Geophysics, Lecture Notes- Physics - Prof Gavin Bell 7

# Geophysics, Lecture Notes- Physics - Prof Gavin Bell 7 -...

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Lecture 5 Handout – the Method of Isochrons Dr. Gavin Bell               1 R N R D R D now now 0 0 now now t e Isochron method for radiometric dating 1. Take several mineral samples from a rock, maintaining complete purity. 2. Choose decay system such that D and R are the same element, N is different. 3. Measure isotopic ratios D/R and N/R (mass spectrometer) and plot. 4. If they lie on a straight line, the system has been “closed” a. Gradient = m gives age of rock (we know ) b. Intercept = “initial ratio” (isotopic ratio in general environment) 5. Scatter means poor closure, i.e. isotopes have been exchanged with environment (chemical reaction, thermally activated diffusion, etc.) 6. Line of best fit through many points reduces random error. Quick exercise on Rb-Sr isochron               1 Sr Rb Sr Sr Sr Sr now 86 now 87 0 86 0 87 now 86 now 87 Find the initial ratio for Rb/Sr and age of the rock. What has happened to mineral C?

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Geophysics, Lecture Notes- Physics - Prof Gavin Bell 7 -...

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