MAGMA TRANSPORT AND METASOMATISM IN THE MANTLE - A CRITICAL-REVIEW OFCURRENT GEOCHEMICAL MODELS

Conflicting geochemical models of metasomatic interactions between man tle peridotite and melt all assume that mantle reactions reflect chrom atographic processes. Examination of field, petrological, and composit ional data suggests that the hypothesis of chromatographic fractionati on based on the supposition of large-scale percolative processes (Navo n and Stolper, 1987) needs review and revision. In the hypothesis, mel ts develop enrichment fronts of incompatible elements as the melt perc olates through a porous mantle column of refractory peridotite composi tion and imprint the fractionation patterns on peridotite elsewhere. C urrent models that use Navon and Stolper's (1987) chromatographic frac tionation concept are applied to rocks of the Lherz and Horoman massif s. The calculations produce poor or limited results for the sequence o f compositional variations in time and space, and the assumptions do n ot accord with field relations and estimates of mantle conditions from experiments. In the Lherz model, modest LREE enrichments require melt percolation for as long as 25000 yr, an unrealistic life span for man tle dike conduits that supply metasomatizing melts. Continuous porous flow of melts also requires host peridotite temperatures at or above t he liquidus. Models of regional pervasive porous flow conflict with st ructural and seismic evidence that fractures control fluid transportat ion in the upper mantle. Effects of porous-medium flow have been infer red in studies of mantle peridotite samples on scales of tens of meter s at most, but are well documented only on scales of centimeters or de cimeters. In all these hypotheses, porous flow is fundamentally contro lled by proximity to magma-filled fractures. Well-constrained rock and mineral data from xenoliths indicate that many elements that behave i ncompatibly in equilibrium crystallization processes are absorbed imme diately when melts emerge from conduits into depleted peridotite. Afte r reacting to equilibrium with the peridotite, melt that percolates aw ay from the conduit is largely depleted of incompatible elements. Cont inued addition of melts extends the zone of equilibrium farther from t he conduit. Such a process resembles ion-exchange chromatography for H 2O purification, rather than the model of chromatographic species sepa ration proposed by Navon and Stolper (1987).