Gas reservoirs in the Dead Sea area: evidence from chemistry of combustion metamorphic rocks in Nabi Musa fossil mud volcano

Nabi Musa located at the northern tip of the Dead Sea at 31°48′ N, 35°25′ E is one of fifteen complexes of the Hatrurim Formation or the so-called “Mottled Zone” (MZ) which are fossil mud volcanoes. Self ignition of methane during their eruptions in the Middle–Late Pleistocene caused combustion metamorphism of sediments. Melting foci have been discovered in two craters of Nabi Musa volcano, with numerous veins of paralavas having particular calcic-silicic compositions (Ca₂SiO₄- and CaSiO₃-normative). Their major- and trace-element spectra bear signature of a mixed sedimentary protolith consisting of Cretaceous marine carbonates, marl, and quartz sand. The paralavas inherit high Sr, P, and U enrichments, positive La/La* and Y anomalies, and a negative Ce/Ce* anomaly from calcareous marine sediments, including bituminous and apatite-rich chalks. The presence of quartz arenite in the protolith is responsible for relatively high Ti, Nb, Zr, and Hf while the marl pelitic component accounts for MREE and LREE depletion. The suggested mixing models predict that the Nabi Musa paralavas result from combustion metamorphism of a sediment mixture with 53–60 wt.% chalk, 5–14 wt.% marl, and 27–44 wt.% quartz arenite. The history of mud volcanism at Nabi Musa began with small eruptions that mobilized gas and water from shallow (within 300 m) Turonian carbonate aquifers, and later explosive activity triggered violent gas blowouts from the older terrigenous reservoir of Aptian–Albian Nubian-type sandstone lying as deep as 1300–1500 m.