Flexi Says: Helium is an inert gas that geologists use to study magma and determine its origins. There are two important isotopes of helium. Helium-3 (3He) is stored deep in Earth's mantle where it was trapped during formation of Earth. Helium-4 (4He) is produced during radioactive decay of uranium and thorium, two elements commonly found in Earth's crust. Therefore, geologists can study the isotopes in magma to determine whether it came from the mantle or was formed by melting crust.
