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A new study used single amphibole crystals to unravel the triggering mechanism of volcanic eruptions and the nature of the plumbing system beneath intermediate arc volcanoes. The key observation is that a long-lasting silicic crystal mush body kept in a cold storage could be effectively remobilized by intrusion of fresh magma causing significant reheating. Furthermore, the diverse amphibole compositions often recorded in intermediate volcanic rocks do not necessarily indicate crystallization in distinct magma chambers. These results could help to understand the reactivation of long-dormant volcanoes.
The scientists of the MTA-ELTE Volcanology Research Group in Budapest used single amphibole crystals to describe what processes could lead to reawaken of dormant or seemingly inactive volcanoes. The Ciomadul is a dacitic volcano in the Southeast Carpathians, the youngest one in the Carpathian-Pannonian Region. The volcanic activity commenced around 200 ka and the last eruption occurred at 31 ka. This resulted in the development of a lava dome complex with two explosion craters within it. CO2 enamations, seismic and magnetotelluric studies imply that a magmatic body could still exist beneath the volcano.
Amphibole is a ubiquitous mineral in the dacites and shows a large compositional variation. "This is a usual feature of intermediate volcanic rocks, worldwide and is usually interpreted that it suggests crystallization in magma chambers emplaced at different depths beneath the volcano" said Szabolcs Harangi, a professor at the Eötvös University and the head of the research group. "However, we could demonstrate that this is not necessarily valid and the commonly applied geothermobarometers solely based on the amphibole chemistry could lead to misleading results."
This result was published in the March issue of the Contributions to Mineralogy and Petrology.
The Ciomadul dacites are relatively rich in crystals and contain amphibole with diverse textures and compositional zoning patterns. One group of amphiboles has a close relationship with a mineral assemblage typical of cold (700-800oC) silicic magmas (e.g., quartz, plagioclase, K-feldspar, titanite, biotite and zircon), whereas other amphiboles in the same rock occur often coexisting with mafic minerals (clinopyroxene, olivine) suggesting derivation from a hotter (>900oC) magma. An important finding was, however, the recognition of single composite amphibole crystals, which consist of "cold" core and "hot" margin.
"This feature can be readily explained by reheating of a silicic crystal mush stored in a cold storage." added Balázs Kiss, the senior author of this study. "We carefully compared the compositional range of the amphiboles with experimental data and then applied the most appropriate thermometer. We got a significant increase of crystallization temperature - exceeding 200oC - from the core to the rim of the crystal. That clearly suggests intrusion of a hot basaltic magma batch into a silicic crystal mush body. However, there was a major problem with the estimation of the crystallization pressure applying the most commonly used geobarometer, i.e., the margin of the crystals should have crystallized at least 10 km deeper than the crystal cores."
This is of course, physically unrealistic and required another solution than applying just the widely used amphibole thermobarometer. The researchers concluded that these specific crystals could have formed in a single magma chamber and this condition might be also valid beneath other andesitic-dacitic volcanoes such as Pinatubo, Redoubt, Unzen, Soufriére Hills and Mt. Pelée.
"We emphasize that amphibole perspective studies could provide key-information on the pre-eruptive magma chamber processes and the nature of the plumbing system that are important in the forecast of volcanic eruptions." said Kiss. "But, as our single crystal-based interpretations show, reliable results could be achieved only after careful textural and compositional investigation, comparison with experimental results and the routine applications of thermobarometers solely based on the composition of the amphiboles could lead to false interpretation.
The Ciomadul is a long-dormant volcano, but as this new study suggests the seemingly inactive stage could quickly change. "The 730-750oC temperature calculated for the low-alumina amphiboles implies that the silicic crystal mush might have been in a cold storage for a long time and the magmatic body was close to the solidification. However, an important point is that it still contained about 10-20 vol% melt. This condition enabled this magmatic body to become effectively re-melted when a hot fresh magma body arrived from the mantle." added Harangi.
This means that as long as a crystal mush body with some melt fraction exists beneath dormant volcanoes there is the potential for reawakening. The research group now interprets the data derived from a magnetotelluric survey conducted in the Ciomadul volcano.
Journal Reference:
Kiss, B., Harangi, S., Ntaflos, T., Mason, P.D. and Pál-Molnár, E. Amphibole perspective to unravel pre-eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes: a case study from Ciomadul volcano (SE Carpathians). Contributions to Mineralogy and Petrology 2014, 167/3:986; DOI: 10.1007/s00410-014-0986-6 (http://link.springer.com/article/10.1007/s00410-014-0986-6)
MTA-ELTE Volcanology Research Group - http://vulkanologia.elte.hu/
26th March 2014.
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