Radiogenic isotope initial ratio

The initial ratio, (Nr/Ns)o, is what we hope to determine. We can measure two quantities to extract this information. One is the ratio of the daughter isotope to a stable isotope of the same element. The abundance of the daughter isotope is the sum of the initial abundance, D/i, and the radiogenic component/) ... 

If we know the initial ratio of the daughter isotope (D0/Dref), we can calculate the ratio of the radiogenic daughter isotope to the reference isotope ... 

In the solid Earth, production of nucleogenic Ne is coupled to that of radiogenic " He. This is because production of Ne is directly proportional to the a-particle production ratio from the uranium and thorium series. The Ne/ He production ratio is constant and has been estimated at a value of 4.5 X 10 (Yatsevich and Honda, 1997). In this way, if the Earth accreted with solar helium and neon and initial ratios were modified by production of Ne and " He in a fixed proportion then the present-day He/ He and Ne/ Ne ratios in the mantle should be correlated. Honda et al. (1993) noted a strong correlation between OIB helium and neon isotopes such that steeper trajectories in three-isotope neon space were characterized by samples with high He/ He ratios. Indeed, they showed that it was possible to estimate the He/ He ratio of a suite of OIBs based solely on measurements of the neon isotope composition. 

The first step in evaluating initial Th is to measure the Th/ Th ratio. Th is an abundant radioactive, non-radiogenic isotope with a half-life of 1.401 x lO years and is measured during a typical thorium analysis. Because Th and Th are chemically equivalent, Th is a potential monitor of the initial Th content of a growing speleothem if the 23o contamination can be determined. 

Model age Age of a rock or mineral that is determined by radiogenic isotope methods in which the initial isotopic ratio of the sample is assumed. 

It has been recognized that the Lu-Hf isotopic system in zircon is a powerful tool for deciphering the evolution of the earth s crust and mantle. " Zircon normally contains 0.5-2 wt % Hf, which results in an extremely low Lu/Hf ratio (" Lu/" Hf < 0.002) and consequently a negligible radiogenic growth of Hf due to the decay of Lu. Therefore, the Hf/" Hf ratio of zircon can be regarded as the initial value at the time when it crystaUized. LA-ICP-MS with a multiple ion collector system has also been employed to study the hafnium isotopic composition of zircon and baddeleyite standards in U-Pb geochronology. °°... 

Ar, and " °Ar. The former two isotopes are primordial in origin with no significant contributions by production within the Earth " Ar, however, is produced by decay of ih/2 = 1-40 X 10 yr). The initial (or primordial) " °Ar/ Ar ratio of the solar system lies between 10 " and 10 (Begemann et al, 1976). In contrast, the terrestrial atmosphere has a " Ar/ Ar ratio of 296, and all other terrestrial materials have higher values. Terrestrial " Ar/ Ar ratios therefore are essentially the result of mixing of radiogenic " Ar with primordial Ar. 

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