Tracing the emerald origin by oxygen isotope data: the case of Sandawana, Zimbabwe

Given the wide range of oxygen isotopic composition of emerald from all over the world ( δ 18 O between +6.2 and +24.7‰), the δ 18 O V-SMOW values of emeralds from the Sandawana mines in Zimbabwe ( δ 18 O‰=+6.6 to +8.0), are relatively constant, among the lowest ever measured. These consistently low values can be explained by host-rock buffering in a very narrow emerald-bearing reaction zone between ultrabasic greenstones (metamorphosed komatiites) and albitised pegmatites. δ 18 O values of Sandawana emeralds overlap those of emeralds from Brazil, Austria, Australia and Madagascar, a fact indicating that, in these cases, oxygen isotope composition alone is not sufficient to determine the geographic origin of commercially available emeralds. However, stones with overlapping δ 18 O values may eventually be identified using a combination of physical properties, inclusion characteristics and chemical composition.