Experimental Investigation of Irradiation-Driven Hydrogen Isotope Fractionation in Analogs of Protoplanetary Hydrous Silicate Dust

Abstract

The origin of hydrogen in chondritic components is poorly understood. Their isotopic composition is heavier than the solar nebula gas. In addition, in most meteorites, hydrous silicates are found to be lighter than the coexisting organic matter. Ionizing irradiation recently emerged as an efficient hydrogen fractionating process in organics, but its effect on H-bearing silicates remains essentially unknown. We report the evolution of the D/H of hydrous silicates experimentally irradiated by electrons. Thin films of amorphous silica, amorphous “serpentine,” and pellets of crystalline muscovite were irradiated at 4 and 30 keV. For all samples, irradiation leads to a large hydrogen loss correlated with a moderate deuterium enrichment of the solid residue. The entire data set can be described by a Rayleigh distillation. The calculated fractionation factor is consistent with a kinetically controlled fractionation during the loss of hydrogen. Furthermore, for a given ionizing condition, the deuteration of the silicate residues is much lower than the deuteration measured on irradiated organic macromolecules. These results provide firm evidence of the limitations of ionizing irradiation as a driving mechanism for D-enrichment of silicate materials. The isotopic composition of the silicate dust cannot rise from a protosolar to a chondritic signature during solar irradiations. More importantly, these results imply that irradiation of the disk naturally induces a strong decoupling of the isotopic signatures of coexisting organics and silicates. This decoupling is consistent with the systematic difference observed between the heavy organic matter and the lighter water typically associated with minerals in the matrix of most carbonaceous chondrites.

Kinetic D/H fractionation during hydration and dehydration of silicate glasses, melts and nominally anhydrous minerals

“…The amplitude of the fractionation for =0.5 is of several hundreds of ‰. Such a large diffusion-driven fractionation was indeed recently produced experimentally by electron irradiation of hydrous analogues of the dust present in the protoplanetary disk (Roskosz et al, 2016).…”

“…Nevertheless, molecular hydrogen diffusion also occurs in more exotic environments, namely i) in very reducing conditions, ii) when the hydrogen fugacity (f H2 ) is high, iii) when radiolysis occurs in situ, for instance during ionizing irradiation of silicates. The last process was recently found efficient to separate D and H of water (and hydroxyl) groups dissolved in amorphous and crystalline silicates (Roskosz et al, 2016). For this reason, we provide models describing the spatial redistribution of H and D, in an amorphous silicate, caused by the diffusion of either molecular water or hydrogen atoms ( Fig.…”

New clues to ancient water on Itokawa

“…During accretion, localized irradiation processes within 3 AU could not have modified the primitive hydrogen signatures of the NAMs ( 42 ). Therefore, the accreted pebbles from the NAMs and planetesimals would preserve the D/H ratio acquired during formation in agreement with our observations: Itokawa, Earth, and parent bodies of LAR 12036 and Bishunpur ordinary chondrites, all formed in the inner solar system and exhibit comparable D/H ratios.…”

New Evidence for Wet Accretion of Inner Solar System Planetesimals from Meteorites Chelyabinsk and Benenitra

“…Thus, it is possible that D/H ratios in silicate minerals freely floating in the protosolar nebula prior to accretion or on the surface of the parent bodies are modified by electron irradiation. Previous experiments by Roskosz et al (2016) reported that electrons in the keV regime have the capacity to produce large D-enrichment (fractionation of D/H can be up to 600‰) in analogs of insoluble organic matter that is present in chondrites. However, the electron irradiation effects on silicate minerals are shown to be much smaller (fractionation of D/H is <200‰).…”

“…However, the electron irradiation effects on silicate minerals are shown to be much smaller (fractionation of D/H is <200‰). Particularly, fractionation of hydrogen isotopes caused by the electron irradiation in the silicate minerals larger than 10 μm is supposed to be <20‰ (Roskosz et al 2016). In our case, the measured orthopyroxenes in the OCs are all larger than 10 μm in size.…”