The Tabito composite mass in the Abukuma Mountains was dated for its biotite and hornblende separates by ⁴⁰Ar-³⁹Ar and K-Ar methods. K-Ar biotite ages ranging from 111 to 95 Ma are significantly older than the coexisting hornblendes in two of four rock samples examined.
     ⁴⁰Ar-³⁹Ar age spectra of biotite and hornblende show argon-loss and convex-upward patterns. However, higher temperature fractions of hornblende yielded well defined plateau ages of 103.1±1.4 Ma for the Iritabyuto granodiorite and 101.7±1.2 Ma for Myojin-ishi tonalite, which are consistent with a conventional K-Ar age of hornblende from the Komuro tonalite by Shibata and Uchiumi (1983). ⁴⁰Ar-³⁹Ar plateau ages of hornblende support the cooling rate (∼15°C/m.y.) of the Tabito mass by Tanaka et al. (1999).

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Distributions of Mg²⁺, Ni²⁺, Co²⁺, Zn²⁺, Mn²⁺(Me²⁺) and Fe²⁺ ions between ilmenite and 2-N aqueous chloride solution were experimentally determined in the temperature range from 500 to 800°C under 1 kb. The experimental results were analyzed thermodynamically taking into consideration the aqueous speciation.
     Distribution coefficient (K_DN=(X_MeTiO3/X_FeTiO2)/(m_MeCl2⁰aq/m_FeCl2⁰aq)) vs. ionic radius (PC-IR) diagrams are drawn for the experimental results. Log K_DN vs. ionic radius curves are parabolic with a maximum between Co²⁺ and Fe²⁺. However, Ni²⁺ and Zn²⁺ deviate from such a tendency. The anomaly of Zn²⁺ and Ni²⁺ may be attributable to their strong preferency of the tetrahedral sites of the aqueous chloro-complexes and the octahedral sites of the ilmenite structure, respectively. The K_DN value for Ni²⁺ decreases with increasing temperature, whereas those for Mg²⁺, Zn²⁺ and Mn²⁺ increase. The optimum ionic radius for ilmenite seems to decrease slightly with increasing temperature.

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