Radiogenic isotopic variations from

Radiogenic Isotope Variations from the Decay oT Long-Lived Radioactive Nuclides... 

Radiogenic isotopic variations for Sardinia Plio-Quaternary volcanics are accompanied by systematic modifications of trace element ratios (Lustrino et al. 2004a). For instance, Ba/Nb is higher in the northern than in the southern outcrops, whereas Ce/Pb is lower (Fig. 9.9). In general terms, the rocks from the southern occurrences have incompatible element ratios close to typical anorogenic magmas such as those of the Sicily Province,... 

It is shown in Fig. 2.57 that the lead isotopic variation of the Besshi-subtype is similar to that of midoceanic ridge basalt, suggesting the lead in the Besshi-subtype was derived from mantle. The data from the Shimokawa, and Yanahara deposits (Group B) are slightly more radiogenic than Group A, suggesting that crustal lead was involved in the formation of the Shimokawa deposit, and lead isotopic values for the Shimokawa and Yanahara plot between MORB and Cretaceous-Tertiary deposits in Japan (Kuroko, skarn, vein-type deposits). 

Sano (1986) and Sano et al. (1986) found He isotopic variations with depth, 3He/4He decreasing toward the surface, in two natural gas wells in northern Taiwan. This relation is interpreted as a mantle flux to the bottom of the well, progressively diluted by radiogenic He released from the surrounding sediment as the gas migrates upward. With a simple mixing model, they obtained mantle He fluxes close to the mean oceanic value (Table 6.4), but the situation in a gas well is rather complicated, and it remains to be seen whether or not the coincidence with the oceanic value is accidental. 

There are few pubhshed Lu-Hf isotope studies of mantle xenohths because of difficulties in efficient ionization of hafnium by thermal ionization mass spectrometers. Multicollector plasma mass spectrometers are a solution to this problem and data are emerging that promise to be a more revealing tool in mantle environments than neodymium isotopes. The variety of Lu/Hf fractionation displayed by mantle minerals (Figure 42) indicates that, as with other isotope systems, isotopic variation should be considerable and initial results are confirming this. Salters and Zindler (1995) found very radiogenic Hf/ Hf at relatively unradiogenic neodymium isotope compositions in spinel peridotites from Salt Lake Crater, Hawaii. Radiogenic Hf/ Hf also characterizes low-T circum-cratonic... 

Radiogenic isotopes are, of course, commonly measured in metamorphic minerals to obtain mineral isochron ages (see Chapter 3.08). However, sample size requirements restrict most isotopic measurements to bulk mineral separates core versus rim isotopic variability is rarely investigated. Nonetheless, some inferences regarding isotopic trends are obtainable for measured concentration variations in parent isotopes, or from direct measurements of two or more isotopic compositions from large crystals that exhibit systematic zoning of other elements. 

There are various terrestrial reservoirs that have distinct volatile characteristics. Data from midocean ridge basalts (MORBs) characterize the underlying convecting upper mantle, and are described here without any assumptions about the depth of this reservoir. Other mantle reservoirs are sampled by ocean island basalts (OIBs) and may represent a significant fraction of the mantle (Chapter 2.06). Note that significant krypton isotopic variations due to radiogenic additions are neither expected nor observed, and there are no isotopic fractionation observed between any terrestrial noble gas reservoirs. Therefore, no constraints on mantle degassing can be obtained from krypton, and so krypton is not discussed further. Comparison between terrestrial and solar system krypton is discussed in Chapter 4.12. 

Mantle-derived materials have a range of elemental-abundance patterns due to various fractionation processes, although the mantle pattern has been inferred from measured isotopic variations and radiogenic isotope-production ratios (see PorceUi and Ballentine, 2002). From the " He/ Ne production ratio and the average coexisting shifts in " He/ He and Ne/ Ne in the mantle relative to the primordial compositions, a ratio of He/ Ne = 11 is obtained. This is greater than the more recent estimate of 1.9 for the solar nebula (see discussion in Porcelli and Pepin (2000)). Similarly, using the mantle " °Ar/2 Ar... 

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