Geoscience Reference
In-Depth Information
min
=
A
min
T
−2
+
C
min
for
Table 3.1
Temperature coefficients of 1000 ln
α
18
O in minerals from the same
gneiss samples
different minerals. Third column: values of
δ
10
−
6
A
min
18
O
Mineral
C
min
δ
Feldspar
3.38
−
2.92
9.2
Plagioclase
2.76
−
3.49
7.6
Magnetite
−
1.47
−
3.70
0.3
Muscovite
2.38
−
3.89
6.6
min
=
AT
−2
BT
−1
Table 3.2
Coefficients of 1000 ln
C
for the liquid
water-vapor oxygen and hydrogen isotope exchange reactions
α
+
+
10
−
6
A
10
−
3
B
C
O
α
1.137
−
0.4156
−
2.0667
D
α
24.844
−
76.248
52.612
18
O
calcite
−
δ
18
O
seawater
=
10
6
T
−
2
the equation
δ
2.78
×
−
2.91, where
T
is the abso-
lute temperature.
4. Explain why
18
O in ice cores can be used to trace local precipitation
δ
D and
δ
temperatures.
5. Oxygen isotope thermometry of a metamorphic rock. The oxygen isotope composi-
Let us call
min
the value of the
18
O/
16
O fractionation coefficients between min-
erals and water. The temperature dependence of this coefficient can be written as
1000 ln
O
α
O
A
min
T
−
2
C
min
in which
A
min
and
C
min
are mineral-specific constants
and
T
is the absolute temperature. The values of
A
and
C
are reported in
Table 3.1
for
the different minerals. What is the quartz-magnetite apparent equilibration tempera-
ture? Plot (
α
min
=
+
18
O
δ
−
C
) for each mineral vs.
A
: what is the slope of this alignment? What
18
O value of the water in equilibrium with this mineral assemblage?
6. Explain why even subtle variations of
is the
δ
18
O values in fresh basaltic glasses are said to
indicate a source component processed at low temperature.
7. Let us call
δ
D
the ratio
for D/H fractionation. Using the values listed in
Table 3.2
calculate these coefficients at
5
◦
C and 25
◦
C. Calculate the
O
the value for
18
O/
16
O vapor/liquid fractionation in water and
α
α
18
O and the
D values of water vapor in equilibrium with
seawater at 25
◦
C. This water vapor now condenses as rain at 5
◦
C. Using the Rayleigh
fractionation law, calculate the
δ
δ
18
O and the
D values of rain water when 10, 20, 50,
and 80 wt% of the water vapor have condensed. Plot the corresponding
δ
δ
18
O
δ
Dvs.
δ
values and compare the slope of the alignment with the meteoric water line.
