Deciphering crust, mantle, and slab controls on arc magma compositions: A global perspective

Abstract:

Global correlations between segment-averaged arc lava compositions and over-riding plate structure have now been verified by a series of studies along the margins of the Southern Andean Arc of Chile. Each global and regional study demonstrates a strong relationship between over-riding plate thickness, magmatic enrichment in incompatible elements, and variations in Nd and Hf isotopes ratios. This finding is again reinforced by a comparison of global arc-segment compositional averages, Chilean volcano averages, and a global compilation of raw data from several high-Mg# continental arc-front stratovolcano samples. These variations in arc magma compositions persist to high Mg#, and therefore likely originate in the mantle. Even though all erupted arc lavas have cooled and crystallized, and in many cases assimilated crustal materials, most compositional variability among mafic to intermediate arc lavas appears to originate at greater depths.

Here we interpret arc geochemistry under the lens of recent work in experimental petrology and a systematic framework for geochemical modelling. This analysis builds on a recent interpretation of Sr systematics which found that the mixed mantle sources of all arc magmas requires 5-10% ocean crust+sediment melt. In addition to ubiquitous melting of the subducting slab, trace element and isotopic data require that ambient mantle compositions span from more depleted than any MORB source to as enriched as OIB mantle, with extents of melting that vary globally by a factor of ~4 and are lowest in continental arcs. A geochemical model motivated by these observations and consistent with experimental data and thermo-mechanical models reproduces global trends in arc geochemistry. This same model provides internally consistent solutions to outstanding questions about regional arc geochemical variations from Mexico and the Marianas.