WHAT CAN WE LEARN FROM ISOTOPES? - Chemical Fundamentals of Geology and Environmental Geoscience - page 178

10

WHAT CAN WE LEARN FROM
ISOTOPES?

Nearly all of the elements discussed in Chapter 9 consist
of more than one isotope . To say that an element con-
sists of several isotopes means that - for the unique
value of the atomic number Z that identifies the element
(Box 6.1) - nuclei exist in nature with two or more alter-
native values of the neutron number N . This is illustrated
for the trace elements Rb (rubidium) and Sr (strontium)
in Figure 10.1a: each square represents a naturally occur-
ring isotope of the relevant element. Natural Rb
comprises two isotopes, a stable isotope 85 Rb ( Z = 37,
N = 48, A = 85) and a radioactive isotope 87 Rb ( Z = 37, N = 50,
A = 87). Of these, 85 Rb is the more abundant in nature
(Figure 10.1b). 87 Rb decays slowly to 87 Sr (Figure 10.1b),
although its half-life is long enough (Table 10.1) for
some 87 Rb still to be present in the Earth, the remnant of
an episode of heavy-element production that preceded
the formation of the Solar System (see Chapter 11).

Natural Sr, on the other hand, consists of four
isotopes, 84 Sr, 86 Sr, 87 Sr and 88 Sr, all of which are sta-
ble, and of these 88 Sr is the most abundant
(Figure 10.1b). Although 87 Sr is a stable isotope (it
doesn't itself undergo radioactive decay), its abun-
dance in the Earth increases with time owing to decay
of radioactive 87 Rb.
The isotopes of an element share the same electron
configuration (Chapter 5) and thus nominally have
identical chemical properties, yet their relative
abundance in Earth materials varies to a small yet
measurable extent. As we shall see, studying how
the isotopic compositions of particular elements
vary in the natural world - the subject of the present
chapter - has proved an immensely fertile source of
information on how geological and environmental
processes operate.