Importance of the thermal structure of the mantle wedge to global arc lava compositions

The processes that lead to the fourfold variation in arc-averaged compositions of mafic arc lavas remain controversial. Control by the mantle-wedge thermal structure is supported by chemical correlations with the thickness of the underlying arc crust, which affects the thermal state of the wedge. Control by down-going slab temperature is supported by correlations with the slab thermal parameter. The Chilean Southern Volcanic Zone (SVZ, Figure 1) provides a test of these hypotheses. Our chemical data demonstrate that the SVZ and global arc averages define the same chemical trends, both among elements and between elements and crustal thickness. But in contrast to the global arc system, the SVZ is built on crust of variable thickness with a constant slab thermal parameter. This natural experiment, along with a set of numerical simulations, shows that global arc compositional variability is dominated by different extents of melting that are controlled by the thermal structure of the mantle wedge. Slab temperatures play a subordinate role. Variations in the subducting slab’s fluid flux and sediment compositions as well as mantle-wedge heterogeneities produce second-order effects that are manifested as trace element and isotopic signatures; these can be more clearly elucidated once the importance of wedge thermal structure is recognized.