Analog experiments on melting and contamination at the roof and walls of magma chambers

A series of analogue laboratory experiments were carried out to model melting and mixing processes occurring at the roof and sides of a magma chamber when hot, dense mantle-derived magmas are emplaced in the deep crust. The experiments investigated the influence of a viscosity contrast between crustal melts and input magma, the presence of crystals in the input magma, and roof geometry. The crust was modelled by aqueous ice or a cold, low-melting point wax, arranged as a flat roof or in an upside-down U. The input magma was modelled by a hot, dense aqueous solution of NaNO3 or FeSO4, which was injected into the cavity between the walls. The experiments revealed the potentially important role that side-wall processes may have in crustal melting and mantle magma contamination in this environment. The walls melted back much faster than the roof and the melts from the opposite walls ponded next to each other under the roof and mixed with each other only in the central region. When the input solution was able to crystallise, it did so preferentially in the active side-wall boundary layers. The tiny, dense crystals, which were dragged up in the melt boundary layer and then dropped into the interior fluid, enhanced hybridisation of the melt and contamination of the input fluid and, through their influence on the boundary layer structure, reduced the melting rate.