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The formation process of myrmekites in granitic rocks can help us to understand the mass transfer between minerals and hydrothermal fluids during the deuteric stage. The Toki granite, central Japan, has three types of myrmekites. Type A myrmekite is defined as a single layer. Type B myrmekite shows a composite texture consisting of two layers, namely, a myrmekite layer and an albite (Ab)-rich layer that is free of vermicular quartz. Type C has a composite texture with the following three layers: two myrmekite layers separated by one Ab-rich layer. Micropores are found in these myrmekites in the undeformed granite, which enable quantitative determinations of the volume decrease during myrmekite formation by measurement of the area of micropores. The areal relationship between the micropores and vermicular quartz in the myrmekites exhibited a high correlation (R2 = 0.8352), thus indicating that the genesis of the micropores is evidently related to myrmekite formation. We derived the reaction equations for myrmekite formation based on the singular value decomposition method. The matrices for singular value decomposition involve the following volume factors: volume change during the reaction and volume ratios of the product minerals. The singular value decomposition indicates that the myrmekites are produced through the consumption of plagioclase and K-feldspar with an inflow of H4SiO4, Na+, and H+ from the hydrothermal fluid, accompanied by an outflow of Al3+, Ca2+, and K+ into the fluid, which constitute essential mass transfers during myrmekite formation. The difference of pH in the hydrothermal fluid and inflow amounts of H4SiO4 and Na+ can explain the reason why micropores occur in the myrmekites but not in the Ab-rich rim (layer); the smaller inflow of H4SiO4 and Na+ from the hydrothermal fluid with lower pH conditions yielded micropore production during myrmekitization, and the larger inflow of H4SiO4 and Na+ from the hydrothermal fluid with higher pH conditions yielded the formation of the Ab-rich rim (layer) with few micropores. The sequential variations in the chemical characteristics of the hydrothermal fluid during the sub-solidus conditions were characterized by a gradual decrease in H4SiO4 (Si4+), Fe2+, Mn2+, Mg2+, and Na+ and a gradual increase in Ca2+, K+, H+, and F–.

This study, along with our previous studies (Yuguchi et al. 2015, 2019a), reveals the hydrothermal alteration processes in a pluton, with a focus on the mass transfer between minerals and hydrothermal fluid. It also depicts the sequential variations in fluid chemistry as alteration progresses. Hydrothermal alteration of the Toki granite in Tono, Japan—the study area of this research—progressed through the successive processes of chloritization, plagioclase alteration, and precipitation of a carbonate. This paper describes the alteration processes of hornblende chloritization, K-feldspar chloritization, and the formation of fracture-filling chlorite through petrography and mineral chemistry. A set of singular value decomposition analyses was conducted to obtain reaction equations for the chloritization processes, which facilitates the quantitative assessment of mass transfer between the reactant and product minerals, and the inflow and outflow of components through the hydrothermal fluid. Several types of chloritization reactions (including biotite chloritization) can be characterized by their reaction with the inflow of Al3+, Fe2+, Mn2+, and Mg2+ and the outflow of H4SiO4, Ca2+, K+, and F–. The age and thermal conditions of hornblende chloritization (64–54 Ma and 330–190 °C), K-feldspar chloritization (68–53 Ma and 350–210 °C), and precipitation of fracture-filling chlorite (66 and 63 Ma, 340 and 320 °C) overlap with those of biotite chloritization (68–51 Ma and 350–180 °C). The chloritization reactions (this study and Yuguchi et al. 2015) and plagioclase alteration (Yuguchi et al. 2019a) represent sequential variations in fluid chemistry at temporal conditions from 68 to 51 Ma as the temperature decreased from 350 to 180 °C. As the alteration proceeds, the concentrations of aluminum, iron, manganese, and magnesium ions in the hydrothermal fluid decrease gradually, and those of calcium, hydrogen, and fluorine ions increase gradually. Hornblende chloritization is associated with formation of magnetite and ilmenite. The thermal conditions of the hydrothermal fluid yielding the formation of magnetite and ilmenite can be interpreted by the chemical characteristics of chlorite around their associated minerals. The formation temperature of magnetite was higher than that of ilmenite, implying a decrease in oxygen fugacity in the hydrothermal fluid with the decrease in temperature from 280–310 to 220–250 °C.