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Radiogenic Isotopes

Isotope geochemistry has a markedly different viewpoint from radiometric dating, while it is based on the same physical laws and the same equations. It does not seek an age calculation but at the isotope ratios measured and how their natural variations could be interpreted. 

 

Examination of the variations in isotopic ratios will lead us to develop a methodology in which these ratios are used as tracers for major geological and geodynamic phenomena. The purpose of these studies is therefore to determine the major structures and exchanges of matter occurring (or has occurred) among the major terrestrial reservoirs (crust, mantle, core, and atmosphere).

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Sr Isotopes:

 

Systematic collation of 87Sr/86Sr isotope compositions of terrestrial basalt and granite (basalt being an isotopic ’‘messenger’’ from the mantle while granite is a sample of continental crust) reveals very different distributions. Strontium isotope ratios for granites from the various continents are highly variable, ranging from 0.705 to 0.850 and more. Strontium isotope ratios for basalts are much more uniform. They range from 0.7020 to 0.7070 and for oceanic basalts from just 0.7022 to 0.7045; Oceanic basalt reflects the isotopic compositions of the present-day mantle. 

 

Nd Isotopes:

 

Qualitatively, the phenomenon is the same as for 87Sr/86Sr ratios, except that the variations are in the opposite direction. The most radiogenic strontium ratios are for granites, while the most radiogenic neodymium ratios are found in MORBs, which have143Nd/144 Nd isotope ratios of about 0.51320; OIBs have 143Nd/144Nd isotope ratios of about 0.5128; the isotope ratios of granitic rocks range widely from 0.508 to 0.511 depending on the age of the rock.

 

To make the variations easier to read, the Nd variations are expressed as variations relative to a reference meteorite, aka εNd. In short, it is a relative variation expressed per 10 mil, which is more convenient for expressing variations and can be used to manipulate numbers like 2,10, or 20 whether positive or negative.

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Hf Isotopes:

 

Lutetium is a heavy rare earth element, while hafnium has geochemical properties close to those of light rare earth elements. Lutetium-176 disintegrates into176Hf at a decay ratel λ = 1.9410-11 yr -1. Similar to Nd, εHf is defined.

 

Pb Isotopes:

 

Three of the isotopes of lead are produced by the final decay of radioactive chains: 206Pb by 238U, 207Pb by 235U, and 208Pb by 232Th. Assuming the chains are in secular equilibrium, we can suppose that U and Th decay directly to the Pb isotopes. The advantage with lead isotopes is that the results are naturally correlated since the two decay schemes are chemically similar and it is only the decay constant that differs. It should also be remembered that as the Pb isotope ratios have the same denominator(204Pb), mixtures are always shown by straight lines on the various plots.

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For more information on different isotopic systems decoupling and their implications of the earth’s components evolution, please refer to Isotope Geology by Allegre. 

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Using TIMS and ICPMS Multi-Collector, Nordwest laboratory services provide: 

 

  • Rb-Sr

  • Sm-Nd

  • Sr-

  • Nd-

  • Pb-

  • Lu-Hf

 

Cautious shall be taken prior to choosing such methods, as each system has its own use. We are ready to consult you what method shall be used based on the objective of your project. 

 

The prices will be provided on demand.

 

The text and plot are modified after cAllègre, 2008.

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