HR: 08:20h
AN: V41G-02 INVITED    [Abstracts]
TI: Concurrent Evaluation of Magma Production, Volcano Growth, and Geochemical Structure in Mantle Plumes: Hawaii Drilling Project (HSDP) Results
AU: * DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Department of Earth & Planetary Science, University of California, MS 4767, Berkeley, CA 94720-4767, United States
AB: The Hawaii Scientific Drilling Project (HSDP) completed its drilling and coring of the northeast flank of the Mauna Kea volcano in early 2007. The project obtained a nearly continuous core consisting of lava flows, hyaloclastite, minor intrusives and sediment from a 3260 m section of the Mauna Kea volcano, covering an age range from 200 to over 600 ka. It also recovered a 280m section of the Mauna Loa volcano. When combined with surface and dredge samples, there now is a 600-700 ky record of the lava output from Mauna Kea as well as a 200 ky record from Mauna Loa. These records can be interpreted in terms of the geochemical structure of the Hawaiian plume, given a model for the sampling of the plume by melting and melt transport. The continuous nature of the HSDP core, with the implied continuous monitoring of the lava output from the volcano, has dictated that we develop models for the plume behavior just below the lithosphere, and for how magma is collected from the plume melting region and supplied to an individual volcano. Although there are as yet no detailed physical models for the melt collection and transport, we have experimented with simple geometric models. These models can be constrained by the volume and volume-age structure of the Hawaiian volcanoes, and by available geodynamic models for the Hawaiian plume. Using these models we can interpret geochemical data from the lavas in terms of plume structure. Any systematic variability in Hawaiian lavas with depth (age) in the drillcore can be attributed to structure in the plume, and one of the interesting results is that there is such structure even though melting within the plume samples only the innermost third or so the plume radius. The data show that there is radial geochemical zoning of the melting region of the plume in terms of He, Pb, Nd, Sr and Hf isotopes. This geochemical structure represents the hot core of the plume and does not reflect entrainment of ambient lower or upper mantle. To first order, the radial component of the geochemical structure of the plume represents the vertical structure at the thermal boundary layer from which the plume originates. In the case of Hawaii, all of the lavas are derived from melting of mantle that originates from within 20-50 km of the base of the mantle. One of the most striking characteristics of the HSDP data is that the high 3He/4He anomaly is nested within the innermost core of the melting region of the plume and is much larger in amplitude and much smaller in diameter than the Nd, Sr and Hf anomalies. The He-3 anomaly apparently has a different origin than the other anomalies, and is restricted to the lowermost 10-20 km of the mantle plume source. The helium signal is therefore likely to come either directly from the Earth's core via leakage across the core-mantle boundary, or from a dense layer separating the main mantle from the outer core. The Hawaiian plume contains little evidence for a hidden light- REE enriched reservoir near the core-mantle boundary, but could potentially be considered to indicate the presence of a primitive layer if the bulk Earth is not chondritic with respect to REE.
UR: http://www.icdp- online.de/sites/hawaii/
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting