Abstract
The thermal history of a mafic xenolith from the Beaunit maar (Massif Central, France) is reconstructedatthe basisofa transmissionelectronmicroscopystudy. The protolith isa meta-microgabbro(opx- 1,cpx-1, pl-1)sampledinthelowercontinentalcrust(T=870-970°C,P 0.7-0.8 GPa).The incorporation inthe basalticmagmaproducedfivereactionsaround orthopyroxene: opx-1 ficpx-2fi cpx-3(augite+highpigeonite) fi liq fi cpx-4. The final reaction is the transformation of residual cpx-3 (augite + high pigeonite) into cpx-5 (augite+lowpigeonite).The calculationofthetimerequiredforeachtransformationyieldsaminimumresidence 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 decreaseas the xenolith is brought up to the surfacein the host basalt. Fracturesobserved in primary minerals are interpreted as a consequence of xenolith shocks against the wall of the magma conduit.
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