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This paper describes the biotite chloritization process with a focus on mass transfer in the Toki granitic pluton, Central Japan, and also depicts the temporal variations in chemical characteristics of hydrothermal fluid associated with chloritization during the sub-solidus cooling of the pluton. Singular value decomposition (SVD) analysis results in chloritization reaction equations for eight mineral assemblages, leading to the quantitative assessment of mass transfer between the reactant and product minerals, and inflow and outflow of components through the hydrothermal fluid. The matrices for SVD analysis consist of arbitrary combinations of molar volume and closure component in the reactant and product minerals. The eight reactions represent the temporal variations of chemical characteristics of the hydrothermal fluid associated with chloritization: the progress of chloritization results in gradual increase of silicon, potassium, and chlorine and gradual decrease of calcium and sodium in the hydrothermal fluid with temperature decrease. The biotite chloritization involves two essential formation mechanisms: chlorite formation (CF) mechanism 1, small volume decrease from biotite to chlorite and large inflow of metallic ions such as Al 3+ , Fe 2+ , Mn 2+ , and Mg 2+ from the hydrothermal fluid, and CF mechanism 2, large volume decrease and large outflow of the metallic ions into hydrothermal fluid. Chlorite produced with CF mechanism 1 dominates over that of CF mechanism 2, resulting in the gradual decrease of the metallic components in the hydrothermal fluid with chloritization progress. The chloritization reactions also give the temporal variations in physicochemical parameter of the hydrothermal fluid: a gradual decrease of pH and a gradual increase of redox potential in the hydrothermal fluid as chloritization proceeds. The combination of continuous reactions based on compositional variations in chlorite together with corresponding continuous Al IV variations gives an indication of the temporal variations in rates of decreasing and increasing concentration of chemical components in the hydrothermal fluid associated with chloritization. The biotite chloritization and resultant temporal variations of chemical and physicochemical characteristics in hydrothermal fluid act as a trigger for the successive dissolution-precipitation process of a granitic rock.

The three dimensional spatial variations in the cooling pattern of the Toki granitic body, a zoned pluton in Central Japan, have been evaluated quantitatively by thermochronology using cooling age determination based on the different closure temperatures for target mineral species. The Toki granite has hornblende K-Ar ages of about 74.3±3.7 Ma (n = 2; closure temperature of 510±25 o C), biotite K-Ar ages of 78.5±3.9 to 59.7±1.5 Ma (n = 33; 300±50 o C), and zircon fission-track ages of 75.6 ±3.3 to 52.8±2.6 Ma (n = 44; 240±50 o C).The spatial variation in the biotite K-Ar age is similar to that in the zircon fission-track age in samples collected from 11 boreholes and seven outcrop sites in the Toki granite, indicating that cooling was effectively from the roof and also from the northwest margin. This cooling pattern shows a strong correlation with the Alumina Saturation Index (ASI) distribution of the body.Larger ASI values correspond to earlier and more rapid cooling after emplacement and smaller value to slower cooling. Toki granite was effectively cooled from the peraluminous regions where assimilation of country sedimentary rock was most extensive.

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Anorthositic series apatites of the Duluth Complex, Minnesota, USA, have high spontaneous fission‐track densities of up to ~107 cm–2 and a homogeneous age of ~900 Ma, allowing high‐precision fission‐track dating based on LA–ICP–MS U analysis. Absolute fission‐track dating, track‐length measurement and chemical composition analysis were performed to evaluate a cooling history, which is essential for age reference materials. Preliminary inverse modelling for a sample with a shortened track‐length distribution yielded a monotonic cooling history from ~100°C at 925 Ma. The apatites incur an over‐etching problem when employing the commonly used etching protocol involving 5.5 M HNO3.