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: (K 0.05 Na 0.40 Ca 1.55 ) 2 (Mg 1.85 Mn 0.03 Fe 2+ 245 Fe 3+ 0.66 Al 0.09 Ti 0.92 ) 6 (Si 3.88 Al 2.12 ) 6 O 20 in tephrite and (K 0.01 Na 0.58 Ca 1.41 ) 2 (Mg 1.93 Mn 0.03 Fe 2+ 2.26 Fe 3+ 0.71 Al 0.17 Ti 0.90 ) 6 (Si 3.96 Al 2.04 ) 6 O 20 in limburgite. Mg/(Mg + Fe 2+ ) ratios range between 0.40 and 0.49, and the Fe 2+ -rich rhonite can be classed as an unnamed Fe 2+ -dominant analogue of rhonite with XMg < 0.5 as defined by Grew et al. (2008). Compared with other Fe 2+ -dominant rhonites in terrestrial and extraterrestrial rocks, the Kaiserstuhl compositions have less Ti and are richer in Si + (Na,K). The Kaiserstuhl rhonite coexists with subsilicic Al―Ti-bearing augite and titanian magnetite as in other basaltic occurrences of Fe 2+ -dominant and Mg-dominant rhonite, and is a quench phase in glass with liquidus temperatures of 1137―1068°C.