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Check out this new entry describing how to pick and select apatites for (U-Th)/He Thermochronology!At CU we are equipped to measure (U-Th)/He ages on a variety of materials, and are always interested in pursuing new research directions developing new collaborative relationships.He is constantly being produced by decay, when the rocks are at hot He will not be retained in the crystals, and will instead diffuse out of the system (and into the atmosphere, where it makes up about 1 ppm, or 0.0001%, of what we breathe).However, once a mineral is sufficiently cool, the He will be retained.During the last 10 years the understanding of the diffusion behavior of He in apatite has been increased (Lippolt et al., 1994; Wolf et al., 1996, 1998, Farley, 2000).The closure temperature of mineral grains that accumulate radiogenic He during the α-disintegration of 238U, 235U, 232Th and daughter products is dependent on: activation energy (E), a geometric factor for the crystal form (A), thermal diffusivity (D0), the length of the average diffusion pathway from the interior to the surface of the grain (a) and the cooling rate at closure temperature (d T/dt; Dodson, 1973).A correction has to be applied for the loss of radiogenic He generated within an outer rim of the mineral grain by the α-stopping distances (apatite: 25m).The most important requirements for (U-Th-Sm)/He thermochronology are 1.) minerals free from inclusion and cracks, 2.) idiomorphic crystals, 3.) a homogeneous distribution of U and Th.
Data are also corrected for down-hole laser fractionation, elemental fractionation, and common Pb correction.Therefore, when you calculate the (U-Th)/He age of a rock or mineral, you are in effect calculating the time that has elapsed since the system switched from open (hot) to closed (cool) behavior.