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Ti-substitution mechanism in plutonic oxy-kaersutite from the Larvik alkaline complex, Oslo rift, Norway

H. Satoh^*,, Y. Yamaguchi and K. Makino

Department of Geology, Faculty of Science, Shinshu University, Asahi, Matsumoto 390-8621, Japan

^* E-mail: hsatoh{at}ganko.tohoku.ac.jp

Amphibole in the Larvik alkaline plutonic complex in the Oslo rift, Norway, has Ti-rich compositions from edenite through pargasite to kaersutite, and has a large H⁺ deficiency (0.7–1.1 atoms per formula unit: a.p.f.u.) with a large oxy component in the amphibole OH^– site (O^2– = 2 – (OH + F + Cl) = 0.2–0.9 a.p.f.u.), similar to the mantle-derived kaersutites. Their compositions reveal a characteristically low Fe³⁺/(Fe³⁺+Fe²⁺) ratio (<0.23) and a high F concentration (0.3–0.9 a.p.f.u.). Correlation with the Fe³⁺ ratio caused by Fe²⁺ + OH^– = Fe³⁺ + O^2– + 1/2H₂ substitution is negligible, which is supported by H and O isotope compositions. A possible substitution, ^[6]Al³⁺ + OH^– = ^[6]Ti⁴⁺ + O^2– may be operative for Larvik kaersutites when the O^2–/Ti is 1.0. A relatively larger O^2–/Ti ratio (1.2–2.0) suggests an another kaersutite substitution mechanism, ^[6]R²⁺ + 2OH^– = ^[6]Ti⁴⁺ + 2O^2–, where ^[6]R²⁺ = Fe²⁺ + Mg + Mn. These effects might result in the limited O^2–/Ti ratio value from 1.0 to 2.0.

A negative correlation between Ti and F, suggesting F incorporation into kaersutite may diminish the O^2–/Ti ratio, not only due to the occupation of this non-oxy species in the O3 site, but also due to F–Ti avoidance. Composition-dependent H and O isotope variations (D = –106 to –71 and ¹⁸O = 4.6–5.2) suggest equilibrium in the closed-system magma with differentiation. The mineral chemistry of Larvik oxy-kaersutitic amphibole could reflect the crystallization in a closed-system magma during rifting with passive crustal thinning at the Oslo palaeorift.

KEYWORDS: Ti-substitution, amphibole, Larvik alkaline complex, Oslo Rift, Norway

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