Mantle Plumes and Intraplate Volcanism Origin of Oceanic Island Volcanoes EAS 302 Lecture 20 Volcanism on the Earth • Mid-ocean ridges (>90% of the volcanism) – “constructive” plate margins • Subduction-related (much of the rest) – “destructive plate” margins • Volcanism in plate interiors (usually) – , e.g., Yellowstone, Hawaii not explained by the plate tectonic paradigm. Characteristics of Intra-plate Volcanoes • Not restricted to plate margins. • Occur at locations that are stationary relative to plate motions, “hot spots”(pointed out by J. T. Wilson, 1963). • Distinctive isotopic and trace element composition. Hot Spot Traces on the Pacific Ocean Floor The Mantle Plume Model • “ Hot spot” volcanoes are manifestations of mantle plumes: columns of hot rock rising buoyantly from the deep mantle – This idea proposed by W. J. Morgan in 1971. • Evidence – Maintain (almost) fixed positions relative to each other; i.e., they are not affected by plate motions – A number of “hot spots” are associated with “swells”, indicative of hot mantle below – Their magmas are compositionally distinct from mid-ocean ridge basalts and therefore must be derived from a different part of the mantle Current Mantle Plumes The Hawaiian Mantle Plume Age of Hawaiian Volcanism The Hawaiian “Swell” Plumes at the surface • In the last 100-200 km, the plume begins to melt. • Once it reaches the base of the lithosphere, it can no longer rise and spreads out. Isotopic Compositions of Oceanic Island Basalts • Nd and Sr isotope ratios 12 DMM distinct from MORB: 10 derived from separate MORB 8 reservoir which is less 6 depleted (and Society ε Nd 4 sometimes enriched) in HIMU incompatible elements. 2 0 • Isotopic compositions 0 St. Helena EM II fall into just a few -2-2 Kerguelen groups: there are only a -4-4 EM I few “flavors” of plumes. -6-6 .702.702 .703.703 .704.704 .705.705 .706.706 .707.707 .708.708 8787Sr/8686Sr Chemical Histories of Mantle Plumes • Apparently, the material in mantle plumes has evolved through just a few pathways. What are these? No one is certain. Some ideas: – 1. Mixing between primitive and depleted mantle – 2. Recycling of oceanic crust and sediment – 3. “Delamination” of the mantle lithosphere beneath the continents, which could be enriched in incompatible elements by melts or fluids migrating into it from below – 4. Component of core material in some plumes? In ideas 2 & 3 above, the material, being dense, would sink to the core-mantle boundary where it would be slowly reheated until it is buoyant enough to rise. a “Primitive” plume source • Mantle Plumes come from a deep “primitive” (i.e., chemically unprocessed since the Earth formed) reservoir. • But their isotopic and trace element compositions are not “primitive”. Recycling of Oceanic Crust • Oceanic crust and and Sediment sediment subducts and sinks to the deep mantle. – This is because it is cold, and rich in iron. • This material is heated by the core, and eventually becomes hotter than overlying mantle. • When hot enough, it rises buoyantly as plumes. Recycling of Subcontinental Lithosphere • Lithosphere beneath continents enriched in incompatible elements by melts from below. • Collisions “delaminate” this lithosphere, which sinks to deep mantle. Where do mantle plumes come from? • must be generated at thermal boundary layer: two possibilities – 660 km discontinuity – the core- mantle boundary D’’ as the source of plumes? • Hot spots seem to be located over regions of high seismic wave velocity gradient in the lower mantle. Seismic Confirmation of Deep Mantle Plumes Montelli et al., Science 303: 338, 2004 Mantle Plumes, Large Igneous Provinces, and Climate • Theory says that new plumes need large heads to initiate buoyant rise. • When these “heads” reach the surface, they produce large pulses of volcanism, know as “flood basalts”, “plateau basalts”, “oceanic plateaus” or “large igneous provinces”. • CO2 released by these events may change climate. Flood Basalts and LIPS (Large Igneous Provinces) Cretaceous Flood Basalts • Numerous new plumes surfaced in the Cretaceous, creating “oceanic plateaus” and flood basalts. • Were these plumes responsible for the warm Cretaceous climate?.