Figure 3: Example of a Re-Os isochron from the Botneheia Formation, Svalbard generated in this project.
Rhenium (Re) and Osmium (Os) are two heavy metals, both very rare in the geological environment. One of the isotope of Re (E=mc187Re) decays slowly to one of the isotope of Os (187Os) with a half-life of 4160 millions of years. The 187Os content of any given material increases with time depending on the quantity of 187Re present in this material. This forms the basis for the Re-Os isotopic method. Variations of the quantities of 187Re and 187Os, expressed using the 187Re/188Os and 187Os/188Os ratios, can be measured in rocks and minerals that have a wide range of geological age using an elaborate analytical protocol that involves measurement of isotopic ratios with Negative Thermal Ionization Mass Spectrometry (NTIMS).
The Re-Os method intrinsically consists of constructing isochrons in the 187Re/188Os versus 187Os/188Os diagram (Figure 3). A set of samples characterized by an homogeneous 187Os/188Os ratio at the time of formation will be distributed along an isochron today, if the system behaved as a close system since the time of formation. The position of each sample along the isochron is dependent on the 187Re/188Os ratio of this sample.
The isochron is a straight line in the 187Re/188Os versus 187Os/188Os space. It has the following equation: (187Os/188Os) = (187Os/188Os)i+ (187Re/188Os) (eat-1) where (187Os/188Os) and (187Re/188Os) are ratio measured today, (187Os/188Os)i is the initial ratio at the time of formation of the system, a is the decay constant of 187Re (a187Re = 1.666x10-11 year-1)and t is the time passed since the samples formed.
A black shale from the Botneheia formation, Svalbard.Rhenium and osmium have interesting chemical properties. They are siderophile-chalcophile elements enriched in sulfide minerals relative to silicate minerals. They are immobile in reduced environment but mobile in oxidized environment. They are enriched in reduced organic material. This is the reason why they are useful for petroleum geology.
Black shales are fine-grained sedimentary rocks (mudstone) characterized by high content of organic material. They are deposited in oxygen-poor waters (anoxic to euxinic conditions). In such reducing environments, degradation of organic material of biologic origin is slow. Redox-sensitive heavy metals dissolved in the water precipitate at the water-sediment interface and are largely associated with the organic material (kerogen).
Black shales are enriched in heavy metals like uranium, molybdenum and copper. They are also enriched in rhenium and osmium, and these two elements will remain largely associated with the organic material as long as reducing conditions are prevailing. In a well-behaved section of marine black shale, different samples collected in a narrow stratigraphic interval will define an isochron in 187Re/188Os versus 187Os/188Os space. A dissolution procedure using a CrO3-H2SO4 mixture is used to selectively digest the organic material and sulfides in the whole-rock while minimizing analysis of Re and Os hosted in detrital minerals. The initial (187Os/188Os) ratio of the isochron is interpreted as the (187Os/188Os) ratio of the seawater from which the metals precipitated, while time records the timing of sedimentation of the black shale.
These two fundamental parameters, the age and the initial (187Os/188Os) ratio fingerprint, are combined with other stratigraphic, biostratigraphic, geochemical and geochronologic information to characterize the black shale and elaborate the study.
When buried and heated above ca. 60 °C, a black shale matures and becomes a source rock producing hydrocarbons. Upon migrating, the hydrocarbons will carry with them some of the Re and Os from the source rock. Re-Os analyses derived from samples of migrated oil can provide an isochron, if they derive from a single batch of homogenous hydrocarbon. In such a simple system, the isochron will provide the age of migration of the oil and the initial ratio a fingerprint to link the oil batch with potential source rocks.
The Re-Os team along Isfjorden in Svalbard in 2007. From left to right: Judith Hannah, Holly Stein, Bernard Bingen and Wolfgang Weitschat.