EarthRef.org Reference Database (ERR) -- Cawthorn 1996

In layered intrusions the capacity of trapped residual liquid to reset the major element composition of minerals is well established. This paper extends the principle and provides a mathematical treatment of incompatible trace-element distribution in minerals during crystallization of trapped interstitial liquid. The effects of variations in mineral proportions, trapped liquid content, partition coefficients, and accessory minerals crystallizing from the trapped liquid are explored. These calculations show that the concentrations of incompatible elements in minerals present in low abundance in the rock can show extreme enrichment relative to that in the originals cumulus mineral. Concentrations of an incompatible element in the equilibrated mineral may exceed that in the parent liquid, even though the partition coefficient may be considerably less than unity. For such incompatible elements, the assumption 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. These principles are applied to published analyses for Rb in plagioclase and rare earth elements (REE) in pyroxenes from the Merensky Reef interval 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 contents in clinopyroxene may exceed twice that in the liquid from which it formed. Infiltration metasomatism by a liquid rich in incompatible elements is not required to produce these high concentrations. The calculations indicate that the parental liquid to the Bushveld Complex was a typical continental tholeiite in terms of these incompatible trace-element contents.