Which of the following is NOT a radio isotopic dating method that is currently u

Question

Which of the following is NOT a radio isotopic dating method that is currently used (circle best answer)? (a) U-Th/He (Uranium-Thorium/Helium) dating, (b) K-Ar (Potassium-Argon) dating, (c) Fe-Mg (iron-magnesium) dating; (d) Rb-Sr (Rubidium-Strontium) dating, (e) Lu-Hf (Lutetium-Hafnium) dating, (f) U-Pb (Uranium-Lead) dating Sedimentary rocks are seldom directly datable by radio isotopic means because: (circle best answer); (a) they are highly radioactive, disrupting the radio isotopic "clock" of the isotopic systems used for absolute dating, (b) the only datable minerals are soluble in petroleum so they have been destroyed; (c) they lack datable minerals that grew at the time the sediment turned to rock, (d) they form too quickly to be dated, (e) they form too slowly to be dated, (f) it is too expensive to collect a sample of a sedimentary rock, (g) it is illegal to collect sedimentary rocks. Most radio isotopic systems are associated with a closure temperature that is the temperature below which daughter product does not escape from the mineral. Suppose an eclogite crystallized at temperature of 750 degree C and this rock was dated by the Ar-Ar technique using muscovite, which has a closure temperature for Ar diffusion of about 400 degree C. It is most likely that the age obtained will be YOUNGER or OLDER than the actual crystallization age (circle best answer (ie Younger or Older) and IGNORE the possibility of the phenomenon known as excess Argon). In many regions of Earth the typical geothermal gradient in the upper 20 km of crust is about 20 degree C per km meaning that the temperature increases progressively with greater depth from about 0 degree C at the Earth's surface to 400 degree C at a depth of 20 km. If you were looking at a granite rock in this region and you wanted to estimate when the rock was exhumed to a depth of less than 5 km, which of the following dating techniques would be best to apply (circle best answer and note you will have to do some background reading and possible internet searching for this)? (a) fission track dating of apatite in the granite, (b) Ar-Ar dating of hornblende in the granite, (c) U-Pb dating of zircon in the granite.

Explanation / Answer

Question 12
Answer: (c) Fe- Mg ( iron-magnesium) dating.
Fe- Mg (iron-magnesium) dating is not a radioisotropic method that is currently used.
Fe- Mg (iron-magnesium) dating is not much useful to trace the dating of rocks than other methods so Fe- Mg (iron-magnesium) dating is not used currently.

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