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SIMS21, Poland 2017 - Laurent Remusat abstract

Laurent Remusat oral presentation (OA1-Tue2-2-3)

NanoSIMS Investigation of Geological Processes

Laurent Remusat

IMPMC - UMR CNRS 7590, Sorbonne Universit├ęs, MNHN, 57 rue Cuvier, CP52, 75005 Paris, France

Thanks its high sensitivity at high spatial resolution, NanoSIMS is a relevant instrument to study melting and differentiation in geology. This technique is one of the few suitable for the detection and quantification of volatiles and trace elements at the micron scale, in natural and experimental samples. Two examples will be presented to show the advances that NanoSIMS can bring to the understanding of crucial geological processes.

Planet Earth results from the accretion of planetesimals that underwent subsequent melting. Its current internal structure is the consequence of a dramatic chemical partitioning that occurred early in the planet history. Depending on their affinity with metal phases or silicate minerals, chemical elements could sink in the dense metallic core, or they could remain in the silicate mantle. Only the mantle composition can be probed, the core being unreachable; hence the S budget of Earth remains debated. To unravel the origin of sulfur in the mantle and improve our understanding of the Earth formation, high pressure (up to 80 GPa) and high temperature (up to 4200K) experiments were performed in diamond anvil cell [1]. They were extracted using the focused ion beam technique as a few microns large slices. Thanks to NanoSIMS mapping (see figure 1), the S partition coefficient can be determined at the temperature and pressure conditions relevant for planetary differentiation and scenarios for Earth formation can be refined.

Differentiation processes still occurs today, leading for instance to the formation of granitic rocks constituting most of the continental crust. Granitic magmas are formed by the partial melting of lithosphere rocks. This melting is assisted by the occurrence of volatiles, including water and CO2. The characterization of melt inclusions in some specific rocks, namely migmatites, provides insights on the melting conditions required for granite formation [2]. The NanoSIMS was used to determine the water content in various types of melt inclusions, being smaller than 10 microns. The results are useful to infer the thermodynamic conditions of partial melting in the deep lithosphere and to constrain the influence of water in the formation of continents [3].

[1] T.-A. Suer et al., Earth and Planetary Science Letters, In press, 2017.

[2] O. Bartoli et al., Earth and Planetary Science Letters, 395, 2014, 281-290.

[3] A. Acosta-Vigil et al., Contributions to Mineral and Petrology, 171, 2016, 24.