Mantle - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2006 by Jeffrey S. Barker) 11. The Mantle Composition We can't dig down into the Mantle and analyze what we find. Instead, we develop models of mantle composition which explain as much of our observations as possible without becoming too complicated. Our models need to satisfy a number of constraints: We must be able to generate oceanic crustal material by partial melting of mantle material. This is simply the outermost process of fractionation which began with the magma ocean stage of Earth's accretion. The model must be solid for all pressures and temperatures within the Mantle (S waves propagate through the Mantle). The model compositon must have physical properties which agree with our observations of the P-wave velocity at the top of the Mantle (Mohorovicic's observations), must generate the 2nd-order velocity discontinuities of the Transition Zone (at 400 and 700 km depth), and must generate a Low-velocity Zone, at least at some locations in the Earth. The model compositon must agree with our density models of the Earth. The model must be consistent with the indirect observations we have of mantle materials. These are indirect simply because the process of bringing these materials to the surface alters them, and we must account for this alteration before interpreting them for mantle composition. Kimberlite Pipes - These are believed to be the remnants of volcanic pipes or conduits which rapidly brought materials to the surface from depths within the mantle. Evidence for this is that several Kimberlite pipes in older continental areas contain diamonds, which could have GEOL 114 1 11. The Mantle
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formed only at pressures found at depths within the Mantle. Thus, other materials delivered to the surface with these diamonds should also represent Mantle materials, although the process of bringing them rapidly to the surface may well have altered them somewhat. In many of these Kimberlites, the mineral assemblages (rocks) are garnet peridotites and eclogites. Nodules in Hawaiian volcanic rocks - Active volcanos in Hawaii are caused by "hot spots" in the Mantle (we'll discuss hot spots in more detail in a later lecture). Found in the volcanic rocks of Hawaii are small pieces (nodules) of rock which may be Mantle material delivered to the surface. These are similar to the rocks found in Kimberlites: spinel peridotites. Oceanic fracture zones - Once again, we'll discuss these in more detail when we cover Plate Tectonics, but these are faults on the ocean floor, some of which expose deeper rocks to the surface. Because they are underwater, severe chemical alteration takes place, but this can be simulated in the lab. In some locations, serpentinized peridotites are found. Ophiolites
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This note was uploaded on 11/02/2010 for the course GEOLOGY 111 taught by Professor Howardr.naslund during the Spring '10 term at Binghamton.

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Mantle - GEOL 114 The Earth's Dynamic Interior Lecture...

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