Magma storage conditions as key factors in the genesis of pure Plinian vs. caldera-forming eruptions: Experimental constraints

Understanding the physico-chemical conditions governing magma plumbing systems is one of the central objectives in volcanological research, as eruptive styles and associated phenomena are strongly influenced by pressure, temperature, and volatile content in magmatic reservoirs. However, precisely constraining pre-eruptive pressure conditions remains challenging. Experimental investigations on phase relationships and stability fields of pressure- and volatile-sensitive mineral phases can provide insightful information to address this issue. Recent geological surveys on Ventotene Island (Pontian Islands, Tyrrhenian Sea) revealed the presence of primary analcime in the Cala Battaglia Unit (UCB), a sequence of pure Plinian fallouts  lacking pyroclastic density currents (PDCs) deposits. In contrast, analcime is absent in both Plinian fallout and PDC deposits related to the Parata Grande caldera-forming eruption. Since analcime stability is generally constrained to PH₂O > 200 MPa, these observations point to significant differences in pre-eruptive storage pressure conditions, highlighting the fundamental role of pressure in controlling phase relations, mineral stability, and therefore eruptive style. Phase equilibria experiments were performed using a piston-cylinder apparatus to investigate the role of pressure and volatile content (H₂O) on phase relations in differentiated alkaline magmas. Two starting compositions representative of two eruptive units were selected for the experimental runs: a tephriphonolite (MD1, Parata Grande) and a trachy-phonolite (UCB2, Cala di Battaglia). Experiments were performed under H₂O-undersaturated to H₂O-oversaturated conditions at pressures of 150, 300, and 600 MPa, and temperatures between 700 and 1000 °C. For the MD1 tephriphonolite at 600 MPa, the mineral assemblage consists of clinopyroxene + apatite + oxides at 1050 °C, followed by biotite, plagioclase, and K-feldspar with decreasing temperature, whereas under H₂O-oversaturated conditions at 950 °C the assemblage is dominated by biotite, clinopyroxene, oxides, and apatite. At 300 MPa, all experiments were conducted under H₂O-saturated conditions, and the mineral assemblages are dominated by clinopyroxene, biotite, and oxides. For the UCB2 trachy-phonolite, experiments at 600 MPa show the crystallization of a hydrous feldspathoid associated with plagioclase and biotite at 900 and 850 °C, followed by K-feldspar at lower temperature (750 °C). In contrast, hydrous feldspathoids do not crystallize at 150 MPa, where the mineral assemblage is limited to K-feldspar, plagioclase, biotite, and oxides. At 300 MPa, the assemblage is dominated by K-feldspar and plagioclase, with subordinate biotite and oxides. These results suggest that hydrous feldspathoid cannot crystallize from H₂O-saturated trachy-phonolitic magmas at pressures ≤150 MPa, emphasizing how pressure variations can affect phase equilibria. This evidence supports the hypothesis of a polybaric differentiation path for the Cala Battaglia plumbing system, leading to pure Plinian events, in contrast to a shallower and isobaric evolution for the Parata Grande system, leading to under-pressure caldera-forming events.