Radiogenic production

The amount of He produced in a closed system, at secular equilibrium and over time t is given by 

respectively Moreira et al. 1995 Sarda et al. 2000), the Southwest Indian Ridge near Bouvet Island (14.9 Ra Kurz et al. 1998) and the Manus Basin back-arc spreading center (15.1 Ra Shaw et al. 2001). In each of these areas, the high He/ tae signal can be attributed to introduction of high He/ He material from a nearby mantle hotspot. The apparent upper limit to the measured He/ He in these cases is usually presumed to stem from dilution with ambient upper mantle having He/ He between 7 and 9 Ra. 

No relationship between He isotope composition and ridge spreading rate is evident. 

However, the variance of He/ He along ridges may be inversely related to spreading rate 

In the equatorial Atlantic, the range of He/ He is 8.6-8.9 Ra (Graham et al. 1992), where the combined Sr, Nd and Pb isotope systematics reveal a MORE mantle source that is one of the most highly depleted areas along the global ridge system ( Sr/ Sr = 

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Figure 8.18 Mean concentration in a slab (0

The first term on the right-hand side is the melting term the second term on the right-hand side is the radioactive decay of the nuclide and the last term on the right-hand side represents the radiogenic production by the parent of the nuclide. and Cj" are the concentrations of the nuclide... 

Recently another precursor-lifetime-controlled isotopic anomaly is reported for the case of 4He/3He by Tolstikhin et al. (1999). They found anomalously low 4He/3He in some sedimentary rocks, down to 1/100 lower than those expected from radiogenic production (about 10 7 cf. Section 5.6). They attributed the anomalously low 4He/3He ratio to a shorter residence time of 4He than that of a precursor nucleus 3H of 3He in rocks. 

Assuming a homogeneous element distribution in the rocks, the radiogenic production rate... 

The noble gas geochemistry of natural waters, including formation waters in sedimentary basins, has been used to determine paleotemperatures in the recharge areas, to evaluate water washing of hydrocarbons, and to identify mantle-derived volatiles (Pinti and Marty, 2000). The dissolved noble gases, helium, neon, argon, krypton, and xenon in sedimentary waters, have four principal sources the atmosphere, in situ radiogenic production, the deep crust, and the mantle. These sources have characteristic chemical and isotopic compositions (Ozima and Podosek, 1983 Kennedy et al., 1997). 

The relative abundances of primordial noble gases can be calculated from the considerations in the previous sections. The exercise will be carried out here for MORE mantle represented by the popping rock, and for OIB mantle represented by Loihi and Iceland, because basalt glasses from these localities show the least fractionation of their radiogenic production ratios from expected mantle source values. 

Distinctive lithospheric mantle He. As discussed in the Possible xenolith noble gas components section, radiogenic production within the lithosphere can lower the He/ He ratio of He derived from the convecting mantle. Therefore, the He represents a distinctive lithospheric component. This may be created in the xenolith source region, or may involve He that is remobilized in the lithosphere. As discussed below, regional studies often contend with distinguishing between distinctive asthenospheric and lithospheric sources for such He. 

Overall, samples that have been erupted recently and have had minimal surface exposure to cosmic rays therefore can be evaluated for subsurface He components. Outside of areas where subducted crustal material may be important, the sources of He are generally limited by the narrow range in xenolithic He/ He ratios to be the convecting upper mantle as sampled by MORB, in some cases with contributions from radiogenic production within the lithosphere or from a distinctive OIB component. The significance of these components is addressed in the more detailed discussion in the Regional studies section. 

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