Crystallization is a fundamental process driving magma evolution and the eruptive styles of active volcanoes. Among other factors, the degree of undercooling (∆T), defined as the difference between the liquidus temperature and the actual crystallization temperature (Td), is a critical parameter that governs crystallization. Clinopyroxene (Cpx), a common constituent of basaltic magmas, is widely used to reconstruct the architecture of plumbing systems and to decipher magma dynamics.
This study experimentally investigates the effect of different ∆T/t paths on Cpx crystallization in Stromboli volcano (Italy), an open-conduit basaltic system characterized by persistent activity regulated by the interplay of two magmas: a volatile-rich (low-porphyritic) LP-magma ascending from depth and a shallower, degassed, (high-porphyritic) HP-magma. The starting material is a basaltic glass (PST9) representing the most primitive LP-magma, whose Di-rich sector-zoned Cpx crystals record the ascent conditions in the conduit.
Time-series kinetic experiments (t = 2.5-20 h) were conducted in an IHPV at ~250 MPa, Td = 1100-1140 °C, under a fO2 near NNO. Both fluid-absent and fluid-present charges were investigated, with melts being systematically hydrous (H2O = 2.3-3.2 wt.% and CO2 = 1581-1717 ppm). The liquidus temperature of PST9, refined from prior experimental studies using phase equilibria experiments, yielded ∆T from 16 to 68°C. However, in kinetic experiments, crystals only formed at ∆T > 20 °C, regardless of t and XCO2. This demonstrates that ∆T, preferentially to t or dissolved CO2, controls the undercooling of volatile-bearing basaltic melts,and that the LP-magma can be significantly undercooled. The formation of Di-rich, sector-zoned Cpx macrocrysts at relatively small ∆T (20-40 °C), resembling natural sector-zoned crystals, confirms previous kinetic interpretations for the crystallization of Stromboli LP-magma.