Answered: The K-Ar and Rb-Sr methods are applied…

Applications and Investigations in Earth Science (9th Edition)

9th Edition

ISBN:9780134746241

Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Chapter1: The Study Of Minerals

Section: Chapter Questions

Problem 1LR

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The K-Ar and Rb-Sr methods are applied to a rock sample. For the K-Ar method, the concentration [40Ar/40K] = 0.096 ± 0.005 is obtained. For the Rb-Sr method, the best-fitting isochron has a slope of m = 0.021 ± 0.001 and a y-intercept 0.71 ±0.05. a) Compute the “ages” of the sample, including uncertainties, from both methods. What is the initial strontium isotope ratio? Assuming the Rb-Sr age is the correct one, how can you tell that argon diffusion has occurred? b) Assume the sample has undergone a very simple cooling process, i.e., it was initially formed above the closure temperature Tc for the mineral being examined in the K-Ar method, after which it cooled steadily with time, dropping below Tc at some point, until the present day. Also assume that all the radiogenic argon was lost above Tc, and that all of it was retained below Tc. If the Rb-Sr age is the correct one, compute the percentage of all the argon produced radioactively that has been lost by diffusion.

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