V41G-01 INVITED
Dynamics and internal structure of a mantle plume conduit
Plumes are often thought of as thermal structures, the key parameter being their excess temperature with respect to the surrounding mantle, while the associated velocity field is frequently neglected. However, in order to understand the structure of a plume conduit and the internal distribution of geochemical heterogeneities we change perspective and focus on velocities and strain rates. Like the hands of an invisible sculptor, velocity gradients constantly modify the shape of heterogeneities rising in the plume conduit. Numerical simulations of a vigorous thermal plume provide a high resolution velocity field that is used to advect deep-seated passive heterogeneities. Our first objective is to investigate the relation between initial length-scales of heterogeneities across D" and the length- and time-scale of geochemical variations induced in the plume conduit. We also explore dynamical differences between the central and peripheral part of the plume conduit, and calculate the strain rate, the buoyancy flux and the elongation as a function of radial distance from the plume axis. Our results clearly show that most of the plume buoyancy flux occurs in highly sheared parts of the conduit. We then concentrate on a 'Hawaiian' plume sheared by a fast moving oceanic plate. Our fully three-dimensional numerical model allows us to study the flow trajectories inside a sheared plume conduit and the deformations undergone by passive heterogeneities. Although our approach is simplistic, we consider the lifetime of a volcano carried by the plate over different parts of the plume conduit and we investigate the relation between heterogeneous structures in the mantle and the spatio-temporal geochemical variability registered by the volcano.
V41G-02 INVITED
Concurrent Evaluation of Magma Production, Volcano Growth, and Geochemical Structure in Mantle Plumes: Hawaii Drilling Project (HSDP) Results
The Hawaii Scientific Drilling Project (HSDP) completed its drilling and coring of the northeast flank of the Mauna Kea volcano in early 2007. The project obtained a nearly continuous core consisting of lava flows, hyaloclastite, minor intrusives and sediment from a 3260 m section of the Mauna Kea volcano, covering an age range from 200 to over 600 ka. It also recovered a 280m section of the Mauna Loa volcano. When combined with surface and dredge samples, there now is a 600-700 ky record of the lava output from Mauna Kea as well as a 200 ky record from Mauna Loa. These records can be interpreted in terms of the geochemical structure of the Hawaiian plume, given a model for the sampling of the plume by melting and melt transport. The continuous nature of the HSDP core, with the implied continuous monitoring of the lava output from the volcano, has dictated that we develop models for the plume behavior just below the lithosphere, and for how magma is collected from the plume melting region and supplied to an individual volcano. Although there are as yet no detailed physical models for the melt collection and transport, we have experimented with simple geometric models. These models can be constrained by the volume and volume-age structure of the Hawaiian volcanoes, and by available geodynamic models for the Hawaiian plume. Using these models we can interpret geochemical data from the lavas in terms of plume structure. Any systematic variability in Hawaiian lavas with depth (age) in the drillcore can be attributed to structure in the plume, and one of the interesting results is that there is such structure even though melting within the plume samples only the innermost third or so the plume radius. The data show that there is radial geochemical zoning of the melting region of the plume in terms of He, Pb, Nd, Sr and Hf isotopes. This geochemical structure represents the hot core of the plume and does not reflect entrainment of ambient lower or upper mantle. To first order, the radial component of the geochemical structure of the plume represents the vertical structure at the thermal boundary layer from which the plume originates. In the case of Hawaii, all of the lavas are derived from melting of mantle that originates from within 20-50 km of the base of the mantle. One of the most striking characteristics of the HSDP data is that the high 3He/4He anomaly is nested within the innermost core of the melting region of the plume and is much larger in amplitude and much smaller in diameter than the Nd, Sr and Hf anomalies. The He-3 anomaly apparently has a different origin than the other anomalies, and is restricted to the lowermost 10-20 km of the mantle plume source. The helium signal is therefore likely to come either directly from the Earth's core via leakage across the core-mantle boundary, or from a dense layer separating the main mantle from the outer core. The Hawaiian plume contains little evidence for a hidden light- REE enriched reservoir near the core-mantle boundary, but could potentially be considered to indicate the presence of a primitive layer if the bulk Earth is not chondritic with respect to REE. http://www.icdp- online.de/sites/hawaii/
V41G-03
What, When, Where, and Why of Secondary Hawaiian Hotspot Volcanism
Secondary hotspot volcanism occurs on most oceanic island groups (Hawaii, Canary, Society) but its origins remain enigmatic. A 28-day marine expedition used multibeam bathymetry and acoustic imagery to map the extent of submarine volcanic fields around the northern Hawaiian Islands (Kauai, Niihau and Kaula), and the JASON2 ROV to sample many volcanoes to characterize the petrology, geochemistry (major and trace elements, and isotopes) and ages of the lavas from these volcanoes. Our integrated geological, geochemical and geophysical study attempts to examine the what (compositions and source), where (distribution and volumes), when (ages), and why (mechanisms) of secondary volcanism on and around the northern Hawaiian Islands. A first-order objective was to establish how the submarine volcanism relates in space, time, volume, and composition to the nearby shield volcanoes and their associated onshore secondary volcanism. Our surveying and sampling revealed major fields of submarine volcanoes extending from the shallow slopes of these islands to more than 100 km offshore. These discoveries dramatically expand the volumetric importance, distribution and geodynamic framework for Hawaiian secondary volcanism. New maps and rock petrology on the samples collected will be used to evaluate currently proposed mechanisms for secondary volcanism and to consider new models such as small-scale mantle convection driven by thermal and melt-induced buoyancy to produce the huge volume of newly discovered lava. Our results seem to indicate substantial revisions are needed to our current perceptions of hotspot dynamics for Hawaii and possibly elsewhere.
V41G-04
Non-hotspot volcano chains originating from small-scale sublithospheric convection
Although most of the intraplate volcanism in ocean basins is expressed in linear chains, not all of these can be attributed to a stationary hotspot. Many ridges do not show linear age progressions as predicted by this model (e. g., Cook-Austal, Magellan or Line Islands, and Pukapuka ridges). The well-studied Pukapuka ridges reside among other short-lived seamount-chains in the eastern part of the South Pacific Superswell. They are aligned by plate motion and by topography and gravity lineations with a wavelength of ~200 km. In order to account for these observations, three types of models have been put forward yet: lithospheric cracking, channelized return flow, and SSC. Gravity and tomography studies have rejected the lithospheric cracking model [Harmon et al., 2007], which furthermore presumes a reservoir of pre-existing partial melt in the asthenosphere. Channelized return flow might be a good explanation for the volcanism and the associated density anomalies. But it fails to explain, why many of the lineations are not associated with volcanism at all. The SSC-hypothesis is instead able to explain the gravity data and the volcanism. In the Earth's uppermost mantle SSC is likely to develop due to instabilities of the thickened thermal boundary layer below mature oceanic lithosphere (usually ~70 Ma). It is characterized by convective rolls aligning plate motion. Their onset is earlier (i.e. beneath younger and thinner lithosphere) for lower mantle viscosities (e.g. for hot or wet mantle) or adjacent to lateral thermal or compositional heterogeneity. In these cases, partial melt potentially emerges in the upwelling limbs of SSC. Partial melting changes the compositional buoyancy owing to melt retention and depletion of the residue. Therefore, it promotes upwelling and further decompression melting. In this study, we take the step towards fully thermo-chemical 3D-numerical models of SSC (using the FEM-Code CITCOM) with a realistic, temperature- and depth-dependent rheology in order to quantitatively test the SSC-hypothesis on intraplate volcanism. We explore the 3D-patterns of melting associated with SSC, the age of seafloor over which it occurs, and the rates of melt generation by varying the key parameters mantle viscosity and temperature, Tm. We also investigate the effect of lateral heterogeneity that locally reduces the onset age of SSC, and the effect of a rheology dependent on water and melt content. Melting due to SSC is predicted to emerge in elongated features (~750 km) parallel to plate motion and not just at a fixed spot. Therefore, irregular age progressions of the associated volcanism are predicted - contrary to the hotspot model. The seafloor age at which volcanism occurs is sensitive to Tm. For moderate Tm (1350 °C), volcanism develops beneath a relatively young lithosphere (~30 Myr), and higher Tm retards the onset of SSC and volcanism because of the stabilizing influence of a thicker residue from previous mid-ocean ridge melting (e. g., ~50 Myr for Tm=1410 °C). Mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism. Effective viscosity required to obtain km-high seamounts is ~2·1019 Pa·s, or significantly lower if stiffening due to exhaustion of water is considered. Our calculations predict many of the key observations of the Pukapuka ridges, and the volcano groups associated with the Cook-Austral, Line and Marshall Islands.
V41G-05
Ridge jumps associated with plume-ridge interaction: Mantle plume-lithosphere interaction and hotspot magmatism
Interaction of mantle plumes and young lithosphere near mid-ocean ridges can lead to changes in spreading geometry by shifts of the ridge-axis toward the plume as seen at various hotspots, notably Iceland and the Galapagos. Previous work has shown that, with a sufficient magma flux, heating of the lithosphere by magmatism can significantly weaken the plate and, in some cases, could cause ridge jumps. Upwelling hot asthenosphere can also weaken the plate through thermal and mechanical thinning of the lithosphere. Using the finite element code CITCOM, we solve the equations of continuity, momentum and energy to examine deformation in near-ridge lithosphere associated with relatively hot upwelling asthenosphere and seafloor spreading. The mantle and lithosphere obey a non-Newtonian viscous rheology with plastic failure in the cold part of the lithosphere simulated by imposing an effective yield stress. Temperatures of the lithospheric thermal boundary region are initially given a square-root of age thermal profile while a hot patch is placed at the bottom to initiate a mantle-plume like upwelling. The effect of upwelling asthenosphere on ridge jumps is evaluated by varying three parameters: the plume excess temperature, the spreading rate and the distance of the plume from the ridge axis. Preliminary results show plume related thinning and weakening of the lithosphere over a wide area (100's of km's) with the rate of thinning increasing with the excess temperature of the plume. Initially, thinning occurs as the plume approaches the lithosphere and asthenospheric material is forced out of the way. As the plume material comes into contact with the lithosphere, thinning occurs through heating and mechanical removal of the thermal boundary layer. Thinning of the lithosphere is one of the primary factors in achieving a ridge jump. Another is large tensile stresses which can facilitate the initiation of rifting at this weakened location. Model stresses induced by the buoyant asthenosphere are significant fractions of the lithospheric yield strength near the plume and reach a maximum at the center of plume upwelling. Models predict that ridge jumps are not likely to occur by lithosphere interaction with the hot upwelling plume alone but require the added effects of magmatic weakening at the hotspot.
V41G-06
Dynamics of flowline Hotspots
Numerous linear chains of volcanoes are perpendicular to the isochrons of oceanic plates. As lithospheric thickness increases with plate age, these "flowline" hotspots are frequently attributed to the flow of buoyant plume material along the base of the lithosphere. Here the base of the lithosphere forms an upside-down drainage pattern for plume material away from the plume orifice toward the ridge axis. The flowline pattern forms by viscous fingering when there is insufficient plume material to form a continuous flow front. The hot plume material within the fingers cools by conduction and small-convection as it flows away from the plume. This process locally thins the lithosphere, channelizing the flow. It also cools the plume material, increasing its viscosity. The thickness and viscosity of the plume material self-organize to conserve material along the flow line and so that convection and conduction carry comparable amounts of heat flow into the overlying lithosphere. The flux of plume material (in m 3 s-1 per meter along the flow front) scales to a critical quantity that is ~ Racrit κ S Δ T / Tη, where Racrit is the critical Rayleigh number for convection, κ is thermal diffusivity, S is the slope of the base of the lithosphere, Δ T is the excess temperature of the plume material, and Tη is the temperature to change viscosity by a factor of e. The plume material cools rapidly along the flowline until the heat flow into the lithosphere scales with the surface plate-age heat flow. The conditions for the formation of flowlines channels are not particularly restrictive, compatible with the common occurrence of flowline hotspots on the seafloor.
V41G-07
Seismic Evidence for the Hainan Plume
We present a high-resolution tomographic image in the upper mantle for the Hainan plume determined by using both local and teleseismic data. The arrival time data from teleseismic events are measured precisely from seismograms recorded by nine permanent seismic stations in Hainan island and Leizhou peninsula. Our results show that striking low-velocity (low-V) anomalies of up to -5 percent in the crust and -2 percent in the mantle are visible under the Hainan hotspot from the surface down to 300 km depth. The Hainan plume is imaged as a continuous, NW-SE tilting, low-V column with a diameter of about 80 km. This image of the Hainan plume is much improved over the results of the previous regional and global studies. Such an inclination of the plume is in good agreement with the results of numerical simulations and paleomagnetic studies. The Hainan plume deflection may be partly attributed to the upper mantle shear flow resulted from the complex subduction and interaction of the Indian, Philippine Sea, Pacific and Eurasian plates. http://www.eq-icd.cn