The quartz-eye trondhjemite and associated hornblendite near Rio Brazos, Brazos Peak quadrangle, New Mexico, and the Kroenke Granodiorite of the Mount Harvard quadrangle and Sawatch Range, Colo., lie on an approximate 1,700-m.y. The Rio Brazos intrusive contains more Si OO than does the Kroenke intrusive.
Six samples of the trondhjemite of Rio Brazos contain 8-43 ppm Rb and 57-194 ppm Sr, and six of the Kroenke intrusive, 53-99 ppm Rb and 314-595 ppm Sr.
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Rubidium is a chemical element with symbol Rb and atomic number 37.
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.
During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase.