Water in Arc Magmas
The water content of magmas is arguably the most important chemical component controlling their formation, evolution, and eruption. Yet, few direct measurements exist because magmas lose virtually all of their water to bubbles during ascent and eruption. Volcanic rocks are thus largely devoid of water, with the exception of melt inclusions - tiny bits of undegassed melt trapped at pressure inside crystals. Micro-analytical measurements of such melt inclusions are finding H2O as a major species (2-7 wt%) dissolved in primitive magmas from subduction environments. An explosion of new data are providing new clues as to how water drives melting in the mantle and magma evolution in the crust, and even the balance of water in the ocean through time.