Oxygen isotopes: the fingerprint of origin

Oxygen makes up nearly half of most rocks by mass, and its three stable isotopes carry a signature that distinguishes parent bodies. Oxygen isotopes are among the most powerful tools in classification.

The three-isotope plot

Oxygen has three stable isotopes: oxygen-16, oxygen-17 and oxygen-18. Results are expressed as δ17O and δ18O, the deviation in parts per thousand, or per mil, of the 17O/16O and 18O/16O ratios from a standard, usually Vienna Standard Mean Ocean Water.

Most physical and chemical processes, such as evaporation, crystallisation and alteration, fractionate isotopes according to their mass. Because oxygen-18 differs from oxygen-16 by two mass units and oxygen-17 by one, such processes change δ17O by roughly 0.52 times the change in δ18O.

All samples from a well-mixed body such as Earth therefore plot along a line of slope about 0.52 on a graph of δ17O against δ18O, called the terrestrial fractionation line, or TFL. The Moon falls on the same line, which is one of the strongest constraints on lunar origin.

Δ17O and mass-independent signatures

In 1973, Robert Clayton, Lawrence Grossman and Toshiko Mayeda discovered that anhydrous minerals in CAIs from Allende do not lie on the TFL. Instead they plot along a line of slope close to 1, enriched in oxygen-16. This line, defined by carbonaceous chondrite anhydrous minerals, is called the CCAM line, with a slope of about 0.94.

A slope near 1 cannot be produced by ordinary mass-dependent processes. It reflects mixing between reservoirs of different, mass-independent isotopic composition in the early Solar System. One leading explanation is self-shielding of carbon monoxide by ultraviolet light in the solar nebula or its parent molecular cloud.

To quantify distance from the TFL, scientists use Δ17O, approximately δ17O minus 0.52 times δ18O. For Earth, Δ17O is close to zero. Because mass-dependent processes on a body do not change Δ17O, each parent body tends to have a characteristic value.

Reading the fingerprints

Different groups occupy distinct regions of the three-isotope plot. Ordinary chondrites lie above the TFL, with Δ17O increasing from H, about +0.7 per mil, to L, about +1.1, and LL, about +1.3. R chondrites lie higher still, near +2.7. Enstatite chondrites and aubrites lie on or very close to the TFL, like Earth and the Moon.

HED meteorites have a tightly defined Δ17O of about −0.24 per mil, supporting a single parent body. Martian meteorites lie on their own fractionation line with Δ17O of roughly +0.3. Most carbonaceous chondrite groups plot below the TFL, towards the CCAM line, though CI chondrites are an exception and lie above it.

These differences allow a small sample to be placed with confidence in, or excluded from, a group, and they are essential for ungrouped achondrites.

The Sun's oxygen

Which oxygen composition did the Sun inherit? For decades this was uncertain, because the solar photosphere cannot be measured to the required precision.

NASA's Genesis mission collected solar wind on ultrapure targets between 2001 and 2004. Despite a hard landing when its parachute failed to open, its samples were analysed successfully. The results, published in 2011, showed that the Sun is strongly enriched in oxygen-16 compared with Earth, with a Δ17O of about −28 per mil, close to the most oxygen-16-rich CAIs.

This implies that the rocky planets and most meteorites are depleted in oxygen-16 relative to the bulk Solar System. Whatever process produced the heavy oxygen in planetary materials must have operated early and widely, consistent with self-shielding models in which water ice became enriched in oxygen-17 and oxygen-18 and later exchanged with dust.

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