CONSTRAINTS ON EARTH EVOLUTION FROM ANTIMONY IN MANTLE-DERIVED ROCKS

Abstract

We have analyzed Sb in a variety of mantle-derived volcanic rocks, peridotites, and in the CI chondrite Orgueil by spark source mass spectrometry. Concentrations vary from 0.02 to 0.8 ppm in oceanic basalts (mid-ocean ridge basalts, MORB; oceanic island basalts, OIB). Antimony is a moderately siderophile element which behaves like the incompatible lithophile element Pr during igneous processes in the mantle.Both MORB and OIB samples have similar Sb/Pr ratios of about 0.02, which are different from those in continental-crustal rocks. Antimony resembles Pb in that it behaves like a highly incompatible element during formation of continental crust, whereas it behaves only moderately incompatible during formation of oceanic basalts (MORB or OIB). Consequently, Sb/Pb ratios of oceanic basalts agree within error limits with those of the continental crust and with the CI chondritic value, indicating that Sb/Pb is not strongly fractionated during crust-mantle-core differentiation.From the Sb/Pb ratios we estimate a Sb concentration of 11 +/- 5 ppb for the primitive mantle. An alternative estimate for the primitive-mantle abundance is obtained by assuming the Sb/Pr ratio of the primitive mantle to be intermediate between the MORB-OIB value (0.02) and that of the continental crust (0.05). This approach yields Sb = 8 +/- 4 ppb for the primitive mantle. Antimony is depleted in the bulk silicate Earth by a factor of 45. The volatility-corrected depletion factor of 7 is similar to other moderately siderophile elements.