Barometry based on mineral phases is potentially a powerful tool to investigate the feeding system of active volcanic edifices, however, this approach is severely limited by large uncertainties in existing models (6-10 km for clinopyroxene-based models). Part of the issue comes from the inadequacy of the experimental database used to calibrate models, and the very large range in compositions that may not be fit with a single model. Alternatively, experimentally constructed phase diagrams provides a much better precision and accuracy, but need to be constructed for every sample of interest, and only for bulk compositions that are true liquids.

We will present an hybrid approach that combines experimental petrology with classical barometric equations to improve our knowledge of magma storage depth underneath a monogenetic volcanic field, the Chaîne des Puys (Massif Central, France). Experiments were designed to crystallize equilibrium clinopyroxene crystals from three compositions representative of the volcanic field (a basanite, a trachybasalt and a trachy-andesite) over a large range of pressure, from 0.5 to 3.0 GPa. Special care was taken to investigate and reduce the sources of pressure uncertainties in piston-cylinder experiments, down to 30-90 MPa depending on pressure. Existing barometric equation were then tested against this new experimental dataset to determine their capacity at reproducing the experimental data. The equation that provided the best fit, in particular regarding the slope of the pressure effect, was then tweaked to best fit the experimental data. The new equation can retrieve pressures with an accuracy of 90 MPa, i.e. 3.4 km in a granitic crust. The analytical precision is 30 MPa.

Application to natural samples confirms the existence of a main storage near the MOHO, with additional mid-crustal storage and possible deeper storage in the shallow mantle.