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 (O2− = 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+ + O2− + 1/2H2 substitution is negligible, which is supported by H and O isotope compositions. A possible substitution, Al3+ + OH− = Ti4+ + O2− may be operative for Larvik kaersutites when the O2−/Ti is 1.0. A relatively larger O2−/Ti ratio (1.2–2.0) suggests an another kaersutite substitution mechanism, R2+ + 2OH− = Ti4+ + 2O2−, where R2+ = Fe2+ + Mg + Mn. These effects might result in the limited O2−/Ti ratio value from 1.0 to 2.0.
A negative correlation between Ti and F, suggesting F incorporation into kaersutite may diminish the O2−/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 (δD = −106 to −71‰ and δ18O = 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.