LECTURE 05 minerals in geology

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Evolution of the
Earth
Seventh Edition

Prothero • Dott

Chapter 5


NUMERICAL DATING OF THE EARTH

• Rocks contain radioactive minerals which are
constantly disintegrating at a steady rate
• Under certain circumstances, these atomic
“clocks” can be red to give a “time”
• The meaning of the “time” depends on what
has happened to the rock since the “clock” was
set


Fig. 5.1

Establishing
absolute geologic
age.

sandstone

shale

e
k
i
d

Example of crosscutting 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 maximum
age for the sandstone.
Note the combination
of “Geologic” age and
absolute age
techniques.


Radioactive elements
• Not all elements are radioactive. Those that are
and are the most useful for geologic dating are:
• U-238
• K-40
• C-14

Half-life = 4.5 By
Half-life = 1.25 By
Half-life = 5.73 years

• Also, Sm-147, Rb 87, Th-232, U-235


U-238 DECAY
• Often elements decay according to a complex decay
scheme in which a host of intermediate products,
many themselves radioactive, are produced.
• U-238 is such and element, and given its importance
to geologic dating, it is worthwhile to examine it
decay scheme.
• Keep in mind that u-238 has a half-life approximately
equal to the age of the earth, 4.5 By.


Fig. 5.3

Half-life for decay from U-238 all the
way to Pb-206 is 4.5 b.y. (billion years).

U-238 Decay Series
Decay rates for intermediate daughter
products range from <1 sec (polonium)
to 1,622 years (radium 226).


Fig. 5.4

Schematic diagram showing decay of radioactive parent isotope (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.


Fig. 5.5

Simple arithmetic plot of a universal isotopic decay curve. After 1 halflife 50% of parent isotope remains; after 2 half-lives, 25% remains.
What happens if the vertical axis is changed from linear to logarithmic?


BLOCKING TEMPERATURES
• The “Blocking Temperature” is an important concept; it refers
to processes that result in a “resetting” of the atomic clocks in
a rock.
• Essentially, it is possible to heat igneous and metamorphic
rocks to high enough temperatures that they no longer behave
as “closed systems”. That is some of the daughter products can
“leak” 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 in
which the blocking temperature has been reached?)


Fig. 5.6

Blocking temperatures for some common minerals and decay series.

The blocking temperature is
the temperature above which
a 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.


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.


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 halflife of about 5,730 years.
Plants and animals ingest this
radioactive C-14 while they
are alive. When they die, the
ingestion stops, and the
radioactive C-14 clock begins
to count down.


Fig. 5.9

Fission tracks in an
apatite crystal.
They are produced when
an atom of U-238
disintegrates 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 U238 to Pb-206 method
directly in such cases?)


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 redistributes
daughter 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).


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.