Abstract
The theoretical basis for a method of determining chemical activity-composition relations for fast diffusing trace components from multicomponent diffusion experiments in silicate melts is developed. The key idea is that in a diffusion couple where two melts of different composition are brought into contact and allowed to begin equilibrating, fast diffusing trace components will closely approach an “equilibrium” state with no gradient in their chemical activity on time scales short compared to that required to erase chemical gradients of the major components of the system. Thus, there is a period of time in the evolution of a suitably designed diffusion couple when the dependence of the tracer's chemical activity on melt composition can be directly determined by probe analysis of the compositional gradients that correspond to there being no gradient in the tracer's chemical activity. Isotopes of the tracer can be used to insure that the tracer has reached “equilibrium” by recognizing that the equilibrium state will have uniform isotopic ratio even when concentration gradients of the tracer are still present. The use and accuracy of the proposed method for determining activity-composition relations is illustrated first by numerical experiments and then applied to some recently published diffusion couple data to obtain the chemical activity of Sr and Nd in silicate melts as a continuous function of silica content. The activities of both Sr and Nd increase by about a factor of two as the silica content of the melt increases from 50 to about 70 wt% SiO 2, and then increase more rapidly in the vicinity of 75 wt% SiO 2. The activity-melt composition relations found for Sr and Nd are in good agreement with earlier two-liquid partition experiments and also explain why these elements are more highly enriched in certain minerals in equilibrium with high-silica rhyolitic melts compared to their partitioning behavior between these same minerals and less silica-rich melts.
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