Amphiboles as indicators of mantle source contamination: Combined evaluation of stable H and O isotope compositions and trace element ratios

Abstract

Stable isotope and trace element compositions of igneous amphiboles from different tectonic settings (ocean island basalts, intraplate alkaline basalts, subduction-related andesitic complexes) were compiled to help understand the role of fluids and melts in subduction-related mantle metasomatism and to evaluate the use of selected trace element ratios (Pb/Pb*(N)=Pb/(√(Ce·Pr)) and Ba/Nb(N), normalized to primitive mantle) to help detect possible metasomatism. Comparisons of stable H and O isotope compositions and trace element ratios of amphiboles from ocean island basalts (Canary Islands), intraplate basalts, and subduction-related calc-alkaline andesitic series (Carpathian–Pannonian Region, CPR) indicate systematic distributions in δD–δ18O–Pb/Pb*(N)–Ba/Nb(N) diagrams that are related to metasomatic processes in the mantle and the migration of fluids and melts derived from subducted crustal slabs. In order to interpret these data for the amphiboles from the CPR, ophiolites of the Penninic and the Meliata–Vardar complexes as potential sources of subducted crustal melts and fluids in the mantle of the Carpathian–Pannonian Region were also analyzed. On the basis of published fluid/rock partition coefficients the compositions of fluids emanating from subducted ophiolites were calculated. The calculated fluid compositions—especially for blueschists of the Meliata complex—fit the amphibole trends, indicating that such fluids could have been responsible for the mantle metasomatism beneath the CPR.