HR: 10:50h
AN: V52B-03    [Abstracts]
TI: U-series Isotopic Evidence for Remelting of Kilauea Volcano's Mantle Source Region During the Puu Oo Eruption
AU: * Pietruszka, A J
EM: apietrus@geology.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, San Diego, CA 92182-1020 United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Garcia, M O
EM: garcia@soest.hawaii.edu
AF: Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822 United States
AB: Partial melting of the mantle to produce ocean-island and mid-ocean ridge basalts is generally assumed to be a steady-state process in which the continuous flux of upwelling solid into the melting region is balanced, after melt generation and extraction, by the permanent removal of the residue from the system. Here we present high-precision measurements of the U-series isotope abundances of lavas from the current Puu Oo eruption of Kilauea Volcano to investigate the dynamics of partial melting within the actively upwelling Hawaiian mantle plume. Our results show that Puu Oo lavas display small, but significant, temporal decreases in their 230Th-238U and 226Ra-230Th disequilibria from 2.5 to 1.4% excess 230Th and 14 to 12% excess 226Ra (relative to our +/-2 sigma analytical uncertainty of 0.3%). These changes in the (230Th)/(238U) and (226Ra)/(230Th) activity ratios of the lavas correlate systematically with larger decreases in the abundance ratios of highly versus moderately incompatible trace elements from a maximum of 23% for Ba/Yb to a minimum of 4% for Nd/Sm. These geochemical signatures cannot be explained by an increase in the degree of partial melting during the eruption. Instead, this volcano has tapped a mantle source component that was depleted of incompatible elements by "recent" prior melting. The systematic correlation between the 230Th-238U and 226Ra-230Th disequilibria and these trace element ratios shows that this depletion must have occurred within several half-lives of 226Ra ($<$8 kyr ago) because 226Ra will return to secular equilibrium with 230Th on this time scale. Furthermore, modeling indicates that the depletion of the volcano's source region may have occurred during the Puu Oo eruption itself. Thus, we propose that relatively fertile mantle within the Hawaiian plume may have been progressively exhausted during the eruption, allowing the increasingly depleted, and thus, more refractory, residue to continue melting. Our results suggest that the dynamics of melt extraction from the mantle may have a dramatic effect on the geochemical signatures of basaltic lavas.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting