In many regions of Earth the typical geothermal gradient in the upper 20 km of c
ID: 283535 • Letter: I
Question
In many regions of Earth the typical geothermal gradient in the upper 20 km of crust is about10 C per km meaning that the temperature increases progressively with greater depth from about 0 C at the Earth's surface to 400°C at a depth of20 km. If you were looking at a granitic 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.
please explain
Explanation / Answer
The granitic rock is an igneous rock with at least 20% Quartz and up to 65% of Alkali Feldspar by means of volume and which is granular and phaneritic structure. The melting temperature of dry granite at ambient pressure is 1215-1260-degree celsius.But it is strongly reduced in the presence of water down to 650-degree celsius. The Geothermal Gradient increase temperature along with depth in the Earth's interior. It is about 25-degree celsius per km of depth.
Ar-Ar dating of hornblende in the granite is the best possible way to estimate the temperature of the rock was exhumed.
Because hornblende is a complex inosilicate of minerals which is not recognized in its own right. The name is used as field term which refers to the dark amphibole.
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