Radiometric Dating Study Notes
Radiometric Dating Overview
Introduction to Radiometric Dating
Radiometric dating is a technique used to estimate the age of Earth and fossils.
It enables us to age materials that are millions or billions of years old.
Age Estimation Concepts
Relative Age: Understanding that deeper fossils are older than those found higher up in geological layers.
Absolute Age: Determining a specific age using methods such as radiometric dating.
Radiometric Dating Mechanism
Utilizes the decay of radioactive isotopes to estimate absolute age.
Radioactive isotopes decay at a consistent rate over time.
Definition of Decay: The process in which a radioactive substance loses particles (like neutrons) to achieve a lower energy state.
Half-Life: The time required for half of the isotopes in a sample to decay into a more stable form. It varies based on the isotope.
Common Radioactive Isotopes and Their Half-Lives
Table 3.1: Some Naturally Occurring Radioactive Isotopes and Their Half-Lives
Uranium-235 decays to Lead-207.
Other examples include:
Neodymium-143: 106 billion years
Samarium-147: 48.8 billion years
Rubidium-87: 42 billion years
Thorium-232: 14 billion years
Carbon-14 decays to Nitrogen-14: 5,715 years.
Process of Uranium-235 Decay
Uranium-235 (U-235) is unstable; it emits an alpha particle (2 protons and 2 neutrons) as it decays into a stable lead isotope Pb-207.
Graphical representation shows the loss of U-235 over time and gain of Pb-207 based on a predictable decay rate (half-life = 704 million years).
Different Isotopes for Different Ages
Various isotopes are suitable for dating samples from different time spans:
Uranium isotopes for ancient rocks.
Carbon-14 for more recent organic materials.
Carbon Dating
Carbon dating utilizes the abundance of carbon isotopes, particularly 14C, in living organisms.
When organisms die, they stop taking in carbon, leading to a decrease in the 14C ratio in their remains.
Example: Cave paintings can be dated between 32,000 to 30,000 years ago using the carbon-14 method due to the carbon from burnt sticks.
Half-Life Calculations for Rocks
Example Problem:
Given how to estimate the age of a rock containing 3g of 87Rb.
Problematic aspects of half-life dating include the unknown initial amounts of isotopes.
The age is instead determined using the ratio of remaining radioactive isotopes to stable isotopes.
Younger Fossils
Carbon-14 is effective for dating materials less than 50,000 years old.
Creation of 14C occurs consistently in the atmosphere via cosmic radiation, maintaining a ratio of about 1:1,000,000,000,000 in living organisms.
Upon death, organisms no longer absorb carbon, resulting in a gradual decrease of 14C and maintaining a constant ratio of 14C to 12C while alive.
Older Dating Techniques
For materials significantly older than 50,000 years, isotopes with longer half-lives such as Uranium-238 (to Lead-206) with half-life of 4.5 billion years are used.
Aging Rocks with Uranium-238
By measuring today's amounts of U-238 and Pb-206 in a rock, we can calculate its age based on the decay ratio, without needing the original quantity of U-238 we started with.
The Rock Cycle and Its Relevance
The rock cycle plays a significant role in forming new rocks and incorporating radioactive isotopes.
Old rocks can melt and produce new magma which includes isotopes like U-238, which can be used for dating.
It is important to note that we age igneous rocks surrounding fossils and not the sedimentary rocks directly containing fossils.
Combining Relative and Absolute Dating
To estimate fossil ages, a combination of relative and absolute dating techniques is utilized.
Zircon Crystal Dating Method
Different temperature and pressure conditions yield various rock types; Zircon crystals (ZrSiO4) often contain Uranium but not lead (Pb).
When the rock forms, Uranium is incorporated into the Zircon, but there is no lead initially present.
Over time, radioactive U-238 decays into Pb-206 within these crystals, allowing age calculations based on the current ratio.
Isochron Method of Dating
Isochron method employs isotopes like Rubidium-87 decaying to Strontium-87, which maintains stable isotopes (e.g., 86Sr).
This method plots ratios of isotopes to determine the age of rocks based on slope rates derived over time.
The slope of the line indicates time since rock formation synchronized with the decay rate of 87Rb.
Implications of Radiometric Dating
Isotopes are inherently unstable and decay at known constant rates, permitting age determination even for ancient samples.
Similar techniques yield concise age estimates for the oldest Earth rocks and meteorites, around 4.5 billion years.
Extraterrestrial materials such as meteorites and moon rocks provide additional insight with less alteration than terrestrial rocks.