Partition coefficients for Sr and Ba between plagioclase and melt for metaluminous and peraluminous rhyolite have been determined for samples from several volcanic systems. Samples were analyzed by WD-XRF (Baylor Univ.) and ICP-AES (Texas Tech.). In these samples, DSr and DBa correlate positively.

For Sr partition coefficients, there are two clear trends correlated with the variation of plagioclase compositions. One has a gentle slope with DSr between 1.2 to 7.6. The other has a steep slope with DSr between 5.6 to 15.8. DSr shows a positive relationship with An content of the plagioclase. Ba partition coefficients show similar patterns. The gentle-slope trend is formed by samples with DBa less than 1, the steep-slope trend by samples with DBa between 1.6 and 8.8.

Large DSr and DBa are characteristic of samples from low-CaO concentration (<1 wt%) in peraluminous rhyolite systems. In systems with low Ca concentration, there may not be enough Ca cations to fill all of the available M2+sites in plagioclase. Sr and Ba then will enter the M2+ plagioclase sites to balance the charge loss caused by Al-Si substitution. Since these large DSr and DBa samples also have relatively low Sr and Ba concentration, the insufficiency of 2+ cations in high-Al magma systems may have caused the high partitioning of 2+ charged trace elements, given reasonable ionic radii.