[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-10] Compositional heterogeneity of trace elements at each scale and the distribution of accessory minerals in Inada granite -towards deep earth observation using geoneutrinos-

*Satoru Haraguchi¹, Tsuyoshi Iizuka², Kenta Ueki³, Nozomu Takeuchi¹, Akiko Tanaka⁴, Hiroko Watanabe⁵, Sanshiro Enomoto⁶ (1.Earthquake Research Institute, The University of Tokyo, 2.The University of Tokyo, 3.JAMSTEC, 4.Geological Suivey of Japan, 5.Research Center for Neutrino Science, Tohoku University, 6.University of Washington)

Keywords:Inada granite, accessory minerals, radioactive elements, heterogeneity

In the Grants-in-Aid for Scientific Research from 2015 to 2019 “Interaction and coevolution of the core and mantle”, for the purpose of detecting geoneutrinos derived from radioactive elements in the mantle at the antineutrino detector “KamLAND” located in Kamioka, Hida-city, Gifu Prefecture, we have been working on accurate modeling of the distribution of radioactive elements in the Earth's crust In order to accurately classify geoneutrinos originating from the earth's crust, which has a much higher concentration of radioactive elements than the mantle. Continuing from this project, we are examining the "scale-by-scale" homogeneity and heterogeneity of radioactive elements in the earth's crust and rocks in the GASR from 2024 to 2026 “Advanced modeling of 3-D compositional distribution in the Japan crust for geoneutrino flux prediction to constrain the bulk silicate Earth compositional model”.
As a host phase for radioactive elements, on a macroscopic to magnifying glass scale (about cm to 1/10 mm), accessory minerals containing trace elements are considered likely. Therefore, we performed quantitative and qualitative analysis of thin sections of Inada granite (Kasama-city, Ibaraki Prefecture) using an electron probe microanalyzer (EPMA), focusing on accessory minerals. Regarding trace element analysis using EPMA, Banno (2023) has published a method using standards of trace elements. In this report, major elements were quantified, which is standard method in EPMA analysis. In minerals rich in trace elements, major element analysis shows that the total major element content is significantly lower than 100%, suggesting that trace elements occupy the missing areas. As a result, in addition to well-known minerals containing radioactive elements such as zircon and apatite, allanite was confirmed as a potential host phase for radioactive elements. Allanite is a colored mineral, and when it is associated with biotite, a halo derived from radioactive elements is formed, so it can be easily distinguished by plane polarized in thin-section observation. In addition, allanite in Inada granite is on the order of mm. Therefore, allanite can be easily observed distribution because it can be observed with a low-magnification microscope. In addition, the total amount of major elements in allanite is 60 to 70%, and the rest is mainly LREE in the order of %, but Th oxide are also contained a few %, which can be confirmed by qualitative analysis. Because lizard has these characteristics, it is attracting attention as a host phase for rare earth elements in “granite as a rare earth resource” as pointed out by Ishihara and Murakami (2006) and others, and also as a host phase for radioactive elements. It is noteworthy that mineral amounts and elemental abundances are easy to estimate. In this report, based on the quantitative results of accessory minerals, mainly allanite, we would like to discuss the contribution of accessory minerals as a host phase for radioactive elements, their distribution in rocks, etc.