Final answer:
The closure temperature is when the parent/daughter ratio of radioactive elements is locked into a mineral's structure, allowing geologists to date rocks and minerals by measuring this ratio. This concept is foundational in geochronology and depends on the assumption that the system remains closed post-solidification.
Step-by-step explanation:
The temperature at which the parent/daughter ratio of radioactive elements is effectively locked into the mineral structure is known as the closure temperature. At this temperature, the crystal lattice of the mineral is formed to the point that the isotopes can no longer escape or move within the mineral, and hence the radiometric 'clock' starts. This is a critical concept in geochronology, as it enables geologists to determine the age of the rock or mineral since it last cooled below this pivotal temperature. By using isotope systems like potassium-40 on hornblende grains or uranium-235 on zircon grains, geologists can measure parent-daughter ratios to date events such as metamorphism and igneous intrusions. The assumption that the system remains closed after solidification is fundamental to obtaining accurate dates. However, geologists are aware of potential complexities and employ methods to correct for any system open to migration of isotopes post-closure.