Origine, dynamique et multi-compositions isotopiques du soufre dans le manteau terrestre
IPGP - Îlot Cuvier
Soutenances de thèses
Sulfur content and isotope composition of the terrestrial mantle can provide major constraints on the internal dynamic of this element and on nature of mantle heterogeneities. The mean S isotope composition of this reservoir can also characterize the potential mantle-core equilibrium and hence help to decipher the question of sulfur origin on Earth. In this study, we report S content and relative abundances of 32S, 33S, 34S and 36S in more than 120 glassy basalts coming from 3 ocean ridges and a transform fault. We propose a new sulfur extraction protocol for silicate glasses based on hydrofluoric acid. We evidenced an analytical bias affecting the previously published dataset, probably due to the use of the ”Kiba reagent”. The re-determined d34S are approximately 1‰ lower than the previously published data. We also provide the first D33S and D36S data for ocean ridge basalts. These data are, within uncertainty, indistinguishable from our CDT estimate, confirming the idea that terrestrial mantle has a D33S and D36S close to 0.000‰ with respect to this international standard. The d34S variability encompasses a 3‰ range in ocean ridge basalts and correlations between d34S and radiogenic isotopes demonstrate that S isotopes are tracers of mantle heterogeneities. On the other hand, the mean d34S of MORB mantle is -0,80 ± 0,38‰, significantly lower than the chondritic value of 0,04 ± 0,31‰. The Pb-S isotope systematic in Pacific Antarctic basalts indicates that recycled oceanic crust bears a positive d34S. As a corrollary, this result implies that the depleted part of the mantle is even more negative than the mean MORB value. We propose a d34S value of -1,40 ± 0,20‰ for this reservoir. In South Atlantic basalts, the d34S are correlated with Sr isotopes, trends being consistent with recycled sediment feeding the mantle source of these basalts. Taken together, the d34S variability associated to the D33S and D36S constancy indicates that the recycled components are of post- archean age, in agreement with independant estimation of their mean age using Pb isotopes. Finally, the negative d34S of MORB mantle can be reconcilied only with a mantle-core equilibrium, implying an internal origin of sulfur as opposed to a late origin.