Abstract. In order to better understand both the fixation and
migration of gases in evaporites, investigations were performed in five
horizontal boreholes drilled in an underground potash seam. One of the five
boreholes was pressurised with Ar and the pressure signal and chemical gas
composition were then monitored in the other holes. A further gas sample
from a separate borehole was characterised for the chemical composition and
for noble gas and carbon isotopic compositions to conclude on the origin and
evolution of the gas in the salt rocks. Additionally, in order to determine
the total gas amount in the salt rocks, a potash-bearing salt sample was
dissolved in water and from the mass of 1 kg salt sample,
9 cm(STP)3 gas was liberated.
Due to the relatively large permeability of the disturbed salt rocks
(4×10-17 to 4×10-18 m2), which is about 3–4 orders
of magnitude higher than in undisturbed salt rocks, we assume that the
migration of injected Ar most likely takes place along micro-cracks produced during the mining process. The geogenic gas concentrations found in the
observation holes correlate directly to the Ar concentration, suggesting
that they were stripped from the rocks in between the holes.
According to the He-isotopes (0.092 Ra), a small contribution of mantle gas can be found in the geogenic salt gas. The δ13CCO2-isotopic composition (−7.8 ‰ to 6.7 ‰) indicates a magmatic source, whereas
13C∕12C of CH4 (−22.2 ‰ to −21.3 ‰)
is typical for a thermogenic gas. We assume that
CO2 and CH4 are related to volcanic activity, where they
isotopically equilibrated at temperatures of 513
to 519 ∘C about 15–16 Ma ago.
The Heavy Noble Gas (Kr, Xe) Composition of the Galapagos Hotspot: Insights into the Origin and Evolution of Mantle Volatiles
