Radiogenic isotopes chondrites

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

Radiogenic isotopes have proved a most powerful tool for understanding mantle processes. By studying mantle peridotites and mantle-derived melts from a variety of different geological time periods it is possible to define evolutionary curves for the different isotopic systems within the mantle. These curves, when plotted on isotope ratio versus time diagrams, can be used to characterize the chemical evolution of the mantle over time. Deviations from the chondritic trend are used to identify chemical fractionation events in the mantle during Earth history. Of particular... 

Variability in Mg isotope ratios among chondritic meteorites and their constituents is dominated by mixing between a radiogenic CAI-like reservoir and a reservoir resembling ordinary chondrites. The mixing is evident in 5 Mg and 8 Mg, Al/Mg, and A 0 values but... 

The W isotopic compositions of Martian meteorites range from chondritic to slightly radiogenic (ew = 0 to +3) relative to the mean of carbonaceous chondrites (Fig. 8.9) (Lee and Halliday, 1997). The W isotopic compositions of these... 

He, Ne, Ar, C and N between cosmochemical potential precursors (PSN and chondrites) and terrestrial reservoirs (the atmosphere and the mantle source of MORB) is given in Figure 2. Volatile abundances are normalized to Ne and the Sun, which, in this figure, results in a flat pattern for the solar abundance. Neon is used for normalization as its isotopic (non-radiogenic) composition in the mantle is clearly different from that of the atmosphere (see below). 

Nonradiogenic Xe isotopes in the atmosphere. The isotopic compositions of Xe components in the solar system have been more difficult to unravel. There is no suitable widespread solar system Xe component to use as a reference composition (see Ott 2002, this volume). The light isotopes of atmospheric Xe are related to both bulk chondritic and solar Xe by fractionation of -4.2% per amu (first noted by Krummenacher et al. 1962), with a clear radiogenic excess of -7% in Xe from decay of However, these components cannot be precisely related to the composition of the heavy Xe isotopes in... 

A suitable initial composition for the atmosphere has been deduced from meteorites. Multi-dimensional isotopic correlations of chondrite data have been used to constrain a range of compositions that, when mass-fractionated, yields the light-isotope ratios of terrestrial Xe. In order to match the terrestrial heavy Xe isotope ratios, addition of radiogenic I and a heavy Xe isotope component is required. Constraining the composition of the heavy isotope component to known fission spectra then defines the U-Xe composition and identifies " Tu-derived fissiogenic Xe as the heavy isotope component (see Primordial Xe section). This is compatible with meteorite data that... 

The Sm-Nd method can be used to date mafic igneous rocks (basalt and gabbro) which are not suitable for dating by the Rb-Sr method. Magma of basaltic chemical composition originates by decompression melting in the mantle of the Earth which contains radiogenic Nd that has formed by decay of Sm. The isotopic evolution of Nd in the mantle of the Earth is represented by a model in Fig. 3.19 that is based on the isotope composition of neodymium in chondrite meteorites and which is therefore known as the Chondritic Uniform Reservoir (CHUR). The present value of the Nd/ Nd ratio in CHUR is ... 

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