be considered as representing the time of emplacement or formation of a rock. The system must have remained closed to Rb and Sr diffusion from the time at which the rock formed or fell below the closure temperature (generally considered to be 650 °C); The minerals which are taken from a rock to construct an isochron must have formed in chemical equilibrium with one another or in the case of sediments, be deposited at the same time; The rock must not have undergone any metasomatism which could have disturbed the Rb-Sr system either thermally or chemically One of the major drawbacks (and, conversely, the most important use) of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids. Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism. Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock (generally during potassic alteration or calcic (albitisation) alteration. Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock. Uses Geochronology The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the 87Sr/86Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same 87Sr/86Sr as the mineral. Although this is a potential source of error for terrestrial rocks, it is irrelevant for lunar rocks and meteorites, as there are no chemical weathering reactions in those environments. Isotope geochemistry Initial 87Sr/86Sr ratios are a useful tool in archaeology, forensics and paleontology because the 87Sr/86Sr of a skeleton, sea