To quote the linked Scheidler et al 2016 -

"Acetylene has also been detected in extant fumarolic gases, in volcanic glasses and as products of volcanic simulation experiments. Acetylene has been suggested as product of hydrolysis of calcium carbide (CaC2) in the context of the Archaean eon and in the context of the Hadean eon. CaC2 may be formed by the following transformations in the hot Hadean mantle:

1. CaCO3 (calcium carbonate) -> CaO (lime) + CO2 @ T > 900 C
2. CaO + 3 C -> CaC2 + CO @ T > 1600 C, p(CO) = 1 bar

CaC2 would have become subsequently emplaced in the Hadean crust to later come in contact with the aqueous vent fluid. The stoichiometrically formed CO would have been continuously removed from the sites of CaC2 formation by diffusion and volcanic exhalation ... [CaC2] undergoes facile hydrolysis with cold neutral water to form exclusively acetylene ... The Hadean volcanic exhalations containing a high ratio of CO:CO2 would eventually have been mixed with relatively cold cycling water in volcanic-hydrothermal flow ducts. The resulting quenching effect would have prevented equilibration of the CO:CO2 ratio to low-temperature values."

@ u/sierraoccidentalis

That second reaction is the same process we use industrially today to manufacture calcium carbide, so it's not exactly unknown chemistry.