Imperfect melt separation, finite increment size and source region flow during fractional melting and the generation of reversed or subdued discrimination of incompatible trace elements

Abstract

Imperfections in the perfect fractional melting (PFM) process can ameliorate the drastic “stripping” of incompatible trace elements from the residue which characterises ideal PFM, and the consequent catastrophic collapse of the relative concentrations of these elements in all further aliquots of melt which may be extracted. Once the numerical value of the mass fraction of partial melt extracted in PFM exceeds the numerical value of the crystal-liquid distribution coefficient for an element, then reversed discrimination (reversed, that is, relative to the initial concentrations in the system undergoing PFM) appears in all further aliquots of melt extracted, i.e. the relative concentration of an incompatible element in the next drop of liquid becomes lower than that of a slightly more compatible element, contrary to the effect of equilibrium (batch) melting. However, in PFM there is only an extremely narrow range of (very low) values of the melt fraction extracted within which reversed discrimination occurs in the next drop of liquid to be extracted while the relative concentrations of both highly incompatible elements are still high relative to the source region. Thereafter, as melting proceeds, these relative concentrations collapse to extremely low values. Subdued discrimination, in which the relative concentration of the more incompatible element is lower than expected (from the equilibrium melting equation) relative to that of a more compatible element, given the ratios of these two elements in the source material, is only achieved while the relative concentrations of the two elements are still high, but the range of mass fraction of previous and current melt extraction permitted are again extremely small — necessarily lower even than those required to produce reversed discrimination with high relative concentrations. Any admixture of these aliquots of liquid with earlier derived aliquots (which have high relative concentrations but do not show reversed discrimination) obliterates the reversed or subdued discrimination. Any excess of melt extraction in PFM above the critical values produces further aliquots of liquid with negligible concentrations of the incompatible trace elements concerned (such as have never been observed in natural basalts). Nevertheless, many basic eruptives display reversed or subdued discrimination of Rb relative to Sr combined with high relative concentrations. This paper explores the effects of imperfections in the PFM process to find ways to relax the severe restrictions on the generation of reversed discrimination which characterise pure PFM. Flux of source material through the melting region, accompanied by mixing within the region on short time scales relative to the rate of melt extraction, is by far the most potent factor in this respect.