Rhenium-osmium isotope systematics in MORB from the Southern Mid-Atlantic Ridge (40°-50° S)

We present 27 osmium isotopic analyses and Re-Os contents in MORB glasses from the South Atlantic ridge (40°-50° S), an area showing an important mantle source heterogeneity expressed by large variations in trace element and Sr, Nd and Pb isotope compositions. Os concentrations are very low and range from 1.37 to 12.47 ppt and Re concentrations range from to 0.44 to 2.13 ppb. The 187Os / 188Os ratios vary from 0.1314 to 0.2316 and define a global positive correlation with 1 / Os. A multidimensional analysis of our dataset reveals that 187Os / 188Os ratio is independent from the other isotopic systems or that it provides complementary information on the nature of the processes affecting the upper mantle. Using trace element and Sr isotopic ratios to distinguish the different processes affecting the South Atlantic MORB source, our results enable an assessment of their effects on the Os isotopic composition. The variable 187Os / 188Os ratios of N-MORB samples could be attributed to metasomatism of the asthenospheric mantle by melt percolation related to an ancient subducting zone. The 187Os / 188Os ratio appears also to be very sensitive to plume-ridge interaction and its latitudinal variation along the 47.35°-48.5° S segment confirms the migration of material from the Discovery plume southward the Agulhas FZ. Finally, associated to Pb-Sr-Nd isotopic ratios, the 187Os / 188Os ratios measured in E-MORB from the 48.5°-49.2° S segment confirm that delaminated lower continental crust is responsible for the DUPAL signature observed in the South Atlantic and Indian Oceans.