THE INFLUENCE OF WHOLE ROCK CAO CONTENT ON PLAGIOCLASE/MELT SR AND BA PARTITIONING IN RHYOLITE

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.