Rb-Sr isotope evolution in Earth and the preferred decay constant of 87Rb

Abstract Any model trying to define the Rb-Sr isotope evolution in the planet Earth/Solar/System has to specify the beginning 87Sr/86Sr isotope ratio of the planet Earth at the time of its formation, its present-day Sr isotope ratio and the 87Rb/86Sr ratio. Furthermore, such a model attempting to describe this evolution has to take into account several other factors such as age of the Earth, decay constant of 87Rb, present-day Rb/Sr ratio, and isotopic characteristics of the mantle as seen through the initial Sr isotope ratios of certain ancient mantle-derived rocks because these factors impose significant constraints on this evolution. The present study shows that one model for the isotopic evolution in the Earth/Solar System which stipulates beginning (initial) and present-day Sr isotope ratios of 0.69877 and 0.7047, respectively, and a present-day 87Rb/86Sr ratio of 0.09, satisfies all the abovementioned constraints. However, for this model to be feasible, the age of the Earth must be assumed to be similar to the mean age of the meteorites, that is, 4.555 Gyr, and the decay constant of 87Rb must be revised to a lower value of 1.4087(10− 11 yr− 1).