The origin of large zoned ignimbrites: the case of Aso caldera, Japan
Abstract
Silicic magmas are the most evolved, most viscous and potentially most explosive magmas present on Earth. Despite their importance, the processes leading to accumulation of large amounts of silicic magma in the crust are still a matter of debate. Ignimbrite sheets of large caldera-forming eruptions are interpreted to be unique snapshots of upper crustal magma reservoirs just prior to eruption and hence represent an exceptional possibility to study pre-eruptive magmatic conditions within silicic reservoirs.
The Aso System, in Central Kyushu (Japan), is an archetypical example of a multicyclic caldera-forming volcanic edifice; it was built by four catastrophic caldera-forming eruptions, with the latest (Aso-4) taking place approximately 86.4 ±1.1 ka ago. The ignimbrite sheets produced during the Aso eruptions are some of the first ever described compositionally zoned pyroclastic flow deposits and are up-to-date interpreted to be the result of extensive magma mixing of two compositionally distinct magmas in an upper crustal reservoir. However, new studies imply that magma mixing alone is not sufficient to fully explain strong compositional, mineralogical and thermal gradients as observed in the Aso-4 deposits. Here, we re-evaluate the Aso-4 deposits in light of latest findings relating the origin of voluminous zoned ignimbrites to the reactivation of extracted melt pockets and complementary cumulate left-overs. The relatively scarce presence of mafic minerals indicates recharge of hot, mafic magmas occurring shortly prior to eruption. However, the large amount of crystal-poor, felsic material in early erupted units in combination with late-erupted, crystal-rich units, which are enriched in compatible elements and rich in compositionally highly evolved minerals, again lead to the conclusion that magma mixing alone is not able to explain the complexities observed in Aso-4 deposits. Evidence for crystal accumulation in late-erupted clasts implies the formation of melt-rich lenses within a crystal-rich residue due to significant crystal-melt separation. 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 pockets within a cumulate mush.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV004.0017K
- Keywords:
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- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3640 Igneous petrology;
- MINERALOGY AND PETROLOGY;
- 8410 Geochemical modeling;
- VOLCANOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY