HR: 11:10h
AN: U12A-04    [Abstracts]
TI: Earth's Various Recipes for Making Lherzolites
AU: * Becker, H
EM: hbecker@zedat.fu-berlin.de
AF: Inst. f. Geol. Wiss, Freie Universität Berlin, Malteserstrasse 74-100, Berlin, 12249, Germany
AU: van Acken, D
AF: Inst. f. Geol. Wiss, Freie Universität Berlin, Malteserstrasse 74-100, Berlin, 12249, Germany
AB: Petrological and cosmochemical arguments suggest that the convecting upper mantle overall should have a lherzolitic composition, otherwise, continous production of MORB would not be feasible. The predominance of harzburgites among ocean floor peridotites fits this picture because harzburgites are commonly believed to be the residue of high degrees of partial melting at shallow depths, with fertile components lost during polybaric partial melting. Implicitly, it is commonly assumed that the deeper parts of the asthenosphere and new-formed lithosphere should be residues of low-degree partial melting. This view has been supported by the abundance of lherzolites among mantle xenoliths and orogenic peridotite massifs. But is this model really correct? Data and observations on oceanic and continental peridotites accumulated over recent years hint that reality is more complicated. On the basis of mineral and whole rock compositions, and isotopic data, it has long been suspected that many continental peridotites have undergone some form of pyroxene addition via percolating melts, yet the efficacy of these processes has been uncertain. Novel combination of structural and chemical work by Le Roux et al. (2007) indicates that melt influx may have converted deformed harzburgitic rocks of the Lherz peridotite massif into little-deformed spinel lherzolites. Refertilization by MORB-like sub-lithospheric melts, and marble cake style stretching of pyroxenites have been implicated as major processes that affected the composition of peridotites from the Totalp spinel lherzolite body, a fragment of Jurassic ultra-slow spreading Thetys ocean floor in the Swiss Alps (van Acken et al., 2007). Refertilization by melts has been associated with lherzolites from oceanic fracture zones (e. g., Seyler and Bonatti, 1997) and may be responsible for lherzolites alternating with harzburgitic domains at the Arctic Gakkel ridge (Liu et al. 2007). Evidence for compositional transformation of depleted peridotites into fertile rocks, both in young oceanic and in continental settings brings up questions that need to be addressed in the future: How common are truly residual lherzolites? Are lherzolites suitable to constrain the composition of the primitive mantle? How are fertile components in the asthenosphere distributed? Mantle rocks may have more surprises in stock.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Fall Meeting