Isotopes, stable terrestrial abundances

Table 19.1. The stable isotopes of hydrogen, carbon, oxygen, and sulfur, and their approximate terrestrial abundances (Wedepohl et al., 1978)...

Atomic number Electronic structure Mean atomic weight Terrestrial abundance Stable isotopes... 

Nitrogen has two stable isotopes N (relative atomic mass 14.003 07, abundance 99.634%) and (15.000 11, 0.366%) their relative abundance (272 1) is almost invariant in terrestrial sources and corresponds to an atomic weight of 14.00674(7). Both isotopes have a nuclear spin and can be used in nmr experiments. though... 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

Because nitrogen possesses only two stable isotopes, it is a matter of semantics to assert which isotope is varying in meteoritic presolar-grain samples that have different isotope ratios (see 14N for that data in presolar grains). Identifying the correct solar abundance ratio for N, whether terrestrial or Jovian (see Abundance, above) will facilitate interpretation of yet other ratios found in presolar grains. 

There are two stable isotopes of N N and N. Since the average abundance of N in air is a very constant 0.366% (Junk and Svec, 1958), air (AIR) is used as the standard for reporting 6 N values. Most terrestrial materials have 6 N values between —20%o and - -30%o. The dominant source... 

Mercury has a relatively even distribution of its seven stable isotopes (196, 0.15% 198,10.0% 199, 16.7% 200,23.2% 201,13.2% 202,29.8% 204,6.8% Friedlander et al., 1981 Lauretta et al., 2001). This pattern presented cosmochemists with a formidable task when mercury isotopic distributions in meteorites were examined (e.g., Jovanovic and Reed, 1976 Thakur and Goel, 1989). Analytical difficulties apparently resulted in inaccurate determinations of the bulk abundance and isotopic composition of some meteorites, leading to the so-called mercury problem examined meteorites did not show the same bulk abundance and isotopic distribution as terrestrial material (Grevesse, 1970 Lauretta et al., 1999). Subsequent advances in mass spectrometry, and especially the development of multi-collectors. 

Probably the most informative objects in meteorites are the refractory, calcium-aluminum-rich inclusions (CAIs). They are sub-millimeter- to centimeter-sized objects found in all types of primitive (chondritic) meteorites. On the basis of their uranium/lead radiometric ages, they are believed to be the first-formed rocks in the Solar System 4). Their chemical compositions are consistent with equilibrium condensation as solids from a gas of solar composition at high temperatures 1700 K). The major mineral phases are spinel (MgAl204), pyroxene (Mg, Ca, Al, Ti silicate), melilite (another Mg, Ca, A1 silicate), and anorthite (CaAl2Si20s). They are enriched in refractory (less volatile) trace elements, such as the rare-earth elements, by a factor of 15-20 (5), reflecting their high temperature of condensation. The abundances of the three stable isotopes of oxygen exhibit a pattern not seen in any terrestrial rocks (6). On earth, ratios of abundances of isotopes, such as and vary by... 

Manganese has a naturally occurring stable isotope, Mn, and 12 radioactive isotopes. The one of interest is Mn that has a half-Ufe of 3.7 x 10 years and decays by K-capture into the stable Cr. It may be possible to use it in assessing the terrestrial ages of meteorites and extraterrestrial dust abundances in ice and sediments. 

Long-lived radioisotope analysis is based on the measurement of a very small isotopic ratio. Usually, small amounts of chemically prepared pure samples (in the form of elements or compounds) rather than the original samples are used in the ion source. The element-preconcentration is performed during the preparation of the AMS sample. Since the natural abundances of long-lived radioisotopes are usually very low in terrestrial materials, the contamination by the radioisotope itself from the environment is not a severe problem. The background may come from interferences of atomic and molecular species, stable isobars and isotopes, which can be discriminated effectively by AMS. Consequently, high sensitivities can be achieved in the AMS measurements of the radioisotopes ( °Be, Al, Si, Cl, Ca, etc.) for tracer studies. The... 

The most abundant isotope of rhenium, Re, decays to stable Os, one of the less abundant (<2%) Os isotopes. The decay constant for Re is 1.666 x 10 year [6]. Despite this rather small decay constant, the contrast in relative abundances between Re and Os in terrestrial reservoirs means that observed variations in Os/ Os are large. The other radiogenic isotope of Os, Os, is produced by the decay of Pt, with a decay constant that is an order of magnitude smaller than that of Re. This, combined with the fact that Pt is the least abundant (0.0129%) of the Pt isotopes, means that observed variations in Os/ Os are rather small, unless dealing with Pt alloys. 

Nitrogen is present primarily as N2 gas in the atmosphere and dissolved N2 in oceans. In terrestrial systems, nitrogen is found in minor amounts bound to H, C and O in both reduced and oxidized states. Nitrogen has two stable isotopes N and /N of -99.64% and -0.36% abundance, respectively. Isotope abundance variations measured for nitrogen are summarized in Figure 10. 

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