HR: 0800h
AN: V51B-0530    [Abstracts]
TI: The Importance of Being Plumes: Entrainment, Isotopes, Melting and the Vertical Structure of the Earth's Mantle
AU: * DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Weaver, K L
EM: karrie@eps.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720 United States
AB: The standard approach to hot spots has been to associate their geochemical characteristics with the lower mantle. This, along with arguments about material balance, has led to the notion that the composition of the mantle beneath some depth horizon (e.g. 670 km) is different from that above. This particular picture is difficult to reconcile with ideas of whole mantle convection. Plumes, however, are generally imagined as originating from a thermal/compositional boundary layer, and probably at or near the base of the mantle. Thus the fact that plume volcanic rocks are different from, e.g. MORB, may mean only that the base of the mantle is distinct from the rest of the mantle, and that there need be no major compositional discontinuity suspended somewhere in the mid-mantle. This point can be generalized - we should be able to get more information from hot spots with better models and more systematic sampling of lava flows. Theoretically, if plumes originate near the bottom of the mantle, and then entrain some ambient surrounding mantle on the way to the surface, the radial structure of a plume should mimic the vertical structure of the mantle. The full cross-section of the plume however, does not necessarily pass through the melting zone, especially under thick lithosphere, so the lavas represent only a fraction of the plume: the hottest part. In areas where the lithosphere is thin, e.g. where plumes impinge on very thin lithosphere, almost the full cross section of the plume (and hence the full vertical section of the traversed mantle) should be represented in the lavas. We use this concept along with available geodynamic models to compare the observations at Hawaii (thick lithosphere) with Iceland (thin or no lithosphere along the ridge). In Hawaii the highest value of 87Sr/86Sr is about 0.7037 near the plume axis (although it varies somewhat with time). In Iceland the value is not much different at 0.7036. However, in Hawaii the lowest values in the shield stage tholeiites are only slightly lower: 0.7035. In Iceland the values extend continuously all the way down to the local MORB values of 0.7027. In Hawaii, the width of the sampled region is about 100 km, whereas in Iceland the width of the sampled region along the ridge is about 600 km. There is consistency between these patterns that is explained by the effect of lithosphere thickness. Other relationships like this apply for Nd, He, and Pb, and the length scales are not all the same. The details of these distributions, if properly placed into a context of plumes, entrainment and melting beneath the lithosphere, could provide more information about the vertical structure of the mantle, and detailed information about the structure of the base of the mantle. A limiting factor at present is geodynamic models to describe the plumes and melting.
DE: 8450 Planetary volcanism (5480)
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7207 Core and mantle
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting