A possible role for low temperature melt in silicic magmatic systems

Abstract:

Observations point to silicic magma mush existing at ~500°C for prolonged periods (cold storage)—the difficulty with this view is that rejuvenation to eruption within the normal framework of igneous petrology is exceedingly difficult because latent heats of melting are large when water has been lost. If melt exists at 500°C, this problem goes away. Lundstrom (IGR, 2016) reported experiments demonstrating that a very hydrous peralkaline melt coexists with quartz and two feldspars at 330-400°C and 0.5-1 kbar, consistent with previous literature (e.g. Tuttle and Bowen, 1958, Friedman, 1951). The melt contains >40 wt% H₂O; its anhydrous composition has ~35 wt% total alkalis, 2 wt% Al₂O₃ and 63 wt% SiO₂. New experiments in the Na₂O-Al₂O₃-SiO₂-H₂O system show a continuum of melts coexisting with quartz and albite from 820-330°C (1 kbar). At <550°C, melt w/ 28% H₂O is undersaturated while vapor bubbles exist in an experiment at 695°C with 8.4% H₂O. If such melt exists in upper crustal silicic magma bodies (both plutons and magma chambers), it would have profound impact on views of magmatic processes including water storage and eruption triggers of mushes.

The presence of low temperature melt could strongly affect interpretations of heat flow, electrical conductivity and uplift at silicic volcanic systems such as Maule. Buoyant advection of this melt upward through granitic mush will increase heat loss from magma bodies. Thermal modeling of the Torres del Paine granitoid features (e.g. a perfectly horizontal gabbro-granite contact that crosses the Valles Frances) indicates that heat conductivity must be >5x greater in the vertical direction than the horizontal direction. Interstitial water and alkali-rich melt within silicic mush would also explain observed high electrical conductivities that are currently difficult to reconcile with seismic constraints. Finally, the dramatic change in H₂O solubility with temperature means that small increases in temperature will cause undersaturated low temperature melt to readily form a vapor phase, possibly explaining rapid uplift in upper crustal systems.