Using high-resolution Pb isotopes to unravel the petrogenesis of Sakurajima volcano, Japan

Abstract

Sakurajima volcano on the rim of Aira caldera erupts daily, threatening the major nearby population centre of Kagoshima. Before 1955, eruptions have typically consisted of intermittent Plinian and effusive activity, but since 1955, frequent Vulcanian explosions have occurred, indicating a change in pre-eruptive processes. High-resolution Pb isotopes are used here to constrain the components, including crustal assimilants, and petrogenetic processes contributing to the composition of both pre and post 1955 magmas. Sakurajima eruptive products have more radiogenic Pb (206 Pb/ 204 Pb = 18.40-18.42) than nearby Sumiyoshiike basalts (206 Pb/ 204 Pb = 18.24), a proxy for the primitive magma feeding the Sakurajima-Aira system. Sakurajima samples lie along a mixing line between these basalts and locally exposed crustal compositions. Their Pb isotopes are consistent with addition of~5% average crustal melt to the primitive basaltic magma. The narrow range of Pb isotope ratios, despite variable SiO 2 contents (56.6-72.3 wt%), suggests that the final erupted magmas are derived from fractional crystallization of a mafic precursory magma displaying relatively consistent levels of crustal contamination. Andesites erupted between 4-3.7 ka and the 1995 eruption are contaminated to a greater extent, indicating that magmas with distinct compositions can feed phases of activity or individual eruptions. Post 1955 andesitic pyroclastics have lower SiO 2 and higher MgO than older lavas, yet equivalent Pb isotope ratios. The more mafic composition of post 1955 eruptive products can be attributed to increased throughput of mafic magma to the system.

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

“…The lead (Pb) was isolated using a column of precleaned Bio‐Rad AG‐1 X8 (200–400 mesh) using HBr and HCl to elute (Brown et al, 2020). The collected fractions were dried down before being redissolved in 3% HNO 3 .…”

Sulfur Isotope and Trace Element Systematics in Arc Magmas: Seeing through the Degassing via a Melt Inclusion Study of Kyushu Island Volcanoes, Japan

“…S5ab). One component appears to be granite, added via crustal assimilation, as already identified by Pb isotope systematics (Brown et al, 2020). The other two endmembers are the low K2O/TiO2 with the lowest Ba/Nb endmember (S-II), and the low K2O/TiO2 with the highest Ba/Nb (AA-I).…”

“…Reported whole rock compositions of Kyushu Island volcanic rocks are shown as small solid circles for comparison: pink, volcanic rocks along the volcanic front; grey, other volcanic rocks of Kyushu. WR data: Sawamura and Matsui, 1957;Shinno, 1966;Nakamura, 1971;Nakada, 1986;Nakada and Kamata, 1988;Kakubuchi and Matsumoto, 1990;Ohta et al, 1990, 1992, Ohta and Aoki, 1991Iwamori et al, 1991Iwamori et al, , 1992Nagao et al, 1992;Kakubuchi et al, 1995;Imura and Kobayashi, 2001;Kita et al, 2001Kita et al, , 2012Uto et al, 2004;Kawanabe and Sakaguchi, 2005;Sugimoto et al, 2005Sugimoto et al, , 2006Miyoshi et al, 2005Miyoshi et al, , 2008aMiyoshi et al, , b, 2010Miyoshi et al, , 2011Sakuyama et al, 2009Sakuyama et al, , 2014Hoang et al, 2013;Tajima et al, 2013Tajima et al, , 2014Shibata et al, 2013Shibata et al, , 2014Kawanabe et al, 2015;Yamasaki et al, 2016;Kuritani et al, 2017;Brown et al, 2020;Nche et al, 2021. Composition of the primitive mantle is taken from Sun and McDonough (1989).…”