Oxygen Isotope Plot

--- A primer from PSRD on how cosmochemists use these plots and interpret the data.

Written by Edward Scott and G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Oxygen has three stable isotopes: ¹⁶O, ¹⁷O, and ¹⁸O. The mixtures of oxygen isotopes in the Earth, Mars, and the asteroids differ slightly. Understanding why they differ is an active topic of cosmochemical research and helps us in our quest to understand the origin of planets and asteroids and the formation of our Solar System.

One informative way to plot oxygen isotopic data is to use all three isotopes by plotting the ¹⁷O/¹⁶O ratio against the ¹⁸O/¹⁶O ratio, as shown in the diagram below. In general, rocks in and on a given planet fall along a well-defined line with a slope of about ½; the line for terrestrial and lunar rocks is labeled Earth and Moon in this plot. A striking discovery made more than three decades ago by Robert Clayton (University of Chicago) and coworkers was that primitive materials in chondrites (CAIs and chondrules) plot along a line that suggests addition or subtraction of ¹⁶O.

Plot showing the 17O/16O and 18O/16O ratios in Earth and Moon rocks, and chondrules and CAIs in meteorites in parts per thousand. Data have been standardized to standard mean ocean water (SMOW) and plotted as deviations from that value (expressed in delta notation on the axes). The meteorite particles define a line with much steeper slope than the Earth-Moon line, which is consistent with loss or addition of 16O. A shorthand way to show the deviation from the Earth-Moon line is to plot the vertical displacement of any point from it, as indicated graphically in purple. This parameter (Δ17O) is called "big delta O-17" by cosmochemists.

In the standard used for laboratory analyses, the ratio of the number of ¹⁷O atoms to ¹⁶O atoms is 0.000380. The ratio of ¹⁸O to ¹⁶O in the standard is 0.002005.

A sample with an ¹⁸O/¹⁶O ratio of 0.002009 has a value higher than the standard by 4 parts in 2000, i.e. higher by 0.2 per cent (%) or 2 per mille (‰). Such a sample would plot at +2 on the horizontal axis.

If a sample has an ¹⁸O/¹⁶O ratio of 0.002001, it's value is lower than the standard by 4 parts in 2000, i.e. 0.2 per cent (%) or 2 per mille (‰) lower. It would then plot at -2 on the horizontal axis.

In other words, a positive delta value (δ) means the ratio is higher than the standard, and a negative delta value (δ) means the ratio is lower than in the standard.

For example, a data point at -30 on the horizonal axis means that the ¹⁸O/¹⁶O ratio is 30 per mille or 3% lower than in the standard. Read the vertical axis in the same way.

It is much easier to plot the variations in the isotopic ratios using the deviations in per mille from the standard rather than using the actual ratios.

A shorthand way to show the deviation from the Earth-Moon line is to plot the vertical displacement of any point from it, as indicated graphically in purple. This parameter (Δ¹⁷O) is called "big delta O-17" by cosmochemists. In our example plot, the Δ¹⁷O for the lowest data point on the CAI-chondrule line is -21 per mille.

The CAI-chondrule line has a slope of 1, meaning that if two samples that plot on the line differ by 2 per mille in ¹⁸O/¹⁶O they will differ in ¹⁷O/¹⁶O by 2 per mille also.

Samples from the Earth and Moon plot on the Earth-Moon line, which has a slope of 0.5. Hence, if two samples on the Earth-Moon line differ in ¹⁸O/¹⁶O by 2 per mille, they will differ in ¹⁷O/¹⁶O by 1 per mille.

Here are a few references:

• Clayton R. N. (1993) Oxygen Isotopes in Meteorites, Annual Review of Earth and Planetary Sciences, v. 21, p. 115-149, doi: 10.1146/annurev.ea.21.050193.000555. [ abstract ]


• Greenwood, R. C., Burbine, T. H., Miller, M. F., and Franchi, I. A. (2017) Melting and Differentiation of Early-formed Asteroids: The Perspective from High Precision Oxygen Isotope Studies, chemie der Erde, doi: 10.1016/j.chemer.2016.09.005. [ abstract ]


• Oxygen Isotope Plots, many examples from the Meteoritical Bulletin Database.


• Scott, E. R. D. (December 2001) Oxygen Isotopes Give Clues to the Formation of Planets, Moons, and Asteroids, Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Dec01/Oisotopes.html.


• Taylor, G. J. (November 2016) Revealing the Secrets of Asteroid Melting by Precise Oxygen Isotopic Analyses, Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/CosmoSparks/Nov16/high-precision-Oisotopes.html .


• Taylor, G. J. (August 2010) New View of Gas and Dust in the Solar Nebula, Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Aug10/gas-dust-Oisotopes.html.