Fe2+ dominant rhonite in undersaturated alkaline basaltic rocks, Kaiserstuhl volcanic complex, Upper Rhine Graben, SW Germany

Abstract

Fe-dominant rhonite with between 25 and 28 wt% FeO (as total iron) occurs as a late crystallizing phase in tephrite and limburgite from the Kaiserstuhl volcanic complex, Upper Rhine Graben, Germany. Average compositions are: (K0.05Na0.40Ca1.55)(2) (Mg1.85Mn0.03Fe2.452+Fe0.663+Al0.09Ti0.92)(6)(Si3.88Al2.12)(6)O-20 in tephrite and (K0.01Na0.58Ca1.41)(2)(Mg1.93Mn0.03Fe2.262+Fe0.713+ Al0.17Ti0.90)(6)(Si3.96Al2.04)(6)O-20 in limburgite. Mg/(Mg + Fe2+) ratios range between 0.40 and 0.49, and the Fe2+-rich rhonite can be classed as an unnamed Fe2+-dominant analogue of rho " nite with XMg,0.5 as defined by Grew et al. (2008). Compared with other Fe2+-dominant rho " nites in terrestrial and extraterrestrial rocks, the Kaiserstuhl compositions have less Ti and are richer in Si + (Na, K). The Kaiserstuhl rho " nite coexists with subsilicic Al-Ti-bearing augite and titanian magnetite as in other basaltic occurrences of Fe2+-dominant and Mg-dominant rho " nite, and is a quench phase in glass with liquidus temperatures of 1137-1068 degrees C.