LECTURE 05 minerals in geology

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

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

dik e

Radioactive elements

• Not all elements are radioactive Those that are and are the most useful for geologic dating are:

• 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 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