Common origin of plagioclase in the last three eruptions of Unzen volcano, Japan

Abstract

Megacrysts (large crystals of 2–15 mm in length) of plagioclases extracted from the lavas of the last three eruptions of southwestern Japan's Unzen volcano (AD 1663, AD 1792 and AD 1991–1995) have limited 87Sr/ 86Sr ratios of 0.70439–0.70454. Results of micro-drilling analyses indicated that three of eight megacrysts showed a gradual decrease in 87Sr/ 86Sr ratios from the core to the rim, whereas other megacrysts showed a homogeneous 87Sr/ 86Sr ratio. This relative homogeneity contrasts sharply to the wide variation of 87Sr/ 86Sr ratios of matrixes of the lavas of the last three eruptions (0.71417, 0.70467, 0.70447–0.70450, respectively). Most megacrysts show isotopic disequilibrium between their outer rims and their matrixes, suggesting that they did not grow in the host magmas. In addition, seven of the eight megacrysts of plagioclase also have similar trace element abundance ratios (La/Nd and Sr/Ba). Their similar chemical and isotopic compositions suggest that they crystallized from the same parent magma, which suggests to us that they were formed prior to or during the eruption in 1663. The isotopic ratios of the 1663 eruption lavas of 0.71417, however, rules out the possibility that it is the parent magma for the plagioclase megacrysts. The lavas erupted 4000 and 5000 years ago have 87Sr/ 86Sr ratios of 0.70454 and 0.70442, respectively, [Chen, C.-H., DePaolo, D.J., Nakada, S., Shieh, Y.-N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831–834]; they are inferred to be of the parent magma because of their Sr isotope ratios. Consequently, the micro-analytical results seem to suggest that plagioclases with a single origin can be supplied to volcanic products of several different eruptions surviving several eruption events.