Non-chondritic sulphur isotope composition of the terrestrial mantle

J. Labidi (1) , P. Cartigny (1) , M. Moreira (1)

1 IPGP - Institut de Physique du Globe de Paris

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, which implies that the residual silicate mantle should exhibit fractionated ³⁴S/³²S ratios according to the relevant metal-silicate partition coefficients, together with fractionated siderophile element abundances. However, Earth's mantle has long been thought to be both homogeneous and chondritic for ³⁴S/³²S, similar to Canyon Diablo troilite, 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 by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous ³⁴S/³²S ratios, directly correlated to the strontium and neodymium isotope ratios ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd. These isotope trends are compatible with binary mixing between a low-³⁴S/³²S ambient mantle and a high-³⁴S/³²S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ³⁴S of -1.28 +/- 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic ³⁴S/³²S ratio for the silicate Earth could be accounted for by a core-mantle differentiation record in which the core has a ³⁴S/³²S ratio slightly higher than that of chondrites (δ³⁴S = +0.07‰). 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.

Domaines: Planète et Univers [physics]

J. Labidi, P. Cartigny, M. Moreira. Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature, 2013, 501, pp.208-211. ⟨10.1038/nature12490⟩. ⟨insu-03581766⟩