Apatite u thhe dating
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.
For example, He is only retained in apatite when the crystal cools below about 60–70°C, and in zircon when it cools below ~185°C.
Because the earth gets hotter as you go deeper, the cooling history of a rock often tracks its path from the deep earth up to the surface.
The concept of He-diffusion in apatite assumes that the diffusion path (a) is the grain size.
Therefore, the geometry (A) of the mineral grain is very important (Meesters and Dunai, 2002a, b).
Check out this new entry describing how to pick and select apatites for (U-Th)/He Thermochronology!
Four single-crystal sanidine-anorthoclase Ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31.44±0.18 Ma (2σ) for the apatite itself.
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.
The transitional temperature is called the closure temperature, and is different depending on what mineral you are interested in.
4) Down-hole laser fractionation Down-hole fractionation in apatite during lasing can be treated similarly to zircon. 5) New apatite matrix-matched elemental fractionation standards A previous limitation to using in situ LA-ICMPS for U-Pb dating of apatite has been the lack of a well characterized matrix-matched standard to correct for the differential fractionation of U, Th, and Pb during laser ablation.
Finding a suitable standard is made difficult by the tendency of apatite to have low and variable U (and hence radiogenic Pb) concentration, variable common Pb, and to lose Pb by thermally induced diffusion at temperatures ca. Despite this, we have identified two seemingly reliable natural standards: apatite from the Mc Clure Mountain syenite, Colorado and our now preferred gem rough apatite from Madagascar.