COMPOSITIONAL DEPENDENCY OF FE-MG EXCHANGE BETWEEN OLIVINE AND MELT

Extensive experimental work with basaltic compositions at low as well as high pressures have shown that KD for Fe-Mg exchange between an olivine and coexisting melt is 0.3 0.03 and many publications in the literature indicate that this value is being used regardless of composition of melts. New experiments with tholeiitic, alkalic and silicic melts, however, indicate that KD depends upon the melt composition outside the tholeiitic range.

Experiments we conducted at low-pressures with alkalic rocks from the K-rich alkali basalts from the Wudalianchi Volcanoes in China and alkaline rocks from the Mt. Erebus, Antarctica indicate that KD varies from 0.22 to 0.26. These values are similar to those (0.17-0.24) calculated by Gee and Sack (1988) in their low pressure experiments with melilitites and nephelinites. Results of these studies suggest that when the melt composition shifts from tholeiitic to alkalic KD varies as a function of silica activity in the melt.

We also conducted experiments with high silica melts and calculated the KD values. For example experiments with silica-rich compositions (75.5% SiO2) yielded KD of 0.43 0.02 and a rhyolitic composition at 875C and 0.5 KD yielded a value of 0.83.

A total of 300 KD values obtained from our experiments and from the literature were stepwise regressed and an equation representing KD as a function of S-CA-M was developed.