GEOCHEMICAL CONSEQUENCES OF MELT TRANSPORT IN 2-D - THE SENSITIVITY OF TRACE-ELEMENTS TO MANTLE

Dynamic models are presented to investigate the consequences of meltin g and melt transport for stable trace element geochemistry in open sys tems. These models show that including explicit melt transport in 2-D adds non-trivial behaviour because melts and residues can travel and m ix along very different trajectories. Calculations are presented for b oth equilibrium and disequilibrium transport, and passive and active m id-ocean ridge flows. These calculations demonstrate that trace elemen ts are sensitive to mantle dynamics and can readily distinguish betwee n different end-member flow fields. Passive, plate-driven flow with st rong melt focusing produces enrichments of incompatible elements. Acti ve small-scale solid convection within the partially molten region, ho wever, can lead to extreme dilution of incompatible elements, suggesti ng that this form of convection may not be significant beneath normal ridges. These calculations provide additional predictions about across -axis trends of geochemical variability and estimate the variation in concentrations that can occur even for a constant source. Many of thes e results are not seen in geochemical models that neglect melt transpo rt and we discuss how this new behaviour affects the inferences drawn from simpler models.