Trace element geochemical effects of imperfect crystal-liquid separation

Abstract

Abstract The strongly contrasted effects of melting or crystallization under perfect equilibrium conditions; of perfect fractional partial melting; and of perfect fractional crystallization upon the relative concentrations of the highly incompatible and highly compatible elements are well known and are surveyed briefly at the start. A major role for either of the two latter processes in the genesis of natural basaltic magmas and peridotitic residues is apparently excluded provided that the liquids or crystals are removed in infinitesimally small batches, the crystals and melts are perfectly separated and there is no flux of source material or residue in or out of the greater system during the process (either liquid or crystals must be removed from the immediate, or lesser, system if it is undergoing perfect fractional melting or perfect fractional crystallization). The consequences of small departures from (i) infinitesimal size of the batches of liquid or crystals removed, (ii) perfect separation of the crystal and liquid phases, or (iii) perfect closure of the greater system to fluxes of the source material and the residue are explored. These departures can very substantially reduce the differences between the trace element concentrations generated in the products and those expected in a perfect equilibrium process. These effects are illustrated here by means of 3D plots of the logarithm of relative concentration in the liquid phase as a function of the logarithm of the distribution coefficient and the logarithm of the mass fraction of liquid involved in each process. There are also significant complementary effects on the compositions of the residues or precipitates from these processes which are not illustrated in this paper, although the necessary equations and some worked examples are provided. With the values of parameters chosen in these examples, the fluxing of liquid or solid through a ‘box’ in which fractional crystallization or fractional melting is taking place proves to be by far the most important factor in modifying the results towards those of the equilibrium processes. Although much trace element behaviour in basalts can be quite successfully modelled using the perfect equilibrium melting relationships, it is unsafe to exclude on these grounds a major role for physical processes of fractional melting and fractional crystallization in the genesis of these magmas or to conclude that equilibrium partial melting is the principal physical process involved in their genesis.