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Mantle Plumes and Geodynamics of Earth's Interior

Mantle Plume and Geodynamics of Earth's Interior Figure 1

Click image to enlarge. Cartoon depicting 2 possible modes of convection in Earth's mantle (left-zoned convection, right-whole mantle convection). Mantle plumes provide the fuel for many volcanic hotspots such as Hawaii. Iceland and Galapagos. Mid-ocean ridges tap the shallow upper mantle. Subduction zones are areas where tectonic plates are recycled into Earth's interior.

The debate in recent years over the existence of mantle plumes and models for the structure of the mantle have produced a resurgent interest in noble gas geochemistry. Fluids and magmas expelled at the Earth's surface, such as hydrothermal gases and volcanic rocks, provide key samples for investigating these topics. Notably, the isotopes of the noble gases provide unique clues bearing on structure and dynamics of the mantle. Terrestrial noble gases differ in their isotopic makeup from the primordial values found in early solar system material because they have been modified by radioactive decay of uranium (U), thorium (Th) and potassium (K), the major heat-producing elements in the Earth. The ratio of primordial to radiogenic noble gases in the mantle therefore reflects the time-integrated ratio of primordial noble gas to U, Th and K. For example, the relatively high 3He/4He ratio observed in ocean island basalts (OIBs) from localities such as Hawaii and Iceland indicate a mantle source that is characterized by high 3He/(U+Th). This OIB source has higher 3He/4He compared to mid-ocean ridge basalts (MORBs), and is therefore less degassed and generally considered to lie below the upper mantle source for MORBs. Additional evidence comes from Ne isotope studies. Elevated 21Ne/22Ne is found where enhanced 21Ne production occurs by nucleogenic processes.; the most significant production of 21Ne in the mantle is from alpha-particles (4He atoms produced by the radioactive decay of U and Th) that collide with 18O in mantle silicates. However, elevated ratios of 20Ne/22Ne cannot be explained by this type of process, and instead are best accounted for by incorporation of solar-derived Ne in the early Earth. The fact that OIBs from Hawaii, Iceland and Galapagos have elevated 20Ne/22Ne along with low 21Ne/22Ne, when compared with MORBs, is consistent with the presence of a deep, relatively undegassed mantle plume source for some ocean islands.

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