HR: 1340h
AN: V33A-1168    [Abstracts]
TI: Ultra-Refractory Domains in the Oceanic Lithosphere: Evidence from Major Element and Modal Relationships in Mantle Xenoliths from Ocean Islands
AU: * Neumann, E
EM: e.r.neumann@geo.uio.no
AF: PGP, University if Oslo, P.O.Box 1048, Oslo, 0316, Norway
AU: Simon, N S
EM: n.s.c.simon@fys.uio.no
AF: PGP, University if Oslo, P.O.Box 1048, Oslo, 0316, Norway
AU: Bonadiman, C
EM: bdc@unife.it
AF: Earth Science Department, University of Ferrara, Via Saragat, 1, Ferrara, 44100, Italy
AU: Coltorti, M
EM: clt@unife.it
AF: Earth Science Department, University of Ferrara, Via Saragat, 1, Ferrara, 44100, Italy
AU: Delpech, G
EM: delpech@geol.u-psud.fr
AF: Faculté des Sciences, Université Orsay-Paris Sud, BÂT.504, Orsay Cedex, Paris, 91405, France
AU: Gregoire, M
EM: Michel.Gregoire@dtp.obs-mip.fr
AF: Observatoire Midi-Pyrénées, Université Toulouse III, Avenue E. Belin, Toulouse, 31400, France
AB: We have compiled a database for major element and modal data of mantle harzburgite and lherzolite xenoliths from different ocean islands. The xenoliths fall in two main categories. Xenoliths showing no petrographic evidence of metasomatism (OI1) from the Canary Islands, Kerguelen, Cape Verde and Samoa are ultra-depleted spinel harzburgites. These xenoliths formed by high degrees of partial melting leading to total exhaustion of cpx. Small amounts of cpx observed in these rocks (mainly <3 vol. percent) have partly formed through exsolution from opx followed by recrystallization, and partly through reactions between mantle wall-rocks and LREE-enriched metasomatic melts. Peridotites from the same islands (and the Azores) which show petrographic evidence of metasomatism (OI2) partly fall within the major element and modal range of OI1 xenoliths, partly they are less refractory. On average, the OI1 xenoliths are significantly more refractory than peridotites collected along mid- ocean ridges and fracture zone (MORP), although the most refractory mid-ocean ridge peridotites fall within the OI1 range. Although the OI1 xenoliths show indications of metasomatism in trace elements, Na2O, and radiogenic isotopes, their major element and modal compositions seem to be essentially unaffected by metasomatic processes. The ultra-depleted nature of the OI1 xenoliths is also reflected in the Fo-contents in ol, and in the HREE and MREE in opx. OI2 xenoliths are OI1 rocks, many of which have been metasomatized to the degree that major element and modal phase compositions have been modified. The other main category, consisting of harzburgite and lherzolite xenoliths collected in Hawaii, Tahiti and Grande Comore (OIcpx) are significantly less refractory that the OI1 xenoliths and show similarities to MORP. Many of these xenoliths contain significant amounts of cpx (up to 20 vol. %), and contain primary cpx, indicating only moderate amounts of partial melting (or extensive re-fertilization). Unradiogenic Os isotopes and high Re-Os model ages suggest that OI1 and OI1cpx rocks from some islands are much older than the abyssal lithosphere in which they are found. P-T estimates of 0.7-1.3 GPa and 850- 1200„aC indicate that the OI1 xenoliths last equilibrated at depths corresponding to >80 Ma old oceanic lithospheric mantle. We interpret the ultra-depleted xenoliths as fragments of ¡§exotic¡¨ material trapped in the oceanic mantle lithosphere. Major element similarities to some series of oceanic sub-arc mantle, and significant differences from continental xenolith series, makes it most likely that the ultra-refractory OI1 peridotites represent recycled abyssal mantle. The large proportion of ultra-refractory peridotite xenoliths in ocean islands, and the presence of such rocks also along some mid-ocean ridges and in sub-arc mantle, suggest that ultra-refractory material may be important ingredients in the convecting mantle. Because of their ultra-depleted chemistry the OI1 type harzburgites will have relatively low densities and are therefore buoyant relative to less refractory mantle rock types. Their high solidus temperatures make the OI1 xenoliths immune to further partial melting (sterile).
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
DE: 3037 Oceanic hotspots and intraplate volcanism
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting