Rubidium-Strontium Isochrons Click on the illustration to step through the discussion. Rubidium-strontium isochrons can be used to calculate the last time of complete melting of a rock. The complete melting of the rock is a necessary condition, because that is what accomplishes the equilibrium of the isotopes of strontium. The isotopes of an element are chemically identical, and any chemical process will treat them identically. That's why we know the ratio of the strontium isotopes in the melt is a horizontal straight line in the illustration above. The isotope 86Sr is non-radiogenic in origin and does not change, but 87Sr is produced by the radioactive decay of 87Rb. There is no way of anticipating what the 87Sr is at the time of melt, but if there is 87Rb present then it will increase with time as the rubidium isotope decays. That is what makes this a useful clock. Clocks in the Rocks

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Rubidium-Strontium Isochrons Click on the illustration to step through the discussion. Rubidium-strontium isochrons will be formed at any time after crystallization of a rock provided the initial conditions are met. Different minerals which make up the rock will in general include different amounts of rubidium (87Rb) in their structures, and those which have more rubidium at the time of crystallization will have more radioactive decays and gain more of the daughter product 87Sr. The precise nature of the radioactive decay process predicts that all the minerals should lie along a straight line, an isochron. The longer the time interval, the more the decay and the steeper the slope of the isochron line. Clocks in the Rocks

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Rubidium-Strontium Isochrons Click on the illustration to step through the discussion. Rubidium-strontium isochrons can be used to calculate the last time of complete melting of a rock. The slope of the isochron line gives a measurement of the time since the last complete melting of the rock. It also gives the initial concentrations of strontium at the time of melting by projecting the isochron line to the point of zero 87Rb concentration. If the strontium isotope ratios for the various minerals do not form a straight line, then the assumptions of the analysis are invalid. Perhaps some things can be learned from the data, but it does not give an unambiguous date since melting. Clocks in the Rocks

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Rubidium-Strontium Isochrons Rubidium-strontium isochrons are not fool-proof, but they are to some extent self-checking. Only to the extent that the different mineral components within a rock lie along a straight isochron line can you have confidence in the measured time since the last complete melt. If the rock did not completely melt so that the initial assumptions are not met, then there will be minerals for which the strontium isotope ratio does not lie along the line. This is not uncommon. There are minerals like zircon which have a high melting point and may not melt when the other minerals in the rock do. The Rb-Sr isochron method is usually not applicable to recent lava flows on or near the surface because the temperatures involved are often not high enough to produce complete melts. Clocks in the Rocks

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