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

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- + 1/2H(2) substitution is negligible, which is supported by H and O isotope compositions. A possible substitution Al-[6](3+) + OH- = Ti-[6](4+) + 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, R-[6](2+) + (OH-)-O-2 = Ti-[6](4+) + 2O(2-), where R-[6](2+) = 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 (deltaD = -106 to -71parts per thousand and delta(18)O = 4.6-5.2parts per thousand) 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.