V33G-01 13:40h
Regional and Local Flexure Zones in Iceland: Upper Crustal Structures in Magmatically Robust Spreading Systems
Extensively uplifted and glaciated exposures of the Tertiary crust of Iceland provide a window into the architecture of oceanic crust formed near hot spots on ridges and may provide analogs for magmatically robust ridge segments in general. Upper crustal rock units on Iceland consist mainly of laterally continuous, sheet-like, lava flows cut by sparse dikes. These units reveal a surprising degree of structural complexity with asymmetrical flexure zones marking the sites of former spreading axes. In regional flexure zones ($>$10 km across), near-horizontal lava flows overlie inward-dipping (toward the rift) lavas in a downward -steepening geometry. Associated dikes increase in (outward) dip, but in some places steeply dipping dikes cut this entire assemblage. Paleomagnetic data on lavas and dikes help quantify these rotations. Along-strike variations in regional-scale flexures appear to show segmentation patterns with spreading segments a few tens of kilometers long separated by accommodation zones. Local flexures (1-2 km across) are characterized by packages of dipping lava flows unconformably overlain by later, less tilted flows. In some places, multiple episodes of subsidence, tilting and burial by later lavas have occurred. These areas document dramatic, 3-dimensional, subaxial subsidence (few hundred meters) near magmatically robust segment centers. Rapid subaxial subsidence during crustal construction may be a hallmark of spreading in magmatically robust systems.
V33G-02 13:55h
The North Atlantic Igneous Province: Computer Modeling of Hot Spots for the Icelandic Convective System
North Atlantic Igneous Province can be regarded as unique natural laboratory. Besides, it is a one of the most studied regions. The paper deals with the dynamics of rock melting in the upper mantle under Iceland, when a mantle plume develops above a hot spot. Our 2D convection codes `Maix2D' allow us to simulate the evolution of magmatic system. Mathematical model is based on the extended Boussinesq approximation. Four solid-state phase transitions have been taken into account in the equation of state. This allowed us to give a realistic description for the ashenosphere development and further evolution of the magmatic system. For the Icelandic Convective System heat-mass transfer processes of the magmatic system and the data of the melting zone have been computed. Temperature distribution with depth in the upper mantle was taken in such a way, that convection was developing without melting in the absence of a hot spot. For the lowest degree of melting the temperature inside the hot spot was taken $1850-1900 {\deg}C$. Several numerical experiments for various temperatures and sizes of hot spots as well as the lithosphere thicknesses have been computed. The results show that in the presence of a hot spot a zone of melting (asthenosphere) develops. The existence time for this zone is about $10-25 Ma$ (depending on the initial conditions), which is in agreement with the observed data for the Icelandic Convective System. The codes allowed us to compute the degree of melting, the location, size and geometry, the chemical composition, and the existence time of the asthenosphere. This work was supported by the RFBR (Grant No. 04-05-64107), the Ministry of Science of Russian Federation (the State Contract No. 43.043.1.1.1601), the Presidium of SB RAS (grant No. 106), the President Grants (NSh-2118.2003.5, NSh-1573.2003.5).
V33G-03 14:10h
Geochemistry of Alvarado and Sarmiento Ridges Suggests Widespread Galapagos Plume-Upper Mantle Interaction in the Miocene Eastern Pacific?
Alvarado and Sarmiento are 1-2 km high, ~400 km-long, NE striking volcanic ridges in the Peru Basin that lie 150 km and 250 km, respectively, southeast of the Grijalva Scarp. The latter marks the southern boundary between the younger Cocos-Nazca and older EPR-Farallon crusts. The two ridges were originally proposed as transform fault trails on the Farallon plate, but new evidence suggest that they were the result of fissure eruption during an initially (pre-24Ma) broad zone of Farallon plate rupture. The rupture later became focussed along the Grijalva fracture, splitting the Farallon plate at 23Ma to create the Cocos and Nazca plates and initiating the Cocos-Nazca spreading center. Lava samples from the ridges were dredged during the DANA 02 expedition aboard R/V Revelle in Fall 2003. The lavas are invariably basalts ranging from tholeiitic to alkalic in composition. The basalts have flat to highly incompatible element-enriched trace element patterns and although their isotopic ratios are not as radiogenic as those of some of the Galapagos hotspot lavas, they are neither as depleted as those of normal-MORB (87Sr/86Sr[25Ma] $>$0.7027; 143Nd/144Nd[25Ma] $<$0.5130; 206Pb/204Pb[25Ma] $>$18.66). Some of the ridge lavas are compositionally similar to enriched-MORB erupted along the GSC section close to the Galapagos hotspot, but the majority are more akin to the relatively depleted hotspot lavas erupted at the center of the Galapagos Archipelago. The new geochemical data suggest that the volcanic ridge lavas came from a fairly large, anomalous region of the upper mantle that may have been created by the interaction between a Galapagos plume and the depleted source of MORB.
V33G-04 14:25h
A Lot of Melt Beneath the Large Axial High Along the Hotspot-Influenced Western Galapagos Spreading Center
The hotspot-influenced western Galapagos Spreading Center (GSC) spreads at an intermediate rate (45-55 mm/yr) and has an axial high that is appreciably larger in amplitude than many sections of fast spreading ridges such as the East Pacific Rise. Moving westward from $91\deg$W away from the Galapagos hotspot, the amplitude of the axial high decreases as the depth of the axial magma lens reflector increases (observed in our multi-channel seismic data). We investigate the cause of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere (Shah and Buck, 2001). In this model, low-density material underlying the ridge axis was originally assumed to be hot and partially molten crust but we now extend it to include partial melt in the mantle. The low-density material rapidly cools and becomes denser away from the ridge axis imparting downward loads on the lithosphere. These loads, combined with thermal contraction stresses, depress the flanks of the axis downward such that the ridge axis stands on a topographic high. Using this model, we are able to predict the decrease in amplitude of the axial high with increasing magma lens depth by either decreasing the amount of low-density material beneath the ridge axis or by allowing the crust to cool more slowly as it moves off axis. Previous applications of this model to other axial highs show that both the observed topography and gravity can be created by low-density material near the ridge axis and melt contained entirely within the crust. However, results of our calculations reveal that the unusually large axial high of the GSC requires that either the crust below the magma lens contains an extremely large amount of melt (up to $\sim$35$%$), or alternatively, the melt extends well below the crust (up to $\sim$70 km) in a narrow region below the ridge axis. It thus appears likely that the elevated mantle temperature and crustal production associated with the Galapagos hotspot maintains a significant amount of melt in the mantle. Using multi-channel seismic imaging of the axial magma lens, seismic refraction data, gravity and bathymetry measurements we constrain the amount of melt needed in the crust versus the mantle. Future mantle seismic studies could be used to further test our models.
V33G-05 14:40h
The Seismic Low Velocity Zone West of the EPR Where an Off-Axis Plume may be Feeding the Ridge
We present seismic Rayleigh wave data from the GLIMPSE experiment to investigate the upper mantle beneath intra-plate seamount chains on young (4 - 9 Ma) Pacific seafloor which may be the surface manifestation of mantle flow between an off-axis plume source (e.g. the hotspot Super Swell region) and the EPR. Shear wave inversion of phase velocity data collected from the study area surrounding the Sojourn ridge and Hotu Matua seamount complex indicate a high velocity lithosphere extending to 60 km $\pm$ 20 km is underlain by anomalous low velocities reaching a minumum of $\sim$3.9 km/s at 70 km depth. Sufficient long period data is available to resolve a steep positive velocity gradient at the base of the low velocity zone (LVZ) at about 110 km depth. Rayleigh waves also demonstrate significant attenuation for periods between 40 s and 70 s with a minimum quality factor, 65 ${ < Q_{LR} < }$ 110. Recent studies that suggest variations in the elastic properties of the oceanic LVZ can be explained exclusively by the effects of temperature and pressure are able to predict the seismic velocities that we observe but indicate stronger attenuation than is shown in our data. Conversely, models that consider only the effects of dehydration are consistent with the range of seismic attenuation but indicate seismic velocities which are too high. We propose a model beneath young seafloor where decompression melting begins at $\sim$110 km and "dries out" at $\sim$60 km. The low velocities and moderate attenuation can be explained by the combined effects of partial melt and dehydration. The unusually low seismic velocities may be associated with asthenospheric flow from a volatile-rich off-axis plume to the East Pacific Rise.
V33G-06 14:55h
Hotspot-Ridge Interaction: Shaping the Geometry of Mid-Ocean Ridges
Surface manifestations of hotspot-ridge interaction include geochemical anomalies, elevated ridge topography, negative gravity anomalies, off-axis volcanic lineaments, and ridge reorganization events. The last of these is expressed as either "captured" ridge segments due to asymmetric spreading, such as at the Galapagos, or as discrete jumps of the ridge axis toward the hotspot, such as at the Iceland, Tristan de Cuhna, Discovery, Shona, Louisville, Kerguelen, and Reunion hotspots. Mid-ocean ridge axis reorganizations through discrete jumps will cause variations in local volcanic patterns, lead to changes in overall plate shape and ridge axis morphology, and alter local mantle flow patterns. It has been proposed that discrete ridge jumps are a product of interaction between the lithosphere and a mantle plume. We examine this hypothesis using thin plate theory coupled with continuum damage mechanics to calculate the two-dimensional (plan-view) pattern of depth-integrated stresses in a plate of varying thickness with weakening due to volcanism at the ridge and above the plume center. Forces on the plate include plume shear, plate parallel gravitational forces due to buoyant uplift, and a prescribed velocity of plate motion along the edges of the model. We explore these forces and the effect of damage as mechanisms that may be required to predict ridge jumps.
V33G-07 15:10h
Rare gas isotopic compositions in single vesicles from oceanic basalt glasses using UV laser ablation
A 193nm laser (ArF) was installed on the mass spectrometer ARESIBO II in order to analyze single vesicles in oceanic basalt glasses. Pieces of 1/2 cm in size of glasses (MORB and OIB) were loaded in a cell under vacuum, connected to the purification and extraction line. After purification, rare gases were introduced in the mass spectrometer for analyze with the standard procedure. Measurement of the CO2 abundance was performed using a MKS Baratron before gas purification, by assuming that the gas in MORB vesicles is mainly CO2. We want to address three issues. One is the origin of the atmospheric component observed in some oceanic basalts, that is similar to modern air for both isotopic and elemental ratios. The second is to constrain the vesiculation process when the magma is en route to the surface as well the nature of the degassing (e.g. distillation vs. closed system vesiculation depending of the tectonic context). The last issue is to constrain the mixing process between MORB and OIB magmas when a hotspot is located on the ridge (e.g. Shona and Discovery ridge anomalies in the south atlantic). The preliminary results show important variations for the isotopic ratios and gas abundances in the vesicles of a single sample, suggesting that the air component is located in vesicles, associated with mantle derived helium and CO2.
V33G-08 15:25h
Thin Spherical Shell Model of Global Asthenosphere Flow: Applications to Geochemical Segmentation of MORs and Seismic Anisotropy
Asthenosphere plume-to-ridge flow has often been proposed to explain both the existence of geochemical anomalies at the mid-ocean ridge segments nearest an off-axis hotspot, and the existence of apparent geochemical `provinces' within the global mid-ocean spreading system. We have constructed a thin-spherical-shell finite element model to explore the possible structure of global asthenosphere flow and to determine whether plume-fed asthenosphere flow is compatible with present-day geochemical and seismic observations. In this model, lubrication theory approximations are used to solve for the flow profile in the vertical direction, and a ~100-km-scale mesh is used to solve for the mean horizontal asthenosphere flow. At each mesh node, the asthenosphere thickness is set according to the age/thickness of overlying lithosphere. Asthenosphere is assumed to be brought up by mantle plumes, with `sinks' of asthenosphere at spreading centers where compositional lithosphere is made, at trenches (where some, but not much asthenosphere is entrained and dragged down by subducting lithosphere), and also a distributed sink of asthenosphere due to its cooling and attachment to the base of the aging and thickening oceanic lithosphere. Important model boundary conditions are plate velocities and the changing thickness of the asthenosphere/lithosphere at continental margins. We also assume that the strength of all the plume (hotspot) asthenosphere sources is equal to the sum of all the asthenosphere sinks, i.e. that the asthenosphere has a present-day steady-state thickness and hotspot fluxes have remained constant through time. In spite of these evident oversimplifications, the model appears to show considerable promise as a possible mechanism to explain observed patterns of MOR geochemical segmentation. Atlantic, Indian, and Pacific MOR isotope geochemistry can be fit well at both medium and long wavelengths by the predicted global asthenosphere flow pattern from distinct plume sources. Matching recent observations of seismic anisotropy is currently more problematic. Fossil spreading directions and present-day plate motions also appear to correlate with seismic observations - we are currently trying to isolate each of these effects in an inverse formulation. The model makes specific and sometimes surprising predictions about the global segmentation of ridge geochemistry.