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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