Non-chondritic sulphur isotope composition of the terrestrial mantle

Core-mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth's sulphur should at present reside in the core(1), which implies that the residual silicate mantle should exhibit fractionated S-34/S-32 ratios according to the relevant metal-silicate partition coefficients(2), together with fractionated siderophile element abundances. However, Earth's mantle has long been thought to be both homogeneous and chondritic for S-34/S-32, similar to Canyon Diablo troilite(3-6), as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled bymid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous S-34/S-32 ratios, directly correlated to the strontium and neodymium isotope ratios Sr-87/Sr-86 and Nd-143/Nd-144. These isotope trends are compatible with binary mixing between a low-S-34/S-32 ambient mantle and a high-S-34/S-32 recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative delta S-34 of -1.28 +/- 0.33 parts per thousand that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic S-34/S-32 ratio for the silicate Earth could be accounted for by a core-mantle differentiation record in which the core has a S-34/S-32 ratio slightly higher than that of chondrites (delta S-34=+0.07 parts per thousand). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core-mantle differentiation.