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Evidence for a ‘third’ endmember of the Unzen 1991–1995 eruption from amphibole thermometry and crystal clots
Journal of Volcanology and Geothermal Research ( IF 2.9 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.jvolgeores.2020.106833
Kurumi Iwahashi , Hidemi Ishibashi , Atsushi Yasuda , Natsumi Hokanishi

Abstract Amphibole phenocrysts and crystal clots composed of amphibole, plagioclase, and interstitial glass were observed in dacitic products of the Unzen 1991–1995 eruption and analyzed in order to understand pre-eruptive magmatic processes. Amphibole phenocrysts in the samples erupted at different times are classified by composition as either magnesiohornblendes or tschermakites. Temperatures of 770–830 °C are estimated from magnesiohornblende phenocrysts and amphiboles in the crystal clots, which are consistent with the proposed temperature of the silicic endmember magma. The average approximate composition of the interstitial melt is 67–73 wt% SiO2, ~0.25 wt% TiO2, ~12 wt% Al2O3, ~1.1 wt% FeO, 0.14 wt% MgO, 0.80 wt% CaO, 3.0 wt% Na2O, and 4.0 wt% K2O. The SiO2 plus H2O content (~8.8 wt%) of the melt estimated from these amphiboles (SiO2melt = 71–72 wt%) is consistent with the compositions of interstitial melts observed in the crystal clots. The H2O saturation depth estimated for the interstitial melt and plagioclase compositions is consistent with that of the magma chamber where the silicic endmember magma was likely stored (11–15 km). These results suggest that interstitial melts in crystal clots represent the silicic endmember melt. Temperatures estimated from tschermakite phenocrysts are 870–950 °C, which is lower than the proposed temperature of the mafic endmember magma. In addition, a temperature gap is observed between the tschermakites and magnesiohornblendes. These results suggest the contribution of a previously unrecognized third endmember magma to the 1991–1995 magma, which we term the mid-temperature magma (MT magma).

中文翻译:

从闪石温度测量和晶体凝块证明云仙 1991-1995 喷发“第三”端元的证据

摘要 为了了解喷发前的岩浆过程,在云仙 1991-1995 喷发的英安岩产物中观察到了闪石斑晶和由闪石、斜长石和间隙玻璃组成的晶体凝块。在不同时间喷发的样品中的角闪石斑晶按成分分类为镁角闪石或闪闪石。从晶块中的镁角闪石斑晶和角闪石估计温度为 770–830 °C,这与硅质端元岩浆的建议温度一致。间隙熔体的平均近似组成为 67–73 wt% SiO2、~0.25 wt% TiO2、~12 wt% Al2O3、~1.1 wt% FeO、0.14 wt% MgO、0.80 wt% CaO、3.0 wt% Na2O 和4.0 重量% K2O。SiO2 加 H2O 的含量(~8. 从这些角闪石(SiO2melt = 71–72 wt%)估计的熔体的 8 wt% 与在晶体凝块中观察到的间隙熔体的组成一致。为间隙熔体和斜长石成分估计的 H2O 饱和深度与可能储存硅质端元岩浆的岩浆房(11-15 公里)一致。这些结果表明晶体块中的间隙熔体代表硅质端元熔体。从 tschermakite 斑晶估计的温度为 870-950 °C,低于镁铁质端元岩浆的建议温度。此外,在tschemakites 和magnesiohornblendes 之间观察到温度差距。这些结果表明以前未被认识的第三端元岩浆对 1991-1995 岩浆的贡献,
更新日期:2020-05-01
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