HR: 11:40h
AN: V22A-06    [Abstracts]
TI: Upper Mantle Geochemistry at Peridotites of Site 1274 (ODP Leg 209): Relation to Melt-Rock Reaction and Processes at the Base of the Lithosphere
AU: * Suhr, G
EM: suhr@min.uni-koeln.de
AF: Dept. of Mineralogy, University Koeln Zuelpicher Str. 49b, Koeln, 50674 Germany
AU: Paulick, H
EM: holger.paulick@uni-bonn.de
AF: Dept. of Mineralogy, University Bonn Poppelsdorfer Schloss, Bonn, 53115 Germany
AB: ODP Leg 209, Site 1274, has penetrated 156 m of upper mantle rocks plus minor gabbro intrusives north of 15\deg20' N Fracture Zone at the Mid Atlantic Ridge. The core has an unusually high amount of dunite (21%) and contains an unusual suite of opx-depleted harzburgites and dunites between 70 and 90 m depth. Between 0 and 70 m depth, harzburgites are chemically highly depleted (Cr\# in spinel 37 to 52, TiO2 in cpx $<$ 0.07% with negative correlation to Cr\#). The chemistry of these harzburgites appears to preserve evidence for progressive melt extraction. A section of 3 m dunite occurring within this interval largely shares the depleted chemical signature, though TiO2, Na2O and REE in cpx are somewhat elevated relative to the local host rocks. In the interval between 70 and 90 m depth, Cr\# are 45-57 and cpx is enriched in TiO2 (0.09 to 0.24%) and Na2O (up to 1%). Microstructural evidence for melt-rock reaction (e.g. cpx rims on opx) is pervasive throughout the core. The pristine preservation of these microstructures argues for efficient freezing during a rapid uplift. The percolating melts appear derived relatively locally because chemical indicators for foreign melts are weak. In the interval between 70 and 90 m, melt infiltration from a more remote source is suggested by the presence of strong reactive tendencies (opx depleted peridotites and dunites) and a more enriched chemical signature in all rocks. Probably, the opx-depleted rocks formed when focused melt channels failed near the lithosphere-asthenosphere boundary. In the upper part of the core, the depleted chemical signature of the dunite is interpreted as derived by melt extraction from the host into the dunite at a late stage, perhaps triggered by a fracture in the overlying lithosphere. Since locally generated melts are only weakly reactive towards opx, the formation of the dunite itself (by dissolution of opx from harzburgite) cannot be explained by locally generated melt. Instead, the early passage of more reactive melts derived from larger depth seems likely. Chemical clues to the former passage of different melts in this dunite are the occurrence of an enriched cpx inclusion in spinel in dunite and preliminary data suggesting that the internal part of the dunite may record the passage of a melt with a more radiogenic Os signatures than the highly unradiogenic harzburgites and marginal dunites. It thus appears that melt infiltration and extraction can occur in close proximity, depending on the local physical properties of the lithosphere-asthenosphere boundary.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting