  Volcanism is a major and long-term source of volatile elements such as C and
H to Earth's atmosphere, likely has been to Venus's atmosphere, and may be for
exoplanets. Models simulating volcanic growth of atmospheres often make one of
two assumptions: either that atmospheric speciation is set by the
high-temperature equilibrium of volcanism; or, that volcanic gases
thermochemically re-equilibrate to the new, lower, temperature of the surface
environment. In the latter case it has been suggested that volcanic atmospheres
may create biosignature false positives. Here, we test the assumptions
underlying such inferences by performing chemical kinetic calculations to
estimate the relaxation timescale of volcanically-derived atmospheres to
thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry
and reaction catalysis. We demonstrate that for planets with volcanic
atmospheres, thermochemical equilibrium over geological timescales can only be
assumed if the atmospheric temperature is above ~700K. Slow chemical kinetics
at lower temperatures inhibit the relaxation of redox-sensitive species to
low-temperature thermochemical equilibrium, precluding the production of two
independent biosignatures through thermochemistry alone: 1. ammonia, and 2. the
co-occurrence of CO$_2$ and CH$_4$ in an atmosphere in the absence of CO. This
supports the use of both biosignatures for detecting life. Quenched at the high
temperature of their degassing, volcanic gases also have speciations
characteristic of those produced from a more oxidized mantle, if interpreted as
being at thermochemical equilibrium. This therefore complicates linking
atmospheres to the interiors of rocky exoplanets, even when their atmospheres
are purely volcanic in origin.
