Explain how the slab component is generated in island arc igneous rocks. Consider both the fore-arc and back-arc basin environments
Island arcs form as oceanic plate subducts under oceanic plate. Volcanism is concentrated in an arc of volcanoes, generally approximately located above the leading edge of the subducting plate. A trench often forms where the slabs meet and subduction begins. On the non subducting slab a series of basins form, with a fore-arc basin nearest the subduction/trench, then the main arc, and a back-arc basin on the far side (Mitchell and Reading, 1971; Frisch, Meschede and Blakey, 2010).
An island arc subduction zone. Modified from Frisch et al. (2010).
Magmas in island arc settings consist primarily of components from two different origins, the slab component, and the mantle wedge. The mantle wedge may melt due the descent of the slab, giving the main portion of the non-slab component. The slab component is derived from the subducting slab as it descends. This may consist of melting of the crustal portion of the slab, but also melting of the mantle wedge due to addition of water driven off the slab. Since the descending slab is composed of old, cold oceanic crust, there is a slight paradox in the idea of melts forming from it. However, frictional heat, as well as the heat of the underlying mantle, can drive hydrous fluids off the slab. (Machado, Chemale Jr., Conceição, Kawashita, Morata, and Van Schmus, 2003; Kimura and Yoshida, 2006).The addition of water to the mantle wedge results in the lowering of melting temperatures, allowing melts to be formed at much lower temperatures than might otherwise be expected. Also, the circulation of hot fluids allows materials dissolved in them to migrate upwards into areas of greater melt generation, thus enriching the resulting melt with elements characteristic of the slab and lower mantle.
If slab material is melted, this is more likely to occur in the back-arc environment, where greater temperatures are present making partial melting possible. Most studies however, have suggested that little of the melt is actually from the slab, instead being from the mantle wedge with the addition of fluid phases driven off the slab (Davies and Stevenson, 1992; Kimura and Yoshida, 2006; Smith, Price, Stewart, and Worthington, 2009).
In the fore-arc environment, there is less likelihood of the necessary temperatures being achieved to form melt from the descending slab. Water is likely therefore to play a much bigger part in the transport of slab related material, with dehydration of the slab material allowing wet melts of the mantle wedge to form. These melts are enriched in elements from the slab which can be carried in the water.
Location of melt generation, and the slab component
In both environments then, the most common slab contribution is an aqueous fluid, driven out of the crustal section of the subducting plate. This fluid allows the enrichment of the arc magmas with slab...