High SiO2 (Rhyolitic) Melt Inclusions in Augite Phenocrysts from an Aleutian High Alumina Basalt: Evidence for a Magmatic Component or a Petrologic Process?

A high alumina basalt lava from the Aleutian Island of Kanaga has phenocrysts of sector zoned augite that contain numerous melt inclusions of highly evolved (greater than >70% SiO₂) rhyolitic glass. The inclusions range from 10 to 100 microns in their maximum dimension. All inclusions are anhedral and contain no evidence for post entrapment crystallization suggesting that they represent the composition of the liquid at the time of entrapment. A complete lack of textural or mineralogic evidence for a mixing origin coupled with a measured groundmass of 56% SiO₂ removes the possibility that the melt inclusions reflect either a rhyolitic body of magma (i.e. magmatic component) or the residual liquid during progressive basalt crystallization. These same augites have been previously explained (Brophy et al., 1999) as a consequence of rapid crystal growth due to decompression- induced volatile exsolution. This suggests that the melt inclusions might represent a thick (up to 100 microns) compositional boundary layer that developed adjacent to the crystal-liquid interface during rapid crystal growth. Given this possibility, analysis of the relative growth rates of the (111) and (010) sectors and crystallization-liquid diffusion modeling of the compositional boundary layer independently yield augite growth rate estimates of 10^-6 to 10^-7 cm/sec. Combining these estimates with a single augite CSD profile for the host lava yields an estimated "residence" time of 35 to 350 days. This implies that the entire process of magma ascent, volatile ex-solution, augite crystallization and eruption could have occurred quickly or even very quickly. This study also indicates that melt inclusions should be used with caution for deducing the presence of discrete magmatic components as they could very well be the reflection of a petrologic process.