# Building Isochrons and Determining the Ages of Three Rocks

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For each of the three rocks listed below follow these steps to determine the rock's age of formation.

1. Prepare an isochron by plotting the points that represent the amounts of isotopes Rb-87 and Sr-87. (The values are RATIOS of parent and daughter to the stable isotope, Sr-86. Mass spectrometers measure ratios and not absolute amounts.) Plotting Instructions

2. Click "Verify Points" and then use the "regressor" to find the slope and intercept of the line that Best Fits the points. The process, called "Least Squares," will result in the best value for the slope. Regressor Instructions

3. The age of the rock will be calculated for you. After you minimize the error term, click the "Verify Age" button, and if age is OK, transfer the rock's age to the table on the right. After completing table, click the "Next" button to receive your CERTIFICATE of completion as a Geochronologist.

Sample Locality Age (Million years)

Granite Scottish Highlands

Gabbro Sudburry, Canada

Gneiss Greenland

## Granite Scottish Highlands - Age: "Old as dirt, but still looking rock-some!" (Million years)

Gabbro Sudbury, Canada - Age: "Rock and Roll, it's a heavy metal!" (Million years)

Gneiss Greenland - Age: "Straight outta Greenland, rockin' since forever!" (Million years)

## To determine the age of formation for each of the three rocks (Granite in the Scottish Highlands, Gabbro in Sudbury, Canada, and Gneiss in Greenland), you need to follow these steps:

1. Prepare an isochron by plotting the points that represent the ratios of isotopes Rb-87 and Sr-87. The values are ratios of parent and daughter isotopes to the stable isotope Sr-86. You will need to have the data for the ratio of Rb-87/Sr-86 and Sr-87/Sr-86 for each rock. Plot these points on a graph with Rb-87/Sr-86 on the x-axis and Sr-87/Sr-86 on the y-axis. The x-axis represents the parent isotope ratio, and the y-axis represents the daughter isotope ratio.

2. Once you have plotted the points, click on "Verify Points" to ensure that the points are correctly plotted on the graph.

3. Next, use the "regressor" to find the slope and intercept of the line that best fits the points on the graph. This process is called "Least Squares." The regressor will calculate the best value for the slope.

4. After determining the slope and intercept, the age of the rock can be calculated. To minimize the error term, click the "Verify Age" button. If the age calculation is accurate, transfer the rock's age to the table on the right.

5. Repeat these steps for each of the three rocks, recording the calculated ages in the table.

6. Once you have completed the table with the ages of the three rocks, click the "Next" button to receive your Certificate of completion as a Geochronologist.

By following these steps, you will be able to determine the age of formation for each rock using the isochron method.

## 23

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## Sample Locality Age (Million years)

Granite Scottish Highlands

Gabbro Sudburry, Canada

Gneiss Greenland

## 458

1751

2894