An inversion approach to assimilation and fractional crystallisation processes

Abstract

A numerical inversion program has been developed to investigate isotope and trace element variations in suites of related rocks in terms of assimilation and fractional crystallisation processes. Major element compositions constrain the degree of fractionation, and the program may be used to calculate the isotope and trace element composition of either the crustal contaminant, or of the parental magmas once the contaminant is known. The ratio of the rate of assimilation to the rate of fractional crystallisation, and the bulk partition coefficients, can be varied with changes in magma composition. The approach is illustrated with a suite of calc-alkaline rocks from the Cerro Galan centre in NW Argentina, and a suite of continental flood basalts from the southern Parana in Brazil. Both exhibit striking increases in 87Sr/86Sr with increasing SiO2 consistent with progressive contamination during differentiation within the continental crust. In detail, both suites contain a subset of samples which have undergone relatively little contamination, and a subset characterised by relatively high SiO2 and 87Sr/86Sr reflecting larger crustal contributions. Individual samples within the more contaminated subset can be modelled in terms of assimilation with fractional crystallisation (AFC) processes, although no clear progressively contaminated liquid lines of descent are preserved. At both Cerro Galan and the southern Parana, the associated dacites and rhyodacites appear to have similar compositions to the crustal contaminants. Thus the inversion program has been used to investigate the amount of contamination, the relationship between the amount of contamination and the amount of crystals removed by fractional crystallisation, and the isotope and trace element contents of the parental magmas prior to AFC. The latter are estimated to have had 87Sr/86Sr=0.7055 and 143Nd/144Nd=0.51256 at Cerro Galan, and 0.7090 and 0.51226 respectively in the southern Parana.