HR: 0800h
AN: V41D-1501    [Abstracts]
TI: Highly Siderophile Element Systematics in Mantle Rocks from the Oman Ophiolite: Evidence for Ancient Crustal Components in the Upper Mantle
AU: Kurth-Velz, M
EM: michaela.kurth@uni-koeln.de
AF: Institute for Mineralogy and Geochemistry, University of Koeln, Koeln, 50674 Germany
AU: * Bruegmann, G E
EM: bruegman@mpch-mainz.mpg.de
AF: Max-Planck-Intitut fuer Chemie, Becherweg 27, Mainz, 55020 Germany
AU: Palme, H
EM: palme@min.uni-koeln.de
AF: Institute for Mineralogy and Geochemistry, University of Koeln, Koeln, 50674 Germany
AB: This study describes Os isotope systematics and the distribution of highly siderophile elements in mantle peridotites from different tectonic levels along the paleo-ridge of the 90 Ma-old Oman ophiolite. It reveals a large spread in Os isotopic composition indicating an along-ridge and a bottom to top isotope heterogeneity in the upper mantle. The Maqsad diapir is considered as a fossil zone of focused upwelling of melt beneath a spreading center. The deep mantle harzburgites belong to the most depleted members of oceanic lithosphere (Yb_N in cpx: 1.5; Ti_N in cpx: 0.5; Cr Number of spinel: 0.5) when compared with abyssal peridotites. The chondrite-normalised PGE_N patterns are flat or depleted towards Pd and Re (PGE_N: 0.01-0.005). Their Os isotopic compositions indicate a moderately to highly depleted upper mantle (¹⁸⁷Os/¹⁸⁸Os: 0.1248-0.1150). Plag-bearing dunites and chromitites of the mantle-crust transition zone (TZ) have clinopyroxenes with MORB-type trace element abundances (Yb_N: 2-6; Ti_N: 0.7-5). The PGE_N patterns of the dunites are melt-like in that they are enriched in Pd and Re relative to Ir, Os and Ru and they are similar to pyroxenite dikes (PGE_N: 0.0003-0.01). The radiogenic samples have lower Os concentrations (0.55 and 2.42 ppb) than the deep mantle peridotites The Os isotope composition of the TZ samples is radiogenic (¹⁸⁷Os/¹⁸⁸Os: 0.1317-0.1455). In a ¹⁸⁷Os/¹⁸⁸Os vs 1/Os diagram the radiogenic samples define a mixing line having depleted mantle and radiogenic pyroxenite dykes (¹⁸⁷Os/¹⁸⁸Os > 0.21510) as end members. It is suggested that the isotopic heterogeneity within a single diapir is the result of two processes. Firstly unradiogenic peridotites formed during an ancient melting process (> 2 Gyr). Afterwards its isotope signature has been overprinted by the migration of radiogenic melts 90 Ma ago. During partial melting Cu-Pd-Re-rich (relatively radiogenic) sulfide liquid preferentially enters the melt leaving behind unradiogenic, Ir-, Os-, Ru-enriched Fe-rich sulfides. On the assumption that the Sumail diapir did not form above a subduction zone the origin of radiogenic melts could be related to the melting of radiogenic dikes incorporated within the mantle during ancient melting events or remains of old recycled oceanic crust.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005