Helium,Neon,and Argon in the Iron Meteorites Dongling,Nantan and Ningbo

The light noble gases He, Ne and Ar have been measured in the iron meteorites Dongling, Nantan and Ningbo. Dongling and Ningbo show a deficit of cosmic-ray that produced ~3He of ca. 30% and 10%, respectively, which is argued to be caused by the loss of ~3H (tritium) from the meteoroids during the time of their exposure to the cosmic radiation. Nantan has the lowest eontent of noble gases as yet reportcd for any iron meteorite. Cosmogenic ~3He and ~(38)Ar are only about 1/5000 of those in Dongling which has particularly interesting implications if the two meteorites belong to the same fall. In addition, Nantan contains non- spallogenic ~4He which we believe to be of radiogenic origin. This radiogenic~4He, together with a U-content of 2.6×10~(-11)g/g yields a ~4He retention age close to the cosmic-ray exposure age of Dongling. If Dongling and Nantan were part of the same meteoroid, this result would indicate that He retention in the meteoroid started only at or after onset of the exposure to the cosmic radiation. On the other hand, if the He retention age were 4,500 Ma, a U-content of less than 7.2×10~(-13)g/g is required to explain the non-cosmogenic _4He present. An upper limit to the number of transuranium or superheavy-element atoms which have deeayed by x-emission in Nantan since onset of He retention is 2×10~(10) per gram.