A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

Abstract

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H 2 O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and viceversa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and more evolved amphiboles present in the lava flows. Mineral thermobarometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710-790˚C; 5.6-6.5 wt% H 2 O) than their explosive counterparts (815-850˚C; 4.2-4.6 wt% H 2 O). At storage pressures of 1.5-2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at > 5 wt% H 2 O. It is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved gas in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions.

Deep versus shallow sources of CO2 and Rn from a multi-parametric approach: the case of the Nisyros caldera (Aegean Arc, Greece)

“…In order to determine the exhalation rate of 222 Rn- 220 Rn in laboratory, several rock and soil samples from Nisyros caldera were also collected: (1) a rhyolitic pumice belonging to the Upper Pumice succession (sample UP), one of the caldera-forming Plinian eruptive cycles 53 , 54 , (2) a rhyodacitic lava (sample RD) from the post-caldera domes (in particular, the small dome of Lofos), following the Upper Pumice eruption 55 , 56 , (3) the soil from site A7 (sample SA7), (4) the soil from site A24 (sample SA24), and (5) the soil from site C2 (sample SC2). The closed-loop experimental setup is described in ref.…”

The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

“…The presence of a MVP is commonly predicted in recently developed models of mush-dominated magma reservoirs (Edmonds and Wallace, 2017;Parmigiani et al, 2017;Degruyter et al, 2019) and is fundamental in controlling differentiation processes and, in some cases, explosivity in magmatic systems (Anderson et al, 1984;Sisson and Bacon, 1999;Pistone et al, 2015;Degruyter et al, 2017;Bachmann and Huber, 2018;Cassidy et al, 2018;Popa et al, 2019). Decades of careful work describing solubilities and H 2 O-CO 2 concentrations in magmas (Tuttle and Bowen, 1958;Burnham and Jahns, 1962;Dingwell et al, 1984;Holtz et al, 1992;Newman and Lowenstern, 2002;Papale et al, 2006) or tracking exsolution through fluid-mobile trace elements (Webster and Rebbert, 1998;Webster et al, 2020) provide a clear answer to the question of when volatile saturation occurs in a magmatic system's history; intermediate to silicic mushes in the mid to upper crust will become saturated with a MVP starting at low crystal volume…”

The Role of Crystal Accumulation and Cumulate Remobilization in the Formation of Large Zoned Ignimbrites: Insights From the Aso-4 Caldera-forming Eruption, Kyushu, Japan

“…To monitor the accuracy of the measurements, glasses, which were analyzed via electron probe micro analyzer (EPMA), were used as secondary standards (EDS calibrated SEM and EPMA analyses can be found in Supplementary Table S1). A detailed description of the method can be found in (Popa et al, 2019).…”