Global Na8-Fe8 Systematics of MORBs: Implications for Mantle Heterogeneity, Temperature, and Plumes D. Presnall (1,2,3), G. Gudfinnsson (2,3) (1) Department of Geosciences, University of Texas at Dallas, Richardson, TX, USA, (2) Geophysical Laboratory, Washington, D. C., USA, (3) Bayerisches Geoinstitut, Bayreuth, Germany ([email protected] / 972-883-2405 / 972-883-2444) In a global examination of Na8-Fe8-axial depth variations of mid-ocean ridge basalt (MORB) glass compositions from the Smithsonian database, we find that modeling of MORBs as the product of large variations in potential temperaure (Klein and Langmuir, 1987, JGR, 92, 8089) is not supported by Na8-Fe8-depth data for any ridge segment of any length. However, the observed inverse and positive Na8-Fe8 variations are in excellent agreement with the systematics of solidus melts in the plagioclase/spinel lherzolite transition in the CaO-MgO-Al2O 3-SiO2-Na2O-FeO system at 0.93-1.5 GPa, and 1240-1260˚C (Presnall et al., 2002, GCA, 66, 2073). This system contains all the main mantle mineral phases in this pressure range (ol, opx, cpx, pl, sp) and 99% of both the extracted melts and _ the source. Thus, chemical systematics of melts at the solidus provide very strong constraints on the major-element chemistry of melting processes. On the surface that defines solidus melt compositions in the pl/sp lherzolite transition interval, contours of constant pressure are nearly parallel to contours of constant MgO, but are at a high angle to contours of Na2O. An inverse Na8-Fe8 correlation occurs for melting at constant pressure. In contrast, positive Na8-Fe8 correlations occur when short diapirs progressively melt as they rise. Melts having the highest MgO/FeO are extracted last from the tops of these diapirs. In this modeling, we find that the southern Atlantic mantle from Bouvet to about 26˚N is relatively homogeneous, whereas the Atlantic mantle north of about 26˚N shows significant long-range heterogeneity. The mantle between the Charlie Gibbs and Jan Mayen fracture zones is strongly enriched in FeO/MgO, perhaps by a trapped fragment of basaltic crust in a Caledonian suture (Foulger et al., 2005, Spec. Paper 388, 595). The strongly enhanced melt productivity at Iceland is explained as the product of this enrichment, not a hot plume. The East Pacific Rise and Galapagos Ridge sample mantle that is heterogeneous over short distances. The mantle beneath the northern part of the Indian Ocean is fairly homogeneous and is depleted in FeO/MgO relative to the mantle beneath the Red Sea. Our model replaces plumes and large variations in potential temperature with mantle heterogeneity and uniformly low temperatures of MORB generation. 2 Intraplate volcanism due to small-scale convection -a 3D numerical study M. D. Ballmer (1), J. van Hunen (2), P. J. Tackley (1) (1) Institute of Geophysics, ETH Zurich, (2) Department of Earth Sciences, University of Durham ([email protected]) Some volcanic chains in the south pacific show fast and unsteady age progressions towards the East Pacific Rise (300 mm/yr) inconsistent with the plume hypothesis. They have yet been attributed to plate processes (cracking due to extension [Sandwell et al. 1995] or thermal contraction [Sandwell and Fialko 2004]) and dynamic mantle processes (return flow [Conder et al. 2002] or small-scale convective instabilities [Buck and Parmentier 1986]). The latter is supported by recent gravity studies [Harmon et al. 2006]. In the Earth’s uppermost mantle small-scale convection (SSC) is likely to develop due to instabilities of the thickened thermal boundary layer below mature oceanic lithosphere (usually 70 Ma). _ They are characterized by convective rolls aligning with plate motion, whose onset is earlier for higher Rayleigh numbers (e.g. hot or wet mantle) and adjacent to lateral inhomogeneities, such as fracture zones or hotspot tracks. Beneath hence young (< 40 Ma) and thin lithosphere partial melt is potentially emerging in the upwellings of SSC. Melting changes particularly the compositional buoyancy by melt retention and additional depletion of the residue, and therefore promotes upwelling and allows for further melting (buoyant decompression melting (BDM) [Raddick et al. 2002]). These processes allow only for a few percent of partial melting. Here we present results of a fully thermo-chemical 3D-numerical mantle flow study on the interaction of SSC and BDM with a realistic, temperature- dependent rheology. We explore depth, duration, degree and amount of melting and melt extraction of the BDM, and study the 3D-geometry of SSC and melting. We vary parameters such as mantle temperature, plate speed, thermal and compositional Rayleigh numbers and melt extraction scheme. We compare patterns of SSC and melting with purely thermal cases. This study reveals, that 3D-patterns (and onset) of melting due to SSC are mainly controlled by thermal buoyancies. They are slightly modified by compositional buoyancies, latent heat of melt and melt extraction scheme. Moreover, we put constraints on the SSC hypothesis for intraplate volcanism. Positive thermal anomalies (> 100 K) would be required to trigger off-ridge melting in a static mantle (no SSC). However, we find, that a 18 few percent of partial melt are emerging due to SSC for low mantle viscosities ( 5x10 Pas) _ and ambient temperatures. We postulate small lateral thermal or compositional anomalies to obtain early onset of SSC locally yielding the observed patterns of volcanism (i.e. chains). REFERENCES Buck, W. R., and E. M. Parmentier (1986), Convection Beneath Young Oceanic Lithosphere: Implications for Thermal Structure and Gravity, J. Geophys. Res., 91(B2), 1961 Conder, J. A., D. W. Forsyth, and E. M. Parmentier (2002), Asthenospheric flow and asymmetry of the East Pacific Rise, MELT area, J. Geophys. Res., 107(B12), 2344 Harmon, N., D. W. Forsyth and D. S. Scheirer (2006), Analysis of topography and gravity 3 in the GLIMPSE study region: compensation and uplift of the Sejourn and Hotu Matua ridge systems, J. Geophys. Res., 111(B11406) Raddick, M. J., E. M. Parmentier, and D. S. Scheirer (2002), Buoyant decompression melting: A possible mechanism for intraplate volcanism, J. Geophys. Res., 107(B10), 2228 Sandwell, D. T., E. L. Winterer, J. Mammerickx, R. A. Duncan, M.A Lynch, D. A. Levitt, and C. L. Johnson (1995), Evidence for diffuse extension of the Pacific plate from Puka Puka and cross-grain gravity lineations, J. Geophys. Res., 100, 15078 Sandwell, D., and Y. Fialko (2004), Warping and cracking of the Pacific plate by thermal contraction, J. Geophys. Res., 109(B10411) Testing the volcanic record for evidence of broad hotspot melting anomalies J.M. O’Connor (1), P. Stoffers (2), J.R. Wijbrans (1), T.J. Worthington (2) and W. Jokat (1) Department of Isotope Geochemistry, Vrije University Amsterdam, The Netherlands, (2) Institute for Geosciences, Christian-Albrechts-University, Kiel, Germany, (3) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany ([email protected]) The notion of a mantle plume has long been that of a mushroom-like ‘head’ and thin ‘tail’ structure rising from a deep thermal boundary layer, generally depicted as the core-mantle boundary. Drifting of tectonic plates over the narrow, presumably fixed, hotspots created by such plume ‘tails’ has long provided an elegant explanation for time-progressive lines of islands, seamounts and ridges. But a major problem cited for this ‘standard’ plume model is a lack of evidence in the volcanic record for head-andtail upwellings. However, recent thermo-chemical numerical modeling is exploring scenarios where upwelling structures are more irregular in shape and behaviour compared to a classic thermal plume ‘head-tail’ (Farnetani and Samuel, 2006). Furthermore, possible entrainment of a deep chemical boundary can vary greatly with strongly temperature-dependent viscosity from nearly stagnant large plumes in the lower mantle to fast episodic pulsations travelling up the pre-existing plume conduit (Lin and van Keken, 2005, 2006). Thus, the classic ‘head- tail’ may not be the only possible upwelling structure. New age data show that the Galapagos Volcanic Province developed via the progression of broad regions of concurrent dispersed volcanism that we link to a correspondingly broad mantle melting anomaly (O’Connor et al., submitted, 2007). Thus, evidence from direct dating of the oceanic hotspot record is also suggesting that hotspot melting anomalies might be much broader than commonly inferred from the ‘headtail’ plume model and the dimensions of individual seamount chains and aseismic ridges. New strategies are therefore needed for investigating the hotspot volcanic record in order to better test the plume hypothesis. To this end we have recently sampled multiple seamount chains and ridges scattered across a broad region of the southern South Atlantic. Our focus 4 is on investigating multiple hotspot chains stretching across a very broad region of the South Atlantic seafloor as a potentially useful way of testing 1) the new thinking that plume upwellings may differ from the classic ‘head-tail’ structure and 2) our evolving hypothesis that hotspot melting anomalies are much broader than suggested by regions of active volcanism marking the young ends of individual hotspot trails. Formation of the North Atlantic Igneous Province: what is the role of the Iceland mantle anomaly? R. Meyer (1), J. van Wijk (2), L. Gernigon (3) (1) Katholieke Universiteit Leuven, Leuven, Belgium, (2) Los Alamos National Laboratory, Los Alamos, USA, (3) Norges Geologiske Undersøkelse, Trondheim, Norway ([email protected]) Several studies have recently challenged the arrival of the Iceland mantle plume around breakup time as an explanation for North Atlantic Igneous Province formation, as some unexplained aspects remain. Alternative processes have been suggested for the wide-spread and excessive magmatism, including delamination, small-scale or edge driven convection, and chemical mantle heterogeneities. We review available datasets and compare them with predictions from the mantle plume and alternative models.