Plagioclase–hosted melt inclusions as indicators of inhibited rhyolitic melt beneath a mafic volcano: a case study of the Izu–Omuroyama monogenetic volcano, Japan

Abstract

We conducted textural and chemical analyses of melt inclusions and their host plagioclase crystals in the scoria of the Izu – Omuroyama monogenetic volcano, erupted at ~ 4 ka in the Higashi – Izu monogenetic volcanic fi eld, Japan. The groundmass melt was andesitic with ~ 59 – 61 wt% SiO 2 , and it contained abundant microphenocrysts of olivine and plagioclase. In contrast, ~ 59% of the plagioclase – hosted melt inclusions have rhyolitic compositions with ~ 70 – 75 wt% SiO 2 . The host plagioclase phenocrysts have cores with An# of 44.7 ± 4.2 [An# = 100Ca/(Ca + Na) in mol] and rims with An# of 68 – 78, and the calcic rims have compositions similar to the microphenocrysts. The cores of the host plagioclase phenocrysts have FeO* and K 2 O contents that are in equilibrium with the rhyolitic melt inclusions. Using the plagioclase – melt geohygrometers and assuming temper-atures of 790 – 850 °C, we estimated the H 2 O contents of the rhyolitic melt inclusions to be ~ 4.4 – 10.2 wt%, indicating H 2 O – saturation depths of >4.5 km. Our results suggest that an inhibited reservoir of plagioclase – bearing rhyolitic melt existed beneath the monogenetic volcano at the time of the scoria eruption, which was ~ 800 years earlier than the fi rst rhyolitic eruption in the volcanic fi eld. Plagioclase content in the silicic reservoir is estimated to be less than 35.8%, suggesting the magma was eruptible. Our results demonstrate the potential usefulness of plagioclase – hosted melt inclusions for indicating the existence of such an inhibited silicic magma.