Deep Roots of Island Arc Crust: Experimental Constraints on Formation, Composition, Element Partitioning and Physical Properties

HR: 16:00h
AN: V44C-01 INVITED [Abstracts]
TI: Deep Roots of Island Arc Crust: Experimental Constraints on Formation, Composition, Element Partitioning and Physical Properties
AU: * Muntener, O
EM: muntener@geo.unibe.ch
AF: University of Bern, Baltzerstr. 1, Bern, 3012 Switzerland
AU: Ulmer, P
EM: peter.ulmer@erdw.ethz.ch
AF: ETH Z〉ich, Sonneggstr. 5, Z〉ich, 8092 Switzerland
AU: Perez-Alonso, R
EM: raquel.alonso@erdw.ethz.ch
AF: ETH Z〉ich, Sonneggstr. 5, Z〉ich, 8092 Switzerland
AB: Formation and growth of juvenile island arc crust from hydrous magmas is a polybaric process primarily controlled by density and enthalpy `filters'. Compositions of magmas separating from mantle residues are modified through interaction with the lithospheric mantle and the lower crust and by crystallization-differentiation that drives them towards andesitic to dacitic liquids, typical for more evolved island-arc magmas and rocks (tonalites). We summarize the results of equilibrium and fractional crystallization experiments (0.8-1.5 GPa) that were performed on H₂O-undersaturated picrobasalt to high-Mg andesite representing mantle extraction depths between 90 and 40 km. We investigated the mutual phase relations of the principal phases olivine, cpx, opx, garnet, amphibole, plagioclase and spinel and will present new, experimentally determined mineral/liquid partition coefficients relevant for arc lower crust. At pressures exceeding 0.8 GPa, between 45 and 70% of ultramafic, garnet-bearing cpx and/or hbl dominated cumulates are produced to obtain andesite to dacite compositions. Natural examples of ultramafic-mafic lower crustal cumulates are similar to the experimental results thus supporting the production of important volumes of ultramafic `crust' at the base of an island arc. Calculated densities are comparable to the upper mantle whereas seismic velocities (v_p) vary continuously between 7 (`typical' lower crust) and 8 km/s (`typical' mantle). As a consequence, the sharp transition assigned to the Moho is replaced by a transition zone where v_p vary between 7 and 8 km/s. The lack of a sharp Moho and velocities between 7 and 8 km/s at the crust/mantle transition are observed in several locations (e.g. Central/Southern Andes) supporting the predominance of plagioclase-poor ultramafic plutonic rocks at the base of the crust. We infer that the Moho reflects a `plagioclase saturation boundary' and that much of the ultramafic rocks represent igneous cumulates and hence formed part of the crust and not the mantle. The `real' crust-mantle boundary is not sharp but transitional and difficult to image/detect in seismic velocity profiles.
DE: 1020 Composition of the continental crust
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005