Fe~(2+)-dominant rhonite in undersaturated alkaline basaltic rocks, Kaiserstuhl volcanic complex, Upper Rhine Graben, SW Germany

摘要

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+)_(2.45) Fe ~(3+)_(0.66)Al_(0.09)Ti_(092))_6(Si_(3.88)Al_(2.12))_60_(2o) in tephrite and (K_(0,01)Na_(0.58)Ca_(1.41))_2(Mg_(1. 93)Mn_(o.03)Fe~(2+)2.26Fe~(3+)0,71 Al_(0.17)Ti_(0.90))_6(Si_(3.96)Al_(2.04))_6O_(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 Mgdominant rhonite, and is a quench phase in glass with liquidus temperatures of 1137-1068 °C.