Minor phases as Carriers of trace elements in non-modal crystal-liquid separation processes II: Illustrations and bearing on behaviour of REE, U, Th and the PGE in igneous processes

Abstract

Minor phases which strongly concentrate selected trace elements, termed here 'carrier-phases', release relatively large amounts of those elements to the liquid phase when they are eliminated during partial melting and glean relatively large amounts of those elements when they first appear during progressive crystallization. It is characteristic of such relationships that concentrations of the selected trace elements in the bulk residues of partial melting will rise to a peak somewhat before the last of the carrier-phase is eliminated during progressive melting. In the liquids produced during equilibrium partial melting a corresponding peak in the concentration of the trace element occurs at the point where the carrier-phase is eliminated; the corresponding peak in trace element concentration in the liquids produced by accumulated perfect fractional melting is found somewhat above that point. These peaks become more sharply accentuated as the distribution coefficient of the trace element into the carrier-phase increases. The highest trace element concentration in a partial melt liquid product is found in the small drop of liquid produced during perfect fractional melting at the point where the carrier-phase is eliminated. Still higher concentrations may be found in the first cumulates containing the carrier-phase which precipitate during perfect fractional crystallization but the corresponding liquids do not contain exceptionally high concentrations. Under favourable conditions a large proportion of the available mass of a trace element in a magmatic system may be transferred from the solid to the liquid phases or vice versa with only a small change in the mass fraction of liquid in and energy content of the system. Within that range, separation of otherwise very similarly behaved trace elements becomes possible. Further complexities arise and the opportunities for separation increase when two carrier-phases compete with differing success for the same group of trace elements.