Crystallisation age
A crystallisation age is the time since a rock solidified from melt, measured with radiometric dating.
A crystallisation age is the date when an igneous rock solidified from its melt. It is measured by radiometric methods on minerals that formed as the melt froze and that retain their isotopes well afterwards.
Crystallisation ages vary greatly among meteorites. Some angrites crystallised about 4,564 million years ago, within a few million years of the Solar System's start. Eucrites mostly crystallised between about 4,560 and 4,500 million years ago. Martian shergottites have much younger crystallisation ages, most between about 150 and 600 million years, showing Mars stayed volcanically active much longer. Lunar mare basalts range mostly from about 3.1 to 3.9 billion years.
Crystallisation ages differ from other kinds of age. A rock can crystallise early, be shocked and lose its argon later, spend millions of years in space as a small body and then lie on Earth for thousands of years. Each event has its own age, and meteoriticists combine them to tell the full story.
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