Isotopic relative

Element Isotope Relative Abundance (%) Mass (Da) Isotope Relative Abundance (%) Mass (Da)... 

Figure 1. Number of stable isotopes relative to atomic number (Z) for the elements. Mono-isotopic elements shown in gray diamonds. Elements discussed in this volume are shown as large gray circles. Other elements that have been the major focus of prior isotopic studies are shown in small white circles, and include H, C, O, and S. Nuclides that are radioactive but have very long half-lives are also shown in the diagram.

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Cr at two field sites where other studies had inferred Cr(Vl) reduction from concentration data (Table 8). We are adding here unpublished data from one of those sites and three analyses from a third site where conditions are strongly oxidizing and Cr(Vl) reduction should not occur. All of the groundwater samples are enriched in the heavy isotope relative to SRM-979, indicating that Cr(VI) reduction has taken place. 

If remobilization of Cr(III) in an aquifer were to occur, this process may be detectable using Cr isotope ratios. When Cr(VI) reduction is occurring, Cr(III) in the aquifer matrix should be enriched in the lighter isotopes relative to the Cr(I V) in the water. If part of a Cr(VI) plume were to change over time from enrichment in heavier isotopes to emichment in lighter isotopes, this would suggest remobilization of previously reduced Cr has begun. 

Atomic Name Atomic Mass Isotopic Mass Isotopic Relative... 

Sulfur isotope relative fractionation factors between various minerals and py-rite are shown in figure 11.40. As we can see, the isotopic compositions of sulfates reflect the relative fractionation effects induced by the 804 groups (compare figures 11.40 and 11.39). 

Elements Isotope Relative Abundance Isotope Relative Abundance Isotope Relative Abundance... 

The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 isotope. The relative atomic mass of an element is the weighted average of the isotopes relative to I/12(of jhe carbon-12 isotope. For example, the atomic mass of neon is 20.17 amu and is calculated from the following data neon-19 (amu of 19.99245, natural abundance of 90.92%), neon-20 (amu of 20.99396, natural abu dan c of 0,260%) and ncon-21 (amu of 21.99139, natural abundance ofc 82%) ... 

In stable isotope dilution techniques, an enriched stable isotope is added to the sample to be analyzed. Comparison of the amount of that isotope relative to another isotope of the element in a sample then allows the amount of the element in that sample to be determined. The normal relative amounts of the two isotopes have been altered by the addition of the known amount of the enriched isotope, so allowing the calculation. The technique cannot be used where an element does not have two stable isotopes and so elements such as Na, Mn and Co are excluded. A considerable advantage results from the fact that the determination involves the ratio of two isotopes, in that quantitative recovery of an element is not necessary. 

This isotope of Mg does not always occur in all natural samples from meteorites in its usual proportion. In particular, presolar grains of several different types reveal sometimes a deficiency and sometimes an excess of this isotope relative to the two heavier isotopes. It is common to represent the isotopic abundance ratios with 24Mg in the denominator (traditional, because 24Mg is the most abundant isotope). The... 

Isotope Relative abundance (%) Mass (u) Calculated Measured... 

Hydrogen Isotopes (relative to p-H.2) and Heats of Adsorption Temperature with TTieory... 

Since the phyllosilicates in CM chondrites are a few per mil enriched in heavy isotopes relative to the whole rock, material balance requires that the unanalyzed residual anhydrous silicates must be depleted in heavy isotopes by a comparable amount. This puts their composition into the range of CO chondrites. Figure 8 shows the relationship between CO and CM chondrites, which apparently represents different water/rock ratios in the aqueous alteration of a common CO-like precursor. The genetic association of CO and CM... 

Isotope Relative abundances Percent molar abundance... 

Isotope Relative abundances Principal sources of nuclides ... 

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