RADIATION DAMAGE TRAPS & HELIUM DIFFUSIVITY MECHANISMS IN APATITE & ZIRCON

Here we present an investigation of potential mechanisms that control He mobility in common geochronology minerals. The trapping model for He in minerals (first proposed in Farley, 2000) predicts a temperature-dependent equilibration between He in the trapped and mobile phases of a mineral. Previous experiments have confirmed that apatite exhibits transient responses to heating after being exposed to temperatures above its annealing range (>265 oC) (Farley, 2000), which manifests as anomalously high diffusivity at low temperatures succeeding a high temperature step. This could indicate a kinetic limitation on He being ‘retrapped’ according to the trapping model, and provides opportunity to investigate in a previously unexplored way the manner in which radiation damage controls He diffusivity in minerals.

Though the transient heating response has been established to exist for apatite, the question of whether or not it is exhibited by other minerals then also becomes important for geochronology applications. As minerals very commonly used in geochronology applications, zircon and titanite have also been chosen for this project. Preliminary data reveals that zircon also demonstrates a clear heating response when exposed to maximum temperatures above ~350 oC, and at larger relative magnitudes than apatite. This seems to confirm that trapping also plays a role in the diffusive release of He from zircon, and that it may affect other minerals used in He geochronology. Further data will be presented which documents the extent to which this effect is transient or irreversible in zircon, as well as whether or not titanite also exhibits these properties.