HR: 13:55h
AN: DI43A-02    [Abstracts]
TI: Volatiles and Trace Elements in Rejuvenated-Stage Lavas from Ni'ihau, Hawai'i: Evidence for Silicate Melt and Carbonatite Metasomatism of the Mantle Surrounding the Hawai'ian Plume
AU: * Dixon, J E
EM: j.dixon@miami.edu
AF: University of Miami, PO Box 249176, Coral Gables, FL 33124-9176, United States
AU: Clague, D
EM: clague@mbari.org
AF: MBARI, 7700 Sandholt Rd., Moss Landing, CA 95039, United States
AU: Cousens, B
EM: bcousens@earthsci.carleton.ca
AF: Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
AU: Monsalve, M L
EM: mmonsalve@rsmas.miami.edu
AF: University of Miami, PO Box 249176, Coral Gables, FL 33124-9176, United States
AB: We present new volatile, trace element, and radiogenic isotopic compositions for submarine rejuvenated-stage lavas erupted on the northwest flank of Ni'ihau. These lavas are mildly alkalic and are isotopically similar to rejuvenated-stage lavas from Ni'ihau and other Hawai'ian islands. They display trace element heterogeneity, however, greater than that of rejuvenated-stage lavas from other islands, with enrichments in Ba, Sr, and light- rare earth elements resulting in high and highly variable Ba/Th and Sr/Nd. The highest Ba/Th (354) are in the least silica-undersaturated lavas, implying that the greatest enrichments are associated with the largest extents of melting. Ni'ihau lavas also have high and variable H₂O/Ce and Cl/La (620 and 39, respectively). We model the trace element concentrations of most rejuvenated-stage lavas by small degrees (1-15%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5-1% melting) from the Hawai'ian plume, similar to the model of Yang et al. (2003, J.Pet. 44). Compositions of Ni'ihau rejuvenated- stage lavas are best explained by addition of 0.1 to 0.4% carbonatite, similar in composition of oceanic carbonatites from the Canary and Cape Verde Islands, to a depleted peridotite less enriched in incipient melt than that for other rejuvenated-stage lavas. Primary H₂O and Cl of carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments (Dasgupta et al., 2006, J.Pet. 47) in which 1) carbonated eclogite within the Hawai'ian plume is the first to melt during plume ascent, 2) carbonatite melt metasomatizes the plume itself and the surrounding asthenosphere, 3) as the plume rises, silica-undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is only preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 3621 Mantle processes (1038)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8145 Physics of magma and magma bodies
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting