Complementary Ti and Zr anomalies in orthopyroxene and clinopyroxene from mantle peridotites

Abstract

FROM the observation that clinopyroxenes in Iherzolites from sub-arc, sub-continental and sub-oceanic lithosphere are depleted in titanium and zirconium, Salters and Shimizu1 inferred the existence of a worldwide layer in the upper mantle depleted in these 'high-field-strength' elements. This seems inconsistent, however, with the fact that basalts generated in the upper mantle, except in certain restricted environments (related to subduction processes), do not show depletion of these elements. In an attempt to address this conflict, we have used ion-probe measurements to study Ti and Zr abundances in sub-continental spinel peridotites from Liguria, Italy. We confirm the existence of Ti and Zr depletion in clinopyroxenes, but find that these are compensated by enrichment in the coexisting orthopyroxene. The calculated whole-rock abundance patterns therefore show negligible anomalies. We conclude that Ti, Zr and rare-earth abundances in clinopyroxene alone cannot be used to infer the existence of a global mantle layer depleted in high-field-strength elements, a feature that would place important constraints on models of lithospheric growth2.