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SAMARIUM-NEODYMIUM
Samarium (Sm) is a silvery-white metal with an atomic number (Z! of 62, electronic configuration of [Xe]4f65d 06s 2, atomic we1g~t of 150.3~ ~nd melting point of 1350 K. It is a Group IIIB mner transitiOn element and one of the lanthanide and rare earth elements. Samarium has seven natural isotopes 144 Sm (3.075%), 147 Sm (15.00%), 148 Sm (11.24%), 149 S~ (13.82?'o), 150Sm (7.38%), 152 Sm (26.74%) and 154 Sm (22.75%). Three Isotopes are radioactive (with a half-lives (T112 ) in parentheses): 147 Sm (1.06 x 10 11 years), 148 Sm (8 x 10 15 years) and 149 Sm (> 10 16 years). Discovered in 1879, Sm is named after the mineral samarskite and is used as a radioactive tracer and in magnets. Samarium is typically a trace element in most rocks and minerals. It is refractory and under most conditions is lithophile and found in the trivalent state. The major importance of Sm in geochemistry is that it is one of the middle trivalent rare earth elements (or lanthanides), among the most useful trace elem~nts in all areas of geochemistry and cosmochemistry due to their coherent and systematic behavior as a group. The 147 Sm/143Nd decay system is one of the most important geochronological and isotope tracer systems in geochemistry and cosmochemistry. The short-lived and now extinct 146 Sm isotope decays to 142Nd (T112 = 103 Ma) and this system has recently been used to evaluate continental crustal evolution during early Earth history.
The decay scheme described The Sm-Nd methods are based upon the a-decay of 147 Sm to 143 Nd. The relatively long half-life of 147 Sm makes precise determination of that half-life somewhat difficult. The generally accepted value of the decay constant, Jce 47 Sm), is 6.54 x 10 -l2/year, corresponding to a half-life of 106 billion years (Lugmair and Marti, 1978). Evolution of Nd isotopic composition of a closed system over time period t is described by reference to 144Nd, which is not a stable isotope of Nd, but which decays at such a slow rate (t 112 > 10 14 years) that it can be treated as stable:
Scott M. McLennan
Cross-references Elements: lanthanide series, rare earths Elements: large ion lithophile Elements: lithophile Elements: trace Samarium-neodymium
e43Nd/144Nd), = e43Ndjl44Nd)o
e
e
(Sl)
where 43 Nd/ 44Nd) 0 and 43 Nd/ 44 Nd), are the values of that ratio at the start and end of period t respectively, e 47 Sm/ 44Nd), is the value of that ratio at the end of time period t, and }, is the decay constant for 147 Sm. (In some of the early literature, 146Nd, rather than 144Nd, was used for normalization.) The long half-life of 147 Sm produces very slow changes in 143Nd/144Nd · · most samples. Therefore, high. . over time m precisiOn mass spectrometry is essential to the use of this system, particularly for sufficiently exact determination of the 143 Nd/ 144Nd ratio. Another technical challenge is the separation of Sm and Nd, from each other and from the other rare earth elements (REE ), typically accomplished by ion-exchange chromatography. The fact that both parent and da
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