Mass spectrometry and oxygen isotopes

Mass spectrometers measure isotope ratios to within about ten parts per million. Oxygen's three isotopes then sort meteorites into families and link them to their parent bodies.

Time of flight

In a time-of-flight mass spectrometer, a sample is ionised, for example by electrospray or by electron impact, to form positive ions. The ions are accelerated by an electric field so that all ions of the same charge gain the same kinetic energy, ½mv² = zeV. They then drift through a field-free region. Lighter ions travel faster and reach the detector first; time of flight is proportional to √(m/z). The detector current gives each ion's abundance.

Isotope work on meteorites mostly uses magnetic-sector instruments, which separate ions by deflection rather than flight time, but the principle of measuring abundance by mass-to-charge ratio is the same.

Fractionation

Physical and chemical processes such as evaporation and crystallisation separate isotopes slightly by mass. Ratios are expressed in delta notation, the difference in parts per thousand (‰) from a standard, ocean water: δ¹⁸O = [(¹⁸O/¹⁶O)_sample ÷ (¹⁸O/¹⁶O)_standard − 1] × 1,000, and δ¹⁷O likewise.

Because ¹⁷O differs from ¹⁶O by one mass unit and ¹⁸O by two, mass-dependent processes change δ¹⁷O by about 0.52 times as much as δ¹⁸O. All Earth rocks therefore plot on a line of slope about 0.52, the terrestrial fractionation line.

Δ¹⁷O: the fingerprint

The deviation from that line, Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O, is unchanged by mass-dependent processes, so all rocks from a well-mixed parent body share the same Δ¹⁷O. Earth and the Moon have Δ¹⁷O close to 0‰. Martian meteorites have about +0.3‰, HED meteorites about −0.24‰, and the various chondrite groups each have their own range. CAIs and many carbonaceous chondrites plot far below the line, recording oxygen reservoirs that existed in the solar nebula before it was homogenised.

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