Andean subduction-related mantle xenoliths: Isotopic evidence of Sr-Nd decoupling during metasomatism

Abstract

Sr-Nd isotopic analyses on some mantle xenolith samples from the Northern, Southern and Austral Andean volcanic zones exhibit radiogenic Sr enrichment without dramatic changing of the Nd isotopic composition. This anomalous effect (Sr-Nd decoupling) makes these samples plot displaced to the right side of the "mantle array" trend (here called the "MORB-OIB-BSE trend") in the Sr-87/(86) Sr vs. Nd-143/Nd-144 isotopic diagram. Such behavior reflects processes that took place in the mantle and can be related to: i) the mixture of a depleted mantle and an enriched source (enriched mantle II-EMII); ii) the mixture of a depleted mantle and a mixture of mantle-derived and slab-derived melts; and iii) a chromatographic process that occurs during the percolation of a metasomatic agent through the mantle.