The application of Laser Scanning Confocal Microscopy to the study of coronitic microstructure from a gabbronorite xenolith

Laser Scanning Confocal Microscopy observations have been carried out on a standard polished thin section of a gabbronoritic xenolith recovered from the Tertiary volcanics of the Lessini Mountains (Veneto Volcanic Province, NE- Italy). We obtained high-resolution 2D images of Fe-rich olivine + augitic clinopyroxene + alkali-feldspar aggregates rimming orthopyroxene. These aggregates (corona), which originated during the reaction between orthopyroxene and the host lava, represent partial pseudomorphs after orthopyroxene. Optical sectioning along the Z-axis allowed the acquisition of an image stack eventually used to produce a 3D image of the corona. Digital image manipulation can obtain, with little effort, the micro-topography of the targeted microstructure: compared to the centre of the orthopy- roxene, the corona shows troughs in correspondence of alkali-feldspar grains. These troughs represent voids within the reaction corona, i.e. porosity formed during reaction between orthopyroxene and the host lava. The development of porosity in igneous rock is tentatively related to the molar volume decrease of the corona minerals compared to the parent orthopyroxene. Alternatively, more orthopyroxene dissolved into the host lava than corona phases crystallised. The 3D visualization of microporosity in magmatic (and metamorphic) rocks is potentially important for the charac- terization of unconventional water and/or hydrocarbons reservoirs.