The occurrence of hydrogen in all of the sylvite samples is anomalous. Many samples of other rocks and minerals analysed in the same laboratory yield no hydrogen (not detected at 0.001% level). Most of the halite samples from the same setting at Boulby similarly yielded no detectable hydrogen. Accordingly, the mean value for hydrogen in the sylvite is far greater than for halite. The anomalous value for hydrogen is consistent with the prediction of hydrogen generation in the sylvite due to irradiation-induced radiolysis. The hydrogen content in the sylvite varies by two orders of magnitude, indicating significant sample heterogeneity, but is comparable with the variation found in other studies of hydrogen incorporation in minerals [9
Experimental irradiation of halite crystals, where no natural source of irradiation is available, similarly generates hydrogen from radiolysis of fluid inclusion water [22
]. The hydrogen is measurable despite the much shorter timescale of irradiation compared with that in a geological environment. This confirms that hydrogen from radiolysis should be expected within the fluid inclusions of potassium evaporite minerals.
Oxygen contents in the Boulby sylvite and halite do not show clear trends. Oxygen is routinely detected in the volatiles released from crushing samples, and is derived from a variety of sources including entrapped atmosphere [24
]. Therefore, oxygen formed from radiolysis is not distinguishable against that from other, more abundant, sources. Similarly, argon formed by the decay of potassium in the sylvite is not distinguishable from that contributed by the atmosphere. Nitrogen, carbon dioxide and other components are similarly entrapped with the original water.
The highest helium contents occur in the samples with the highest hydrogen contents (Figure 3
). Helium is another product of subsurface radioactivity, and a similar association between helium and hydrogen is recorded in the radioactive uranium-rich rocks of the Witwatersrand Basin, South Africa [6
]. However, helium is a product of alpha irradiation from the uranium, and would not be a product of beta irradiation from potassium. Deposits of the Permian Zechstein sea, from which the sylvite was precipitated, are anomalously enriched in uranium [27
]. This is especially evident on the European continent, but is also evident in north-east England, where modern groundwaters through Zechstein-age rocks have relatively high uranium contents [29
]. Uranium is readily adsorbed into iron oxides such as haematite [30
]. Thus, the widespread iron oxide in the Boulby sylvite may contain enough uranium to yield helium by alpha irradiation over a geological timescale. By contrast, the halite does not contain iron oxide, and concomitantly does not yield helium. The halite samples that contain no helium reflect the lack of a source of alpha irradiation in the halite, but some halite samples contain limited helium, which must have migrated from adjacent sylvite or another source of alpha irradiation. Some post-depositional fluid migration is implied by the entrapment of two-phase inclusions in the sylvite.