopia. These fossils have provided a better understanding of earliest hominin biology and context. Here, we describe five hominin teeth from two periods (ca. 5.4 Million-years-ago and ca. 6.3 Ma) that were recovered from the Adu-Asa Formation in the Gona Paleoanthropological Research Project area in the Afar, Ethiopia that we assign to either Hominina, gen. et sp. indet. or Ardipithecus kadabba. These specimens are compared with extant African ape and other Latest Miocene and Early Pliocene hominin teeth. The derived morphology of the large, non-sectorial maxillary canine and mandibular third premolar links them with later hominins and they are phenetically distinguishable and thus phyletically distinct from extant apes. PMID:25795338

  20. Caring from Afar: Asian H1B Migrant Workers and Aging Parents.

    PubMed

    Lee, Yeon-Shim; Chaudhuri, Anoshua; Yoo, Grace J

    2015-09-01

    With the growth in engineering/technology industries, the United States has seen an increase in the arrival of highly skilled temporary migrant workers on H1B visas from various Asian countries. Limited research exists on how these groups maintain family ties from afar including caring for aging parents. This study explores the experiences and challenges that Asian H1B workers face when providing care from a distance. A total of 21 Chinese/Taiwanese, Korean, and Indian H1B workers participated in in-depth qualitative interviews. Key findings indicate that despite distance, caring relationships still continue through regular communications, financial remittances, and return visits, at the same time creating emotional, psychological, and financial challenges for the workers. Findings highlight the need for further research in understanding how the decline of aging parent's health impacts the migrants' adjustment and health in the United States. PMID:26267591

  1. Ground deformation near Gada ‘Ale Volcano, Afar, observed by radar interferometry

    NASA Astrophysics Data System (ADS)

    Amelung, Falk; Oppenheimer, Clive; Segall, P.; Zebker, H.

    2000-10-01

    Radar interferometric measurements of ground-surface displacement using ERS data show a change in radar range, corresponding to up to 12 cm of subsidence near Gada ‘Ale volcano in northern Afar, Ethiopia, that occurred between June 1993 and May 1996. This is the area of lowest topography within the Danakil Depression (-126 m). Geodetic inverse modeling and geological evidence suggest a volcanic origin of the observed deformation; it was probably caused by a combined process of magma withdrawal from a larger reservoir and normal faulting. There is no evidence of subaerial eruption. This is the only identifiable deformation event during June 1993-October 1997 in the 80 km long Erta ‘Ale volcanic range, indicating surprising inactivity elsewhere in the range.

  2. 40 Million Years of the Iceland Plume

    NASA Astrophysics Data System (ADS)

    Parnell-Turner, R. E.; White, N.; Henstock, T.; Maclennan, J.; Murton, B. J.; Jones, S. M.

    2011-12-01

    The V-shaped ridges, straddling the mid oceanic ridges to the North and South of Iceland, provide us with a linear record of transient mantle convective circulation. Surprisingly, we know little about the structure of these ridges: prior to this study, the most recent regional seismic reflection profiles were acquired in the 1960s. During the Summer of 2010, we acquired over 3,000 km of seismic reflection data across the oceanic basin South of Iceland. The cornerstones of this programme are two 1000 km flowlines, which traverse the basin from Greenland to the European margin. The geometry of young V-shaped ridges near to the oceanic spreading center has been imaged in fine detail; older ridges, otherwise obscured in gravity datasets by sediment cover, have been resolved for the first time. We have mapped the sediment-basement interface, transformed each profile onto an astronomical time scale, and removed the effects of long wavelength plate cooling. The resulting chronology of Icelandic plume activity provides an important temporal frame of reference for plume flux over the past 40 million years. The profiles also cross major contourite drift deposits, notably the Gardar, Bjorn and Eirik drifts. Fine-scale sedimentary features imaged here demonstrate distinct episodes of drift construction; by making simple assumptions about sedimentation rates, we can show that periods of drift formation correspond to periods of enhanced deep water circulation which is in turn moderated by plume activity. From a regional point of view, this transient behaviour manifests itself in several important ways. Within sedimentary basins fringing the North Atlantic, short lived regional uplift events periodically interrupt thermal subsidence from Eocene times to the present day. From a paleoceanographic perspective, there is good correlation between V-shaped ridge activity and changes in overflow of the ancient precursor to North Atlantic Deep Water. This complete history of the Iceland

  3. The Mantle Seismic Heterogeneities Inferred by USArray Data

    NASA Astrophysics Data System (ADS)

    Ko, J. Y. T.; Zhan, Z.; Hung, S. H.; Li, D.; Helmberger, D. V.

    2015-12-01

    The detailed images of mantle seismic heterogeneities is establishing the link between modern mantle dynamics and past surface geological evolutions. The recent deployment of the USArray network of seismometers rolling from the west coast to the east coast of United States during 2004 to 2015 afford an extraordinary data set to investigate such mantle seismic heterogeneities. Here we first explored the D" structure beneath Caribbean region and found an east-to-west asymmetrical undulation of the D" discontinuity with a V-shaped depression of ~80-150 km over a lateral distance of 600 km, coinciding with a similar trend of shear wave velocity showing the most profound reduction of ~5% at the bottom of the thinnest D" layer. The strong correlation between the D" topography and velocity variations indicates lateral fluctuation in the D" temperature modulated by the reheated slab material has perturbed the phase transition boundary significantly and may reflect a transitional period of the proposed mega-plume scenario. Secondly, these emerging data not only shed light on the lowermost mantle structures but provide new constraints on the mid and upper mantle seismic velocity heterogeneities beneath the United States. We found that frequency-dependent traveltime residuals and amplitudes of S waves from South America events display considerable scatter patterns recorded by USArray stations which can be attributed to upper mantle heterogeneities beneath the U.S. and mid or lower mantle seismic anomalies along the raypaths. The analysis of waveform complexity is utilized in this work and gives complementary constraints on the location and geometry of these mantle heterogeneities such as possible slab remnants below Central and Eastern United States. We further exploited the newly developed 2D finite-difference method with various mantle heterogeneity models to better understand the possible geophysical features producing these anomalies.

  4. Plumes and Earth's Dynamic History : from Core to Biosphere

    NASA Astrophysics Data System (ADS)

    Courtillot, V. E.

    2002-12-01

    The last half century has been dominated by the general acceptance of plate tectonics. Although the plume concept emerged early in this story, its role has remained ambiguous. Because plumes are singularities, both in space and time, they tend to lie dangerously close to catastrophism, as opposed to the calm uniformitarian view of plate tectonics. Yet, it has become apparent that singular events and transient phenomena are of great importance, even if by definition they cover only a small fraction of geological time, in diverse observational and theoretical fields such as 1) magnetic reversals and the geodynamo, 2) tomography and mantle convection, 3) continental rifting and collision, and 4) evolution of the fluid envelopes (atmospheric and oceanic "climate"; evolution of species in the biosphere). I will emphasize recent work on different types of plumes and on the correlation between flood basalts and mass extinctions. The origin of mantle plumes remains a controversial topic. We suggest that three types of plumes exist, which originate at the three main discontinuities in the Earth's mantle (base of lithosphere, transition zone and core-mantle boundary). Most of the hotspots are short lived (~ 10Ma) and seem to come from the transition zone or above. Important concentrations occur above the Pacific and African superswells. Less than 10 hotspots have been long lived (~ 100Ma) and may have a very deep origin. In the last 50 Ma, these deep-seated plumes in the Pacific and Indo-Atlantic hemispheres have moved slowly, but motion was much faster prior to that. This change correlates with major episodes of true polar wander. The deeper ("primary") plumes are thought to trace global shifts in quadrupolar convection in the lower mantle. These are the plumes that were born as major flood basalts or oceanic plateaus (designated as large igneous provinces or LIPs). Most have an original volume on the order or in excess of 2.5 Mkm3. In most provinces, volcanism lasted on

  5. Deformation signals in the currently-rifting Afar (Ethiopia) Rift measured with InSAR

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Wright, T. J.; Wang, H.; Hamling, I. J.; Kier, D.; Belachew, M.; Ebinger, C.

    2008-12-01

    A major rifting episode is currently occurring in the Dabbahu magmatic segment in the Afar region. The rifting episode began in September-October 2005, when continuous seismicity, a volcanic eruption and extensive diking occurred along the 60-km-long magmatic segment. Since then, nine additional dike intrusions occurred in the area and have been detected by radar interferometry (InSAR), with the most recent dike intrusion occurring between July 4-21, 2008 at the southern edge of the segment. We used radar images acquired by the European satellite, Envisat, in both descending and ascending orbits to form interferograms, spanning the time period from October 2005 to the present. The interferograms cover the main magmatic segments in the Afar region including Dabbahu, Hararo, Alayta and Erta Ale. The recorded interferograms were inverted using a least-squares method to obtain average deformation maps and a time series of incremental deformations. Preliminary results indicate that rapid deformation is currently confined to the Dabbahu region with no large deformation signals observed in the nearby magmatic segments. High rates of deformation are observed in the Dabbahu segment associated with shallow magma movements and possible visco-elastic relaxation. Outside the Dabbahu segment, the largest signal is a broad area subsiding at a steady rate of 10 cm/yr, located south east of the Dabbahu segment, and east of the Hararo segment. Our preliminary interpretation is that this represents deep flow of magma away from the Hararo segment and towards Dabbahu. The observed deformation patterns will also be compared to seismicity observed over the same time periods.

  6. Reconstructing recent basaltic fissure eruptions in the Afar Depression, Ethiopia, using satellite imagery

    NASA Astrophysics Data System (ADS)

    Barnie, T. D.; Ferguson, D. J.; Oppenheimer, C.

    2009-12-01

    Since 2005, there have been numerous dike intrusions in the Manda-Hararo rift segment in central Afar, Ethiopia, as part of an on-going rifting event similar to that which occurred at Krafla, Iceland between 1975 and 1984. Two of the dikes have been associated with basaltic fissure eruptions, in August 2007 and in June 2009. A large fissure eruption also occurred in the Erta ‘Ale volcanic range in northern Afar in November 2008, which may be related to the activity further south. These eruptions took place in remote areas and lasted only a few days, making field observation almost impossible. We must therefore rely on the geomorphology and geochemistry of the erupted deposits, and syn-eruption satellite observations of thermally emitted radiance and gas emissions to reconstruct the evolution of these eruptions. Reconstruction of the eruptions is important for i) understanding the relationship between dike intrusion and fissure eruption dynamics ii) understanding the relationship between eruption dynamics and the resulting deposits, iii) interpreting the deposits of pre-historic eruptions from previous rifting events, and iv) estimating the hazard posed to the local population by further activity. In this study we use pre- and post-eruption high resolution ASTER and ALI images to map the morphology of the erupted material, and SEVIRI and MODIS time series of the eruptions to track evolution of the vent dynamics and lava flows. These datasets are augmented by oblique aerial and ground based photographic and FLIR surveys of the cooling lava flows and tephra. ASTER and ALI images are also used to map older erupted materials, which allow us to compare this rifting event with previous events to anticipate future activity.

  7. Mineralogy and composition of the oceanic mantle

    USGS Publications Warehouse

    Putirka, Keith; Ryerson, F.J.; Perfit, Michael; Ridley, W. Ian

    2011-01-01

    The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures (T) of melt generation and processing (by c. 300°C) related to the Hawaiian mantle plume. DNi is low at high T, so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where DNi is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4-5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO2-enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1-1·5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al2O3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO2

  8. Mantle updrafts and mechanisms of oceanic volcanism.

    PubMed

    Anderson, Don L; Natland, James H

    2014-10-14

    Convection in an isolated planet is characterized by narrow downwellings and broad updrafts--consequences of Archimedes' principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties. A mature cooling planet with a conductive low-viscosity core develops a thick insulating surface boundary layer with a thermal maximum, a subadiabatic interior, and a cooling highly conductive but thin boundary layer above the core. Parts of the surface layer sink into the interior, displacing older, colder material, which is entrained by spreading ridges. Magma characteristics of intraplate volcanoes are derived from within the upper boundary layer. Upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets. Their morphology, aspect ratio, inferred ascent rate, and temperature show that they are passively responding to downward fluxes, as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating. Response to doward flux is the inverse of the heat-pipe/mantle-plume mode of planetary cooling. Shear-driven melt extraction from the surface boundary layer explains volcanic provinces such as Yellowstone, Hawaii, and Samoa. Passive upwellings from deeper in the upper mantle feed ridges and near-ridge hotspots, and others interact with the sheared and metasomatized surface layer. Normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism. PMID:25201992

  9. Mantle updrafts and mechanisms of oceanic volcanism

    PubMed Central

    Anderson, Don L.; Natland, James H.

    2014-01-01

    Convection in an isolated planet is characterized by narrow downwellings and broad updrafts—consequences of Archimedes’ principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties. A mature cooling planet with a conductive low-viscosity core develops a thick insulating surface boundary layer with a thermal maximum, a subadiabatic interior, and a cooling highly conductive but thin boundary layer above the core. Parts of the surface layer sink into the interior, displacing older, colder material, which is entrained by spreading ridges. Magma characteristics of intraplate volcanoes are derived from within the upper boundary layer. Upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets. Their morphology, aspect ratio, inferred ascent rate, and temperature show that they are passively responding to downward fluxes, as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating. Response to doward flux is the inverse of the heat-pipe/mantle-plume mode of planetary cooling. Shear-driven melt extraction from the surface boundary layer explains volcanic provinces such as Yellowstone, Hawaii, and Samoa. Passive upwellings from deeper in the upper mantle feed ridges and near-ridge hotspots, and others interact with the sheared and metasomatized surface layer. Normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism. PMID:25201992

  10. Mantle updrafts and mechanisms of oceanic volcanism

    NASA Astrophysics Data System (ADS)

    Anderson, Don L.; Natland, James H.

    2014-10-01

    Convection in an isolated planet is characterized by narrow downwellings and broad updrafts-consequences of Archimedes' principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties. A mature cooling planet with a conductive low-viscosity core develops a thick insulating surface boundary layer with a thermal maximum, a subadiabatic interior, and a cooling highly conductive but thin boundary layer above the core. Parts of the surface layer sink into the interior, displacing older, colder material, which is entrained by spreading ridges. Magma characteristics of intraplate volcanoes are derived from within the upper boundary layer. Upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets. Their morphology, aspect ratio, inferred ascent rate, and temperature show that they are passively responding to downward fluxes, as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating. Response to doward flux is the inverse of the heat-pipe/mantle-plume mode of planetary cooling. Shear-driven melt extraction from the surface boundary layer explains volcanic provinces such as Yellowstone, Hawaii, and Samoa. Passive upwellings from deeper in the upper mantle feed ridges and near-ridge hotspots, and others interact with the sheared and metasomatized surface layer. Normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism.

  11. Investigating Late Cenozoic Mantle Dynamics beneath Yellowstone

    NASA Astrophysics Data System (ADS)

    Zhou, Q.; Liu, L.

    2015-12-01

    Recent tomography models (Sigloch, 2011; Schmandt & Lin, 2014) reveal unprecedented details of the mantle structure beneath the United States (U.S.). Prominent slow seismic anomalies below Yellowstone, traditionally interpreted as due to a mantle plume, are restricted to depths either shallower than 200 km or between 500 and 1000 km, but a continuation to greater depth is missing. Compared to fast seismic anomalies, which are usually interpreted as slabs or delaminated lithosphere, origin of deep slow seismic anomalies, especially those in the vicinity of subduction zones, is more enigmatic. As a consequence, both the dynamics and evolution of these slow anomalies remain poorly understood. To investigate the origin and evolution of the Yellowstone slow anomaly during the past 20 Myr, we construct a 4D inverse mantle convection model with a hybrid data assimilation scheme. On the one hand, we use the adjoint method to recover the past evolution of mantle seismic structures beyond the subduction zones. On the other hand, we use a high-resolution forward model to simulate the subduction of the oceanic (i.e., Farallon) plate. During the adjoint iterations, features from these two approaches are blended together at a depth of ~200 km below the subduction zone. In practice, we convert fast and slow seismic anomalies to effective positive and negative density heterogeneities. Our preliminary results indicate that at 20 Ma, the present-day shallow slow anomalies beneath the western U.S. were located inside the oceanic asthenosphere, which subsequently entered the mantle wedge, through the segmented Farallon slab. The eastward encroachment of the slow anomaly largely followed the Yellowstone hotspot track migration. The present deep mantle Yellowstone slow anomaly originated at shallower depths (i.e. transition zone), a