Isochrone radioactive

Figure 23. Measured ( °Th/ Th) ratios in basalts from Piton de la Fournaise (Reunion Island) as a function of their eraption ages deduced from mineral isochrons. These ratios decrease with increasing emption ages as a result of post-eraptive radioactive decay. The curve shows the theoretical evolution by radioactive decay for a rock with a Th/U ratio of 3.95 and a ( °Th/ Th) ratio of 0.93, similar to the values measured in presently erapted lavas. An approximate age can thus be obtained from the measured ( °Th/ Th) ratio of an old sample. Part of the dispersion around the theoretical curve are due to small source heterogeneities (slightly variable ( °Th/ rh) and Th/U ratios), also evidenced by Sr/ Sr ratios (Condomines et al. 1988, and unpublished results).

The above two methods require knowing the initial amount of either the radioactive parent or the radiogenic daughter. Although several powerful methods have been developed, they are not general because for many systems the initial amount of neither the parent nor the daughter is constrained. Under such circumstances, we need a method to determine all of these unknowns. This method is the isochron method, and it is the most powerful method in dating. 

Geochronological measurements (isochrone methodology) are based on the radioactive decay of the parent nuclide to the daughter nuclide using the fundamental Equation (8.8) for calculating the ages of minerals. 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

Isochron age calculations are commonly made for the Rb-Sr (rubidium-strontium], Sm-Nd (samarium-neodymium], and U-Pb (uranium-lead] radioactive systems. They are most commonly applied to whole-rock systems, that is, a suite of samples thought to have formed at the same time, such as an igneous plu-ton or a suite of lavas. Isochron age calculations may also be made for a suite of minerals in a rock, in which case they date the time at which the minerals lost isotopic contact with each other, that is, became closed systems. This approach can be useful in dating metamorphism. 

The foundations of modern geochronology were laid at the turn of the century in the work of Rutherford and Soddy (1903) on natural radioactivity. They showed that the process of radioactive decay is exponential and independent of chemical or physical conditions. Thus rates of radioactive decay may be used for measuring geological time. Isotopic systems used in age calculations are listed in Table 6.1 and Box 6.1. In this section we discuss two of the most common techniques used in geochronological calculations — isochron diagrams and model age calculations. This is followed by a discussion of the significance of the calculated ages. 

Thus isochron calculations require a suite of cogenedc samples, formed from the same parental material, and assume that there has been no exchange of parent-daughter isotopes other than through radioactive decay. An example of an isochron drawn for the Sm-Nd system from data in Table 6.2 is shown in Figure 6.2. 

Figure 9 An isochron diagram of Mg Mg versus 2rAI Mg for minerais from an Aiiende inciusion. The iinear dependence of the magnitude of the anomaious Mg on the AI Mg ratio can be interpreted as resuiting from the decay of the radioactive nuciide...

---