Δ17O (cap delta O-17)
Δ17O measures how far a sample's oxygen isotope composition lies from Earth's mass-fractionation line, and it identifies meteorite parent bodies.
Δ17O, spoken "cap delta O-17", condenses a sample's oxygen isotope data into one number. A common linear definition is Δ17O = δ17O − 0.52 × δ18O, where the deltas are per mil deviations from standard ocean water. Some laboratories use a slightly different coefficient or a logarithmic form, so precise comparisons need matching conventions.
The value is useful because mass-dependent processes such as melting, evaporation and crystallisation shift δ17O and δ18O together along slope-0.52 lines, leaving Δ17O almost unchanged. It therefore reflects the oxygen reservoir a body formed from, not its later history. Earth, the Moon, enstatite chondrites and aubrites lie at or very close to 0‰. Martian meteorites sit at about +0.3‰, HED meteorites from Vesta at about −0.24‰, and the ordinary chondrite groups are positive, roughly +0.7‰ for H, +1.1‰ for L and +1.3‰ for LL. CV, CO and CK chondrites have strongly negative values, around −4‰.
A new achondrite whose Δ17O matches a known group is strong evidence of shared origin, while an unmatched value can signal a new parent body. Precise values come from laser fluorination analysis of bulk samples.
Related terms
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