Integrating Research and Education > EarthChem > Mafic Rocks > Part 2 - Test the age correction

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.

    Rb-Sr decay equation

  • 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?