3:30 PM - 3:45 PM
[PPS10-01] Hydrogen ion irradiation of various minerals simulating the space weathering.
Keywords:space weathering, asteroid, ion irradiation
The target materials for ion irradiation are forsterite (Fo100, syn.), olivine (Fo92, San Carlos, USA) , enstatite (En99, Tanzania), spinel(MgAl2O4, syn.), corundum (Al2O3, syn.), enstatite glass(MgSi0.97Al0.03O3, syn.), serpentine (Mg#=0.98, South India) , pyrrhotite (Fe0.90S, Chihuahua, Mexico), iron meteorite ((Fe, Ni), Nantan meteorite (ⅢCD)). Samples are mechanically polished and cut into 3×5×0.5 mm rectangles. The damaged layers due to polishing were removed by chemical polishing with colloidal silica.
Experiments were carried out with the low-energy ion implantation equipment in The Wakasa Wan Energy Research Center. The samples were irradiated by 40 keV H2+ ions (corresponding to 20 keV H+ ions) with the doses of 1016, 1017, and 1018 ions/cm2. The cooling stages were used for experiments longer than 60 min to keep the samples at room temperature. We observed the surfaces of the irradiated samples with with an FE-SEM (JEOL JSM 7001F). FIB-lift-out sections were prepared with an FE-FIB (FEI Helios NanoLab 3G CX) and observed with FE-TEM (JEOL JEM 2100F).
We observed forsterite, olivine, and pyrrhotite irradiated by hydrogen ions with a dose of 1018 ions/cm2, and irradiated enstatite with dosed of 1017 and 1018 ions/cm. The TEM observation showed that vesicles were observed under blisters on irradiated enstatite with a dose of 1018 ions/cm. The crystal structure of orthoenstatite remained in the very surface of the blister skin. An amorphous structure was observed just above the vesicles. The irradiated enstatite with a dose of 1017 ions/cm only showed a slight deformation of the crystal structure. These observation shows that the threshold dose of the enstatite amorphization by 20 keV hydrogen ion irradiation is between 1017 and 10-18 ions/cm2. We did not confirm completely amorphous areas in FIB lift-out sections of the irradiated forsterite, olivine, and pyrrhotite, while blister skins of irradiated serpentine were amorphous.
Diffusion rates of hydrogen in silicates and oxides with ionic bonds such as enstatite, forsterite, and olivine are much slower than the experimental duration [e.g., 6]. Thus, the implanted hydrogen may move through vacancies formed by irradiated ions and recoiled atoms, and then bubbles nucleate and grow to form the blisters due to the high pressure of the hydrogen gas . On the other hand, hydrogen diffusion rates in amorphous enstatite and iron meteorite may be very rapid compared to the experimental duration [13, 14]. Hydrogen escaped from the surfaces and could not accumulate to form blisters on enstatite glass and iron meteorites.
We constrained on the threshold dose of the enstatite amorphization by hydrogen ion irradiation. The difference of the threshold doses of blister formation and amorphization, and of the blister structures indicates that we can evaluate the solar-wind irradiation age of the asteroidal regolith more quantitatively form the combination of blister and crystal structures of multiple minerals consisting one regolith.
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