Geoscience Reference
In-Depth Information
y
1
y
2
Sample 1
y
3
Sample 2
Sample 3
Measured
1
87
Sr
2
88
Sr
Slope =
Corrected
8.3752
x
=
88
Sr/
86
Sr
Figure 4.2
87
Sr/
86
Sr) relative to a
Principle of standardization of radiogenic isotope abundances (here
y
=
88
Sr/
86
Sr). The various straight lines represent both natural and analytic
thermodynamic isotopic fractionation depending on mass. The different ordinates between the
straight lines represent the effect of radioactive accumulation of
87
Sr, which varies from one
sample to another. Standardization to
x
reference ratio (here
x
=
8.3752 eliminates thermodynamic fractionation leaving
only radiogenic variability
y
1
,
y
2
,and
y
3
. Note the slope equal to (87-86)/(88-86)
=
87
Sr/
88
Sr.
×
chondritic meteorites, and to use a relative deviation notation, analogous to the
δ
notation in
use for oxygen isotopes:
ε
Nd
(
T
) is defined as:
143
Nd
144
Nd
sample
(
T
)
⎡
⎣
⎤
⎦
×
/
ε
Nd
(
T
)
=
143
Nd
144
Nd
chondr
(
T
)
−
1
10 000
(4.11)
/
which is the deviation in parts per 10 000 of the
143
Nd
144
Nd ratio in the sample relative
to that of chondrites of the same age
T
. In a similar way,
/
ε
Hf
(
T
) can be defined for the
176
Hf
177
Hf ratio.
Lead is an exception because it has only a single stable isotope, which rules out internal
standardization and explains the intrinsically lower precision of measurement of some data
(a few parts per 10 000) compared with that of Sr, Nd, or Hf (10-30
/
10
−6
).
×
element as the radioactive nuclide. As the system is closed, the number of stable nuclides
P
remains constant, which we denote
P
=
P
0
. This gives:
P
P
P
P
e
−
λ
t
t
=
(4.12)
0
The additional condition required to make the decay equation a chronometer is no longer
to assume
P
0
but rather to determine the isotope ratio (
P
P
)
0
when the system formed,
/
