Abstract
Melt impregnation is commonly documented in mantle peridotites which have experienced partial melting, both in present oceans (Dick, 1989) and in ophiolites (i.e. Lanzo, Trinity; Nicolas, 1989). Peridotite-melt interaction is testified by the heterogeneous but widespread occurrence, within the peridotites, of peculiar microstructures: i) plagioclase + pyroxene clots confined along grain boundaries or crosscutting mantle minerals, ii) plagioclase + orthopyroxene intergrowth surrounding mantle clinopyroxene. A major discussion concerns the nature of the trapped melt, whether it is produced in-situ by the same partial melting process responsible of the depletion of the host peridotite, or it represents a transient melt migrating through an upwardly convecting mantle. Petrographic criteria able to discriminate between in-situ melt segregation and exotic melt impregnation have been proposed by Nicolas (1989). Recent literature has concerned numerous petrographic and geochemical studies of peridotite-melt interaction processes (Nicolas, 1989; Dick, 1989; Elthon 1992; Kelemen et al., 1992, and quoted bibliography). However, very little is known about the geochemical signature of the trapped melt. With this aim, we have performed in-situ trace element analyses (by SIMS technique) on plagioclases and clinopyroxenes in peridotites from various ophiolitic massifs (Lanzo South (S), Western Alps; Internal Ligurides (IL), Northern Apennines; Mt.Maggiore, Corsica), which are interpreted as portions of asthenospheric mantle residual after MORB-type partial melting processes.
Published Version
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