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Isochron based on four isotope samples. If the plot is not linear, changes to the isotopes since the formation of rock are assumed.

Idealised graph plotting isotope ratios for four samples. The horizontal line shows the parent element expressed as a ratio of the control isotope. The vertical axis plots the amount of the daughter isotope as a ratio of the control isotope. As the parent decreases so the daughter isotope increases; thus the steeper the line the greater the passage of time. This line is known as the isochron. If the plot is not linear, changes to the isotopes since the formation of rock are assumed.

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