Helium diffusion dating
Helium diffusion dating - esmee 25 polish dating
By irradiating samples with 100 Me V protons at the Northeast Proton Therapy Center, we can transmute some target elements into the rare isotope of helium - 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.
Here we present a series of date and diffusion measurements that document the importance of alpha dose, which we interpret to be correlated with accumulated radiation damage, on He diffusivity.
The role of crystallographic structure in influencing these differences among diffusivities was evaluated using the maximum aperture approach of Cherniak and Watson (2011), in which crystallographic structures are sectioned along possible diffusion directions and the maximum interstitial apertures in each 'slice' in the structure are identified.
Preliminary results show that observed differences in diffusivities are consistent with the size of the smallest maximum aperture along each diffusion direction.
Diffusion in calcite exhibits marked anisotropy, with diffusion parallel to c about two orders of magnitude slower than diffusion normal to cleavage faces.
He diffusivities for transport normal to the c-axis are similar in value to those normal to cleavage surfaces.
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 zircon 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.
In calcite, the smallest maximum apertures are ~0.92 and ~0.66 angstroms for cleavage-normal and c-axis parallel directions respectively.
In dolomite, the smallest maximum aperture is ~0.78 angstroms for the cleavage normal direction.
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 He profile will be "square".
Our model predicts that the bulk zircon (U-Th)/He closure temperature ( above this dose.