Neodymium/strontium values

A number of eclogites have both radiogenic neodymium isotopes and radiogenic strontium relative to bulk Earth. For these samples, some of the Sr/ Sr values exceed modem and Archean seawater compositions (see Chapter 6.17). Hence, in a subduction-type model, the radiogenic strontium isotopic composition of the eclogite protohth is not solely inherited from oceanic cmst but is likely to be a time-integrated response to Rb/Sr enrichment during hydrothermal alteration... 

Earth value (s d = —1.5 to —4.3). They concluded correctly that neodymium in the oceans is mainly derived from the continents, and incorrectly that neodymium-isotope ratios in the oceans are like strontium isotopes in the sense that they are globally well-mixed and display this restricted range. In order to reach this conclusion they ignored a sample from the Indian Ocean having high strontium-isotope ratios and a lower, even more continent-like neodymium-isotope ratio of 8n(J = —8.5. 

Nd/ Nd low 8nneodymium isotopic compositions, the remarkable aspect of the frequency distribution for CLM as a whole is that the pronounced mode is within 58 units of bulk Earth and the mean 8j,jd value is 1.8. On this basis, the dominant neodymium isotopic characteristic of CLM is not enriched but is close to, or slightly more depleted than bulk Earth. Of course, if Depleted Mantle is used as a reference point then the CLM mode is enriched. The strontium isotope frequency distribution has a long tail out to very radiogenic ( enriched ) compositions but the mean Sr/ Sr is 0.7047 very close to estimates of bulk Earth. It is important to bear in mind that this statistical view of CLM geochemistry could... 

Volcanics from the Onverwacht Group, Swaziland and South Africa, comprise the basal part of the Early Precambrian Barberton greenstone belt found both in Swaziland the Transvaal. Data from them gave an age of about 3.54 Ga calculated using an initial [ Nd/ Nd]o ratio of 0.50809 00004 (Hamilton et aL 1979). The strontium and neodymium isotopic record of Apollo-12 basalts from the Moon were examined. Isochrons were derived for an achondrite (eucrite) and another meteorite and gave ages of about 4.58 and 4.562 Ga, respectively, with primordial [ Nd/ Nd]p ratios of 0.50684 0.00008 and 0.506664. An initial [ Sr/ Sr]o ratio was termed ADOR. The isotopic evolution of neodymium in the Earth has been described by means of a model termed CHUR (GHondritic Uniform Reservoir). And the present value of the [ Nd/ Nd] ratio is 0.512638 relative to a [ Nd/ Nd] ratio of 0.7219. The present value of the [ Sm/ Nd] ratio of CHUR is 0.1967, which permits calculation of [ Nd/ Nd] ratio in any past time interval. 

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