Mineral disequilibrium in lavas explained by convective self-mixing in open magma chambers

Characteristic features of many porphyritic andesite and dacite lavas are t hat they are rich in crystals and display a range of disequilibrium feature s, including reversely zoned crystals, resorption surfaces, wide ranges of mineral compositions and minerals which are not in equilibrium with the sur rounding rock matrix. These features are often interpreted as evidence of t he mixing of magmas of contrasting composition, temperature and origin(1,2) . Here, however, we propose that such features can also be caused by convec tion within a magma body with a single composition, that is heated from bel ow and cooled from above. We describe petrological observations of andesite lava erupted at the Soufriere Hills volcano, Montserrat, which indicate a heating event and the intermingling of crystals that have very different th ermal histories. We present experimental data on a representative groundmas s composition of this lava, which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferr ed average temperature. The concept of convective self-mixing allows us to explain the occurrence of compositions of minerals that apparently cannot c oexist under equilibrium conditions.