Isotope thermochronology is an important tool in a variety of geologic studies including, in particular, investigations of uplift-erosion-deformation and basin heating. With a few exceptions, these studies have been focused on minerals with relatively high concentrations of either K or U. In this project, the investigators will study the diffusion of He in the low-U mineral calcite to better understand the potential for the use of calcite as a (U+Th)/He thermochronometer. Although typical marine calcites have [U]<5ppm, problems associated with this low concentration can be overcome by increasing sample size and in many rocks (i.e., limestones and marbles) calcite crystals greater than 1 mm in diameter are common. Preliminary investigations on the diffusivity of He in igneous, metamorphic, and sedimentary calcite indicate a closure temperature for He of ~65C for a cooling rate of 10C/m.y. These extremely promising preliminary results will be confirmed by additional analyses and by comparing lab-induced diffusion with diffusive histories in nature, and calcite versus dolomite diffusion.
The ultimate purpose of this research is to determine the temperature at which He is retained in calcite. This work has great potential because calcite is an essential mineral in both limestones and marbles and these rocks contain no other mineral which can be used to determine time-temperature history. The applications in which this technique might be used include analysis of the history of sedimentary basins (the temperature of sedimentary basins is an important consideration in the search for petroleum), the uplift of mountain belts, and speleology.
