Rubidium-87

Definition

Rubidium-87 is a radioactive isotope of rubidium that has a half-life of approximately 48.8 billion years, making it useful for geochronology and dating geological samples. It is particularly significant in the rubidium-strontium (Rb-Sr) dating system, where it decays to strontium-87, providing a means to determine the age of rocks and minerals based on the ratio of Rb to Sr present.

5 Must Know Facts For Your Next Test

1. Rubidium-87 decays to strontium-87 through beta decay, which is a key process for determining ages in geology.
2. The long half-life of rubidium-87 makes it suitable for dating very old rocks, often those over a billion years old.
3. In the Rb-Sr dating method, samples are typically analyzed using mass spectrometry to measure isotopic ratios accurately.
4. Rubidium is relatively abundant in many types of minerals, particularly mica and granite, which enhances its utility in geochronological studies.
5. The Rb-Sr system is sensitive to processes such as metamorphism and contamination, so careful sample selection and analysis are crucial for obtaining accurate age dates.

Review Questions

• How does the decay process of rubidium-87 to strontium-87 facilitate age determination in geological samples?
  • Rubidium-87 decays to strontium-87 through beta decay, which provides a consistent method for measuring time since formation. By analyzing the ratio of rubidium-87 to strontium-87 in a rock sample, geologists can calculate how long it has been since the rock crystallized. The relationship between these isotopes allows for precise dating because their ratios change predictably over time.
• Discuss the significance of the long half-life of rubidium-87 in geochronology and its impact on understanding Earth's history.
  • The long half-life of rubidium-87, at about 48.8 billion years, allows scientists to date very old geological formations that other isotopic systems cannot effectively date. This capability enables researchers to build a timeline of Earth's geological events and understand processes such as continental formation, tectonic activity, and changes in climate over billions of years. Such insights are crucial for reconstructing Earth's history and evolution.
• Evaluate how factors like metamorphism or contamination can affect rubidium-strontium dating results and what steps can be taken to mitigate these issues.
  • Metamorphism can reset isotopic systems by altering mineral structures and causing diffusion of rubidium and strontium, which may lead to inaccurate age dates. Contamination from external sources can also skew isotopic ratios. To mitigate these issues, geologists must carefully select samples from well-preserved geological contexts and use multiple analytical methods to cross-check results. Understanding the geological history and context of samples helps ensure more reliable data.

Related terms

Strontium-87: Strontium-87 is the stable daughter isotope produced from the radioactive decay of rubidium-87, and its abundance in a sample helps determine the age of geological materials.

Geochronology:

Geochronology is the science of dating geological materials and understanding the timing of events in Earth's history, often utilizing isotopes like rubidium-87.

Isochron Diagram: An isochron diagram is a graphical representation used in radiometric dating that plots the ratios of parent and daughter isotopes, allowing for the determination of age and the initial composition of the isotopes.