TRACE-ELEMENT GEOCHEMICAL EFFECTS OF INTEGRATED MELT EXTRACTION AND SHAPED MELTING REGIMES

The mixing (integration) of liquids obtained as different mass fractions of partial melting from source material of the same bulk composition, travelling along different mantle flow-lines through a melting regime, can result in deficiencies in the relative concentrations of those incompatible elements whose bulk distribution coefficients are numerically approximately equal to the average mass fraction of melt extracted from the total source material involved in the provision of the mixed melts. These deficiencies can be very substantial, exceeding 50% of the concentration which would have been expected to be present in the liquid if that same average mass fraction of melt had been extracted from the whole melting regime by simple equilibrium or accumulated perfect fractional partial melting. The size of the deficit varies with the shape and plan-fern of the melting region, and can be greatly reduced by subsequent perfect fractional crystallization of that liquid Discrimination is increased between all elements whose distribution coefficients are numerically smaller than the average mass fraction of partial melt extracted from the whole region. These effects can lead to steepening of chondrite-normalized REE patterns and to apparent selective light rare earth enrichment in liquid and source.