Sources and processes in arc magmatism: The crucial role of water

Arc magmatism distinguishes itself from magmatism in other tectonic settings by the crucial role that water plays in its petrogenesis. Water influences the geochemistry of the sub-arc mantle, by transferring fluid-soluble elements from the slab into the mantle. Water probably also carries the oxidised signature from the slab into the mantle wedge. The ubiquitous negative Ti-anomaly in arc magmas may reflect an increase of the compatibility of titanium in the solid residue during partial melting in water-bearing peridotite. Finally, water also has a strong influence on the identity of minerals involved in crystal fractionation processes, and gives rise to the low-Fe (calc-alkaline) fractionation trend by lowering the temperature at which the silicates start crystallising, without influencing the crystallisation temperature of iron-oxides.