Evolution of the Earth

Seventh Edition

Prothero • Dott

Chapter 5

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

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

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 maximum age for the sandstone.

Note the combination of “Geologic” age and absolute age techniques.

Establishing absolute geologic age.

shale

sandstone

dike

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

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

U-238 Decay Series

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

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

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 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?

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

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.

Blocking temperatures for some common minerals and decay series.

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 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 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 U-238 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 rock4. Dating of the mineral would now yield the age of the

metamorphic event5. 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.