A model for the simulation of combined major and trace element liquid lines of descent

Abstract

Variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems. Application of a two-lattice melt model greatly reduces compositional dependence for trace elements. This allows the temperature dependence of isobaric trace element partitioning to be evaluated independently, thus making it possible to model an expanded range of conditions and compositions. The first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes, by combining trace element partitioning with a major element phase equilibrium model ( Nielsen, 1985b). The second goal was the application of this model to the simulation of magma chamber recharge, assimilation, eruption and fractional crystallization. These simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system. Recharge or assimilation will drive a magma composition off the cotectic along which it is evolving. However, fractionation will drive the liquid back towards the cotectic. In a recharged magma chamber, as the recharge flux increases, the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma. The bulk partition coefficient for an open system can be considerably different (5–10× or more), for some elements, compared to a closed system. Paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high Al basalts and andesites. However, the addition of an Al-rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that Al in the derived liquids will be effectively buffered, or even decrease. The assimilation of forsteritic olivine by reaction ( Keleman and Ghiorso, 1986), increases the proportions of pyroxene and olivine, thus reducing the Fe-enrichment and Al-depletion characteristic of low pressure basalt fractionation.