Lava Textures, Magma Crystallization History, and the Dynamics of Merapi and Aleutian Mush Columns

Abstract:

The subsurface dynamics of magma in mush columns beneath arc volcanoes is recorded in the textures of the basic lavas. A detailed comparison of lava textures from large mature volcanoes in Indonesia (Merapi) and the Aleutian Islands (Adak and Atka), shows remarkably similar, often indistinguishable, textures of high alumina basalts (HAB) and basaltic andesites (BA). We suggest a systematic characterization of these distinctive textures into a few simple categories reflecting the subsurface history of crystallization within solidification fronts (SFs) and subsequent transport dynamics. The HABs are strongly plagioclase-phyric and of two main groups: A1 consists of large, idiomorphic, mildly zoned, plagioclase (20-30 vol.%) with small amounts of olivine (2-8%) set in a finely crystalline groundmass of these same phases; A2 is similar, but contains an additional pervasive population of large ‘old’ plagioclase, rounded, often highly zoned, and sometimes broken; A1+ is a subclass of A1 where traces of ‘old’ plagioclase are present. Similar categories exist in the BA lavas except overall crystallinity is higher and olivine is replaced by large clinopyroxenes containing or mantled with magnetite. In a temporal stratigraphic sense, the early lavas are generally HAB A1 types and transition into, sometimes alternating, HAB A2 types followed by BA types. The initial establishment of the mush column is by hot, highly mobile primary magmas, followed by increasingly more thermally mature magmas containing debris from disrupted SFs. The detailed nature of this debris, its variation in time, and the volumes give important insights on the size and vigor of the mush column staging chambers.