Isochron

Isochron is a term used in geochronology to describe a line on a plot that arises in isochron dating, a method for determining the age of rocks and meteorites. Each plotted point represents a sample from the same rock body, with axes typically showing ratios of radiogenic daughter isotopes to stable isotopes or to other reference isotopes. If all samples formed at the same time and remained closed to isotopic exchange since formation, the data lie on a straight line known as an isochron. The slope of this line is related to the time elapsed since formation, while the intercept gives the initial isotopic ratio. This approach reduces the need to know the initial isotopic composition a priori and yields a self-consistent age estimate, provided the system has remained closed and the samples were correctly prepared.

Common isochron systems include rubidium–strontium (Rb-Sr), samarium–neodymium (Sm-Nd), lutetium–hafnium (Lu-Hf), and uranium–lead (Pb-Pb) methods, each with its own decay constants and interpretation. Limitations include the need for multiple suitable samples, potential disturbance by metamorphism or weathering, and assumptions about initial isotopic homogeneity.

Outside geology, isochron loosely refers to the concept of equal time intervals in dynamics; isochronism is the property of a system whose period is independent of amplitude, notably in pendulums and clock mechanisms.

Etymology: from Greek isos “equal” and chronos “time.”