Fission-track analysis of meteorites: dating of the Marjalahti pallasite

Chronology is rather a weak point in an investigation of pallasites, the stony-iron meteorites. The results of the Marjalahti pallasite fission-track age determination are presented. To apply this method only uranium-rich phosphates can be used. Extremely rare phosphate (whitlockite) grains were extracted from the silicate sawing residue. It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of 238U; spontaneous fission of extinct, short-lived 244Pu present in significant quantities in the early solar system. Thorough examination of fossil tracks in the whitlockite crystals coupled with U-content determination made it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age. A great track density attributed to the extinct 244Pu testifies to the high fission-track age. The model fission-track age of (4.31±0.02) x 109 yr for the Marjalahti pallasite is calculated. Petrographic studies allows us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.