The role of crystal frameworks in the preservation of enclaves during magma mixing

Abstract

The dacitic lava flows of Nea Kameni, Santorini, Greece, contain a variety of partially-crystalline mafic enclaves. Each enclave type is derived from the break-up of a layer of replenishing magma, emplaced in the dacite chamber prior to eruption. Crystallisation-induced vesiculation of this layer, driven by the temperature contrast between the two magmas, was sufficient to drive upwelling and overturn of the layer, resulting in the formation of mafic enclaves within the host. The replenishing magmas are preserved as enclaves due to the presence of a plagioclase crystal framework. Spatial distribution pattern (SDP) analysis and melting experiments demonstrate that the plagioclase crystallinity and crystal aspect ratios observed in the Kameni enclaves are sufficient to form a touching framework of crystals. Observations on the coherence of the framework during exsolution of volatiles and enclave transport suggest that the morphology of the dominant framework-forming phase places a significant control on the strength and viscosity of the crystal framework, which in turn determines the extent to which the replenishing magmas will fragment and mix with the dacite host. Frameworks composed of high-aspect ratio crystals are weak, and respond to rapid vesicle expansion in a brittle manner. The replenishing magma then disaggregates to become a source of crystals dispersed throughout the host magma. In contrast, low-aspect ratio crystals form strong crystal frameworks that are more resistant to rapid vesicle expansion, resulting in preservation of the replenishing magma as vesicular enclaves.