Building Isochrons and Determining the Ages of Three Rocks 
Page 15 of 17 
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 Rb87 and Sr87. (The values are RATIOS of parent
and daughter to the stable isotope, Sr86. 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. 

