GE-158 Principles of Geological Science In-Class Activity

 

 

P% is the percent of initial parent atoms remaining in the system

Po is the number of parent atoms originally present when the rock formed

Pt is the number of parent atoms today (see table)

Dt is the number of daughter atoms today (see table).

 

Obviously, the total of the parent and daughter atoms today is equal to the number of parent atoms when the rock formed, since each daughter atom came from a parent atom. When you divide the number of parent atoms today by the number of parent atoms originally, you will get a ratio, such as 1/4 (= 25%) or 1/2 (= 50%). This ratio is the percent of parent atoms remaining in the system today, and is directly related to the number of half-lives that have transpired since the rock crystallized from magma.

 

Please answer the following questions. Please show all your work: write your equations, values, and calculations.

 

1. What is the absolute age of the basaltic dike, unit A?

 

 

 

 

 

 

 

2. What is the absolute age of the granite, unit B?

 

 

 

 

 

 

 

3. What is the absolute age of the folded metamorphic rock, unit C?

 

 

 

 

 

 

 

4. Compare your measured ages to the initial chart. If you were to use the principles of relative dating, would the order of the observed layers make sense? Explain using the principles in your answer.

 

 

 

Plot of Parent Amount versus Time (in half-lives).

0 1 2 3 4 5 6 7 100 50 25 12.5 6.25 3.13 1.5 0.75

Half-Lives

 

Remaining Parent Isotope (%)

GE-158  Principles of Geological Science

In-Class Activity

NAME: ___________________________________________     DATE: ______________

PART 1.  Working With Graphs:  How Fast is a Liquid Cooled?  Plot the data on the graph below, with time on the X-axis and temperature on the Y-axis.

1. What information does the graphs contains? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
2. At time 4 minutes after, what is the temperature of the liquid? ____________________
3. At time 12 minutes, what is the temperature of the liquid? _______________________
4. How long will it take to cool this liquid from 40^o C to -30^o C? ______________________
5. At what times will the liquid cool to the following temperatures?

20 ^o C  ____________       -10 ^o C ________________  -35 ^o C ______________

6. How long will it take for the liquid to cool down from 5 ^o C to -5 ^o C? ________________

TIME (Mins.)	TEMP.   (oC)
0	40
1	35
2	30
3	25
4	20
5	15
6	10
7	5
8	0
9	0
10	-5
11	-10
12	-15
13	-20
14	-25
15	-30
16	-35

In this classroom exercise, the students are given the simple logic statements governing radiometric dating (Parent -> Daughter + radiation + heat) and the rate of radioactive decay (dP/dt = ?P) and build an understanding of radiometric decay and how it is used to measure time without mathematical calculations. From these logic statements some important concepts are emphasized:

 

1. First, for each parent isotope decay one daughter isotope is created.
2. Secondly, it is important to notice in the rate expression that the number of decays is not a constant function of time (say for example the way a second is always 1/60th of a minute). Rather, the number of decays over a given time period changes with the number of parents present (simple example: 1/2 the students leave every 5 minutes during this lecture). Thus, decay is not a linear function of time, rather it is ‘curved’. The concept of half-life (t _1/2) is an important concept to remember.  Half-life is the time required for half of the substance to decay to a stable daughter.

Given these basic points students can follow the construction of a Parent and Daughter vs. Time graph. This graph illustrates two major points regarding radiometric dating:

 

1. First, the point of intersection between the parent and daughter curves (both have equal no. of atoms) illustrates the concept of a half-life.
2. Second, this exercise graphically represents the change in Parent-Daughter ratio with time. With every half-life, there will be less parent atoms and correspondingly more daughters.

Finally the concept of an isochron is developed along with a better understanding of how radioactive decay records age. The example used in class: Plot on a P vs D graph three minerals from an igneous rock, each containing different initial parent concentrations (assume no initial daughter present). Define this line as an isochron and then lead students through the change in slope of the isochron over time by following the P/D ratio in the three different minerals as time progresses.

Major Question for students – why can’t all rocks be dated by radiometric methods?

Plot the parent daughter curves on the graph below based on the values of their abundances with time (in half-lives).

Isotope Pair concentration (%)								No. of Half-lives
								1						2						3						4						5							6
Parent								50						25						12.5						6.25						3.125							1.563
Daughter								50						75						87.5						93.75						96.875							98.437

Determination of the Ages of Minerals Using Radiometric Age Dating.  Use the curve you just constructed above to answer the questions below.

Common Radiometric Isotopes	Amount of Parent Isotope Remaining (in %)	Amount of Stable Daughter Isotope Produced (%)	No. of Half-lives Measured	Half -Life (Years)	Age of Mineral
238U& 206Pb	50	50		4.5 billion
235U& Pb207	25	75		713 million
232Th& 208Pb	90	10		14.1 billion
87Rb & 87Sr	75	25		47 billion
40K& 40Ar	40	60		1.3 billion
14C& 14N	10	90		5730

How did you determine the age of the mineral that contains a particular radioactive isotope parent daughter pair,? ________________________________________________________________________

______________________________________________________________________________

PART 2. Using Radiometric Dating to Help Determine the Geologic History of an Area.

RADIOMETRIC DATING EXERCISE

 

This project will introduce you to radiometric dating. You will be asked to calculate the absolute ages of three different rocks shown on the geologic cross-section below. These units are A–the basaltic dike, B–the granite, and C–the folded metamorphic rock (ignore the two sandstone layers for this exercise).

 

Isotopic analyses have been carried out on minerals separated from the three crystalline rocks A, B, and C.  These data are listed below the cross-section in Table 1. To calculate the ages of the units, you will need to understand the principles of radiometric dating that you learned in lecture. In this problem, we will be using the potassium-argon system; potassium-40 has a half-life of 1.25 billion years. Please read the information on the back of this page and answer the questions printed there. Please show all of your mathematical work, and put a box around each of your final answers.

 

 

 

 

TABLE 1.  Results of Isotopic Analyses:

 

Rock Unit              Number of Parent Atoms           Number of Daughter Atoms

 

     A                                      7497                                           1071

     B                                    11480                                           3827

     C                                        839                                           2517

 

You have learned the principle of radiometric dating: radioactive parent isotopes decay at a constant rate and slowly become stable daughter isotopes. The decay rate is stated in terms of half-lives: the time it takes for one-half (50%) of the parent isotope to decay into the daughter isotope is called the half-life. As the rock ages, the amount of the parent isotope will decrease and the amount of the daughter isotope will increase geometrically (not linearly). Graphs of radioactive decay clearly show this geometrical or exponential decay. To determine the age of an unknown rock, you need to measure the number of parent and daughter atoms in a sample (this data is given to you in the table). The ratio of the daughter atoms to the parent atoms is proportional to the age. But if you know the half-life as well as the number of atoms of both isotopes, you can calculate the age in years—an absolute age. You need to use the following formula:

 

 

P% is the percent of initial parent atoms remaining in the system; Po is the number of parent atoms originally present when the rock formed; Pt is the number of parent atoms today (see table); Dt is the number of daughter atoms today (see table). Obviously, the total of the parent and daughter atoms today is equal to the number of parent atoms when the rock formed, since each daughter atom came from a parent atom. When you divide the number of parent atoms today by the number of parent atoms originally, you will get a ratio, such as 1/4 (= 25%) or 1/2 (= 50%). This ratio is the percent of parent atoms remaining in the system today, and is directly related to the number of half-lives that have transpired since the rock crystallized from magma. For example, a single half-life has passed if 50% of the parent atoms remain in a rock sample, while two half-lives have transpired if 25% of the parent atoms remain. If you know the number of half-lives, you can calculate the age of the rock. Using the half-life for potassium-40, these results would give ages of 1.25 billion years and 2.5 billion years for the two rock samples.

 

Please answer the following questions. Please show all your work: write your equations, values, and calculations. You may use a calculator. To answer the relative age question, use the geologic time scale in your textbook. (If a relative age falls on a boundary line between two time units, state this fact and name both units.)

 

1. What is the absolute age of the basaltic dike, unit A? What is the relative age (eon, era, period)?

 

 

 

 

 

 

 

2. What is the absolute age of the granite, unit B? What is the relative age (eon)?

 

 

 

 

 

 

 

3. What is the absolute age of the folded metamorphic rock, unit C? What is the relative age (eon)?

 

 

 

 

 

Plot of Parent Amount versus Time (in half-lives).