Thermodynamic Constraints on Explosive vs. Effusive Onset of Glaciovolcanic Eruptions

Many of the original models for glaciovolcanic (subglacial) eruptions reported an ordered stratigraphy of basal pillow lavas, followed by tephra (hyaloclastite + pillow lava breccia), and capped by subaerial lava flows (e.g. Mathews 1947; Jones, 1969, 1970). These facies were interpreted to represent an effusive eruption onset, followed by slowly building explosive eruptions as water depths decreased, and ending with subaerial lava once the tephra pile was above the level of the enclosing ice-confined lake. In particular the origins of the fragmental deposits (tuff, lapilli tuff and pillow breccia) were very speculative or nebulous; many were viewed mainly as products of dominantly quench fragmentation and the onset and importance of magmatic fragmentation appears to have been underappreciated. Most glaciovolcanic volcaniclastic deposits we have examined show evidence that the magma was saturated with volatiles at the onset of eruption via vesicles preserved in ash and even the rims of pillow fragments. Given recent glaciovolcanic eruptions in Iceland (e.g., 1996, 1998, 2004, 2010) it seems logical that many mafic subglacial eruptions are initially driven largely by magmatic fragmentation. In order to quantitatively explore the role of vesiculation and magmatic fragmentation in glaciovolcanic deposits, we use simple thermodynamic calculations for volatile (H2O-CO2) saturation in silicate melts as a function of pressure (e.g. Papale et al. 1996) and composition to explore the ice conditions needed to suppress vesiculation and magmatic fragmentation. We model the progressive volatile exsolution and expansion of the volatile phase with depressurization to explore the pressure-composition conditions over which typical basaltic magmas a) will become saturated with a fluid phase and b) where the relative volume of the fluid phase exceeds that of the magma by a 3:1 ratio, which is an approximate proxy for the volumetric point where a bubble-rich magma is transformed into a fluid with droplets of silicate melt. Our preliminary calculations suggest the typical basaltic magmas (2 wt % H2O, 0.5 wt. % CO2) will begin to vesiculate at pressures equivalent to crustal thickness of >4 km, which is equivalent to water/ice thicknesses of >10 km. The same magmas will be at the point of explosive fragmentation driven only by magmatic degassing at pressures equivalent to ~1.5 km water/ice thickness. This explains the common presence of vesicles in pillow lavas erupted under thick ice and in almost all fragmental glaciovolcanic deposits considered to be 'Surtseyan'. For eruptions in many thinning ice caps on Earth, explosive glaciovolcanic eruptions should be initiated predominantly by magmatic fragmentation. This has important implications for the thermal impact of glaciovolcanic eruptions on waning ice sheets and for volcanic hazards deriving from generation of ash during glaciovolcanic Surtseyan eruptions.