Radiogenic Isotopes in Geological Sciences

by

Bob Cliff and the University of Leeds Computers and Teaching in Earth Sciences Team

This courseware module is intended for second- and third-year undergraduates and covers the basic principles of radiogenic isotopes. It assumes little or no background in chemistry and physics. Extensive use is made of animations to ensure active involvement of students and cater for the wide range of physical science backgrounds anticipated. The aims of the module are to:

Review the underlying physical principles on which radiogenic isotope geology is based;
Introduce the major parent-daughter systems and the quantitative relationships which describe changes in isotopic composition;
Present the geological concepts on which inter- pretation of isotopic data is based;
Give practice in interpreting real data and selecting appropriate methods to solve specific geological problems.

The module occupies about 4 Mb, and consists of four main units. Users are free to work through as many or as few units as they want, and in any order they want. They are accessed from this menu.

1. Basic Principles

Isotopes - what they are, and how relationships between them are shown on the chart of nuclides. Radioactivity - what types of decay there are and how are parents and daughters related. Geological applications - how changes in the abundance of parent or daughter can be used to address questions about when things happened and where materials come from.

This part of the Basic Principles Unit explains decay mechanisms

2. Quantitative relationships illustrated by specific parent-daughter systems

Measuring the age of minerals with little or no daughter isotope initially present.

How we can date samples that contain significant amounts of daughter initially? Isochron diagrams, growth curves.

What is the significance of initial isotopic abundances? Use as tracers for earlier geochemical fractionation.

Information is given in a variety of ways, with extensive use of graphics

3. Geological interpretation of geochronological data

How can we date the intrusion of an igneous rock which has been metamorphosed? Problems with systems that have been partially opened. Using the more robust behaviour of whole rock systems in comparison to minerals.

The mechanism of isotopic exchange is discussed as part of the Geological Interpretation unit

What can mineral ages tell us about cooling histories? - making use of differing kinetic behaviour of individual minerals. Closure temperature concept.

The Dodson diagram is explained

4. A Geochronological Challenge

A geological scenario with a partial data set which requires the student to make a preliminary geological interpretation and propose further analyses to solve outstanding problems. Throughout the module the basic principles are illustrated using animations of decay processes. The students is prompted to plot and process data on screen and/or paper, with the software providing feedback. Geological interpretations are based on examples of real data and the module concludes with an invented case study requiring student suggestions for further investigation. A file containing tutor's notes provides background to help the tutor with feedback on the "Challenge".

In the Geochronological Challenge users enter answers to test their understanding of the material

Running the software

The different parts of the module are accessed via a main menu and navigation around the module is by means of buttons along the base of the screen.

Part of the self-assessment section in the Quantitative Relations unit