ALBERT

Description: Petrological data have been acquired on natural trachytes from the Chaîne des Puys, French Massif Central, and on experimental products from phase equilibria to (1) constrain the storage conditions of trachytic magmas that lead to explosive eruptions (dome destructions as block-and-ash flows or pumice-and-ash flows) and (2) provide phase relationships and chemical compositions for differentiated alkaline liquids in an intraplate continental context. Phase assemblages, proportions, and compositions have been determined on six trachytes with SiO 2 contents varying from 62 to 69 wt % and alkali contents of 10·5–12·0 wt %. The samples contain up to 30% of phenocrysts, mainly consisting of feldspar (15–17%; plagioclase and/or alkali-feldspar), biotite (2–6%; except in the most SiO 2 -poor sample), Fe–Ti oxides (1–3%) ± amphibole (〈5%), ± clinopyroxene (~1%). All samples have apatite and zircon as minor phases and titanite was found in one sample. Pristine glasses (melt inclusions or residual glasses) in pumice from explosive events are trachytic to rhyolitic (65–73 wt % SiO 2 and 10·5–13·0 wt % alkali). H 2 O dissolved in melt inclusions and the biotite + alkali feldspar + magnetite hygrobarometer both suggest pre-eruptive H 2 O contents up to 8 wt %. These are so far the highest H 2 O contents ever reported for alkaline liquids in an intraplate continental context. Melt inclusions also contain ~3400 ppm chlorine, ~700 ppm fluorine, and ~300 ppm sulphur. Crystallization experiments on the six trachytes were performed between 200 and 400 MPa, 700 and 900°C, at H 2 O saturation, and oxygen fugacity of NNO ± 1. A comparison between the natural and experimental phase assemblage, proportions, and composition suggests magma storage conditions at a pressure of 300–350 MPa (~10–12 km deep), melt H 2 O content ~8 wt % (close to saturation), an oxygen fugacity close to NNO ~0·5, and temperatures increasing from 700 to 825°C with decreasing bulk SiO 2 content of the trachyte. The high H 2 O contents of the trachytes show that wet conditions may prevail during the differentiation of continental alkaline series. Regardless of the size of the magma reservoir assumed to have fed the trachyte eruptions, calculation of thermal relaxation timescales indicates that the tapped magma reservoir(s) are likely to be still partially molten. The four northernmost edifices may correspond to a single large reservoir with a lateral extent of up to 10 km, which could be possibly reactivated in weeks to months if intercepted by new rising mafic magma batches.