Volatile dilution during magma injections and implications for volcano explosivity

Abstract

Magma reservoirs underneath volcanoes grow through episodic emplacement of magma batches. These pulsed magma injections can substantially alter the physical state of the resident magma by changing its temperature, pressure, composition, and volatile content. Here we examine plagioclase phenocrysts in pumice from the 2014 Plinian eruption of Kelud (Indonesia) that record the progressive capture of small melt inclusions within concentric growth zones during crystallization inside a magma reservoir. High-spatial-resolution Raman spectroscopic measurements reveal the concentration of dissolved H 2 O within the melt inclusions, and provide insights into melt-volatile behavior at the single crystal scale. H 2 O contents within melt inclusions range from ∼0.45 to 2.27 wt% and do not correlate with melt inclusion size or distance from the crystal rim, suggesting that minimal H 2 O was lost via diffusion. Instead, inclusion H 2 O contents vary systematically with anorthite content of the host plagioclase (R 2 = 0.51), whereby high anorthite content zones are associated with low H 2 O contents and vice versa. This relationship suggests that injections of hot and H 2 O-poor magma can increase the reservoir temperature, leading to the dilution of melt H 2 O contents. In addition to recording hot and H 2 O-poor conditions after these injections, plagioclase crystals also record relatively cold and H 2 O-rich conditions such as prior to the explosive 2014 eruption. In this case, the elevated H 2 O content and increased viscosity may have contributed to the high explosivity of the eruption. The point at which an eruption occurs within such repeating hot and cool cycles may therefore have important implications for explaining alternating eruptive styles.