# Radioactive method for dating rock

All of the **dating** **methods** rely on the fundamental principles of **radioactive** decay, but the specific materials that can be dated and the exact procedures **for** calculating a date are very different from one **method** to the next.The rest of this activity is about using the Rb-Sr **method**.

Which minerals and *rocks* can be dated with the Rb-Sr *method*?It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer **for** analysis. Analyze the isotopic compositions of the whole **rock** and mineral separates on a mass spectrometer.A Mass Spectrometer is used to Measure Isotopic Ratios A Mass Spectrometer is a very powerful and sophisticated instrument. Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument.These materials are then used to prepare a "whole-**rock**" sample and several "mineral separate" samples.The whole **rock** sample will yield the weighted average isotopic composition of all the minerals in the **rock**.Numerical ages have been added to the Geologic Time Scale since the advent of *radioactive* age-*dating* techniques. In theory, the age of any of these minerals can be determined by: 1) counting the number of daughter isotopes in the mineral, and 2) using the known decay rate to calculate the length of time required to produce that number of daughters.

It illustrates how the amount of a **radioactive** parent isotope decreases with time. **For** example when 42% of the parent still remains, 1.23 Half-Lives of time has passed.

The Geologic Time Scale was originally laid out using relative *dating* principles.

Numerical *dating*, the focus of this exercise, takes advantage of the "clocks in *rocks*" - *radioactive* isotopes ("parents") that spontaneously decay to *form* new isotopes ("daughters") while releasing energy.

**For** example, decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus).

("87" is the atomic mass number = protons neutrons.

Understanding how a mass spectrometer functions is beyond the level of this activity.