Dear_Sir

Mass Independent Fractionations of Sulfur (S-MIF, whom amplitude is expressed by the ?33S notation) result mainly from photochemical process(es) in an atmosphere devoid of oxygen. Production of MIF-S in the Archean atmosphere is evidenced by the ubiquitous presence of S-MIF in the vast majority of sediment older than 2.5 billion years (Ga). Identifying S-MIF in the mantle brings strong evidence for early transfer of crustal Archean sedimentary material into the mantle. However, the application of S-MIF in the mantle is still in its infancy, especially owing to both methodological and analytical limitations. The present project's main objective is to fill these gaps that presently limit the application of this promising tracer of early Earth's geodynamics. We stress in particular that: 1- reports of sulfur isotope fractionation in the mantle remains limited to a few samples from geodynamic contexts that are too specific to establish a quantitative budget of the global mantle sulfur cycle. 2- the rather small size (ie <100 microns) of these samples (sulfide inclusions in diamonds or olivines) requires the use of in-situ technique (SIMS) at high spatial resolution. Yet little available data obtained using more precise gas source mass spectrometry (i.e. analysis of the SF6 molecule) do not corroborate the first results obtained by ion microprobe. 3- the amplitude S-MIF in the mantle-derived samples analyzed to date is restricted (?33S ranging from -1 to 1 ‰) compared to Archean sediments (?33S -3 to + 12 ‰). Mixing with mantle sulfur or re-homogenization within the subducting sediments are potential scenarios that need to be tested. 4- Analytical precision of in-situ techniques does not presently allow to measure the predicted 36S/32S-ratio anomaly; so it is possible to speculate on the existence of other mechanisms than recycling of Archean sedimentary sulfur to explain the occurrence of isotopic anomalies in the mantle. 5- in the absence of additional geochemical constraints, mechanisms (and their timing) such as melting and metasomatism governing the redistribution of isotopic anomalies can not be discussed. At this point, the transfer of Archean sedimentary sulfur in the mantle remains qualitative and S-MIF mantle are interpreted in binary mode (detectable presence of an Archean sedimentary component or not). To overcome these methodological and analytical limitations, we formed a consortium involving four national laboratories and foreign partners to develop an integrated geochemical approach both in terms of tracers than geological settings. From an analytical point of view, we will use a series of rarely gathered equipments for such a project (e.g. high-resolution gas source mass spectrometry, high resolution ion probes) to improve the accuracy of analysis with the aim of accurately measure the isotope ratio of the rare 36-S isotope. This analytical development will subsequently benefit to the whole community. We will develop a new methodology, coupling sulfur stable isotope geochemistry, radiogenic isotopes and chalcophile elements to elucidate the mechanisms and processes controlling the origin, history and fate of sulfur in the mantle. Ultimately, we will deduce a budget of the mantle sulfur cycle and shed new light on the early exchanges between surface and internal envelopes of our planet.