Depth dependence of spallogenic helium, neon and argon in the St. Severin chondrite

Concentration and isotopic composition of He, Ne and Ar have been measured in 14 samples from 12 locations along a 35 cm long drill core through the geometrical center of the 271-kg amphoterite St. Severin. The concentrations of spallogenic 3He and 21Ne increase with depth by about 14% and 35%, respectively. The observed data are in agreement with the model for production of spallogenic nuclides given by Trivedi and Goel (1973). The spallogenic 20Ne/22Ne ratio is constant (0.84+-C.01), whereby the 22Ne/21Ne correlates with the 3He/21He. This relation can be used to correct the depth effect of noble gas production rates. All measurements of the core are thus in agreement with an exposure age of 10.8+-0.2 m.y. (Auth.)