Isotope ratios in rocks

Figure 17 Nd-Hf isotope ratios in rocks and Fe-Mn deposits. In mantle and continental rocks these two isotope ratios a positive covariation, reflecting correlated fractionations of Sm/Nd and Ln/Hf during mantle melting and continent formation over Earth history. Seafloor Fe-Mn deposits are distinct, and lie off the continent-mantle array, at high Hf isotope ratios for a given Nd isotope ratio. Possible explanations are given in the text. The figure is modified from van de Flierdt et al. (2002). The crust-mantle array is based on Vervoort et al. (1999). (Hf isotope data on Fe-Mn samples White et al., 1986 Godfrey et al., 1997 Alhare(c)de et al., 1998 Burton et al., 1999 ...

In a similar vein, mean seawater temperatures can be estimated from the ratio of 0 to 0 in limestone. The latter rock is composed of calcium carbonate, laid down from shells of countless small sea creatures as they die and fall to the bottom of the ocean. The ratio of the oxygen isotopes locked up as carbon dioxide varies with the temperature of sea water. Any organisms building shells will fix the ratio in the calcium carbonate of their shells. As the limestone deposits form, the layers represent a chronological description of the mean sea temperature. To assess mean sea temperatures from thousands or millions of years ago, it is necessary only to measure accurately the ratio and use a precalibrated graph that relates temperatures to isotope ratios in sea water. 

Although GC-C-IRMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study [614,677,678]. Measurement of chlorine stable isotope ratios in natural samples such as rocks and waters has become routine [626,679,680], but few measurements of chlorine isotopes in chlorinated aliphatic hydrocarbons have been reported [614]. A chlorine isotope effect was found in ferf-butyl chloride [681], demonstrating that 37Cl is more strongly bound to carbon than is 35Cl. Significant differences in the <5i7Cl values of some atmospheric chlorinated... 

From the preceding discussion it should be clear that the level of isotopic fractionation for a major rock-forming element like Mg in an object that was once molten in the early Solar System is a barometer of either total pressure (large F) or partial pressures relative to saturation (small F). In order to use Mg as a cosmobarometer, high-precision analyses of Mg isotope ratios in both chondrules and C AIs are required. 

Hagiwara (2000) completed a reconnaissance survey of Se isotope variation in marine sediments and sedimentary rocks (Table 4). The most important observation was a lack of strong enrichment in lighter isotopes in most shale samples and three Black Sea sediments. It appears that near-surface alteration has altered Se isotope ratios in some cases. All of the Phosphoria formation samples were probably altered by deep groundwater or hydrothermal... 

Becker R. H. (1971). Carbon and oxygen isotope ratios in iron-formation and associated rocks from the Hamersley Range of Western Australia and their implications. Ph.D. diss., University of Chicago. 

The Sr isotopes show high initial Sr /Sr ratios in chalcocites, consistent with a high Rb/Sr ratio source. These high ratios are typical of upper crustal rocks. The isotopic ratios in carbonates showed... 

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

Variations in the 8 34S values for plants have been summarised by Krouse et al. (1991), these range from -20 to +34%o and are comparable to those reported in sedemintary rocks. It has been found that the sulphur uptake by the plant is mainly from soil sulphate where ambient S02 levels are low. In some cases ambient S02 dominates and the 8 34S values are close to those of the S02. These two uptake mechanisms are a factor in the differing isotope ratios in different parts of an individual plant (Chukhrov et al., 1978). Thus sulphur isotope ratios are helpful in the determination of sulphur sources in the environment. 

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