Amorphization and D/H fractionation of kerogens during experimental electron irradiation: Comparison with chondritic organic matter

•Experimental electron irradiation of kerogens in the TEM.•D/H increase (δD=1000‰) and hydrogen loss observed by NanoSIMS.•Progressive amorphization observed by STXM–XANES.•Radiolysis and recombination occur at equilibrium and defect diffusion appears as the rate limiting step.•Electron irradiation at 200kV alone may be too slow to account for the deuterium enrichment in chondrites. Irradiation is common in the interstellar medium and the protosolar nebula. We have investigated the effects of electron irradiation on kerogens of type I and III in a 200kV transmission electron microscope (TEM), at 293K and 92K, using various fluences. Using synchrotron-based scanning transmission X-ray microscopy (STXM) and NanoSIMS, we have demonstrated a progressive amorphization coupled with hydrogen loss and a significant deuterium to hydrogen ratio (D/H) fractionation, with δD increasing by up to 1000‰. Hydrogen loss is non-linearly related to the fluence. Irradiation under cryogenic conditions (92K) hinders amorphization and D/H elevation. We suggest that these effects are controlled by radiolysis (carbonhydrogen bonds are broken and hydrogen is lost), coupled with recombination. The amorphization and hydrogen loss are rate-limited by defect diffusion which controls the recombination probability. The D/H increase appears to follow a Rayleigh distillation law with an apparent fractionation factor similar to the equilibrium fractionation factor of the isotopic exchange reaction CH4+HD↔CH3D+H2. This study represents a first step to estimate the kinetics and timescales of D/H fractionation under ionizing radiation. Extrapolatation of this fractionation behavior to natural environments remains difficult at this point because simultaneous irradiation by protons and other cosmic rays at various energies also occurs. However, the present results show that isotopic fractionation by electron irradiation at 200kV alone might have been kinetically inhibited at the low temperatures of the interstellar medium and the outer region of the protosolar nebula. In addition, we show that STXM or NanoSIMS experiments should not be performed on organic samples that have already been investigated using TEM, even under low flux electron irradiation conditions.