Helium radiogenic

Damon, P. E., Kulp, J. L. (1957) Determination of radiogenic helium in zircon by stable isotope dilution technique. Trans. Amer. Geophys. Union, 38(6), 945-53. 

Rocks contain uranium and thorium in small concentrations, and their radioactive decay results in the production of radiogenic helium-4 (4He). The helium reaches the groundwater and is dissolved and stored. With time,... 

Other examples of noble gas paleotemperatures have been reported by Rudolph et al. (1983). They calculated temperatures as low as 1-2 °C 22,000 years ago (dated by 14C) in Germany, accompanied by relatively negative <5D and <5lsO values and a measurable excess of radiogenic helium. 

The production rate of the radiogenic helium in each of these decay series is determined by the respective half-lives, and the overall helium production per unit rock is determined by the concentration of uranium and thorium in the rock (common rocks containing uranium and thorium in parts per million (ppm) concentrations). 

The equation of the accumulation of radiogenic helium in ground-water is based on a number of assumptions ... 

The accumulation of radiogenic helium in confined groundwaters is mainly determined by the duration of water entrapment. Applying the data available on uranium and thorium concentrations in the host rocks and rock water ratios, reliable, though semiquantitative, dating is possible in the range of 104 10s years. 

Internal checks are necessary for reliable water dating—for example, correlations of helium concentration with distance from the recharge area, water depth, temperature, or concentration of accompanying radiogenic 39Ar. 

Exercise 14.3 How can the radiogenic helium be sorted out of the total helium measured in a sample ... 

Exercise 14.4 On what arguments is it claimed that most of the produced radiogenic helium is released from the rocks and dissolved in the associated groundwater (i.e., the helium emanation coefficient is close to 1) ... 

Exercise 14.6 How can the patterns seen in Fig. 14.1 be understood in terms of the parameters controlling radiogenic helium contribution to ground-water ... 

Answer 14.3 The maximum helium concentration expectable in newly recharged water is read from the curve in Fig. 13.1 to be 5 x 10-8 cc STP/cc water. Excesses over this concentration are of radiogenic helium. In groundwaters that have an age of over 10,000 years, this excess is significant. 

Answer 14.4 There are two basic arguments (1) in most rocks the helium age is found to be significantly lower than the corresponding age defined by all other isotopic methods, indicating much of the radiogenic helium left the rock and (2) as the partial pressure of helium in the rock crystals increases with age, the amount expelled increases as well, until a steady state is reached at which the newly produced helium is fully balanced by the helium expelled into the associated groundwater. 

Although most arcs are dominated by MORB-like hehum, segments of two arcs (the Banda arc of eastern Indonesia and the Campanian Magmatic Province of southern Italy) are highly unusual in that they emit predominantly radiogenic helium (Hilton and Craig, 1989 Marty et al., 1994). 

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. 

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