MODELS FOR INCOMPATIBLE TRACE-ELEMENT ABUNDANCES IN CUMULUS MINERALS AND THEIR APPLICATION TO PLAGIOCLASE AND PYROXENES IN THE BUSHVELD COMPLEX

In layered intrusions the capacity of trapped residual liquid to reset the major element composition of minerals is well established. This p aper extends the principle and provides a mathematical treatment of in compatible trace-element distribution in minerals during crystallizati on of trapped interstitial liquid. The effects of variations in minera l proportions, trapped liquid content, partition coefficients, and acc essory minerals crystallizing from the trapped liquid are explored. Th ese calculations show that the concentrations of incompatible elements in minerals present in low abundance in the rock can show extreme enr ichment relative to that in the originals cumulus mineral. Concentrati ons of an incompatible element in the equilibrated mineral may exceed that in the parent liquid, even though the partition coefficient may b e considerably less than unity. For such incompatible elements, the as sumption that the concentration in the immediate parental liquid from which the specific mineral crystallized is equal to the concentration in a mineral divided by its partition coefficient is not justified. Th ese principles are applied to published analyses for Rb in plagioclase and rare earth elements (REE) in pyroxenes from the Merensky Reef int erval of the Bushveld Complex. Observed Rb values in plagioclase of 1- 4 ppm can be produced from a liquid with only 5 ppm Rb, and REE conten ts in clinopyroxene may exceed twice that in the liquid from which it formed. Infiltration metasomatism by a liquid rich in incompatible ele ments is not required to produce these high concentrations. The calcul ations indicate that the parental liquid to the Bushveld Complex was a typical continental tholeiite in terms of these incompatible trace-el ement contents.