Sulfur-rich melt inclusions in pyroxene phenocrysts of Unzen 1993 dacite: Evidence for excessive volatile supply from mafic magma

In the latest Unzen eruption during 1991-1995, excess volatile supply has been pointed out based on comparison of bulk SO₂ contents of the magma (130-450 ppm) estimated from SO₂ flux by COSPEC measurement and magma extrusion rate ^[1] with S contents of melt inclusions (MI) in quartz and plagioclase phenocrysts and matrix glass (< 50 ppm) ^[2]. In order to reveal the source of excessively supplied volatile, we carried out the microanalysis of MI in pyroxene phenocrysts in 1993 dacite clast. Major compositions for coarse calcic and low-Ca pyroxene phenocrysts in 1993 dacite were ~Wo₃₅En₅₅Fs₁₀ and ~Wo₄En₈₄Fs₁₂, respectively. Pyroxene geothermometer ^[3] yielded 900-980° C for typical pairs of the small grains and 1000-1100° C for large ones. Elemental profiling on a large calcic pyroxene ( ~500 μ m) showed high-Al₂O₃ core ( ~3.5 wt%), suggesting derivation from Al₂O₃-rich less-evolved magma. We found that MI especially in the most Al₂O₃-rich calcic pyroxene core have relatively less SiO₂ (67-68 wt%) and much higher S (600-870 ppm) contents than those in other phenocrysts (SiO₂ = 72-76 wt%; S < 80 ppm). The S-rich MI in pyroxene shows negative correlation in SiO₂ vs. Al₂O₃, K₂O, and S plots, suggesting that MI entrapment could have occurred during magma differentiation by mixing between S-rich less evolved and S-poor evolved magmas. These results suggest that the pyroxene MI could be a likely proxy of volatile source for the latest Unzen eruption. Based on a mass-balance calculation, bulk SO₂ content of extruded magma based on COSPEC results (< 450 ppm) could be explained by mixing between ~70% of near S-free dacitic and ~30% of S-rich mafic magmas. [1] J. Hirabayashi, T. Ohba, K. Nogami, Geophys. Res. Lett. 22, 1709 (1995). [2] Y. Yamaguchi, Proc. Unzen International Workshop, 117 (1997). [3] D. H. Lindsley, Ame. Min. 68, 477 (1983).