[S-VC30] 火山・火成活動および長期予測

[SVC30-07] Latent thermodynamic parameter in the chemical compositional variation of basalts of the Fuji volcano, central Japan, identified by unsupervised machine learning

*勝木 悠介^1,2、坂田 周平³、西澤 達治⁴、高橋 正樹⁵、中井 俊一³、中村 仁美²、原口 悟³、岩森 光³ (1.東京大学大学院理学系研究科地球惑星科学専攻、2.国立研究開発法人産業技術総合研究所・地質調査総合センター、3.東京大学地震研究所、4.山梨県富士山科学研究所富士山火山防災研究センター、5.日本大学文理学部自然科学研究所)

キーワード：沈み込み帯火成作用、化学組成、教師なし学習、富士山

Mt. Fuji has erupted voluminous basaltic to basaltic-andesitic lava flows for more than one hundred thousand years. The limited range in composition makes it difficult to elucidate magma processes beneath the Fuji volcano. To resolve the characteristics of such limited compositional variation and to extract statistically independent features hidden in the dataset, we applied an unsupervised machine learning technique, Independent Component Analysis, to the major element compositional dataset of whole rock samples. We identify three independent components (IC1-3) as the compositional vectors. Crystal fractionation simulation by MELTS with a variable H₂O content, pressure, and oxygen fugacity of crystallization was also performed to deduce the geological processes/sources corresponding to the individual ICs. As a result, IC1 represents a vector with increasing Si and decreasing Ti, Fe, which broadly reflects chemical compositional changes with temperature descendant in any pressure, water content, and oxygen fugacity condition. IC2 represents a vector with increasing Al and Ca and decreasing Ti, Fe, K, and P, which broadly reflects the compositional variability associated with different H₂O contents at a similar temperature and oxygen fugacity condition along the crystallization paths. IC3 represents a vector with increasing Si, Ti, Al, Na, K, and P and decreasing Fe and Mg, which reflects the compositional variability associated with different oxygen fugacity at high water content conditions with more than ca. 1wt%. Three independent components identified in this study may correspond to temperature, water content, and oxygen fugacity in the magma reservoirs of Mt. Fuji, respectively.