Noble gas isotopes and mineral assemblages of Antarctic micrometeorites collected at the meteorite ice field around the Yamato Mountains

摘要

From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large-scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser-gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X-ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entiy. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long-time storage in Antarctic ice. Jarosite-bearing AMMs have relatively low concentrations of ~4He, which is suggestive of loss during the alteration. High ~3He/~4He ratios are detected for AMMs with high ~(20)Ne/~4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)-He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean ~(20)Ne/~(22)Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3-component mixing of air, Q, and SEP-Ar. Ar isotopic compositions can not be explained without significant contributions of Q-Ar. SEP-Ne contributed more than 99% of the total Ne. As for ~(36)Ar and ~(38)Ar, the abundance of the Q component is comparable to that of the SEP component. ~(84)Kr and ~(132)Xe are dominated by the primordial component, and solar-derived Xe is almost negligible.