Radiocarbon Dating for Igneous Rocks: Clarifying Common Misconceptions

Radiocarbon Dating for Igneous Rocks: Clarifying Common Misconceptions

Introduction to Radiometric Decay

To determine the ages in years of Earth materials and the timing of geological events such as exhumation and subduction, geologists utilize the process of radiometric decay. This method involves measuring the rates of decay of radioactive isotopes to carbon-14 (radiocarbon dating), and other elements like potassium-40 (potassium-argon dating), uranium-238 (uranium-lead dating), and samarium-neodymium dating. These processes allow geologists to further refine the boundaries of the geologic periods shown on the geologic time scale.

Radiocarbon Dating: Limited to Living Materials

Commonly, radiocarbon dating is used to date once-living material, such as organic carbon, which is effective for periods of 50,000 to 70,000 years. However, it is not suitable for igneous rocks because they do not contain organic carbon. Nonetheless, there are other radioactive dating methods that can be used for rocks ranging from thousands to billions of years old, depending on the available elements.

Using Radiocarbon Dating on Interbedded Plant Debris

Despite the inapplicability of radiocarbon dating to igneous rocks, researchers have developed a workaround. For geologically ‘young’ volcanic rocks (less than 50,000 years old), radiocarbon dating of interbedded plant debris can constrain the age of the interbedded volcanic flows. This method involves the dating of charcoal that is interbedded with volcanic flows in Hawaii. This approach, while indirect, provides valuable constraints on the age of the volcanic rocks in question.

Different Radiometric Dating Methods for Igneous Rocks

Several radiometric dating methods can be applied to igneous rocks, each with its own unique advantages and applicability:

1. Uranium-Lead Dating

The uranium-lead radiometric dating scheme is highly accurate, with error margins of two to five million years within two-and-a-half billion years. This method is often used on zircon, which forms multiple crystal layers during metamorphic events. The closure temperature of zircon is very high, making it resistant to mechanical weathering and highly inert chemically. The main isotopes used are uranium-235 decaying to lead-207 and uranium-238 decaying to lead-206, providing a built-in crosscheck for precise age determination.

2. Samarium-Nodymium Dating

This method involves the alpha-decay of 147Sm to 143Nd, with a half-life of 1.06 x 10^11 years. It is highly accurate, with errors of less than twenty million years within two-and-a-half billion years. This makes it suitable for dating the oldest rocks, including those from lunar samples.

3. Potassium-Argon Dating

This method relies on the electron capture or positron decay of potassium-40 to argon-40. With a half-life of 1.3 billion years, it is ideal for dating very old rocks, including those containing common elements like micas, feldspars, and hornblendes. The closure temperature for these materials is about 125°C for mica and up to 450°C for hornblende.

4. Rubidium-Strontium Dating

This method is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. It is another reliable method for dating old igneous and metamorphic rocks, though it can have errors of 30 to 50 million years for a 3-billion-year-old sample.

Conclusion

While radiocarbon dating is not directly applicable to igneous rocks, other radiometric dating methods offer accurate age determinations for these materials. Understanding the different methods available can provide geologists with a comprehensive toolkit for studying the Earth's geological past accurately and precisely.