Part 2 - Test the age correction

Will the difference between the initial and measured isotopic ratios be negligible for the samples in our dataset? We need to test this assumption before proceeding further. In order to do this, you will search your dataset for the sample with the largest Rb/Sr, and determine how its ⁸⁷Sr/⁸⁶Sr ratio would have changed over 5 million years. Since this result was chosen to maximize the radiogenic growth of ⁸⁷Sr compared to the other samples in the dataset, if it results in a change which is below the analytical precision of Sr isotopic analyses, our assumption of a neglible age correction will prove valid for the entire dataset.

• Open the Excel file you downloaded in Step #1.
• Find the column labelled "RB87_SR86". This is the whole-rock ⁸⁷Rb/⁸⁶Sr ratio. Note that not every sample has data.
• Copy this column to a empty column on the spreadsheet and sort the values in descending order. Locate the largest value.
• Use the equation below to calculate the change in the ⁸⁷Sr/⁸⁶Sr ratio over a time period of 5 million years.
  
  
  
  
  
• Question 1. What was your result? The precision of ⁸⁷Sr/⁸⁶Sr ratios is generally significant to the 4th or 5th decimal place. Is your result below this threshold?