For the last decade my group has worked to develop techniques for establishing very low temperature cooling histories of rock masses. Thermal histories are important for example for studying tectonics, especially associated with mountain range exhumation, for paleogeomorphology, and for resource exploration (e.g., hydrocarbon maturation). The technique we have been developing is based on the radioactive decay of uranium and thorium to 4He, a dating method proposed more than a century ago but never widely utilized. The key to the He thermochronometry method is the recognition that at elevated temperatures characteristic of the crust at a depth of a few km, helium diffuses from common U,Th-bearing minerals as rapidly as it is supplied by radioactive decay. As rocks cool, the rate of helium diffusion drops exponentially. As a result He-ages record time since cooling, rather than time since mineral growth. In the case of apatite (calcium fluorophosphate), lab experiments44 and field observations36,47 demonstrate that above about 80°C, helium is almost instantly lost, while it is quantitatively retained by 40°C19. By measuring He, U and Th concentrations in apatite grains we can calculate how long it has been since the crystal cooled through the critical interval 40-85°.Other minerals we have investigated include zircon60, titanite38,76, and monazite65. Details of the techniques we use, including laser extraction, and a review of He-datable minerals and their temperature sensitivities have been published 48,65,66.
Because of its widespread occurrence and sensitivity to very low temperatures (about 25°C cooler than the apatite fission track method), the apatite (U-Th)/He dating method has now been applied in many different places and with many different objectives. The most straightforward applications are in tectonics, in which the timing of fault motion can be deduced, e.g., in the White Mountains of California47. Along with Brian Wernicke, Jason Saleeby and several students and post-docs we have investigated helium ages of the Sierra Nevada26, 33, 88 (above, at the Kern-Kaweah Divide) as part of a major program for understanding the paleogeomorphology of this range. The basis for interpreting cooling ages in terms of paleogeomorphology and surface processes was recently reviewed68. Other detailed studies include work in the Coast Mountains of British Columbia 52,83,87 and on meteoretic phosphates which constrain the early history of asteroidal parent bodies71.
Most recently we have been developing a different approach to thermochronometry, by measuring the 4He concentration profile in mineral grains. Consider a sample that has cooled quickly, with no time for He diffusion. That sample will have a concentration profile that is unmodified by diffusion, i.e., ignoring other phenomena the 4He profile will be "square". In contrast a sample that has resided at temperatures where diffusion is active will have a "rounded" concentration profile, that is, lower concentrations at the grain edge, where diffusive loss occurs, than in the grain interior. We have forward-modelled the effect, and find that the concentration profile, coupled with the absolute (U-Th)/He age, can provide very restrictive limits on cooling history74. It is not presently possible to directly measure the 4He concentration at the requisite spatial scale, so we have developed an alternative approach. By irradiating samples with 100+ MeV protons at the Northeast Proton Therapy Center, we can transmute some target elements into the rare isotope of helium - 3He. This isotope will be uniformly distributed within the grain. By subjecting the irradiated sample to step heating, in which the helium is progressivly degassed from the sample, we can image the 4He/3He ratio within the grain, and hence the 4He distribution within the grain. We have applied this technique in several different ways, including a destiled study of glacial incision in the Coast Mountains75, 87.
In the summer of 2004 former post-doctoral fellow Marin Clark (now at U. Michigan) and I sampled granites across the Tibetan plateau to better assess when this region achieved its high elevation. The picture at right is of the Kunlun Shan, on the northern margin of the plateau. More pictures from our work in China and Tibet can be found here.