Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

Abstract

We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO 2 57 wt%) with a small amount of more silicic pumice (SiO 2 62-63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (preeruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.

Old magma and a new, intrusive trigger: using diffusion chronometry to understand the rapid-onset Calbuco eruption, April 2015 (Southern Chile)

“…Diffusion chronometry has often been used to calculate magmatic timescales associated with mush reservoir processes (e.g., Nakamura 1995;Martin et al 2008;Hartley et al 2016). The rapid interdiffusivity of Fe-Ti in ilmenite and titanomagnetite, in comparison to ionic diffusion in silicate mineral phases (Freer and Hauptman 1978;Rutherford et al 1993; Van Orman and Crispin 2010) at the same intensive magmatic conditions (P, T, and fO 2 ), allows us to use Fe-Ti oxides to understand processes occurring shortly before eruption (Nakamura 1995;Coombs et al 2000;Devine et al 2003;Chertkoff and Gardner 2004;Tomiya et al 2013).…”

“Volcanism in the Chugoku region: a review for the first UNESCO field school on Geoenvironmental disaster reduction”

“…While it is possible to sample and analyze true liquid magma compositions during volcanic eruptions, information about the magmatic plumbing system is only available via geophysical methods. Such information is generally of low resolution, and although at the frontier of current research (Kahl et al 2013;Tomiya et al 2013), it remains difficult to link geophysical information to distinct magmatic processes. On the other hand, many igneous intrusions are the remnants of active arc magma chambers.…”

Petrology of weakly differentiated alkaline, high-level intrusive rocks in the Zahořany-Chotiněves Belt near Litoměřice (Czech Republic)

“…Even though few individual clinopyroxene phenocrysts display weak reverse zoning, commonly indicative of magma mixing (e.g., Nakagawa et al 2002;Streck 2008;Tomiya et al 2013;Rapprich et al 2017b), most of the clinopyroxene crystal population evidenced common trend from Mg-rich (cores of larger phenocrysts) towards Fe-rich (phenocryst rims and groundmass microcrysts) members (Fig. 4).…”