Rubidium-strontium geochronologic systematics in igneous contact zones: analog for 90Sr and 137Cs behavior in the near field

The behavior of 90Sr and 137Cs in the near field of a HLW package can be investigated by use of Rb-Sr geochronologic information from igenous contact zones. The chemical behavior of 90Sr is identical to other Sr isotopes, and Cs behavior is very similar to that of Rb. During igneous intrusion, the effect of heat from the igneous rock, coupled with fluid action, is to perturb Rb-Sr systematics of minerals in the intruded rock. Rb-Sr redistribution diagrams are useful not only for indirectly dating the time of intrusion, but also for determining the extent of closed versus open system conditions in the contact zones. Once open or closed system behavior is determined, the Rb-Sr systematics can be evaluated in terms of diffusion, fluid-induced metasomatism, selective dissolution, other, and coupled processes. All these can be discussed in terms of time-temperature-fluid compositional parameters. Rb migration is controlled predominantly by fluid flow; while diffusion models better explain some Sr isotopic behavior Cs, from geochemical arguments, should be less mobile than Rb. In most cases, even when temperature of the intrusive is 800 to 12000C the cooling period on the order of 104 to 105 years, and in a highly convective system, Sr and Rb mobility is on the order of only microns to meters. 17 ref.; 2 figs., 1 tab