Preliminary experimental determination of the partitioning of lithium between plagioclase crystals of different anorthite contents

The role of plagioclase anorthite content in controlling the partitioning of Li has been investigated experimentally. Plagioclase crystals of different anorthite contents were equilibrated with a single plagioclase powder at 1000 °C. The results show that the partition coefficient of lithium is a strong function of the anorthite content of the plagioclase, decreasing by a factor of ~ 3.5 from An60 to An80. This has important implications for the interpretation of the distribution of Li in plagioclase in natural samples. An example of the importance of considering the change in partition coefficient with plagioclase anorthite content is given using published plagioclase Li contents in the products of the Soufriere Hills volcano. It is shown that the distribution of Li in these plagioclase phenocrysts can be reproduced using a model in which Li diffuses into, and out of, the plagioclase crystals if the change in partition coefficient with An content is accounted for. Because Li diffusion in plagioclase is very rapid, the distribution of Li in plagioclase crystal rims can record changing conditions during eruption and pyroclastic flow emplacement (second to minute timescales).