Os isotope heterogeneity of the upper mantle: Evidence from the Mayari-Baracoa ophiolite belt in eastern Cuba

In an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle, 27 chromite separates from the 90 My old chromite deposits in the Mayari-Baracoa ophiolite belt in eastern Cuba have been investigated for platinum group element (PGE) concentrations and Re-Os isotopic systematics. The samples are characterized by systematically subchondritic initial Os-187/Os-188 ratios and substantial heterogeneity. The initial Os-187/Os-188 ratios vary with chromite chemistry and with geographical distribution, reflecting differences in the Os isotopic evolution for the different upper mantle sections represented by the ophiolite. Accordingly, the Os isotope data might be divided into three groups. In the Moa-Baracoa district, where the chromite bodies are located in the mantle-crust transition zone, the calculated initial gamma Os values average - 0.97 +/- 0.69 (n = 13). In the Sagua de Tanamo district, where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear, the initial gamma Os values are intermediate, with an average of - 1.77 +/- 0.80 (n = 7). In the Mayari district, where the chromite bodies are located in the lower part of the mantle sequence, initial gamma Os values average -2.66 +/- 0.29 (n = 7). These subchondritic (i.e. negative) initial gamma Os values are most simply explained by Re depletion during ancient partial melting and/or melt percolation events.