¹L. Remusat, ^1,2L. Piani, ¹S. Bernard
¹Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), UMR CNRS 7590 – Sorbonne Universités – UPMC – IRD – Muséum National d’Histoire Naturelle, 57 rue Cuvier, Case 52, 75231 Paris Cedex 5, France
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan

Carbonaceous and ordinary chondrites (CCs and OCs) contain insoluble organic matter (IOM) with large D-excess compared to other objects in the solar system. The higher the temperature experienced by CCs, the lower the D/H ratio of their IOM. It seems to be the opposite for OCs. Here, we report NanoSIMS H- (and N-) isotopic imaging of IOM of three OCs that experienced thermal metamorphism in the sequence Semarkona, Bishunpur and GRO 95502. In addition, we performed flash heating experiments on the IOM of GRO 95502 at 600 °C and characterized the residues using NanoSIMS, Raman and XANES spectroscopy. The present study shows that, in contrast to IOM of CI, CM and CR, IOM of OCs exhibits very few D-rich (or 15N-rich) hotspots. Furthermore, although the evolution of the molecular structure of OC and CC IOM is similar upon heating, their D/H ratios do not follow the same trend: the D/H of OC IOM drastically increases while the D/H of CC IOM decreases. In contrast to CC IOM, the D-rich component of which does not survive at high temperatures, the present results highlight the thermal recalcitrance of the D-rich component of OC IOM. This suggests that CCs and OCs did not accrete the same organic material, thereby challenging the hypothesis of a common precursor on chondritic parent bodies. The present results support the hypothesis that OC IOM contains an organic component that could originate from the interstellar medium.

Reference
Remusat L, Piani L, Bernard S (2016) Thermal recalcitrance of the organic D-rich component of ordinary chondrites. Earth and Planetary Science Letters 435, 36-44
Link to Article [doi:10.1016/j.epsl.2015.12.009]
Published by arrangement with John Wiley & Sons

¹M. J. Loeffler, ²C. A. Dukes, ³R. Christoffersen,²R. A. Baragiola
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²University of Virginia, Laboratory for Astrophysics and Surface Physics (LASP), Charlottesville, Virginia, USA
³Jacobs, NASA Johnson Space Center, Houston, Texas, USA

Pulsed-laser irradiation causes the visible-near-infrared spectral slope of olivine (Fo90 and Fo99+) and SiO2 to increase (redden), while the olivine samples darken and the SiO2 samples brighten slightly. XPS analysis shows that irradiation of Fo90 produces metallic Fe. Analytical SEM and TEM measurements confirm that reddening in the Fo90 olivine samples correlates with the production of “nanophase” metallic Fe (npFe0) grains, 20–50 nm in size. The reddening observed in the SiO2 sample is consistent with the formation of SiO or other SiOx species that absorb in the visible. The weak spectral brightening induced by laser irradiation of SiO2 is consistent with a change in surface topography of the sample. The darkening observed in the olivine samples is likely caused by the formation of larger npFe0 particles, such as the 100–400 nm diameter npFe0 identified during our TEM analysis of Fo90 samples. The Fo90 reflectance spectra are qualitatively similar to those in previous experiments suggesting that in all cases formation of npFe0 is causing the spectral alteration. Finally, we find that the accumulation of successive laser pulses cause continued sample darkening in the Vis-NIR, which suggests that repeated surface impacts are an efficient way to darken airless body surfaces.

Reference
Loeffler MJ, Dukes CA, Christoffersen R,Baragiola RA (2016) Space weathering of silicates simulated by successive laser irradiation: In situ reflectance measurements of Fo90, Fo99+, and SiO2. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12581]
Published by arrangement with John Wiley & Sons