HR: 0800h
AN: V51B-0550    [Abstracts]
TI: Extreme Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence for an Ancient Recycled Lithosphere
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: Dept. Earth Sciences, FIU, 11200 SW 8th Street,, Miami, FL 33199 United States
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: NHMFL and Dept. Geological Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306 United States
AU: Lassiter, J C
EM: lassiter1@mail.utexas.edu
AF: Max Plank Institute f. Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Salters, V J
EM: salters@magnet.fsu.edu
AF: NHMFL and Dept. Geological Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Dept. Earth Sciences, FIU, 11200 SW 8th Street,, Miami, FL 33199 United States
AU: Griselin, M
EM: griselin@mpch-mainz.mpg.de
AF: Max Plank Institute f. Chemie, Postfach 3060, Mainz, 55020 Germany
AB: We report on the first combined Hf-Os isotope systematics of spinel peridotite xenoliths from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents from the island of Oahu, Hawaii. These peridotites are thought to represent the Pacific oceanic lithosphere beneath Oahu, as residues of MORB-type melting at a paleo-ridge some 80-100Ma ago. Clinopyroxene mineral separates in these peridotites have very similar Nd and Sr isotope compositions with the post erosional Honolulu Volcanics (HV) lavas that bring these xenoliths to the surface. This and their relatively elevated Na and LREE contents suggest that these peridotites are not simple residues of MORB-type melting but have experience some metasomatic enrichment by the host HV lavas. However, the SLC and PK xenoliths show an extreme range in Hf isotope compositions towards highly radiogenic values ($\epsilon$$_{Hf}$= 7-80), at nearly constant Nd isotope compositions ($\epsilon$$_{Nd}$= 7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu peridotites show a bimodal distribution in their bulk rock $^{187}$Os/$^{186}$Os ratios: the PK peridotites have similar ratios to the abyssal peridotites (0.130-0.1238), while the SLC peridotites have highly subchondritic ratios (0.1237-0.1134) that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes show a broad negative correlation whereby the samples with the most radiogenic $^{176}$Hf/$^{177}$Hf have the most unradiogenic $^{187}$Os/$^{186}$Os ratios. Based on their combined Hf-Os-Nd isotope and major element compositions, the PK peridotites can be interpreted as fragments of the Hawaiian lithosphere, residue of MORB melting 80-100Ma ago, that have been variably metasomatized by the host HV lavas. In contrast, the extreme Hf-Os isotope compositions of the SLC peridotites suggest that they cannot be the source nor residue of any kind of Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism or melt-rock reaction that has overprinted the Nd and Sr isotope compositions of these peridotites. The ancient ($>$1Ga) melt depletion event recorded by both the low $^{187}$Os/$^{186}$Os and high $^{176}$Hf/$^{177}$Hf ratios in the SLC peridotites can be explained with two different scenarios. First, the SLC peridotites may represent ancient depleted lithosphere that survived subduction, remained "rafting" in the upper mantle and is now sampled beneath Oahu. However, the lack of such unradiogenic Os isotopes in both MORBs and abyssal peridotites suggests that such peridotites are rare in the upper mantle and makes their exclusive presence under Oahu a rather fortuitous coincidence. Alternatively, the SLC peridotites may represent ancient depleted recycled lithosphere brought up by the Hawaiian plume. A recycled oceanic crust origin has been previously invoked for the Koolau shield lavas. It is then conceivable that fragments of the lithospheric portion of that subducted package have remained coupled with the oceanic crust and are being brought up by the plume from the deep, but because they were previously depleted, these peridotites contribute minimally, if at all, to Hawaiian volcanism. The presence of microdiamonds and majoritic garnets in some SLC pyroxenites also corroborates a deep origin. In this case, the SLC peridotites represent the first-ever direct evidence that subducted material actually makes it back on the surface, essentially closing the subduction cycle.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting