Volatile Evolution of Magma Associated with the Solchiaro Eruption in the Phlegrean Volcanic District (Italy)

摘要

More than 1·5 million people live in or near the Phlegrean Volcanic District (PVD) in southern Italy, which represents one of the most carefully studied volcanic hazard areas in the world. Throughout its history, the style of volcanic activity has varied greatly, from relatively quiescent lava flows to explosive phreatomagmatic eruptions. The goal of this study is to develop a more detailed understanding of the physical and chemical processes associated with the Solchiaro eruption in the PVD. The PVD includes three volcanic fields: the Campi Flegrei (CF) caldera and the volcanic islands of Ischia and Procida. The Solchiaro eruption on the island of Procida is one of the few primitive (less evolved) eruptions in the PVD and can provide information on the source of the more evolved magmas associated with this volcanic system. One of the more important chemical parameters that determine the style of volcanic eruptions is the volatile budget of the magma before and during eruption. Melt inclusions (MI) provide the most direct information on the volatile contents of the pre-eruptive melt in the source region for the PVD. The composition of the melt phase before eruption was determined by analyzing the major, minor and trace element and volatile contents of 109 MI in olivine from four samples of the Solchiaro eruption, representing different stratigraphic heights in the deposits and, therefore, different relative times of eruption. Olivine compositions vary from Fo₈₂ to Fo₈₈, with one maximum value of Fo₉₀. The compositions of the MI in olivine were corrected for post-entrapment crystallization (PEC) and for Fe loss by diffusion. Most (97 out of 109) of the MI studied are classified as ‘normal’ MI because they show chemical evolution trends consistent with that of bulk-rocks from the PVD. Two types of anomalous MI were also recognized based on their major and trace element compositions: (1) Sr-rich MI, and (2) enriched MI that are variably enriched in TiO₂, K₂O, P₂O₅, large ion lithophile elements, high field strength elements and rare earth elements relative to ‘normal’ MI. These MI probably originated from dissolution–reaction–mixing processes in the mush zone of the magma body. ‘Normal’ MI include both bubble-bearing and bubble-free (containing only glass ± trapped chromite) types. Bubble-free MI most closely record the pre-eruptive volatile content of the melt over a range of temporal and spatial conditions. The observed trends in CO₂ contents of MI versus crystallization indicators (e.g. Al₂O₃/CaO) support the interpretation that variations in the volatile contents of bubble-free MI reflect real variations in the volatile budget of the melt during the evolution of the magma. The correlation between CO₂ contents of MI and the relative stratigraphic position of each sample is consistent with eruption of a volatile-saturated magma that initially ascended through the crust from an original depth of at least 8 km. The magma ponded at 4–2 km depth prior to eruption and crystallization and the concomitant volatile exsolution from the saturated melt in the shallow chamber triggered the Solchiaro eruption. As the eruption proceeded, the Solchiaro magma continued to ascend through the crust to a final storage depth of about 1 km.