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Caldera-forming eruptions are among the most hazardous natural events on Earth and pose a significant risk for global consequences in the future. Recent petrological re-evaluations of pre-, syn-, and post-caldera deposits in several volcanic systems worldwide suggest a cyclic behavior in the evolution of the subvolcanic reservoirs feeding these eruptions. Such caldera cycles are usually subdivided into maturation, fermentation and recovery phases, each characterized by distinct petrological features (e.g. variations in mineral and glass compositions). Combined with trends in intensive parameters and geophysical information about the eruptive behavior of the system, these characteristics can be used to determine the current state of a reservoir within the caldera cycle framework. Here, we test the application of this caldera cycle model on the Aso volcanic complex in Central Kyushu (Japan) by analyzing pre-, syn-, and post-caldera activity between the Aso-4 and Aso-3 events and evaluate the recent activity of Aso based on mineral and glass geochemistry. Widely overlapping compositions in most minerals (here plagioclase and orthopyroxene) and melt, as well as similar ranges in intensive parameters (e.g. magmatic H2O content), make it difficult to pinpoint chemical differences between pre-or post-caldera eruptions. Tracing the MnO/Al2O3 ratio in titanomagnetite, on the other hand, appears as a useful proxy to trace the level of differentiation within the reservoir prior to eruption. Titanomagnetite is an abundant mineral phase known to chemically re-equilibrate fast with melt and hence records "average" conditions prevailing just prior to eruption, with MnO showing a typical incompatible behavior during magma evolution, while Al2O3 is typically compatible. The assessment of different case studies revealed clear evolutionary patterns common in all these caldera cycles with low Mn (associated with low levels of differentiation) during recovery, progressively increasing during maturation, and culminating at highest levels (and associated levels of differentiation) during the fermentation phase, right before the next caldera-forming event. The MnO/Al2O3 ratio in titanomagnetite is analytically easy and quick to determine by electron microprobe, and gives valuable information about the current position of a system within the caldera cycle framework. Based on the evaluation of recent eruptive products in context with the current volcanic activity, we conclude that Aso is presently still in recovery after the massive Aso-4 eruption. Depending on magma supply from depth, the system can either enter renewed matu-ration or activity could decline leading to the cessation of the system., Journal of Volcanology and Geothermal Research, 436, ISSN:0377-0273

Decompression in the magma chamber due to rapid magma extraction triggers caldera collapse. The roof rock overlying the magma chamber is detached by the development of a ring fault, and then it subsides into the magma chamber by the differential pressure between the lithostatic in the roof and magmatic in the chamber. The subsidence of caldera blocks into the magma chamber causes rapid magma extrusion from the magma chamber resulting the eruption of voluminous ignimbrite. Therefore, magma chamber decompression is the key to understanding the evolution of caldera-forming eruptions (CFE). Here, we examine the decompression process of the magma chamber during CFE based on the sequential variation of the volatile contents in the glass inclusions in the products of several CFEs. We find that some CFEs experienced large magma chamber decompression toward the caldera collapse, whereas some start to collapse with minor decompression in the magma chamber. This variation may represent differences in the fracture strength of the roof rock of the magma chamber. Shallow magma chambers with thin roof rock will collapse with small decompression, whereas deep magma chambers with thick roof rock require larger decompression for caldera collapse. Since the decompression of the magma chamber depends on the volume fraction of magma erupted before the caldera collapse, the structure of the magma chamber, such as the thickness of the roof rock, may be used to predict the variation in the eruption patterns towards the onset of caldera collapse and the eruption of voluminous ignimbrites., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Additional file 1: Figure S1. Whole-rock concentrations of major and representative trace elements of the eruptive products of Miyakejima within the last ~ 4000 years plotted against the SiO2 concentration.

Accepted: 2019-01-21, 資料番号: SA1180368000

The Aso-4 caldera-forming event (86.4 ± 1.1 ka, VEI-8) is the second largest volcanic eruption Earth experienced in the past 100 ka. The ignimbrite sheets produced during this event are some of the first ever described compositionally zoned pyroclastic flow deposits exhibiting clear compositional, mineralogical and thermal gradients with stratigraphic position. Large quantities of the deposits are composed of crystal-poor, highly evolved juvenile pumices, while late-erupted pyroclastic flows are in many cases dominated by crystal-rich and less silicic scoria. These petrological gradients in the Aso-4 deposits have been linked to extensive magma mixing of two compositionally distinct magmas in a complex upper crustal reservoir. However, new studies on several other zoned ignimbrites suggest that magma mixing alone is not sufficient to fully explain such strong compositional gradients in the deposits. These gradients are expected to be dominantly caused by the recharge-induced reactivation of extracted melt caps and their complementary cumulate in the upper crust. Here, we investigate bulk rock and matrix glass data with detailed analyses of mineral chemistry in order to re-evaluate the Aso-4 deposits in light of these latest developments. Reverse chemical zoning in phenocrysts, Sr enrichment in euhedral rims of plagioclase and the presence of mafic minerals (clinopyroxene, olivine) indicate recharge of hot, mafic magmas shortly prior to eruption, inducing a mixing signature. However, the marked enrichment in some elements in bulk-rock analyses and the presence of highly evolved minerals (some in the form of glomerocrysts) in the late-erupted, crystal-rich units, provide clear evidence for crystal accumulation in these scoria. Mass balance modeling of P2O5, Sr and SiO2 supports the extraction of melt-rich lenses within an upper crustal mush zone, leaving a partly cumulative evolved crystal residue. We therefore propose an origin of the compositionally zoned Aso-4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt caps within its associated cumulate mush. This complex reservoir was reactivated by mafic recharge shortly prior to eruption, imparting an additional mixing signature to the deposits., Frontiers in Earth Science, 8, ISSN:2296-6463