Rubidium has two naturally occurring isotopes, 85Rb (72.2%)
and 87Rb (27.8%). 87Rb is radioactive, with a half-life
of 48.8 x 109 years. It readily substitutes for K in minerals,
and is therefore fairly widespread. Rb has been used extensively in dating
rocks; 87Rb decays to stable 87Sr by emission of
a negative beta particle. During fractional crystallization, Sr tends to
be come concentrated in plagioclase, leaving Rb in the liquid phase. Hence,
the Rb/Sr ratio in residual magma may increase over time, resulting in
rocks with increasing Rb/Sr ratios with increasing differentiation. Highest
ratios (10 or higher) occur in pegmatites. If the initial amount of Sr
is known or can be extrapolated, the age can be determined by measurement
of the Rb and Sr concentrations and the 87Sr/86Sr
ratio. The dates indicate the true age of the minerals only if the rocks
have not been subsequently altered. See Strontium
for a more detailed discussion.
Rb has seen scant use in hydrology because of its ready substitution
in minerals and because there is little chance of uptake by the biota.
Furthermore, the half-life is far too long for any practical hydrologic
Source of text: This review was assembled by Dan Snyder
from the references below.
||Jäger, E. (1979). "The Rb-Sr method." In: E. Jäger
and J.C. Hunziker (Eds.), Lectures in Isotope Geology, Springer-Verlag,
New York. pp.13-26.
||Faure, G. (1986)."The Rb-Sr method of dating", In: Principles
of Isotope Geology, Second Edition. John Wiley and Sons, New York,