Dating range equation

07-Oct-2019 07:34

However, due to a number of factors, outlined in the following Sections, a (U-Th)/He age must be interpreted carefully before the true meaning of the measured age can be evaluated. m), a significant proportion of alpha particles produced with an apatite grain may be emitted from the grain, resulting in loss of radiogenic helium. (1996) showed how this effect can be corrected for, by calculation of a correction factor (known as F Calculations of Helium retention over geological timescales, based on laboratory diffusion measurements, suggest that Helium is progressively lost at temperatures between 40 and 90C (for timescales of tens of millions of years), with this temperature range constituting a Helium “ Partial Retention Zone” or He PRZ. The three isotopes represented in the equation represent the only significant contributors of helium in natural samples. By measurement of the amounts of each isotope, the time t can be evaluated by solving this equation iteratively. As with the case of fission track ages, in the absence of other factors, this would provide a measure of the time over which helium has accumulated in the apatite lattice. An empirical test of helium diffusion in apatite: borehole data from the Otway Basin, Australia. Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer.

These values have been used in modelling (U-Th)/He ages for this report. More recently, however, the realisation that the partial loss of radiogenic products could provide quantitative information on the thermal history of mineral grains led to a resurgence of interest in this topic (e.g. In particular, efforts at Caltech through the 1990s led to the development of (U-Th)/He dating of apatite as a rigorous, quantitative technique (Wolf et al., 1996). Studies of the diffusion systematics of Helium in apatite (Wolf et al., 1998; Farley, 2000) also revealed the unique temperature sensitivity of the technique, with all Helium being lost over geological timescales at temperatures as low as 90C or less, and a “closure temperature” as low as 75C. However, effects related to grain size may be significant in the interpretation of apatites from sediments which have been heated to paleotemperatures within the He PRZ, as grains of different radii will give different ages for a particular thermal history. The effects of long alpha-stopping distances on (U-Th)/He ages. While this has yet to be demonstrated in natural samples, this holds considerable promise for obtaining more precise thermal history control in sedimentary basins. Ken Farley of Caltech, based on the systematics presented in Farley (2000) and references therein, allows modelling of the (U-Th)/He age expected from any inputted thermal history, in grains of any specified radius.

These values have been used in modelling (U-Th)/He ages for this report.

More recently, however, the realisation that the partial loss of radiogenic products could provide quantitative information on the thermal history of mineral grains led to a resurgence of interest in this topic (e.g. In particular, efforts at Caltech through the 1990s led to the development of (U-Th)/He dating of apatite as a rigorous, quantitative technique (Wolf et al., 1996).

Studies of the diffusion systematics of Helium in apatite (Wolf et al., 1998; Farley, 2000) also revealed the unique temperature sensitivity of the technique, with all Helium being lost over geological timescales at temperatures as low as 90C or less, and a “closure temperature” as low as 75C.

However, effects related to grain size may be significant in the interpretation of apatites from sediments which have been heated to paleotemperatures within the He PRZ, as grains of different radii will give different ages for a particular thermal history. The effects of long alpha-stopping distances on (U-Th)/He ages.

While this has yet to be demonstrated in natural samples, this holds considerable promise for obtaining more precise thermal history control in sedimentary basins. Ken Farley of Caltech, based on the systematics presented in Farley (2000) and references therein, allows modelling of the (U-Th)/He age expected from any inputted thermal history, in grains of any specified radius.

More recently, measurements of (U-Th)/He ages in samples from hydrocarbon exploration boreholes in the Otway Basin of S. Australia (House et al., 1999) have confirmed this general pattern of behaviour.