Uranium–lead dating facts for kids

Uranium–lead dating, often called U–Pb dating, is one of the oldest and most accurate ways to figure out how old rocks are. It's a type of radiometric dating, which uses the natural decay of radioactive elements like uranium. This method can date rocks that formed and hardened from about 1 million years ago to over 4.5 billion years ago! It's very precise, usually accurate within 0.1% to 1%.

Scientists often use this method on a mineral called zircon. Zircon crystals naturally take in uranium and thorium atoms when they form, but they usually push out lead atoms. This means that when a new zircon crystal forms, it has almost no lead inside it. Over time, the uranium inside the zircon slowly changes into lead through a process called radioactive decay. Because scientists know exactly how fast uranium turns into lead, they can measure the amount of lead and uranium in a zircon sample today to figure out its age.

The U–Pb dating method uses two different "decay chains" or paths:

• One path is from Uranium-238 (²³⁸U) to Lead-206 (²⁰⁶Pb). This process takes a very long time, with half of the Uranium-238 changing into Lead-206 in about 4.47 billion years.
• The other path is from Uranium-235 (²³⁵U) to Lead-207 (²⁰⁷Pb). This one is a bit faster, with half of the Uranium-235 changing into Lead-207 in about 710 million years.

Using both of these paths helps scientists get a more accurate age for the rocks.

How Uranium Changes to Lead

Uranium changes into lead through a series of steps involving alpha and beta decays. Think of it like a chain reaction where one atom changes into another.

• Uranium-238 and the atoms it changes into go through eight alpha decays and six beta decays to become Lead-206.
• Uranium-235 and its changing atoms go through seven alpha decays and four beta decays to become Lead-207.

Having these two "parallel" ways that uranium turns into lead (²³⁸U to ²⁰⁶Pb and ²³⁵U to ²⁰⁷Pb) gives scientists more ways to check their results. When people talk about U–Pb dating, they usually mean using both of these decay paths together.

Sometimes, scientists might use just one of these decay paths. There's also a method called lead–lead dating, which only looks at the ratios of different lead atoms. A famous American scientist named Clair Cameron Patterson used this lead-lead dating method to make one of the first good guesses about the age of the Earth.

Minerals Used for Dating

While zircon (ZrSiO₄) is the most common mineral used for U-Pb dating, scientists can also use other minerals. These include monazite (which has its own dating method called monazite geochronology), titanite, and baddeleyite.

If zircon or other uranium-containing crystals can't be found, scientists can sometimes use U-Pb dating on other minerals. These include calcite / aragonite and other carbonate minerals. These minerals might not give ages as precise as those from igneous (volcanic) and metamorphic (changed by heat and pressure) rocks, but they are often easier to find in the Earth's layers.

How the Dating Works Inside a Crystal

When uranium atoms inside a zircon crystal decay, they release tiny particles. This process, called alpha decay, can cause tiny bits of damage to the zircon crystal. This damage is strongest around the uranium atoms themselves. This damage can push the newly formed lead atoms away from their original spot in the crystal.

In areas where there's a lot of uranium, the damage to the crystal can be quite big. It can even create a network of damaged spots. Tiny cracks, called "fission tracks," can also form within the crystal. These cracks act like tiny tunnels. They can allow lead atoms to escape or "leak" out of the zircon crystal. This "lead loss" can make the dating more complicated, but scientists have ways to account for it.

Undamaged zircon is very good at holding onto the lead created by decaying uranium, even at high temperatures (around 900 °C). However, if a zircon crystal has a lot of radiation damage from too much uranium, it might start losing lead at lower temperatures. Zircon is also very strong and doesn't easily break down. This can be both good and bad for scientists. Sometimes, zircon crystals can survive when their parent rock melts, keeping their original age intact.

This means that a single zircon crystal might have parts with very different ages. For example, the center (core) might be much older than the outer edge (rim). Scientists call this "inherited characteristics." To figure out these complex histories, they use special tools like ion microprobes or laser ICP-MS to analyze tiny spots within the crystal.

Images for kids

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  Figure 1: This is a Concordia diagram. It shows how scientists plot data from zircon samples to find their age. The lines help them see if lead has been lost from the samples.