HR: 16:45h
AN: V42H-04    [PDF]
TI: Volatile-Rich Mineral Phases in the Hawaiian Lithosphere: Phlogopites and Carbonates in 0-age Garnet Pyroxenite Xenoliths From Salt Lake Crater, (Oahu, Hawaii).
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: Dept. Earth Sciences Florida International University, PC 344, 11200 SW 8th St., Miami, Fl 33199 United States
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: NHMFL and Dept. of Geological Sciences, Florida State University, 1800, E. Paul Dirac Dr., Tallahassee, Fl 32306 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Dept. Earth Sciences Florida International University, PC 344, 11200 SW 8th St., Miami, Fl 33199 United States
AU: Salters, V J
EM: salters@magnet.fsu.edu
AF: NHMFL and Dept. of Geological Sciences, Florida State University, 1800, E. Paul Dirac Dr., Tallahassee, Fl 32306 United States
AB: We present the first detailed Hf, Nd, Sr isotope and trace and major element investigation on clinopyroxene (cpx), garnet, phlogopite and carbonate mineral separates from garnet pyroxenite xenoliths from the Salt Lake crater, Oahu, Hawaii. These xenoliths are brought to the surface by the post-erosional lavas that mark the last stage of the evolution of a Hawaiian volcano. Previous studies have suggested that these pyroxenites are either high pressure cumulates associated with the Honolulu volcanics (HV) post erosional lavas, or MORB-related cumulates at a 80-100 Ma paleo ridge. In both Nd-Hf and Nd-Sr isotope spaces all cpx and garnet data are within the range of the HV and other Hawaiian post-erosional lavas. The Lu-Hf and Sm-Nd isotope systematics of the cpx - garnet pairs give essentially 0 ages, providing the first direct evidence that these garnet pyroxenites are indeed 0-age high pressure cumulates from melts isotopically similar to the HVs. Some 0-age pyroxenites also contain veins with volatile-rich mineral assemblages (phlogopite, carbonates and associated glass pockets), that have infiltrated and reacted with the cpx-garnet matrix. These phlogopites have identical $^{143}$Nd/$^{144}$Nd ratios as the cpx (0.51303) but significantly more radiogenic $^{87}$Sr/$^{86}$Sr (0.7032 - 0.7033 in cpx, 0.70365 - 0.70385 in phlogopites, carbonates and glass). These minerals have higher $^{87}$Sr/$^{86}$Sr (for a given $^{143}$Nd/$^{144}$Nd) than MORBs or HVs and fall at the extension of the horizontal displacement of the post shield and post erosional Hawaiian lavas from the Pacific MORB field in Nd-Sr isotope space. Isotopically then, these volatile phases are more similar to the post-erosional lavas than the shield (Koolau) tholeites, providing new evidence for the involvement of volatiles in the generation of the post-erosional volcanism. The association of these volatile-rich phases with garnet pyroxenites and their absence in the more shallow, spinel peridotites points to a deep (i.e. non-lithospheric) origin for these volatiles. We speculate that these volatile-rich melts originate from the cooler periphery of the radially layered Hawaiian plume, either as direct plume melts, or melts from the upwelling asthenosphere that is passively coupled with the rising plume. These volatiles can act as a fluxing agent causing or enhancing melting in the source of the post-erosional lavas, and contribute to their relatively radiogenic Sr isotope compositions.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting