Geology Reference
In-Depth Information
10.2 A horizontal line from 0.707980 intercepts the
curve in Figure 10.7 at 34.0 Ma (see figure).
0.707980 + 0.000022 = 0.708002
0.707980 - 0.000022 = 0.707958 which translates
into ~33.5 Ma.
The age of the carbonate sediment is therefore
34.0 ± 0.5 Ma.
which
translates
into ~34.5 Ma;
0.7082
Contribution of ocean
ridge hydrothermal activity
0.7078
P
P1
M
O
0
10
20
30
40
10.3
Exercise 11.4
0.514
147/144
143/144
0.511814
0.509965
0.513317
0.512628
whole rock
plag
opx
cpx
0.20034
0.09627
0.28428
0.24589
0.513
Max minus min
Gradient
Gradient +1
In (grad + 1)
Decay constant
Age/y
Initial ratio by eye
0.18801
0.00335
0.01783
1.01783
0.01767
0.512
6.54E-12
2.70E+09
0.50825
0.511
0.51
0.509
0.508
0
0.05
0.1
0.15
0.2
0.25
0.3
Draw a line by eye through the data; the extrapo-
lated line intercepts the y -axis at about 0.50826
(=initial ratio). The gradient of the line leads to a
mineral age for the Stillwater gabbro of 2.70 Ga.
10.4 From Equation 3.2.5, half-life and decay constant are
related by t 2
The count rates are proportional to the concentra-
tion of 14 C in each carbon sample. Plugging rele-
vant numbers into Equation 10.13:
1
1 210 10
13 56
58
.
.
0 8493
1 210 10
.
=
t =
ln
4
4
= . λ therefore λ= 0 6931 2
0 6931
. t ,
.
×
.
×
(
) .
=
7019
yearsbeforepresentBP
so in this case λ C = 0.6931/5730 = 1.210 × 10 −4 year −1 .
 
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