This proposal seeks funding for continued investigation and development of (U-Th)/He dating of apatite, a new technique which can provide quantitative insights to an important but previously inaccessible portion of the time-temperature path of many rocks. The ingrowth of He from U and Th provides a method for dating minerals with high analytical precision. In apatite, studies suggest that He is totally lost by diffusion at temperatures >~85oC, but is quantitatively retained at Earth surface temperatures. Apatite He ages are thus "cooling ages" that reflect passage through isotherms located in the uppermost 1-3 km of the crust. This method is sensitive to temperatures substantially lower than are other dating techniques, and may provide fundamentally new information on a variety of geologic problems including studies of mountain range exhumation, river incision, and the thermal evolution of sedimentary basins. Initial studies confirm systematic and meaningful geologic variations of He ages and demonstrate the substantial potential of the method. The overall objective of the proposed work is a rigorous, quantitative foundation for reliable, routine application of the method and interpretation of the resulting ages.
