Excess ³⁶Ar in the Efremovka meteorite: a strong hint for the presence of ³⁶Cl in the early solar system

Abstract

Noble gas studies of fine-grained silicate material of the Efremovka carbonaceous chondrite revealed the presence of excess ³⁶Ar over and above the level expected due to contributions from the trapped and cosmic-ray-produced, both spallogenic and secondary neutron-induced, components. The observed excess in ³⁶Ar can be best explained as due to in situ decay of ³⁶Cl (τ = 0.43 Ma) in the analyzed samples and provides a strong hint for the presence of this now-extinct nuclide in the early solar system. Additional experiments that will confirm our observation are proposed. A conservative estimate of an initial ³⁶Cl/³⁵Cl of (1.4 ± 0.2) × 10^-6 at the time of formation of Efremovka silicates can be made from our data. If we consider a nucleosynthetic origin of ³⁶Cl, the time interval between the last addition of freshly synthesized ³⁶Cl to the solar nebula and the formation of some of the first silicate grains in the nebula could be estimated to be approximately 1 Ma.