Experimental investigation of the effect of Ca, Fe and Ti on cotectic compositions of the rhyolitic system

The effect of the normative anorthite content on the position of cotectic curves in the Qz-Ab-Or-An system has been investigated at 200 MPa and a water activity of 0.5. To simulate compositions as close as possible to those of natural high-silica rhyolites, all investigated compositions also contained approximately 1 wt% FeO and 0.2 wt% TiO (sub 2.) The position of the cotectic curves was deduced from crystallization experiments carried out between 790 and 850 degrees C and using fourteen starting glass compositions containing approximately 3 wt% H (sub 2) O. The liquidus phase of the different starting materials was used to constrain the primary fields of quartz, plagioclase and sanidine. The compositions of residual melts coexisting with solid phases were used to define the position of cotectic curves. Compared to the haplogranite system, the eutectic point is shifted away from the Ab apex, and its composition is estimated to be Qz (sub 42) Ab (sub 21) Or (sub 37) when projected onto the haplogranite system. The implications for the estimation of the depth of magma storage conditions are discussed on the basis of an example from the Snake River Plain high-silica rhyolites.