Chemical characterization of a magma recharging and mixing before an eruption: Insights from chronologically constrained melt inclusions

Despite the many scientific contributions and advances in our understanding of magmatic processes that have been derived from the analyses of melt inclusions (MI), it is difficult to find examples of chronologically constrained MI, tracking a magma mixing event before an eruption. Here, we studied a single, reversely zoned olivine phenocryst with multiple MI from the Solchiaro eruption on Procida Island (Phlegrean Volcanic District, Southern Italy) to understand both the order of appearance of basaltic versus trachybasaltic magmas and the reliability of MI to record a magma mixing event. Based on phosphorous X-ray mapping, MI are characteristic of three populations: early, intermediate, and late MI. Early MI represent trachybasaltic melts which differentiate by fractional crystallization to form the liquids that are represented by intermediate MI. Late MI are hosted within a forsterite-rich growth band near the olivine rim representing a mixing event between basalts and the pre-existing trachybasaltic magmas shortly before the eruption. The compositions of the three MI populations are compared to a database of bulk rocks and MI from the literature on the mafic products of the Phlegrean Volcanic District to test if the MI studied show same geochemical trends. In addition, in this study, the late MI represented a melt inclusion assemblage (MIA) sensu stricto that represents a group of MI trapped all along the same growth band of the olivine. This simultaneous entrapment offers the opportunity to study the reliability of MI based on Roedder's rules. Correlations between major element contents and the size of late MI suggest that MI ≥22 μm are the most reliable in recording the compositions of the melt at specific times and places in the plumbing system.