Three-isotope plots

Fractionation data are usually displayed using a three isotope plot (Fig. 9.5). Meteoric waters lie on or close to the correlation line given by Equation 9.12. 

Three Isotope Plots of Terrestrial and Extraterrestrial Samples... 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Fig. 14.3 Three isotope plot of calcium-aluminum oxide inclusions in chrondrite meteorites compared with the terrestrial fractionation TFL line. For TFL m 0.5, the meteoric line shows m 0.9 (The data are from Clayton, R. N. et al., Earth Planet. Sci. Lett. 34,209 (1977) Geochim. Cosomochim Acta 63, 2089 (1999). Thiemens, M., Ann. Rev. Earth Planet. Sci. 34, 217 (2006))... 

Figure 1. Three isotope plot of O isotopes in Allende inclusions. Deviations are plotted in 6 units which are %o deviations relative to the terrestrial SMOW standard. In a two stage model, normal inclusions had initially a composition close to point A and exchanged with a reservoir poorer in O in the region of point D (Clayton et al. 1973). FUN inclusions underwent an intermediate step along a fractionation line between point A and point C. Then each inclusion exchanged with the same O poor reservoir D (Clayton and Mayeda 1977).

Figure 2. Magnesium three-isotope plot showing terrestrial and extraterrestrial samples relative to the predicted equilibrium and kinetic mass fractionation laws. The slopes in the caption refer to the P values that characterize the mass fractionation laws.

One way to compare data to predicted fractionation laws is to plot the data on the three isotope plot in which 5 "Mg is the ordinate and 5 Mg is the abscissa, and examine how closely the data fall to the different curves defined by the exponent p. However, the differences between the different P values are often evident only with careful attention to the statistics of the data. Ideally, the values of P should be obtained by a best fit to the data. This is most easily accomplished if the problem can be rewritten so that P is the slope in a linear regression. 

Hulston and Thode (1965) showed that the relationship between 5 values on a three isotope plot can be made linear if the definition of the 5 s are modified so that the term - 1) is replaced by ln( i / i , j) where x refers to one of the minor isotopes, R is the ratio of the minor isotope to the major isotope and std refers to the standard ratio. In the case of Mg isotopes, the new definitions for 5 "Mg and 5 Mg are ... 

Figure 18. Magnesium three-isotope plot (relative to SRM 980 0) showing laser ablation data for Allende CAI 3576-1 (after Young et al. 2002a). Ellipses represent the 95% confidence for each datum. The shaded datum is the analysis that included alteration material in the CAI. This point is related to the ofiiers by mass fractionation (dashed line) at constant 5 Mg where 5 Mg is the horizontal deviation from a terrestrial mass-fractionation curve. The 5 Mg value depends on the Al/Mg ratio, indicating in situ decay of A1 in the CAI. The high precision of the MC-ICPMS analyses makes it possible to resolve mass-dependent fractionation from excesses in Mg at the sub-per mil level.

It is a common practice to describe mass dependent isotope fractionation processes by a single linear curve on a three-isotope-plot (Matsuhisa et al. 1978). The resulting straight lines are referred to as terrestrial mass fractionation lines and deviations from it are used as indicating nonmass-dependent isotope effects. The three-isotope-plot is based on the approximation of a power law function to linear format. To describe how far a sample plots off the mass-dependent fractionation line, a new term has been introduced A 0, A Mg, A S, etc. Several definitions of A have been introduced in the literature, which have been discussed by Assonov and Bren-ninkmeijer (2005). The simplest definition is given by ... 

Three-isotope plot of xenon in the Qinghen chondrite, released at different temperatures and illustrating various isotopic components. Modified from Huss and Lewis (1995). 

Figure 2.15 Ne three-isotope plot for a grain-size suite of plagioclase separates from lunar high land soil that were treated by the CSSE treatment (see text). The best fitted line through the data from all etched samples (line p) passes close to the data point GCR (galactic cosmic ray) of cosmogenic Ne. On the left side, the path of mass fractionation of SWC (solar wind composition)-Ne intersects line p at a 20Ne/22Ne ratio of -11.3, which is interpreted to represent SEP (solar energetic particle) Ne (cf. Section 2.8). Open symbols unetched sample. Solid symbols etched samples. SF Solar flare Ne. Reproduced from Signer et al. (1993).

Figure 5.9 shows a neon three-isotope plot (Kennedy et al., 1990) where data were obtained from natural gases and brines in North America representing a broad geographical distribution. A fairly well-defined linear correlation on which air Ne is situated at the upper-left end suggests that the observed neon isotopic data resulted from a mixing between air Ne and another end member Ne characteristic to the crust. It is then reasonable to attribute the latter component to the nucleogenic component produced in the crust. 

Figure 5.9 Neon three-isotope plot for data set from brines in North America. The solid line represents the least-squares fit to the data set, and the solid triangle is the neon isotopic composition in air. After Kennedy et al.

Figure 5.11 Xe three-isotope plot for samples from KTB deep-drilling core in Germany icm = intrusive + cataclastic + mobilized (after Drescher et al., 1998).

This isotope of oxygen does not always occur in all natural samples in its usual proportion. Because ofmass-dependentchemical fractionation itis notpossible to distinguish an anomaly in this one isotope from measurement of a single isotope ratio. One must use the the three-isotope plot (see Glossary). Using it one concludes that the different classes of chondritic meteorites differ in their ratio l80/170. 

The isotopes of Ne do not always occur in all natural samples in their usual proportions. Neon was one of the first elements in which isotopic anomalies were detected in meteorites, and one which subsequent research showed to have many distinct isotopic components. To describe this requires the three-isotope plot (see Glossary). When the abundances of all three Ne isotopes are measured in any sample, it is possible to locate that composition as a point on a graph whose y axis is the ratio 20Ne/22Ne and whose x axis is the other ratio 21Ne/22Ne. Such a graph has been named a three-isotope plot, because three isotopes make only two independent ratios, the two coordinates of the composition. Each bulk sample is represented by its location point... 

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