   Chapter 19, Problem 40E

Chapter
Section
Textbook Problem

# The mass ratios of 40Ar to 40K also can be used to date geologic materials. Potassium-40 decays by two processes: K 19 40   +   e − 1 0   →   A 18 40 r   (10 .7%) t 1 / 2 = 1.27 × 10 9 years K 19 40   →   C 20 40 a     +   e − 1 0   (89 .3%) a. Why are 40Ar/40K ratios used to date materials rather than 40Ca/40K ratios?b. What assumptions must be made using this technique?c. A sedimentary rock has an 40Ar/40K ratio of 0.95. Calculate the age of the rock.d. How will the measured age of a rock compare to the actual age if some 40Ar escaped from the sample?

(a)

Interpretation Introduction

Interpretation: Decay process of potassium- 40 is given. Various questions based upon the decay process are to be answered.

Concept introduction: Radiocarbon dating is based on the radioactivity of the nuclide 614C which decays by beta particle production.

The mass ratios of 40Ar to 40K have been used to date various geologic materials.

To determine: The reason for the use of 40Ar/40K ratios rather than 40Ca/40K .

Explanation

Explanation

Since 1940K decays to form a more stable 1840Ar nuclide than the 2040Ca</

(b)

Interpretation Introduction

Interpretation: Decay process of potassium- 40 is given. Various questions based upon the decay process are to be answered.

Concept introduction: Radiocarbon dating is based on the radioactivity of the nuclide 614C which decays by beta particle production.

The mass ratios of 40Ar to 40K have been used to date various geologic materials.

To determine: An assumption made while using the given technique.

(c)

Interpretation Introduction

Interpretation: Decay process of potassium- 40 is given. Various questions based upon the decay process are to be answered.

Concept introduction: Radiocarbon dating is based on the radioactivity of the nuclide 614C which decays by beta particle production.

The mass ratios of 40Ar to 40K have been used to date various geologic materials.

To determine: The age of a sedimentary rock having an 40Ar/40K ratio of 0.95

(d)

Interpretation Introduction

Interpretation: Decay process of potassium- 40 is given. Various questions based upon the decay process are to be answered.

Concept introduction: Radiocarbon dating is based on the radioactivity of the nuclide 614C which decays by beta particle production.

The mass ratios of 40Ar to 40K have been used to date various geologic materials.

To determine: The effect on the measured age of a rock if some 1840Ar escapes from the sample.

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