Magma–serpentinite interaction as the origin of diatremes: a case study from the Hyblean Plateau (southeastern Sicily)

Some diatremes formed in the Hyblean Plateau in a time span from 6.5 to 9.4 Ma, producing volcaniclastic deposits, which host deep-seated xenoliths. The origin of the explosive activity that has brecciated the magma and the country rocks is controversial, two are the prevalent models: phreatomagmatic and magmatic brecciation. We propose an alternative explanation based on the thermal and mechanical interaction of a rising basaltic magma with serpentinized mantle rocks, representing the main lithologies in the Permo–Triassic Hyblean basement. We suggest that magma–serpentinite interaction could have remobilized fluids from serpentinites that reached the overpressure necessary for the brecciation of country rocks. The possible depth of this interaction has been estimated considering variations of porosity and permeability at different depths in the Hyblean lithosphere. These properties have been taken into account in the dimensionless parameter V e (Jamtveit et al. in Physical geology of high-level magmatic systems, vol 234. Geological Society, London, Special Publications, London, pp 233–241, 2004), which reflects the relative rates of heat and fluid transport, since a high fluid flux is enhanced by a high boiling temperature of pore water raised by magma–rock heat exchange. V e remains less than 1 up to a depth of 7 km, corresponding to the average thickness of the sedimentary-volcanic sequence, characterized by a high permeability (k = 10−12–10−14 m2) that inhibits fluid overpressure built up, in contrast to both phreatomagmatic and magmatic models. At deeper levels (k ~ 10−19 m2), from 8 km downward, the value of V e is greater than 1. Therefore, we propose that the dehydration of serpentinites, upon heating by hot basaltic magma, would have liberated huge amounts of fluids whose accumulation at an average depth of 8 km beneath the impermeable serpentinized level induced supercritical fluid overpressure, strong enough to exceed the lithospheric pressure. Fluid overpressure caused the opening of fractures in the overlying permeable upper crust, triggering the energetic fluid expansion and hence the brecciation of country rocks. Finally, we suppose that aforementioned conditions for supercritical fluid overpressure could have been gained in other areas of the Hyblean region in coincidence with various Mesozoic volcanic events, even though older diatremes may have been obliterated by subsequent sedimentary cycles and volcanic events, as well as in serpentinite diatremes in the Navajo Volcanic Field (Colorado Plateau, SW USA).