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

Noble gas studies of fine-grained silicate material of the Efremovka carbonaceous chondrite revealed the presence of excess Ar-36 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 Ar-36 can be best explained as due to in situ decay of Cl-36 (tau = 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 Cl-36/Cl-35 of (1.4 +/- 0.2) x 10(-6) at the time of formation of Efremovka silicates can be made from our data. If we consider a nucleosynthetic origin of Cl-36, the time interval between the last addition of freshly synthesized Cl-36 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.