Apatite can provide geologists with an exceptionally wide range of ages and temperatures to investigate processes that operate from Earth's surface right down to the lower crust. Apatite is a widespread accessory mineral in igneous, metamorphic, and clastic sedimentary rocks and can be dated using four radioactive decay schemes, each with a different temperature window for isotopic closure: Lu–Hf (675–750 °C); U–Pb (350–550 °C); apatite fission track (60–110 °C); (U–Th)/He (40–80 °C). The fission-track and (U–Th)/He methods are popular for studying upper-crustal and near-surface processes, whereas the U–Pb and Lu–Hf systems are used to investigate the thermal, tectonic, and magmatic histories of the deeper crust.