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Also, by extrapolating backward in time to a situation when there was no lead that had been produced by radiogenic processes, a figure of about 4.6 billion years is obtained for the minimum age of the Earth.
This figure is of the same order as ages obtained for certain meteorites and lunar rocks.
With all these deformation experiments, it is necessary to scale down as precisely as possible variables such as the time and velocity of the experiment and the viscosity and temperature of the material from the natural to the laboratory conditions.
In the 19th century crystallographers were able to study only the external form of minerals, and it was not until 1895 when the German physicist Wilhelm Conrad Röntgen discovered X-rays that it became possible to consider their internal structure.
Experimental petrology also provides valuable data on the stability limits of individual metamorphic minerals and of the reactions between different minerals in a wide variety of chemical systems.Versions of the modern mass spectrometer were invented in the early 1920s and 1930s, and during World War II the device was improved substantially to help in the development of the atomic bomb. By determining the amount of the parent and daughter isotopes present in a sample and by knowing their rate of radioactive decay (each radioisotope has its own decay constant), the isotopic age of the sample can be calculated.For dating minerals and rocks, investigators commonly use the following couplets of parent and daughter isotopes: thorium-232–lead-208, uranium-235–lead-207, samarium-147–neodymium-143, rubidium-87–strontium-87, potassium-40–argon-40, and argon-40–argon-39.The SHRIMP (Sensitive High Resolution Ion Microprobe) enables the accurate determination of the uranium-lead age of the mineral zircon, and this has revolutionized the understanding of the isotopic age of formation of zircon-bearing igneous granitic rocks.Another technological development is the ICP-MS (Inductively Coupled Plasma Mass Spectrometer), which is able to provide the isotopic age of the minerals zircon, titanite, rutile, and monazite.