HR: 14:40h
AN: V22F-05    [PDF]
TI: Isotopic Equilibrium in Mature Oceanic Lithosphere: Insights From Sm-Nd Isotopes on the Corsica (France) Ophiolites
AU: * Rampone, E
EM: betta@dipteris.unige.it
AF: Dipartimento per lo Studio del Territorio e delle sue Risorse, Univ. Genova, C.so Europa 26, Genova, 16132 Italy
AU: Hofmann, A W
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Raczek, I
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Romairone, A
AF: Dipartimento per lo Studio del Territorio e delle sue Risorse, Univ. Genova, C.so Europa 26, Genova, 16132 Italy
AB: In mature oceanic lithosphere, formed at mid-ocean ridges, residual mantle peridotites and associated magmatic crust are, in principle, linked by a cogenetic relationship, because the times of asthenospheric mantle melting and magmatic crust production are assumed to be roughly coheval. This implies that oceanic peridotites and associated magmatic rocks should have similar isotopic compositions. Few isotope studies have been devoted to test this assumption. At mid-ocean ridges, similar Nd isotopic compositions in basalts and abyssal peridotites have been found by Snow et al. (1994), thus indicating that oceanic peridotites are indeed residues of MORB melting. By contrast, Salters and Dick (2002) have documented Nd isotope differences between abyssal peridotites and associated basalts, with peridotites showing higher $^{143}$Nd/$^{144}$Nd values, and they concluded that an enriched pyroxenitic source component is required to explain the low end of the $^{143}$Nd/$^{144}$Nd variation of the basalts. Here we present Sm/Nd isotope data on ophiolitic mantle peridotites and intruded gabbroic rocks from Mt.Maggiore (Corsica, France), interpreted as lithosphere remnants of the Jurassic Ligurian Tethys ocean. The peridotites are residual after low-degree ($<$10%) fractional melting. In places, spinel peridotites grade to plagioclase-rich impregnated peridotites. Clinopyroxene separates from both spinel- and plagioclase- peridotites display high $^{147}$Sm/$^{144}$Nd (0.49-0.59) and $^{143}$Nd/$^{144}$Nd (0.513367-0.513551) ratios, consistent with their depleted signature. The associated gabbros have Nd isotopic compositions typical of MORB ($^{143}$Nd/$^{144}$Nd = 0.51312-0.51314). Sm/Nd data on plag, whole rock and cpx from an olivine gabbro define an internal isochron with an age of 162 $\pm$ 10 Ma, and an initial epsilon Nd value (9.0) indicating a MORB-type source. In the Sm-Nd isochron diagram, the peridotite data also conform to the above linear array, their initial (160 Ma) epsilon Nd values varying in the range 7.6-8.9. Sm/Nd isotopic compositions of the peridotites are therefore consistent with a Jurassic age of melting and melt impregnation, and point to isotopic compositional similarities between depleted peridotites and associated magmatic rocks. In a regional geodynamic context, Sm/Nd isotope data for the Mt.Maggiore gabbro-peridotite association represent the first record of the attainment of a mature oceanic stage of the Ligurian Tethys ocean. Also, the data presented provide striking evidence of the existence of isotopic equilibrium between melts and their mantle residue. References Snow et al. (1994), Nature 371, 57-60. Salters and Dick (2002), Nature 418,68-72.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting