The Dergaon (H5) chondrite: Fall, classification, petrological and chemical characteristics, cosmogenic effects, and noble gas records

Abstract

A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~10^6 cm^(-2) in samples from the largest fragment to approximately (4-9) x 105 cm^(-2) in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite.