1)
Granite is an intrusive, felsic rock, which may be produced by partial melting or fractional crystallization. Explain how these processes can produce felsic magmas from more mafic parent magmas
2)The age of granite can be determined using radiometric dating. Expalin the basis for the determination of a 238^U-Pb radiometric date of 1120 Ma for a granite.
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Sample Locality Age (Million years)
Granite Scottish Highlands
Gabbro Sudburry, Canada
Gneiss Greenland
1) Partial melting and fractional crystallization are processes that can produce felsic magmas from more mafic parent magmas.
Partial melting occurs when rocks are subjected to heat and pressure, causing some of the minerals in the rock to melt while others remain solid. Mafic rocks, which have a high content of magnesium and iron, can undergo partial melting to produce melt that is more felsic in composition. This is because some of the minerals in the mafic rock, such as olivine and pyroxene, have higher melting temperatures compared to the minerals in felsic rocks like granite. When these high-melting minerals melt, they don't contribute to the melt, resulting in a higher concentration of felsic minerals in the magma.
Fractional crystallization is a process that occurs when magma cools and minerals start to crystallize out of the liquid. The crystallized minerals are typically different in composition compared to the remaining liquid magma. In the case of granite formation, fractional crystallization can occur as the magma cools underground. As the magma cools, the minerals with higher melting temperatures start to crystallize out first. These minerals are typically more mafic in composition. Over time, as more and more minerals crystallize out of the remaining magma, the concentration of mafic minerals decreases, resulting in a more felsic composition.
2) The determination of the 238U-Pb radiometric date of 1120 Ma for a granite is based on the decay of uranium-238 (238U) to lead-206 (206Pb).
Uranium-238 has a very long half-life of about 4.5 billion years. Over time, the uranium-238 in the granite will decay into lead-206 through a series of intermediate isotopes. By measuring the ratio of uranium-238 to lead-206 in the granite, scientists can determine how much radioactive decay has occurred since the granite formed.
Radiometric dating relies on the fact that the rate of radioactive decay is constant over time. By knowing the rate of decay and measuring the amount of parent and daughter isotopes in a sample, scientists can calculate the age of the sample.
In the case of the granite with a 238U-Pb radiometric date of 1120 Ma, it means that 1120 million years have passed since the granite crystallized and formed. This date is determined by analyzing the uranium and lead isotopes in the granite sample using techniques such as mass spectrometry.
It is important to note that radiometric dating techniques require careful sample selection and analysis to ensure accurate results.