Geoscience Reference
In-Depth Information
Chapter 17
The other isotopes
Earth has a spirit of growth.
Leonardo da Vinci
mantle components or reservoirs. Isotope studies
derive their power from the existence of suitable
pairs of isotopes of a given element, one a 'pri-
mordial' isotope present in the Earth since its for-
mation, the other a radiogenic daughter isotope
produced by radioactive decay at a known rate
throughout geological time. The isotopic compo-
sition of these isotope pairs in different terres-
trial reservoirs -- for example, the atmosphere,
the ocean, and the different parts of the crust
and mantle -- are a function of the transport
and mixing of parent and daughter elements
between the reservoirs. In some cases the parent
and daughter have similar geochemical charac-
teristics and are difficult to separate in geological
processes. In other cases the parent and daugh-
ter have quite different properties, and isotopic
ratios contain information that is no longer avail-
able from studies of the elements themselves.
For example Sr-isotope ratios give information
about the time-integrated Rb/Sr ratio of the rock
or its source. Since rubidium is a volatile ele-
ment and separates from strontium both in pre-
accretional and magmatic processes, the isotope
ratios of strontium in the products of mantle dif-
ferentiation, combined with mass-balance calcu-
lations, are our best guide to the rubidium con-
tent, and volatile content, of the Earth. Lead iso-
topes can be similarly used to constrain the U/Pb
ratio, a refractory/(volatile, chalcophile) pair. The
40
Ar content of the atmosphere helps constrain
the
40
K content of the Earth; both Ar and K are
considered to be volatile elements in cosmochem-
istry. In other cases, such as the neodymium-
samarium pair, the elements in question are both
Background
The various chemical elements have different
properties and can therefore be readily separated
from each other by igneous processes. The vari-
ous isotopes of a given element are not so easily
separated. The abundances of the radioactive iso-
topes in the crust and mantle, and their decay
products, are not constant in time. Elemental
compositions of magmas and residual mantle are
complementary; isotopic compositions are iden-
tical, but they diverge with time. Therefore, the
information conveyed by the study of isotopes
is different in kind than that provided by the
elements. Each isotopic system contains unique
information, and the radioactive isotopes allow
dating of processes in a planet's history. The
unstable isotopes most useful in geochemistry
have a wide range of decay constants, or half-
lives, and can be used to infer processes occur-
ring over the entire age of the Earth (Table 17.1).
In addition, isotopes can be used as tracers and
in this regard they complement the major- and
trace-element chemistry of rocks and magmas.
Isotopes in magmas and gases, however, cannot
be used to infer the depth or location of the
source.
Studies of isotope ratios have played an impor-
tant role in constraining mantle and crustal evo-
lution, mixing and the long-time isolation of
