GEOSCIENCE SEMINAR: The redox state of the lanthanides Yb and Sm in extraterrestrial materials

GEOSCIENCE SEMINAR #4

" The redox state of the lanthanides Yb and Sm in extraterrestrial materials "

Tahar Hammouda, LMV Clermont Ferrand

Friday, February 20th at 4:15 p.m. in the ESRF Auditorium

In natural terrestrial systems, the REEs usually form REE3+ cations that combine with oxygen in silicate minerals. In some instances, however, Eu may be reduced to the (+2) state, whereas Ce may be oxidized to the (+4) state. Here, we have studied the redox state of Yb and Sm in a collection of meteorites originating from various parent bodies in the Solar System. We have applied x-ray absorption neard-edge spectroscopy (XANES) using microbeam to study polished meteorite sections, so that individual mineral grains (mainly sulfides and phosphates) could be studied. We also analyzed terrestrial phosphates and synthetic material produced under controlled oxygen fugacity conditions. We found that in the most reduced meteorites (the enstatite chondrites), a significant fraction of Yb (50%) and Sm (still to be determined) could be present in the reduced state. In other, less reduced, samples, Yb is exclusively in the (+3) state, whereas some reduced Sm has been detected in other chondrites and achondrites. These results can help us in refining a redox scale in the Solar System as a function of heliocentric distance, based on the redox state of the lanthanides.