Trace element behaviour during anatexis in the presence of a fluid phase

Abstract

If anatexis takes place in the presence of an immiscible volatile-rich fluid phase, the behaviour of a trace element depends not only on partition coefficients D i ℓ between mineral i and silicate liquid, but also on coefficients D i f between mineral i and fluid. The limited experimental data available, as well as theoretical arguments, suggest that for common minerals these coefficients differ, in some cases (REE) notably. Theory has been developed to permit calculation of concentration variations with the fraction F of rock melted, if D-values are known. In its present state this theory neglects the influence of T, P and composition variations in liquid and fluid on the D-values: if such variations can be expressed as functions of F, appropriate modifications can be made. The theory also neglects the effects of T and P on the solubility of fluid in liquid and the resulting modifications to phase petrology, by assuming simply that a given mass of rock melts in the presence of a mass proportion v of fluid. By choosing different values of v, from 0 (dry melting) to 100 (large fluid excess), the response of the trace element concentrations can be followed, beginning with concentration c o in the unmelted rock. Such treatment is highly idealised, but serves to indicate some limits on what can occur.