Radiogenic Ar

The physical transport of mass is essential to many kinetic and d3mamic processes. For example, bubble growth in magma or beer requires mass transfer to bring the gas components to the bubbles radiogenic Ar in a mineral can be lost due to diffusion pollutants in rivers are transported by river flow and diluted by eddy diffusion. Although fluid flow is also important or more important in mass transfer, in this book, we will not deal with fluid flow much because it is the realm of fluid dynamics, not of kinetics. We will focus on diffusive mass transfer, and discuss fluid flow only in relation to diffusion. 

Ar and a measured quantity of Ar (>99% enriched) is added (method of isotope dilution). After purification, the isotope ratios " Ar Ar and Ar Ar are measured by means of MS. From the isotope ratios " Ar Ar and Ari Ar in the added sample and in the air, the number of radiogenic Ar atoms is calculated. The concentration of K in the sample is determined independently by the usual analytical methods. The main problem with the " K/" Ar method is the possibility that some " °Ar may have escaped or that additional amounts of Ar may have entered the system in the course of time. 

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. 

Radiogenic Ar. The atmospheric " °Ar/ Ar ratio has been measured by Viking to be 3,000 400 (Pepin and Carr, 1992), although a lower value of 1,800 has been deduced from meteorite data (Pepin and Carr, 1992 Bogard, 1997 Bogard et al, 2001). Based on the Viking data, there are (7.0 1.4) X 10 atoms " °Ar in the atmosphere. The Mars mantle has been estimated to have 305 ppm K (Wanke and Dreibus, 1988),... 

Irwin JJ, Reynolds JH (1995) Multiple stages of fluid trapping in the Stripa granite indicated by laser microprobe analysis of Cl, Br, I, K, U and nucleogenic plus radiogenic Ar, Kr, and Xe in fluid inclusions. Geochim Cosmochim Acta 59 355-369... 

How did a nearly pure sample of Martian atmosphere end up inside BETA 79001 The trapping mechanism was apparently the shock event that ejected the rock from Mars. Simulation experiments, in which (terrestrial) basaltic samples were shocked in the presence of an atmosphere, showed that shock can produce glass that contains a sample of the ambient noble gas, with little or no elemental or isotopic fractionation (Bogard et al. 1986 Wiens and Pepin 1988). Furthermore, the Martian atmospheric signature is emphasized in BETA 79001. The rock is remarkably young (crystallization age of <200 Ma), so there is little radiogenic " °Ar, and has a remarkably short exposure age (<1 Ma), so there is little cosmogenic noble gas of any kind. 

Figure 5. The - Ar/ Ar ratio of mantle-derived volcanics are gen-erally within error of the atmosphere, while " °Arf Ar ratios are much greater due to addition of radiogenic " °Ar. A mantle component with a solar Ar/ Ar ratio falls outside the array of data, indicating that non-radiogenic mantle Ar was initially trapped after fractionation from the solar composition, or is dominated by Ar subducted after atmospheric fractionation. Data are from Sarda et al. (1985), Hiyagon et al. (1992), Honda et al. (1993), Niedermaim et al. (1997), and Kunz (1999).

In a closed system, the amount of radiogenic " °Ar produced over time t is given by... 

The origin of the radiogenic argon is more problematic as both mantle and crustal reservoirs are characterized by high " Ar/ Ar ratios. Kennedy et al. (1991) attributed radiogenic " °Ar in Honduran geothermal fluids to a crustal source based upon measured... 

He/" °Ar ratios (where " °Ar represents radiogenic " °Ar added to air-like argon to... 

Discussion of the extent of Earth degassing has centered on the K-Ar system, because the atmospheric inventory of radiogenic " °Ar in the atmosphere is well constrained. 

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