  The quantum-to-classical transition through decoherence is a major facet of
the semi-classical analysis of quantum models that are supposed to admit a
classical regime, as quantum gravity should be. A particular problem of
interest is the decoherence of black hole horizons and holographic screens
induced by the bulk-boundary coupling with interior degrees of freedom. Here in
this paper we present a first toy-model, in the context of loop quantum
gravity, for the dynamics of a surface geometry as an open quantum system at
fixed total area. We discuss the resulting decoherence and recoherence and
compare the exact density matrix evolution to the commonly used master equation
approximation {\it \`a la} Lindblad underlining its merits and limitations. The
prospect of this study is to have a clearer understanding of the boundary
decoherence of black hole horizons seen by outside observers.
