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.
So we can determine the age by determining the slope through a set of cogenetic samples. Slide9Assumptions We are assuming each sample analyzed has the same value of R0 and t.
In other words, they are cogenetic: they formed at the same time with the same isotope ratio at the time.
Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite.
Therefore, given sufficient time for significant production (ingrowth) of radiogenic For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar, K-feldspar, hornblende, biotite, and muscovite.
However, because Rb substitutes for K in minerals and these minerals have different K/Ca ratios, the minerals will have had different Rb/Sr ratios.
During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase.
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2.20, we see that t is proportional to the slope on a plot of R vs. Looking at equation 2.19 with this in mind:we see that it has the form y=a xb, where a is the intercept and b the slope.
So we have every reason to think that rocks when they form do incorporate strontium, and Sr in particular.
However, there is still a way to extract a date from the rock.
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.