Stop the Presses! New discovery about the origin of tetrapods!

Transcription

1 Mantle Plumes and Intraplate Volcanism Origin of Oceanic Island Volcanoes Lecture 20 Stop the Presses! New discovery about the origin of tetrapods! Tiktaalik rosaea

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3 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.

4 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 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

5 Current Mantle Plumes The Hawaiian Mantle Plume

6 Age of Hawaiian Volcanism The Hawaiian Swell

7 Plumes at the surface In the last 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 distinct from MORB: derived from separate reservoir which is less depleted (and sometimes enriched) in incompatible elements. Isotopic compositions fall into just a few groups: there are only a few flavors of plumes ε Nd DMM HIMU St. Helena MORB Kerguelen EM I Society EM II Sr/86Sr

8 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. 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.

9 Recycling of Oceanic Crust and Sediment Oceanic crust and 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.

10 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.

11 Finally, 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. CO 2 released by these events may change climate.

12 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?