Neodymium is a rare earth element (REE) that occurs in many silicate,
phosphate, and carbonate minerals by substitution for major ions. It has
seven stable isotopes. Nd and samarium (Sm) are considered "light"
REEs and become concentrated in feldspars, biotite, and apatite. The concentrations
of Nd and Sm increase in minerals during sequential crystallization, but
the Sm/Nd ratio decreases.
147Sm decays by alpha emission to stable 143Nd,
with a half-life of 1.53 x 1011 years. Variations in Nd isotopic
compositions (143Nd/144Nd) are the result of elemental
fractionations occurring between 143Nd and its parent 147Sm
during radioactive decay. This has made Nd useful for age dating terrestrial
and extraterrestrial materials as well as many other geologic applications
(DePaolo, 1988). The growth of 87Sr and 143Nd together
provides useful insight into geochemical evolution. Mafic rocks and associated
suites of more differentiated rocks can be dated by whole-rock or internal
mineral isochrons. These Sm-Nd dates are less susceptible to metamorphic
alteration than Rb-Sr dates.
The concentration of Nd in natural waters is extremely low because the
natural abundance of Nd is very small, and Nd is non-hydrophilic in low
temperature environments. In most geological environments, the isotopic
composition of Nd varies systematically, though inversely, with the isotopic
composition of Sr. Except in very specific environments, Nd will not be
as useful as Sr (due to its higher concentration in natural waters) in
the interpretation of the weathering process in rocks and minerals. And
because of their similar geochemical behavior, Sm and Nd are not fractionated
from one another as severely as Rb and Sr. This fact, combined with the
very long half-life of 147Sm make variations in Nd isotopes
useful in only very old environments.
Source of text: This review was assembled by Eric Caldwell and
Dan Snyder from Nimz (1998), and from the references below.
||Bullen, T.D. and Kharaka, Y.K. (1992). "Isotopic composition of
Sr, Nd, and Li in thermal waters from the Norris-Mammoth corridor, Yellowstone
National Park and surrounding region." In: Water-Rock Interaction.
Proceedings of the 7th International Symposium on Water-Rock Interaction,
Balkema Publishers, Rotterdam, p. 897.
||DePaolo, D.J., 1988. Neodymium Isotope Geochemistry, An Introduction.
Springer-Verlag, Berlin, 187 p.
||Faure, G. (1986). "The Sm-Nd method of dating." In Principles
of Isotope Geology, second edition, John Wiley and Sons, New York,
||Faure, G. (1986). "Isotope geology of neodymium and strontium in
igneous rocks", in Principles of Isotope Geology, second edition,
John Wiley and Sons, New York, pp. 217-238.
||Faure, G. (1986). "Isotope geology of neodymium in sedimentary
rocks", in Principles of Isotope Geology, second edition, John
Wiley and Sons, New York, pp. 239-248.
||Goldstein, S.J. and Jacobsen, S.B. (1987). "The Nd and Sr isotopic
systematics of river-water dissolved material: Implications for the sources
of Nd and Sr in seawater." Chem. Geol. (Isot. Geosci. Sect.),
||Nimz, G.J. (1998). "Lithogenic and Cosmogenic Tracers in Catchment
Hydrology." In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers
in Catchment Hydrology. Elsevier, Amsterdam, pp. 247-290.