EXPERIMENTAL CONSTRAINTS ON THE LOW-PRESSURE EVOLUTION OF TRANSITIONAL AND MILDLY ALKALIC BASALTS - THE EFFECT OF FE-TI OXIDE MINERALS AND THE ORIGIN OF BASALTIC ANDESITES

One-atmosphere, melting experiments, controlled at the fayalitc-magnet ite-quartz oxygen buffer, on mildly alkalic and transitional basalts f rom Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105+/-5-degrees-C, apparently independ ently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystallize plagioclase and olivine as t he first two crystalline phases, augite as the third phase, and an Fc- Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO2 in the liquid, a nd a notable increase in SiO2 and Al2O3, and the minor oxides K2O and P2O5, With decreasing temperature. The most silicic glasses are compos itionally mugearitic and shoshonitic basaltic andesites. Because the s mallest amount of glass that could be analyzed with the microprobe rep resents 20-55 percent liquid remaining, it can be expected that more s ilicic liquids will occur at lower temperatures. On normative, pseudot ernary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in no rmative quartz. This is caused by a strong systematic convergence, wit h the appearance of Fe-Ti oxides, of the bulk solid precipitates towar d the liquid compositions, as projected on the triangle plagioclase-di opside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti o xide crystallization. This causes the liquids to move toward decreasin g normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve tow ard early silica saturation.