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Rb-Sr ages of glauconite pellets commonly fall short of the presumed age of deposition by 10-20%, according to current estimates of the geologic time scale.
In part, the young ages are the result of preferential loss of exchangeable radiogenic Sr.
The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different.
In this study, in situ Rb-Sr geochronology distinguishes the timing of several different fracture-controlled hydrothermal events: 1 and 2) greisen mineralizations and associated far-field hydrothermal veins adjacent to a granite intrusion, 3) reactivation events within a mylonite shear zone and 4) low-temperature precipitation from saline organic-rich brines in thin veinlets.
Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed.
Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
The age of a sample is determined by analysing several minerals within the sample. If these form a straight line then the samples are consistent, and the age probably reliable.
The slope of the line dictates the age of the sample.Consequently, in pellets where tightly bound, nonexchangeable Rb and Sr have remained a closed chemical system since deposition, removal of the exchangeable cations with ammonium acetate (NH OAc) can raise the apparent age to coincide with the expected depositional age.In other cases the age of pelletal glauconite remains younger than the depositional age after chemical treatment. Development of this process was aided by German chemists Otto Hahn and Fritz Strassmann, who later went on to discover nuclear fission in December 1938.