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.
The rock itself gives the integrated, more gradual increase.
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 the geologic past, changes in the activity of these two sources produced varying Sr ratios over time.
The samples are crushed and homogenized to produce a fine representative rock powder from which a fraction of a gram is withdrawn and dissolved in the presence of appropriate isotopic traces, or spikes.
Strontium and rubidium are extracted and loaded into the mass spectrometer, and the values appropriate to the , the data are examined to assess how well they fit the required straight line.
Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium-87 compared with the stable isotopes.
This relative abundance is expressed as the Sr ratio, where strontium-86 is chosen to represent the stable isotopes strontium-88, strontium-86, and strontium-84, which occur in constant proportions in natural materials.
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.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.In contrast, Earth’s most abundant lava rocks, which represent the mantle and make up the major oceanic ridges, have values between 0.703 and 0.705.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.