Oxygen Isotopes and variations of delta 18 O in nature

What are the stable isotopes of oxygen and their abundances?

There are three stable isotopes of oxygen which have the following abundances:

• ¹⁶O: 99.763%
• ¹⁷O: 0.0375%
• ¹⁸O: 0.1995%

The isotope ratio ¹⁸O/¹⁶O is typically measured in oxygen isotope studies, and δ values are calculated from this ratio. There are two main isotopic standards used for these measurements: PDB and SMOW. PDB refers to a belemnite from the Cretaceous Peedee formation in South Carolina, commonly used for carbon isotope measurements. SMOW (Standard Mean Ocean Water) was initially a theoretical water sample with oxygen and hydrogen isotope ratios matching those of standard ocean water. Today, a water standard known as Vienna-SMOW (V-SMOW) is used, distributed by the Atomic Energy Agency in Vienna. V-SMOW has an ¹⁸O/¹⁶O ratio identical to SMOW and a D/H ratio matching the original definition of SMOW.

How are δ¹⁸O values related between V-SMOW and PDB?

δ18OV-SMOW = 1.03091 × δ18OPDB + 30.01

δ18OPDB = 0.97002 × δ18OV-SMOW - 29.98

In addition to SMOW, the Standard Light Antarctic Precipitation (SLAP) is sometimes used, with a δ¹⁸O value of -55.5% relative to SMOW.

How is oxygen liberated for isotope analysis?

Oxygen is liberated from silicates and oxides through fluorination with F₂ or BrF₅, then reduced to CO₂ at high temperatures for measurement in a mass spectrometer. For carbonates, carbon dioxide is liberated using >103% phosphoric acid. When determining oxygen isotope ratios in water, the sample is equilibrated with a small amount of CO₂, and the oxygen isotope ratio in the CO₂ is measured. The known water-CO₂ fractionation factor allows calculation of the ¹⁸O/¹⁶O ratio in water, with a precision of δ¹⁸O values around 0.1-0.2%.

What are the variations of δ¹⁸O in nature?

δ¹⁸O values in nature vary by approximately 100%, with about half of this range found in meteoric water. Chondritic meteorites exhibit a very narrow range of δ¹⁸O values. The Earth’s mantle has a consistent δ¹⁸O value of 5.7 ± 0.3%, which has remained stable over time for both the Earth and the Moon. However, research by Kyser et al. (1982) found that alkali basalts in Hawaii are enriched in δ¹⁸O by 0.5 to 1.0% compared to tholeiites, suggesting distinct mantle sources. There is evidence of minor isotopic heterogeneities within the mantle.

Most granites, metamorphic rocks, and sediments are enriched in δ¹⁸O relative to the mantle, while seawater and meteoric waters are depleted, forming complementary δ¹⁸O reservoirs.