Terrestrial fractionation line
The terrestrial fractionation line is the line on an oxygen isotope graph along which all Earth and Moon rocks plot.
On a three-oxygen isotope graph, scientists plot the change in oxygen-17 against the change in oxygen-18 relative to a standard. All terrestrial samples, from seawater to granite, fall along a straight line with a slope of about 0.52. This is the terrestrial fractionation line, or TFL. Its slope reflects mass-dependent processes acting on a single well-mixed reservoir.
Lunar rocks fall on the same line within very small uncertainties, one of the key observations behind the theory that the Moon formed from material closely related to Earth's. Enstatite chondrites and aubrites also plot on or very near it.
Other meteorites lie off the line, above or below it, according to their parent body's original oxygen reservoir. Martian meteorites, HED meteorites, ordinary chondrites and carbonaceous chondrites each have distinctive positions. The distance from the TFL, called Δ17O, is often the first measurement used to decide what kind of meteorite a new find is.
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