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 (δ18O between +6.2 and +24.7‰), the δ18OV-SMOW values of emeralds from the Sandawana mines in Zimbabwe (δ18O‰=+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. δ18O 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 δ18O values may eventually be identified using a combination of physical properties, inclusion characteristics and chemical composition. To cite this article: J.C. Zwaan et al., C. R. Geoscience 336 (2004).