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 c arbonaceous chondrite revealed the presence of excess Ar-36 over and a bove the level expected due to contributions from the trapped and cosm ic-ray-produced, both spallogenic and secondary neutron-induced, compo nents. 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 provi des a strong hint for the presence of this now-extinct nuclide in the early solar system. Additional experiments that will confirm our obser vation 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 silic ates can be made from our data. If we consider a nucleosynthetic origi n of Cl-36, the time interval between the last addition of freshly syn thesized Cl-36 to the solar nebula and the formation of some of the fi rst silicate grains in the nebula could be estimated to be approximate ly 1 Ma.