Selenium and tellurium systematics in MORBs from the southern Mid-Atlantic Ridge (47–50°S)

Abstract

Selenium and tellurium concentrations along with sulfur and Highly Siderophile Element (HSE) contents as well as 187Os signatures were determined in 20 Mid-Ocean-Ridge Basalts (MORBs) from the southern Mid-Atlantic Ridge (SMAR; 47–50°S), ranging in composition from depleted N-MORBs to mantle plume-related E-MORBs. A comparison between glassy rims and crystalline pillow interiors reveal that secondary processes are only reflected in seawater-overprinted 187Os/188Os signatures and degassing-related low S contents of the crystalline pillow cores but did not affect the Se and Te abundances. In contrast, the segregation of sulfide liquids during MORB differentiation lowers the Se and Te concentrations (∼35% and 60%, respectively) and leads to higher Se/Te ratios. Recomputed primitive melt Se contents broadly overlap for both, N- and E-MORBs, while primitive E-MORB melts have systematically higher Te contents and lower Se/Te ratios compared to those of the N-MORBs (13–14ppb Te and Se/Te≈18 vs. 9–11ppb Te and Se/Te≈25). As suggested by lithophile trace element and Sr–Nd–Pb isotopic constraints, the Se–Te systematics of the E-MORB mantle source traces the involvement of a recycled component likely derived from the nearby Discovery mantle plume. Bulk mixing models suggests an addition of either 20% pyroxenitic melts, or only 10ppm of metasomatic sulfides to account for the Te-richer E-MORB compositions. A conservative correction of our MORB data for sulfide segregation combined with a near fractional melting model predicts a Te-depleted MORB mantle reservoir with a non-chondritic Se/Te of 18–25, significantly higher than the primitive mantle Se/Te estimates (6.3–9.9). The existence of these different Se–Te signatures between the E-MORB mantle source, the N-MORB mantle source and the primitive mantle support an incompatible behavior of both Se and Te during partial melting, with Te being slightly more incompatible. More importantly, this stresses the necessity of considering the full spectrum of the terrestrial silicate reservoirs to realistically constrain the budget of these volatile and highly siderophile elements in the bulk silicate Earth in order to discuss large scale planetary processes.