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Deciphering the Early Archean Recycling through the Multiple Sulfur Isotpic Record – Dear_Sir

Deciphering the Early Archean Recycling through the Multiple Sulfur Isotopic Record

Identifying Mass-Independent Fractionation of Sulfur (S-MIF) in the mantle brings strong evidence for an early transfer of Archean crustal 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 main objective of this project is to fill in the gaps that presently limit the application of this promising tracer of the Earth's early geodynamics.

Analytical difficulties to be overcome by a multi-tool approach

The application of S-MIF was hampered by conflicting observations between whole rock (chemical extraction and fluorination of sulfur and low-resolution gas mass spectrometry analysis, ~10 laboratories worldwide) and in-situ analyses (high-resolution in-situ ion probe analysis, ~5 laboratories) measurements.

As a prelude to this project, we first improved the precision of in-situ analyses of the 3 sulfur isotope ratios by producing/characterizing an unprecedented set of standards that are now made available to the entire community who wishes to undertake these types of measurements at the CRPG in Nancy. The accuracies achieved are without any comparison with previous studies.

We also developed the method of analysis by direct sulfide fluorination (thus avoiding the chemistry step), ultimately allowing analysis of some specific samples using three quasi-independent methods.

Finally, a sequential approach has been developed to analyze sequentially in-situ on the same sulfide grain: major and trace element compositions, S-isotopic compositions, siderophile and chalcophile trace elements, and Re-Os isotope composition.

The dataset supports the idea that the variations (i.e. of sulfur isotope composition) reflect heterogeneity induced by plate tectonics, but the absence of 'mass-independent signatures' suggests that these signatures are the result of recent surface-mantle exchanges. Very few samples record an Archean-type signal (some diamonds, a carbonatite and an EM1-type volcano), a result at odds with assumptions commonly accepted in the scientific literature. These results improve our knowledge of the functioning of the structure, evolution and dynamics of our planet.

Dear_Sir was a fundamental research project in geosciences, coordinated by Pierre Cartigny of the IPG in Paris. It involved Emilie Thomassot from the CRPG in Nancy, Catherine Chauvel from the University of Grenoble Alpes and Olivier Alard from Geosciences Montpellier. The project started in October 2015 and lasted 40 months. It benefited from an ANR grant of €350,000 for a total cost of around €758 500

The project resulted in a first publication (Delavault, Chauvel, Thomassot et al., 2016, PNAS) describing the discovery of isotope anomalies in EM1-type geochemical affinity lavas and a second (Kitayama et al., 2017) dealing with the origin of sulfur in Udachnaya kimberlite. Several publications are already well advanced and several others will follow.

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.

Project coordinator

Monsieur Pierre CARTIGNY (Institut de Physique du Globe de Paris)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CRPG-CNRS Centre re Recherches Pétrographiques et Géochimiques, Nancy
IPGP Institut de Physique du Globe de Paris
Institut des Sciences de la Terre ISTerre
GM Géosciences Montpellier

Help of the ANR 349,996 euros
Beginning and duration of the scientific project: October 2015 - 36 Months

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