Micrometeorites in Antarctic ice detected by Ir: estimation of 120k year old accretion rate

Abstract

The accretion rate of micrometeorites (MMs) was estimated from Ir contents in snow around Dome Fuji Station and ice shards obtained during ice core drilling at Dome Fuji Station, Antarctica. The snow and ice shards were melted and filtered from the residues. Although MMs were not found on filters, we tried to detect them from the residues as Ir peaks determined by instrumental neutron activation analysis (INAA). Although Ir is very rare in the earth’s crust, its concentration is high in extraterrestrial matter (e.g., chondrites). Trace amounts of Ir can be easily detected by INAA, because the cross section of Ir is relatively large (e.g., 309 barn). The accretion rates were estimated for 120k year ago, 5k year ago and at present, as (3.2 ± 0.9) × 102 t/year (8.6 ± 0.18) × 103 t/year and (1.3 ± 0.10) × 103 t/year, respectively. These estimates were comparable to those of previous studies, however the rate of 120k year ago was approximately an order of magnitudes lower than the others.