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Isochron Method for Radiometric Dating: A Geophysics Lecture, Study notes of Physics

University of London (Lond)Physics

An overview of the isochron method for radiometric dating, a geophysics lecture handout by dr. Gavin bell. The method involves taking mineral samples from a rock, measuring isotopic ratios, and plotting them to determine the age of the rock and its thermal history.

Typology: Study notes

2010/2011

Uploaded on 09/10/2011

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PX266 Geophysics (2010/11)
Lecture 5 Handout – the Method of Isochrons
Dr. Gavin Bell
1
R
N
R
D
R
D
now
now
0
0
now
now t
e
Isochron method for radiometric dating
1. Take several mineral samples from a rock, maintaining complete purity.
2. Choose decay system such that D and R are the same element, N is different.
3. Measure isotopic ratios D/R and N/R (mass spectrometer) and plot.
4. If they lie on a straight line, the system has been “closed”
a. Gradient = mgives age of rock (we know
)
b. Intercept = “initial ratio” (isotopic ratio in general environment)
5. Scatter means poor closure, i.e. isotopes have been exchanged with
environment (chemical reaction, thermally activated diffusion, etc.)
6. Line of best fit through many points reduces random error.
Quick exercise on Rb-Sr isochron
1
Sr
Rb
Sr
Sr
Sr
Sr
now
86 now
87
0
86 0
87
now
86 now
87
t
e
Identify N, D and R.
Find the initial ratio for Rb/Sr and age of the rock.
What has happened to mineral C?
pf2

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PX266 Geophysics (2010/11)

Lecture 5 Handout – the Method of Isochrons

Dr. Gavin Bell

R

N

R

D

R

D

now now 0 0 now now

t

e

Isochron method for radiometric dating

  1. Take several mineral samples from a rock, maintaining complete purity.
  2. Choose decay system such that D and R are the same element, N is different.
  3. Measure isotopic ratios D/R and N/R (mass spectrometer) and plot.
  4. If they lie on a straight line, the system has been “closed” a. Gradient = m gives age of rock (we know ) b. Intercept = “initial ratio” (isotopic ratio in general environment)
  5. Scatter means poor closure, i.e. isotopes have been exchanged with environment (chemical reaction, thermally activated diffusion, etc.)
  6. Line of best fit through many points reduces random error. Quick exercise on Rb-Sr isochron

Sr Rb Sr Sr Sr Sr now 86 now 87 0 86 0 87 now 86 now 87    t e  Identify N, D and R. Find the initial ratio for Rb/Sr and age of the rock. What has happened to mineral C?

Example: potassium – argon Actually there are TWO decay channels, but it is possible to derive a simple isochron (see the web site for a proof). The closure properties of Ar are poor but well known – as an inert gas it can easily diffuse through minerals. This diffusion is often thermally activated (i.e. becomes exponentially more rapid as temperature increases), so for different minerals a “closure temperature” can be defined (typically 100°C – 500°C). If the data point for a particular mineral is well off the best-fit isochron we can infer that the rock has been heated past this mineral’s closure temperature at some point. We can therefore learn something about the rock’s thermal history. Further Study Questions 6 – a typical “exam problem”-style question on the method of isochrons. You should be confident with the isochron equation – what the terms mean, how the method of isochrons works. Why is system closure so important? What does scatter in the data points mean? Why are D and R the same chemical element? Why choose different minerals or samples of different chemical composition for a whole-rock isochron? You should be aware of some advantages / disadvantages of the different decay systems – see the web site. The key distinction is half-life, but closure properties (e.g. K-Ar) and chemical reactivity (e.g. U-Pb vs. Th-Pb) are also important. Make sure you understand the simplified form of the gradient m when  t << 1 (i.e. the half life is much longer than the age of the rock): m   t

Summary of Geochronology Topic – what you should know

Basic details on formation of Earth / solar system, and differentiation of Earth. Relative vs. absolute timescales (incl. knowing when Pre-Cambrian ends). Radioactive decay, half-life, decay constant. Elements vs. isotopes, rocks vs. minerals. Whole-rock isochron method: How are isotopic ratios measured? What is the significance of the gradient and intercept? What is the significance of scatter of data points? Systematic vs. random errors for isochron measurements? What is the “t=0 condition”? What does “system closure” mean and what is a closure temperature? Brief / basic advantages and disadvantages of main decay systems. Why is radiocarbon dating more or less useless for geology? How to do a quantitative problem on an isochron.