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Multiple migmatite events and cooling from granulite facies metamorphism withn the Famatina arc margin of northwest Argentina.

This problem was first recognised by Compston and Jeffery (1959), and overcome by the invention of the isochron diagram (Nicolaysen, 1961). Strontium isotope stratigraphy: LOWESS Version 3: best fit to the marine Sr-isotope curve for 0-509 Ma, and accompanying look-up table for deriving numerical age.

Model ages subsequently re-appeared in more specialised aspects of Rb 1), yields the age of the minerals.

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.

Thus, a precise measurement of the Sr ratio in a modern volcano can be used to determine age if recycled older crust is present.

A ratio for average continental crust of about 0.72 has been determined by measuring strontium from clamshells from the major river systems.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.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 strategy now used is to plot an isochron, but to correct for the existence of primordial Sr 87, we plot the ratios (Sr-87)/(Sr-86) versus (Rb-87/Sr-86).

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