Volcanic Arc Magmas: Implications of a Melting-Mixing Model for Element Recycling in the Crust-Upper Mantle System

Abstract

A multicomponent melting-mixing model for the origin of volcanic arc basalt magmas implies signifi cant recycling of K, Rb, Ba, and other elements in the crust-upper mantel system. The model accounts for chemical differences in alkali, alkaline earth, and rare earth content between arc basalts and a wide range of oceanic and intraplate basalt types. In essence, the differences are due to modification of mantle peridotite by recycled continental components (derived from sediment and sea water) at convergent plate margins. In a specific model tested by mass balance calculations, residual periodotite (partly depleted in a basalt fraction) in a subducted lithospheric plate thermally equilibrates with the surrounding hotter mantle at depth beneath island arcs, and being less dense, rises as kilometer size diapires. The diapirs are modified by addition of small proportions of melt or saline supercritical fluid from the denser oceanic basalt (eclogite) and sediment. The diapire serves as a source for arc basalts. Mass balance calculations show that large proportions of the K, Rb, Pb, Ba and to a lesser extent Sr and light REE in "island arc tholeiites" are recycled continental crust materiaL Proportions are less for high-alumina basalts and shoshonites. High La/Sm basalts from volcanic arcs may be quite similar to their oceanic and intraplate counterparts, whereas low La/Sm basalts are distinctly different. The model does not apply to magmas that have been chemically modified during migration through the crust (e.g., some Andean andesites). Mass and element recycling rates can be calculated from the volcanic arc model, which gives the relative contributions of the melting components, and the magmatic production rates of arcs. Mantle-derived magmatism is forming new continental crust at arcs at a rate exceeding subduction of crustally-derived sediment at arcs. Thus, the crust appears to be growing, but at a rate that is perhaps half of the mean past rate. The K content of newly added continental crust, when corrected for recycling, is very much lower than the mean K content of the crust. Thus the dominant present mechanisms of crustal formation appears to yield a very different composition than past mechanisms which were responsible for formation of the bulk of the crust.