HR: 17:20h
AN: V44B-06    [Abstracts]
TI: Recycled Metasomatised Lithosphere as a Source of Alkaline OIBs
AU: * Pilet, S
EM: pilet@gps.caltech.edu
AF: Division of Geological & Planetary Sciences, California Institute of Technology, 1200E California Blvd, Pasadena, CA 91125, United States
AU: * Pilet, S
EM: pilet@gps.caltech.edu
AF: Instititute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, 1015, Switzerland
AU: Baker, M B
EM: mikeb@gps.caltech.edu
AF: Division of Geological & Planetary Sciences, California Institute of Technology, 1200E California Blvd, Pasadena, CA 91125, United States
AU: Stolper, E M
EM: ems@gps.caltech.edu
AF: Division of Geological & Planetary Sciences, California Institute of Technology, 1200E California Blvd, Pasadena, CA 91125, United States
AB: Ocean island basalts (OIBs) are generally thought to provide information on the chemistry of the deep mantle, but the sources of these lavas are still being actively debated. For example, although the trace-element and isotopic compositions of OIBs suggest the presence of recycled components in their source regions, the origin of these recyled components is controversial. Oceanic crust is one possible source of recycled material, and it may be important in the generation of tholeiitic magmas (hy- and qtz- normative magmas) from large oceanic islands and continental lava flows (1, 2); the fact that partial melts of oceanic crust are silica oversaturated (3) makes it difficult to envision a significant role for them in the generation of alkaline (i.e., ne-normative) magmas. Veined/metasomatized oceanic or continental lithosphere in the source regions of OIBs is an alternative source of recycled components in these alkaline magmas (4, 5). We conducted melting experiments on natural amphibole-rich veins and on their dehydrated equivalents at 1.5 and 2.5 GPa to test the metasomatic hypothesis for the origin of recycled components in OIB sources. These experiments demonstrate that melting of metasomatic veins can reproduce key features of the major and trace element compositions of many nephelinites and basanites from both oceanic and continental settings; moreover, these experiments show that reaction of partial melts of amphibole-rich veins (or their dehydrated equivalents) with surrounding lherzolite can explain the observed compositional trends from nephelinites to alkali basalts. These experiments suggest that melting of metasomatized lithosphere (either "in place" or after recycling into the convecting mantle) is a viable alternative to more widely invoked models of alkaline basalt formation by melting of recycled oceanic crust ± sediments. This reinterpretation of the origin of alkaline OIBs has implications for mantle chemistry: First, it suggests that recycled components in the sources of islands characterized by tholeiitic magmas (i.e., Hawaii or Iceland) and in the sources of islands where ne-normative compositions are dominant (i.e. Polynesia, island in the Atlantic Ocean, etc.) are distinct. Second, it suggests that alkaline rocks are produced by large degrees of melting of small volumes of trace-element and volatile-rich material (primarily amphibole-bearing veins) rather then by low degrees of melting of less-enriched material (such as recycled oceanic crust or peridotites). If so, these alkaline rocks do not carry as much chemical information on the large scale composition of the convecting mantle as is often presumed. Third, the range of isotopic composition observed in alkaline lavas from a single oceanic island does not necessarily imply the interaction of distinct mantle components (such HIMU, EM) but could reflect the range and time integrated history of trace element compositions in veins formed by percolative fractional crystallization of metasomatic agents within the lithosphere (5). (1) Sobolev et al. (2007) Science 316, 412-417. (2) Herzberg (2006) Nature 444, 605-609. (3) Green et al. (1967) Earth Planet. Sci. Lett. 2, 41-51. (4) Niu and O'Hara (2003) J. Geophys. Res. 108, 2209. (5) Pilet et al. (2005) EPSL, Earth Planet. Sci. Lett. 236, 148-166.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 3630 Experimental mineralogy and petrology
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting