Helium trapped in historical slags: a search for temporal variation of the He isotopic composition of air

In an attempt to investigate short-term variations of the isotopic composition of atmospheric helium, we have analyzed He isotopic ratios of gases trapped in metallurgical slags from France and United Arab Emirates covering the historical period 900–1991 AD. These slags have been produced in blast- and shaft-furnaces, which are characterized by different temperatures (∼1300°C and ∼1600°C, respectively) and ventilation conditions (open air and compressed air, respectively). The slags contain 100–500 μm vesicles believed to contain air trapped at the epoch of processing. Results for shaft-furnace slags indicate that the melts were not fully equilibrated with atmospheric helium. Crush-extracted He from blast-furnace samples indicate better homogeneity and, in the case of a He-rich sample processed in 1500 AD, show the occurrence of a He component presenting a 3He excess of 3.6±1.2% relative to the isotopic composition of atmospheric helium sampled in 1995. Among possible sources and sinks of He isotopes, the release of crustal, 4He-rich, helium as the result of anthropogenic activity, mainly hydrocarbon exploitation, is consistent with a temporal change of the He isotopic composition of air between the 16th century and the present-day. The present-day rate of change of the atmospheric He isotopic ratio is estimated to be −0.014±0.007%/yr.