THE EFFECTS OF LONG ALPHA-STOPPING DISTANCES ON (U-TH) HE AGES/

A mathematical framework for quantitative evaluation of alpha-stopping effects on (U-Th)/He ages has been developed. Alpha stopping ranges i n the U-238, U-235 and Th-232 chains vary between similar to 10 and si milar to 30 mu m, depending on decay energy and density/composition of the stopping medium. In the case of U- and Th-rich accessory minerals (e.g. apatite, zircon, titanite), the dominant effect of long stoppin g distances is alpha ejection to adjacent minerals. For grains smaller than a few hundred microns in minimum dimension, ejection effects wil l cause measured helium ages to substantially underestimate true ages. For example, a sphere of 100 mu m radius retains only similar to 82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is similar to 1/4 of the mean alpha range m ultiplied by the crystal surface to volume ratio, independent of geome try. Removal of the outer 20 mu m of a crystal prior to dating elimina tes the region which has experienced alpha loss, but may lead to erron eous ages when crystals are strongly zoned with respect to uranium and thorium. By careful characterization of four sieved apatite separates from a single sample, we show that it is possible to accurately corre ct (U-Th)/He ages for alpha ejection even when ejection exceeds 35% of total decays. Our results are useful for identifying the size and sha pe of grains which are best suited for (U-Th)/He dating and provide th e basis for correcting ages when ejection effects are significant. Thi s work underscores that meaningful (U-Th)/He ages require either large crystals, or correction of measured ages for alpha ejection.