HR: 14:45h
AN: V42E-05    [PDF]
TI: Experimental constraints on the role of pyroxenite partial melting in formation of basaltic magmas
AU: * Kogiso, T
EM: kogisot@jamstec.go.jp
AF: JAMSTEC, IFREE, Yokosuka, 237-0061 Japan
AU: Hirschmann, M M
EM: hirsc022@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Pertermann, M
EM: maik.pertermann@erdw.ethz.ch
AF: ETH Zurich, Institute for Mineralogy and Petrography, Zurich, 8902 Switzerland
AB: Mafic lithologies (pyroxenites) are a minor but ubiquitous component in the Earth's mantle, and may play an important role in formation of basaltic magmas. The volumetrically dominant composition of pyroxenite in the mantle may originate from ancient subducted oceanic crust and be similar to MORB, but a wide range of pyroxenite composition may exist in basalt source regions, as indicated by the diversity of pyroxenites sampled from xenoliths and alpine-type peridotite massifs. A variety of magmas can be produced from such diverse pyroxenite compositions, and it is important to reveal systematic relationships between bulk compositions of pyroxenite and their partial melts for understanding the influence of pyroxenite on the chemistry and dynamics of basalt generation. We review experimental phase equilibria associated with partial melting of pyroxenites to examine the relationship between bulk composition and melting relations. An important aspect of pyroxenite phase equilibria is the existence of the garnet-pyroxene thermal divide, defined by enstatite - Ca-Tschermak pyroxene - diopside plane in CMAS projections. This divide appears at pressures above $\sim$2 GPa at which garnet and pyroxenes are the principal residual phases in pyroxenite compositions. Pyroxenites on the silica-deficient (olivine-rich) side of the garnet-pyroxene divide produce low-silica liquids that also plot on the silica-deficient side of the divide, and silica-excess (quartz-rich) pyroxenites generate silica-rich melts that plot on the silica-excess side of the divide. Many of the silica-poor partial melts from silica-deficient pyroxenites have nepheline-normative compositions, and the silica-rich partial melts from silica-excess pyroxenites principally have hypersthene- or quartz-normative compositions. Silica-deficient pyroxenites are generally richer in olivine component, and thus their partial melts generally have higher MgO contents than those from silica-excess pyroxenites. Changes in phase volumes of residual minerals are another key factor controlling partial melt compositions. The phase volume of garnet expands relative to clinopyroxene with increasing pressure, producing liquids enriched in CaO and depleted in Al$_{2}$O$_{3}$ at higher pressures. As many silica-deficient pyroxenites lack olivine during partial melting, they produce high Ca/Al liquids with moderate MgO contents, which have strong similarities to nepheline-normative OIB lavas from many hotspots. Silica-excess pyroxenites also produce high Ca/Al melts with low MgO contents at high pressures, which have similarities to the silica-rich endmember of Hawaiian tholeiites.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting