Original paper

Nano-petrographic investigation of a mafic xenolith (maar de Beaunit, Massif Central, France)

Faure, François; Trolliard, Gilles; Montel, Jean-Marc; Nicollet, Christian


The thermal history of a mafic xenolith from the Beaunit maar (Massif Central, France) is reconstructed at the basis of a transmission electron microscopy study. The protolith is a meta-microgabbro (opx1, cpx-1, pl-1) sampled in the lower continental crust (T = 870-970°C, P ≈ 0.7-0.8 GPa). The incorporation in the basaltic magma produced five reactions around orthopyroxene: opx-1 → cpx-2 → cpx-3 (augite + high pigeonite) → liq → cpx-4. The final reaction is the transformation of residual cpx-3 (augite + high pigeonite) into cpx-5 (augite + low pigeonite). The calculation of the time required for each transformation yields a minimum residence time of the enclave in the host magma of 16 hours and a magma ascent velocity of 1.8 km.h-1. Exsolutions are produced by pressure decrease as the xenolith is brought up to the surface in the host basalt. Fractures observed in primary minerals are interpreted as a consequence of xenolith shocks against the wall of the magma conduit.


xenolithtemexsolutionpyroxeneascent velocity