HR: 0800h
AN: U21B-0418    [Abstracts]
TI: Recycled MORB in the Convecting Upper Mantle? Evidence From Os, Sr and Nd Isotopes in Orogenic Peridotites and Pyroxenites From the Totalp Massif, Eastern Switzerland
AU: * van Acken, D
EM: dvana@zedat.fu-berlin.de
AF: Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, AB Geochemie, Malteserstr. 74 - 100, Berlin, 12249, Germany
AU: * van Acken, D
EM: dvana@zedat.fu-berlin.de
AF: University of Maryland, Department of Geology, College Park, MD 20742, United States
AU: Becker, H
EM: hbecker@zedat.fu-berlin.de
AF: Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, AB Geochemie, Malteserstr. 74 - 100, Berlin, 12249, Germany
AU: Becker, H
EM: hbecker@zedat.fu-berlin.de
AF: University of Maryland, Department of Geology, College Park, MD 20742, United States
AU: Weynell, M
EM: marcwey@freenet.de
AF: Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, AB Geochemie, Malteserstr. 74 - 100, Berlin, 12249, Germany
AU: Wombacher, F
EM: fwo@zedat.fu-berlin.de
AF: Freie Universitaet Berlin, Institut fuer Geologische Wissenschaften, AB Geochemie, Malteserstr. 74 - 100, Berlin, 12249, Germany
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: University of Maryland, Department of Geology, College Park, MD 20742, United States
AB: The presence of recycled mafic material in the MORB source region remains a subject of debate. Some orogenic peridotite massifs include materials from the deep lithosphere and may thus provide information on the asthenospheric sources of melts. The Totalp ultramafic massif in eastern Switzerland, which formed part of the Jurassic ocean floor in the ultraslow spreading Piedmont-Liguria ocean basin, is a site, where the deep lithosphere may be studied. Serpentinized fertile spinel lherzolites are interlayered with folded and stretched websterites and modally layered pyroxenites. In addition, igneous textures and chemical evidence suggest widespread influx of presumably asthenosphere-derived melts into the peridotite via websteritic layers and limited temperature contrasts between melts and host rock. Bulk rock rare earth element patterns in both pyroxenites and lherzolites are depleted in the LREE, suggesting a MORB-like composition of the precurser melt for the residual pyroxenites. Measured Sr and Nd isotopic compositions in clinopyroxene separates from lherzolites and pyroxenites fall into the depleted sector of modern MORB compositions with ⁸⁷Sr/⁸⁶Sr between 0.7022 and 0.7036, and εNd ranges between +8.3 and +12.7. Initial isotopic compositions at 160 Ma yield a similar picture. In contrast, measured and initial ¹⁸⁷Os/¹⁸⁸Os in the lherzolites range from subchondritic to suprachondritic, with radiogenic lherzolite compositions that are clearly affected by melt influx from the pyroxenites, as the latter have very radiogenic measured and initial ¹⁸⁷Os/¹⁸⁸Os. No correlation exists between Os isotopes and the lithophile radiogenic isotopes. In current models advocating recycled materials in the mantle, the combination of radiogenic Os, enhanced Re contents, but MORB-like Sr and Nd isotopic compositions can only be reconciled with a component from recycled mafic oceanic crust. The enhanced Re and Pd contents of some Totalp pyroxenites, however, indicate that the pyroxenites cannot be simple residues of molten recycled MORB. More likely, the pyroxenites may be precipitates from percolating melts derived from the sublithospheric mantle. Thus, the required recycled crust component may have resided in the convecting mantle.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Union [U]
MN: 2007 Fall Meeting