Degrees of contamination in magmas evolving by assimilation-fractional crystallization

Abstract

An equation which gives the degree of contamination in magmas evolving by assimilation-fractional crystallization is derived. In an assimilation-fractional crystallization process which proceeds with constant r and D (De Paolo, 1981), if a trace element in the parental magma is characterized by a different isotope ratio with respect to the contaminant, a linear correlation exists in the magma between the degree of contamination of the element and the isotope ratio. This relationship can be used for easy calculations of the degree of contamination of the element starting from isotope data only. However, estimation of the total contamination for a rock sample which represents instantaneous magma composition is much more difficult, because the major element composition of the parental magma, the bulk partition coefficients of the major elements in the process, ratio r between rates of assimilation and crystallization, and residual liquid fraction F for that instantaneous magma composition must be known.