Insights into the temporal evolution of magma plumbing systems from compositional zoning in clinopyroxene crystals from the Agnano-Monte Spina Plinian eruption (Campi Flegrei, Italy)

The complex sequential compositional zoning of clinopyroxene crystals erupted <5 ka during the Agnano-Monte Spina (A-MS) eruption, which is considered to be the reference event for a future large-scale explosive eruption at Campi Flegrei caldera (Italy), has been characterized in detail. Concentration profiles (20–800 μm long, spacing 2.5–10 μm) of major (Si, Ti, Al, Fe, Mg and Ca) and minor (Mn, Na, Ni and Cr) elements were measured along different directions in the clinopyroxene crystals. The zoning patterns of Fe–Mg and selected elements (e.g., Al, Ti) often consist of two or more compositional plateaus with both sharp and/or slightly diffuse boundaries between the plateaus. Each compositional population results from a growth stage in a distinct Magmatic Environment (ME), which is identified as a defined set of intensive thermodynamic variables (pressure, temperature, bulk composition and fugacity of fluids including oxygen). The large range of chemical compositions of clinopyroxenes reveals the existence of at least four MEs that are characterized by different compositional populations; two of these are dominant. The variation in zoning pattern from one plateau composition to another fingerprints the transfer of the crystal from one ME to another. In combination with Sr isotopic data and thermobarometric estimates, our systematic characterization of clinopyroxene zoning patterns suggests recharge by deep mafic magmas (ME0: Mg# = 85–92) of an evolved shallow reservoir (ME2: Mg# = 70–78). Such a process also led to the formation of a compositionally intermediate environment (ME1: Mg# = 84–80), that is detected in the clinopyroxene zoning pattern.