Isotopes

A number of elements occur in nature as different isotopes: the atomic number (protons) is constant but there are different numbers of neutrons. They therefore have the same chemical properties although their masses are slightly different. Isotopes which are radioactive (unstable) break down at a specific rate characteristic for the isotope species (the disintegration constant). By analysing the reaction products formed in the minerals they can be dated.

The 87Rb −87Sr and the 40K −40Ar methods are the ones most commonly used in determining the age of rocks. The ratios between lead isotopes can also be employed because of the 235U −207Pb, 238U −206Pb and 238Th −208Pb reactions. Dating sedimentary rocks is a complicated procedure and the results are often difficult to interpret. The main problem is that clastic sediments are comprised of fragments and minerals which have been eroded from older rocks and the measured radiometric age may be strongly influenced by the age of these source rocks.

Separating the newly formed (authigenic) mineral to be dated, can be particularly challenging. The fact that isotopes have different masses causes fractionation to take place through both chemical and biological processes. The simplest example is water, H2O, which contains two oxygen isotopes and two hydrogen isotopes. The oxygen isotopes are fractionated through evaporation, with more H16 2 O evaporating than H18 2 O. This is because the 18O isotope has greater mass and a phase change from fluid to vapour therefore requires more energy. H16 2 O has higher vapour pressure than H18 2 O. This is the reason why rainwater and ice contain less 18O than seawater.

Isotope fractionation is a function of temperature, however, and is much more effective with evaporation at low temperatures than at high ones. The explanation for this is that at high temperatures the energy of the molecules are so great that the difference in mass between 18O and 16O is of less consequence. At low temperatures the isotopic separation evaporation is much more selective so that the water evaporated is more enriched in 16O. When water vapour condenses to rainwater, molecules with 18O are most stable.

Rain and snow becomes enriched in the heavier isotope ( 18O), so that the water vapour remaining in the air becomes more enriched in 16O. Most of the evaporation takes place at low latitudes and the water vapour in the air has a progressively lower 18O-content towards higher latitudes as the air cools and it rains. The concentration of oxygen isotopes is expressed in relation to a standard:

This standard may be the average composition of seawater, called SMOW (Standard Mean Ocean Water). Another commonly used standard is PDB (Pee Dee Belemnite), which is the composition of calcite in a Cretaceous belemnite. The calcite (CaCO3) was precipitated in the sea and its composition was in equilibrium with the seawater at normal temperatures (15–20◦C). There is more 18O in calcite than in the water (positive fractionation), but with higher temperatures the less effective fractionation of oxygen lowers the δ18O values.

PDB values are preferred for carbonate minerals while the SMOW scale is mainly used for water samples and silicate minerals. Hydrogen has two stable isotopes, 1H and 2H (deuterium), and an unstable one, 3H (tritium), which has a half-life of 12 years. The hydrogen isotopes are even more strongly fractionated than oxygen isotopes during evaporation. Water molecules with deuterium (heavy water) have lower vapour pressure that water moles with hydrogen.

In meteoric water there is a linear relation between the deuterium/hydrogen ratio (D/H) and the δ18O. The isotopic composition of seawater has varied through geological time, though not so much during the last 200–300 million years. During glacial periods, seawater acquires more positive δ18O values because the water bound as ice has more negative δ18O values.

Rainwater (meteoric water) has normal δ18O values from –2 to –15. The values become more negative towards higher latitudes, and near the poles one can measure δ18O values of about –50 and δD (2H) values close to –350 (see Fig. 3.4). Minerals that form in seawater show decreased 18O/16O ratios with increased ambient temperature during formation. The δ18O/16O ratio in carbonate secreting marine organisms, for example, is thus a function of both temperature and salinity. The seawater changes its δ18O values by around 1–1.5‰. Isotopes can thus provide important proxy evidence for palaeoclimate studies.