WHAT CAN WE LEARN FROM ISOTOPES? - Chemical Fundamentals of Geology and Environmental Geoscience - page 179

(b)

(a)

Sr

84 Sr 86 Sr 87 Sr 88 Sr

38

86 Rb 87 Rb

Isotopes
Z = 37

Rb Rb 37

46 47 48 49 50

Neutron number N

Radiogenic 87 Sr

Non-radiogenic 87 Sr

Figure 10.1 (a) The naturally occurring istopes of the elements rubidium (Rb) and strontium (Sr) plotted by atomic number Z
and neutron number N . The shading of the 87 Rb box signifies its radioactivity. The meanings of 'isotope', 'isotone' and
'isobar' are illustrated. (b) Relative abundance of the naturally occurring isotopes of Rb and Sr. The base of the solid figure is
the Z-N plot shown in (a). The height of each column represents the relative abundance of the isotope (shown here for equal
amounts of the elements Rb and Sr); the number on the top of each column is the mass number A = Z + N that serves to
identify each isotope. Note that the 87 Sr column comprises radiogenic and non-radiogenic parts.

Some nuclear terminology
Figure 10.1a illustrates the jargon used when discuss-
ing relationships between isotopes. Each individual
square in Figure 10.1a - having its own unique comb-
ination of Z and N - represents a specific nuclide . Three
terms are used to describe groupings of nuclides that
share a common attribute (given here with mnemonic
hints to help in memorizing):

age - only emerges when the isotope's abundance
is considered in relation to other relevant isotopes and
elements. In Rb-Sr geochronology, for instance, the
age of a suite of rocks is determined by plotting the
87 Sr/ 86 Sr isotope ratios of several samples against
the corresponding 87 Rb/ 86 Sr ratios (Figures 10.4 and
10.5b). The interrelated isotope measurements and
constants (e.g. decay constant) that allow an age to be
calculated - or other aspects of sample origin to be
inferred - together constitute the Rb-Sr isotope
system .
Isotope systems of current interest in the Earth sci-
ences fall into three categories:

Isoto p es: nuclides that share the same Z value ( i.e.
have the same number of p rotons, forming
a horizontal row in the nuclide chart).

Isoto n es: nuclides that share the same N value (same

number of n eutrons - a vertical column of
squares in Figure 10.1.1, e.g . 40 Ca, 39 K and
38 Ar are isotones).

Radiogenic isotope systems
87 Sr is described as the daughter isotope or daughter
nuclide that results (at least in part) from the radio-
active decay of the parent isotope 87 Rb (Figure 10.1b).
The Rb-Sr isotope system, in which every radioac-
tive 87 Rb nucleus that decays is replaced by a new
radiogenic nucleus of 87 Sr, is one of several so-called
radiogenic isotope systems (Table 10.1), in which relative
isotopic proportions change progressively with the
passage of time. The 87 Rb content of a geological
material decreases with time in relation to 85 Rb,

Isob a rs: nuclides that share the same A value (a diag-

onal array in Figure 10.1.1, e.g . 40 Ca is isobaric
with 40 K and 40 Ar).

Isotope systems

Measuring the abundance of a single isotope (e.g.
87 Sr) in a geological sample reveals little about the
sample's origins. Useful information - such as