HIGH-FIELD-STRENGTH ELEMENT DEPLETIONS IN ARC BASALTS DUE TO MANTLE MAGMA INTERACTION

BASALTS, basaltic andesites and andesites in subduction-related magmatic arcs are all depleted in high-field-strength elements (such as Ti, V, Zr, Nb, Hf and Ta) relative to mid-ocean-ridge basalt (MORB). Here we show that these depletions can be produced in liquids by interaction with depleted mantle periodotite, as would occur during the ascent of an arc magma through the mantle lithosphere of the overriding plate. This process involves extensive reaction between liquid, olivine, orthopyroxene and spinel. High-field-strength element depletions are produced because olivine, orthopyroxene and spinel have higher crystal/liquid distribution coefficients for these elements than for other incompatible trace elements. Liquids modified by mantle-magma interaction will also be depleted in heavy rare-earth elements, Cr and Ni, and enriched in light rare-earth and large-ion lithophile elements, relative to MORB. These characteristics are all common in mafic magmas at convergent plate margins1-7. © 1990 Nature Publishing Group.