Investigation of excess 4He and 40Ar in beryl by laser extraction technique

Abstract

We have investigated the distribution of 4He and 40Ar in a beryl crystal using a laser gas-extraction system. 4He is distributed almost homogeneously in the crystal, but 40Ar is concentrated both near a beryl-quartz boundary and near a large crack. This fact suggests that noble gases diffused into the beryl from the outside. From numerical simulation we infer that Ar in this beryl had two origins: one type had been trapped at the time of crystallization (“trapped”) and the other diffused into the mineral from the outside (“diffuse-in”). Stepwise heating experiments revealed that beryl has two kinds of sites which trap Ar. The amounts of Ar trapped in these sites were comparable to each other for all samples irrespective of their localities within a crystal. These observations suggest that both kinds of sites trapped Ar atoms not only at the time of crystallization but also by “diffuse-in” after crystallization. The present study showed that we should consider two origins, “diffuse-in” and “trapped”, in explaining excess 4He and 40Ar in beryl.