The rock itself gives the integrated, more gradual increase.
At the time of heating, identical Sr ratios are again achieved as described above, only to be followed by a second episode of isotopic divergence.
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.
Dissolved strontium in the oceans today has a value of 0.709 that is dependent on the relative input from the continents and the ridges.
In most cases, the changes in the Sr ratio are so large that an initial value can be assumed without jeopardizing the accuracy of the results.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.A more dramatic presentation of this phenomenon is found when the changes in the Sr ratio over time, whereas the value in such rubidium-rich, strontium-poor minerals as biotite increases rapidly with time.Should a simple igneous body be subjected to an episode of heating or of deformation or of a combination of both, a well-documented special data pattern develops.With heat, daughter isotopes diffuse out of their host minerals but are incorporated into other minerals in the rock.