viscous death in subduction zones

Fun geology term: viscous death

What is controlling water concentrations in arc magmas? 

Water is the weirdest and most important molecule there is. Besides sustaining life on our planet, water affects every part of the evolution of an arc volcano. Water's presence in the deep mantle dramatically lowers its melting temperature, resulting in magmatism in an otherwise cold subduction zone. Water also lowers the viscosity of magmas allowing them to rise, and later expands as a gas fueling explosive eruptions. Despite its importance to the transfer of heat and matter throughout nearly every system on our planet, little is known about the filtering effects of subduction zone water. This is changing with the rise of improved measurements for water concentrations in magmatic inclusions, tiny bits of volcanic glass that were trapped as liquids in growing crystals (usually olivine), prior to degassing and eruption. The new data seem to point to a surprisingly narrow range of water concentrations within each arc, and a fairly uniform global average of about 4 weight percent.

This uniformity implies a process that regulates the water concentration. Plank et al., 2103 suggest two hypotheses.
1) Arc magmas stall at around the same depth (~6 km) beneath the Earth’s surface water is saturated within the melt and melt inclusions form at relatively constant water concentrations. Such uniformity in upper crustal process, however, seems unlikely.

2) Water concentration strongly controls the amount of melting in the mantle, maintaining a general proportion of water to melt: more water = more melting.

The answer will carry important implications on the amount of water that is transferred from the mantle to the surface, and how much magma ultimately gets stuck and never erupts. For example, magmas with more water leads to an earlier crystallization of amphibole, which subsequently lowers the water content of the remaining melt. Lower amounts of water increase the melt viscosity and also raises the melting temperature, bringing on further crystallization. Thus magmas with more water may actually never erupt, but rather succumb to a viscous death. This leads to the counterintuitive idea that plutonic rocks are actually wetter than volcanic rocks. 

Figure 4 from Plank et al. (2013) showing the fairly constant water concentrations of arc magmas. 

Source:

Terry Plank, Katherine A. Kelley, Mindy M. Zimmer, Erik H. Hauri, Paul J. Wallace, Why do mafic arc magmas contain ∼4 wt% water on average?, Earth and Planetary Science Letters, Volume 364, 15 February 2013, Pages 168-179, ISSN 0012-821X, (http://www.sciencedirect.com/science/article/pii/S0012821X1200670X)