The H2O Contents in Melts Involved in the Formation of the Karymshina Caldera (Southern Kamchatka)

Abstract

One of the key aspects of the evolution of powerful explosive eruptions is the study of the nature, composition, and water content of magma during the preparation of caldera-forming events. Water is the predominant volatile component of magmas. In felsic melts saturated with fluid, characteristic of large explosive eruptions, its concentration reaches 4‒6 wt % at 800‒900°C and a pressure of 1‒2 kbar, while the CO₂ content does not exceed 0.1% by weight. The Karymshina caldera (Southern Kamchatka), which formed in the early Eopleistocene (∼1.78 Ma), is associated with the eruption of ∼825 km³ of igneous material, including thick ignimbrites (up to 1000 m) and subsequent rhyolitic extrusions. However, data on the behavior of volatile components in magma before eruption remains poorly understood. In this work, melt inclusions (MIs) in quartz from intracaldera ignimbrites and postcaldera extrusions were studied in order to estimate the water content and reconstruct the composition of the initial melt. Despite postentrapment changes (quartz crystallization on the walls of the inclusion, postentrapment overheating and decrepitation), a significant part of the inclusions retained dissolved water (2.9‒5.8 wt % H₂O). The absence of correlation between water loss and decompression suggests that the maximum estimates are close to the initial concentrations. At the same time, ignimbrite quartz contains practically no water, which is probably due to its loss during an explosive eruption due to sudden changes in temperature and pressure. The obtained values of the H₂O content in MIs in the quartz of the Karymshina caldera are in good agreement with the compositions of MIs in quartz from the pumice of powerful Plinian eruptions accompanied by the formation of calderas, which confirms their reliability for reconstructing the conditions in the magmatic reservoir. The results of this study are important for understanding the role of volatile components in the formation of the Karymshina caldera and similar large explosive eruptions.