In doing a standard geophysical survey of a valley floor, you notice a seismic r

Question

In doing a standard geophysical survey of a valley floor, you notice a seismic reflector at 10 m below the surface. You check your gravimeter and discover that this reflector is the start of a buried lens of low density materials 100 m thick. Intrigued, you hire a crew from a nearby town and dig down to the buried layer where you discover the remains of an ancient landslide. Upon close inspection you notice that the landslide is made up of granite blocks that are separated by a fine interstitial matrix of ground up soil. You obtain samples of the blocks and of the soil matrix and head back to the laboratory for isotopic testing. Once back at the lab, you examine the soil matrix sample first. You examine the material using a mass spectrometer and discover that the ratio of Carbon-14 to Carbon-12 is 2.5 x 10-13. You also use a sample of atmospheric gas to calibrate your equipment and you find that the ratio here is 1.1 x 10-12 (a) What age do you derive for the soil sample? Next, you examine the sample taken from the blocks. Here you use two separate age dating methods. First, you examine the total amount of Argon-40 and Potassium-40 in your sample. You observe that your granite sample contains 4% by weight of Potassium, of which 0.01% is Potassium-40 When you grind up and heat your granite sample to drive off the Argon-40, you find that 1 g of sample gives off 1.56 x 10-9 mol of Argon-40 Secondly, you examine the amount of Rubidium-87, Strontium-87 and Strontium-86 in several of the minerals you find within the block sample. Here you find that for the first mineral 87Sr/86Sr- 0.0529 and 87Rb/86Sr = 0.1 for the 2nd mineral 87Sr/86Sr = 0.0558 and 87Rb/86Sr = 0.2 and for the 3rd mineral 87Sr/86Sr 0.0572 and 87Rb/86Sr 0.25. (b) What age do you acquire from the K-Ar analysis? (c) What age do you obtain from the Rb-Sr analysis? (d) Using the information you found in parts (a) through (c) briefly describe the history of the minerals in the blocks, describing what happened to them at the three different times you found in your analysis.

Explanation / Answer

a) Amount of C-14 left in the soil = ( Ratio in soil ) / (Ratio in standard) = (2.5 x 10-13) / (1.1 x 10-12) = 0.22

Since 22% of the original C-14 remains in the soil sample, the age of the sample is around 11,500 years with limits of standard deviations from all samples.

b) K-40 has a weight of 0.4 x 10-5 g, Ar-40 has a weight of 1.56 x 10-9 x 40 = 6.24 x 10-8 g, so the ratio of K-40 / Ar-40 (=64.1) indicates that the granite is less than 1.3 billion years old.

c) The ratio of Sr-87 / Sr-86 is less than 0.70 in all samples, indicating that the age of minerals is less than 1.09 billion years.

d) The ages from isotopic ratios of the soil and minerals in blocks confirm that the minerals in granite formed in a chronological order. 3rd Mineral formed first, followed by 2nd and 1st as evident in the increasing ratio of  Sr-87 / Sr-86.

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