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The Generation of Melting Anomalies by Plate Tectonic Processes Gillian R. Foulger University of Durham .... based on ideas developed over several years by a working group that includes .... Don Anderson, Warren Hamilton, Jerry Winterer, Jim Natland, Dean Presnall, Peter Vogt, Anders Meibom, Hetu Sheth, Seth Stein, Mike O’Hara & Alan Smith .... etc for more see www.mantleplumes.org Two key elements: 1. Variations in lithosphere stress 2. Mantle inhomogeneity • • Simply put Stress governs location of volcanism Fusibility governs volume of magma Mantle dehomogenising • ridges MELT experiment EPR Mantle dehomogenising • ridges • subduction zones • eclogitisation of subducted crust Eclogite is fusible Pyrolite A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite Eclogite Yaxley (2000) Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup Cantal basalts model, Massif Central, France Pilet et al. (2005) Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup QuickTime™ and a GIF decompressor are needed to see this picture. Schott et al. (2000) Mantle dehomogenising • metasomatism of oceanic and continental mantle lithosphere • delamination of thickened lithosphere, including lower crust • erosion of continental lithosphere during breakup Mantle dehomogenising adapted from Meibom & Anderson (2003) Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength From Natland, 2004 Variations in stress • Lithosphere cooling • Spatial and temporal variations in plate boundary type & tectonics • Variations in lithosphere strength adapted from Lundin & Doré (2005) Proposal “Hot spot” volcanism occurs where – stress is extensional – mantle is highly fusible Examples “Hot spots” on MORs • 1/3 of all “hot spots” are on or near MORs East African Rift • Afar • Other EAR “hotspots”? Basin & Range Province • Broad, intraplate extensional region • Associated with subduction of “Farallon slab” • Widespread volcanism NAVP & Iceland • Formed when continent rifted along Iapetus suture • Diverse data suggest not hot • Recycled Iapetus crust can explain geochemistry & melt volume Closure of the Iapetus Azores – – – – Kinematic models EQ focal mechanisms bathymetry suggest Azores branch is: – oblique, ultra-slow spreading (3-4 mm/yr) – diffuse plate boundary – dextral differential shear motion Lourenço et al. (1998) Time-progressive volcanism Predictions • Melt volumes can be explained by lithosphere extension + source fertility • Vertical motions related to shallow tectonic processes • Upper mantle is inhomogeneous • Migration of volcanism = migration of locus of extension Predictions • Seismic tomography anomalies indicate composition and melt, not only temperature • “Hot spot” lavas not required to be hot • Geochemistry can be explained by inhomogeneities in the shallow mantle That’s all folks! Example: mantle potential temperature, Iceland Temperature (Tp) from petrology • mid-ocean ridges: ~1280 - 1400˚C • Iceland: ~1280 - 1460˚C • Hawaii: ~1560˚C The only place on Earth hot enough for a weak upper-mantle plume is Hawaii Seismology does not reliably detect them in the lower mantle Vertical exaggeration x 10 DT ~ 200˚C DT ~ 60˚C Iceland Ritsema & Montagner (2003) Example: whole-mantle tomography: Iceland Ritsema et al. 1999 Please read our book: Plates, Plumes & Paradigms Iceland: A plume from the core-mantle boundary? Hudson Bay plume? Bijwaard & Spakman (1999) Resolution of the “whole-mantle plume” The data used by Bijwaard & Spakman have no resolution in the lower mantle (courtesy of Karason & van der Hilst) From Foulger et al. (2001)