parent and non-radiogenic isotopes, the slope of the isochron gets steeper with time. The change in slope from initial conditions—assuming an initial isochron slope of zero (a horizontal isochron) at the point of intersection (intercept) of the isochron with the y-axis—to the current computed slope gives the age of the rock. The slope of the isochron, or , represents the ratio of daughter to parent as used in standard radiometric dating and can be derived to calculate the age of the sample at time t. The y-intercept of the isochron line yields the initial radiogenic daughter ratio, . Whole rock isochron dating uses the same ideas but instead of different minerals obtained from one rock uses different types of rocks that are derived from a common reservoir; e.g. the same precursor melt. It is possible to date the differentiation of the precursor melt which then cooled and crystallized into the different types of rocks. One of the best known isotopic systems for isochron dating is the rubidium–strontium system. Other systems that are used for isochron dating include samarium–neodymium, and uranium–lead. Some isotopic systems based on short-living extinct radionuclides such as 53Mn, 26Al, 129I, 60Fe and others are used for isochron dating of events in the early history of the Solar System. However, methods using extinct radionuclides give only relative ages and have to be calibrated with radiometric dating techniques based on long-living radionuclides like Pb-Pb dating to give absolute ages. Application Isochron dating is useful in the determination of the age of igneous rocks, which have their initial origin in the cooling of liquid magma. It is also useful to determine the time of metamorphism, shock events (such as the consequence of an asteroid impact) and other events depending on the behaviour of the particular isotopic systems under such events. It can be used to determine the age of grains in sedimentary rocks and understand their origin by a method known as a provenance study. See also Radiometric dating References External links Basics of radioactive isotope geochemistry from Cornell Isochron Dating at the TalkOrigins Archive Radiometric dating