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Evolution of theEarthSeventh Edition

Prothero Dott

Chapter 5

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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NUMERICAL DATING OF THE EARTH

Rocks contain radioactive minerals which areconstantly disintegrating at a steady rate

Under certain circumstances, these atomicclocks can be red to give a time

The meaning of the time depends on whathas happened to the rock since the clock wasset

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Fig. 5.1

Example of cross-

cutting relationships

that establish relative

ages: an igneous dike

cuts through red shales

and is truncated by

overlying sandstone.

A radiometric date on

the dike will give a

minimum age for the

shale and a maximumage for the sandstone.

Note the combination

of Geologic age and

absolute age

techniques.

Establishing

absolute geologic

age.

shale

sandstone

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Radioactive elements

Not all elements are radioactive. Those that are

and are the most useful for geologic dating are:

U-238 Half-life = 4.5 By

K-40 Half-life = 1.25 By

C-14 Half-life = 5.73 years

Also, Sm-147, Rb 87, Th-232, U-235

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U-238 DECAY

Often elements decay according to a complex decayscheme in which a host of intermediate products,many themselves radioactive, are produced.

U-238 is such and element, and given its importanceto geologic dating, it is worthwhile to examine itdecay scheme.

Keep in mind that u-238 has a half-life approximatelyequal to the age of the earth, 4.5 By.

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Fig. 5.3

U-238 Decay Series

Decay rates for intermediate daughter

products range from

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Fig. 5.4

Schematic diagram showing decay of radioactive parentisotope (e.g. U-238)

to a daughter(e.g. Pb-206). The original isotope was sealed in a mineral

grain at time of crystallization. Note changing ratio of parent/daughter after

2 half-lives. Note that to get an estimate of the geologicc age, you need the

ratio of the parent isotope to the daughter isotope, e.g. two measurements.

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Fig. 5.5

Simple arithmetic plot of a universal isotopic decay curve. After 1 half-

life 50% of parent isotope remains; after 2 half-lives, 25% remains.

What happens if the vertical axis is changed from linear to logarithmic?

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BLOCKING TEMPERATURES

The Blocking Temperature is an important concept; it refersto processes that result in a resetting of the atomic clocks ina rock.

Essentially, it is possible to heat igneous and metamorphicrocks to high enough temperatures that they no longer behaveas closed systems. That is some of the daughter products canleak out of the primary mineral, giving an erroneous

parent/daughter ratio and hence a wrong age.

(Age for what? How could the age be interpreted in a rock inwhich the blocking temperature has been reached?)

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Fig. 5.6

The blockingtemperature is

the temperature above whicha mineral or rock no longer

behaves as a closed system

and the parent/daughter ratios

may be altered from that due

to pure radioactive

disintegration.

This can result in resetting the

isotopic clock and/or give

what are called discordant

dates.

These types of problems have

given opponents of the

radiometric dating of the

Earth ammunition to attack

the 4.5 By age geologists cite.

Blocking temperatures for some common minerals and decay series.

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Fig. 5.7

Use of daughter lead

isotopes for dating. The

ratios of 3 radiogenic lead

isotopes to non-radiogenic

lead-204 all change but at

different rates.

These ratios can also be

used to date a rock or

mineral.

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Fig. 5.8

Constant generation of C-14

in the upper atmosphere by

cosmic particle bombardment

of N (nitrogen).

Nitrogen (N-15) emits a

proton and becomes C-14.

This is radioactive with a half-

life of about 5,730 years.

Plants and animals ingest this

radioactive C-14 while they

are alive. When they die, the

ingestion stops, and theradioactive C-14 clock begins

to count down.

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Fig. 5.9

Fission tracks in an

apatite crystal.

They are produced when

an atom of U-238disintegrates emitting an

alpha particle, a Helium

nucleus (He-4). This

massive atomic particle

causes massive structural

damage in the crystal that

can be revealed by

etching.

The number of tracks in a

given area is proportional

to the age of the mineral.

(Why not just use the U-

238 to Pb-206 method

directly in such cases?)

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Fig. 5.10

Metamorphic redistribution of daughter isotopes.

1. Mineral crystallizes 1000 mya (1 billion yrs ago)

2. After 500 my (million yrs) some parent isotopes have decayed.

3. 480 mya (million yrs ago) metamorphic event redistributesdaughter atoms out of crystal into adjacent rock

4. Dating of the mineral would now yield the age of the

metamorphic event

5. But a whole rock age would provide the original age of the

rock/mineral (1000 mya).

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Fig. 5.11

Illustration of how radiometric dating can establish a geologic time scale.

Fossils establish that the granite is Silurian. (a) A date for the granite establishes that the

Silurian is about 425 my old. (b) The date for the lave flow in the Old Red sandstone

establishes that part of the Devonian is about 370 my old.

Thus the Silurian must be younger than 425 My and older than 370 My.