ASSESSMENT OF IGNEOUS PROCESSES AT THE MICRON SCALE USING MELT INCLUSION

One of the main goals of studying melt inclusions (MI) is to decipher which types of processes have occurred in a magma reservoir at the micron scale. Recently, several studies that focused on magmatic differentiation of volcanic systems have produced extensive data based on MI. This offers the possibility of studying MI trapped at different times and locations in a plumbing system. The reliability of MI should be tested based on the melt inclusion assemblage (MIA) approach (Bodnar and Student, 2006), that consists of studying and analyzing groups of MI trapped at the same time, and, thus, at the same chemical and physical conditions. However, this approach is hampered by the rarity of MIA in juvenile volcanic phenocrysts.

In this study, we focused on MI data associated with the Campi Flegrei and Procida Island volcanic systems in southern Italy, including data from this study and from the literature. The database included MI hosted in sanidine, clinopyroxene, plagioclase and olivine, and, thus, represent melts trapped at different differentiation stages. We developed a protocol to select the most reliable MI of a dataset associated with a single magmatic system. Comparison of MI data with bulk rock data indicates that major element compositions of MI sometimes span a wider range than do the bulk rocks. Some MI show anomalous compositions and are not representative of the melt in equilibrium with the phenocryst host. After considering only “normal” MI, bubble-free MI show much less variability of volatile contents relative to those recorded by bubble-bearing MI. In particular, H₂O/CO₂ of bubble-free MI increases with increasing Al₂O₃/K₂O suggesting crystallization of an H₂O-CO₂ saturated magma. The normal and bubble-free MI were compared to rhyolite-MELTS simulations assuming a variety of initial conditions. Comparison of MI data with data produced by rhyolite-MELTS shows that one group of MI represents the geochemical evolution of a volatile saturated magma, differentiating by fractional crystallization only. The fractional crystallization can occur either at isobaric condition at ≥200 MPa (≥7km below the Earth’s surface), or polybarically from 200 MPa to 30 MPa (from ~7 km to ~1 km beneath the Campi Flegrei caldera). Another group of MI can only be explained by mixing between two different melts.