Radiocarbon dating equation
Sr) was the first widely used dating system that utilized the isochron method.
Rubidium is a relatively abundant trace element in Earth’s crust and can be found in many common rock-forming minerals in which it substitutes for the major element potassium.
Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited.
The rubidium–strontium pair is ideally suited for the isochron dating of igneous rocks.
On the other hand, meteorites that have spent most of their time in the deep freeze of outer space can provide ideal samples.
Potassium-bearing minerals including several varieties of mica, are ideal for rubidium–strontium dating as they have abundant parent rubidium and a low abundance of initial strontium.
At the time of crystallization, this produces a wide range in the Rb/Sr ratio in rocks that have identical − 1).
In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification.
This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70,000, it is quite significant.
This situation is easily visualized on an isochron diagram, where a series of rocks plots on a steep line showing the primary age, but the minerals in each rock plot on a series of parallel lines that indicate the time since the heating event.
If cooling is very slow, the minerals with the lowest blocking temperature, such as biotite mica, will fall below the upper end of the line.
Weathering is a disturbing influence, as is leaching or exchange by hot crustal fluids, since many secondary minerals contain rubidium.
Volcanic rocks are most susceptible to such changes because their minerals are fine-grained and unstable glass may be present.