HR: 0800h
AN: V41C-1456    [Abstracts]
TI: Contrasted Origins For Gabbroic Cumulates Along The Mid-Atlantic Ridge.
AU: * Nonnotte, P
EM: pnonnott@sdt.univ-brest.fr
AF: UBO-IUEM, UMR6538, Place N. Copernic, Plouzane, 29280 France
AU: Benoit, M
EM: mbenoit@univ-brest.fr
AF: UBO-IUEM, UMR6538, Place N. Copernic, Plouzane, 29280 France
AU: Ceuleneer, G
EM: georges.ceuleneer@cnes.fr
AF: OMP, UMR5562, 14, avenue E. Bellin, Toulouse, 31400 France
AB: Gabbroic cumulates sampled along mid-ocean ridges are classically viewed as fractional crystallization products of primitive mid-ocean-ridge basalts (MORB) in shallow axial magma chambers. Recent observations of deep horizons of the ocean crust have shown that, in slow spreading environments, crustal building proceeds by repeated injections of modest amounts of variously evolved melt batches. Such a discontinuous character of magma emplacement favors the development of complex petrogenetic evolutions triggered by the migration of various types of melts and fluids into formerly crystallized - or still crystallizing - gabbroic cumulates and into residual peridotite. In some occurrences, however, gabbros are clearly not issued from fractional crystallization of MORB-like melts. In order to better constrain the origin of various kinds of gabbronorites emplaced along the Mid-Atlantic Ridge (MAR), we performed a geochemical study (trace elements and Sr-Nd isotopes) of clinopyroxene (cpx) separates leached with a severe analytical protocol designed to remove the geochemical effects of post-crystallization water rock interaction. We focus on two sites: MAR off the FAMOUS area (DSDP Site 334) and MAR on both sides of the 15°20 N fracture zone (MODE98-Leg1 cruise). Cumulates sampled at DSDP Site 334 are orthopyroxene-rich gabbronorites which call for parent melts richer in SiO₂, more "andesitic", than MORB at a given MgO content. They also show an extreme depletion in most incompatible major, minor and trace elements compared to MORB. We measured isotopic signatures to trace the source of the DSDP Site 334 cumulates. We find that Site 334 cpx depart from the global mantle correlation: normal MORB values for the ¹⁴³Nd/¹⁴⁴Nd ratio (0.51307-0.51315) are associated to highly radiogenic ⁸⁷Sr/⁸⁶Sr (0.7034-0.7067) ratios. These signatures show that the parent melts of Site 334 cumulates are issued from an ultra-depleted MORB source and that contamination with seawater occurred at some stage of their genesis, during the production of the melt itself (possibly hydrated melting of the lithosphere), and/or during emplacement and crystallization at shallower levels. Gabbros from the 15°N area vary from olivine gabbros to gabbronorites up to hornblende (+/- biotite) gabbronorites. Contrasting with Site 334 gabbros, leached cpx, among other from the gabbronorites, do not present, here, compositions ultra-depleted in incompatible elements nor isotopic signatures departing from those of the associated lavas. The well-known dichotomy in basalt composition according to the position relative to the 15°N fracture zone is also observed in the isotopic and geochemical signature of these gabbros, suggesting a common mantle source. The gabbros located north of the fracture zone can be explained in a classical way, i.e. by variable degrees of fractional crystallization from the associated MORB. In the case of gabbros sampled south of the fracture zone, large amounts of trapped liquid have to be invoked to explain trace element signatures of their cpx. Either in-situ crystallization or late-stage melt circulation may explain the anomalous trace element enrichment for those gabbros. This feature is clearly disconnected from the isotopic enrichment and is thus not related to the contrasted mantle source compositions but can be accounted for in terms of contrasted P-T conditions: colder lithosphere south of the fracture zone will trigger faster crystallization of gabbro plutons and in-situ crystallization will compete there with a pure fractional crystallization process.
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005