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 <2kg 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 ∼ 10⁶cm^-2 in samples from the largest fragment to approximately (4-9)×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.