HR: 17:00h
AN: V42H-05 INVITED     [PDF]
TI: A Physical Description of the Lithosphere and Seismic Low-Velocity Zone beneath Oahu: Perspectives from Hawaiian Mantle Xenoliths
AU: * Sen, G
EM: seng@fiu.edu
AF: Florida International University, University Park, Miami, FL 33199 United States
AU: Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: Florida International University, University Park, Miami, FL 33199 United States
AU: Keshav, S
EM: skesha01@fiu.edu
AF: Florida International University, University Park, Miami, FL 33199 United States
AB: Mantle xenoliths provide a random sampling of a significant portion of the uppermost mantle beneath Oahu, including all of the lithosphere and the lithosphere-asthenosphere transition (LAT). We combine all the petrological, geochemical, and isotopic information on these xenoliths and evaluate existing models of the lithosphere and LAT beneath Oahu. The xenoliths are principally of two kinds - spinel peridotites and garenet clinopyroxenites; and they occur within the post-erosional Honolulu Volcanics (HV). Plagioclase peridotites are rare, and plagioclases in them are exotic, having crystallized from Koolau-like (enriched) magma. Trace element and Nd-Hf isotope ratios indicate recent metasomatic enrichment of the depleted peridotitic lithosphere by HV-like fluids. The pyroxenite suite xenoliths are dominantly composed of clinopyroxene and garnet with variable proportions of olivine, minor spinel and orthopyroxene, and rare phlogopite, carbonate, amphibole, and melt pockets. Phase equilibrium considerations indicate that these rocks form accumulates at the LAT and along the walls of fracture-conduits within the deep lithosphere. Nd-Sr isotope data on whole rocks and garnet and cpx separates show them to be depleted as well, partially overlapping with the host HV lavas and lithospheric spinel peridotite xenoliths. We view the Oahu lithosphere (below the crustal part) as follows: 20-30 km - depleted harzburgite with veins of plagioclase; 30 - 90 km - spinel peridotite layer (we model the vertical modal variation), 90 -130 km - cumulate garnet clinopyroxenites and "piclogites" with small amounts of (5 percent) intergranular H2O+CO2-rich fluids that may be of kimberlitic-carbonatite affinity. Our inferred compsition of the lithosphere and LAT are generally similar to geophysical models but predict a 200-250oC lower temperature within the LVZ beneath Oahu than the latter. In our model, Hawaiian plume-derived shield-stage magmas have little interaction with the lithosphere, suggesting that these magmas are efficiently transported via dikes. During the post-erosional stage, limited interactions occur between the lithosphere and fluids that escaped from the main HV conduits. We estimate that the post-erosional HV magmas were generated from a minimum depth of about 100-130 km, and that Koolau shield magmas were produced from even deeper levels based on the rarity/absence of cumulate/residue type xenoliths with such compositional affinity.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8450 Planetary volcanism (5480)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting