Fig. 4.

Isotopic ratios versus MgO concentration for the Öræfajökull compositional range (13) demonstrating the lack of systematic trends. Samples from the Öræfajökull volcanic basement up to 12 My, formed in the older rift zones further west, are not included. The δ¹⁸O-range for the Öræfajökull rock suite is generally between or in the ranges for midocean ridge basalts (62, 63) and the most primitive olivine tholeiites and picrites of the Theistareykir volcanic system in the northernmost part of the Northern Rift Zone in Iceland (64). The high δ¹⁸O-range for the intermediate and silicic part of the Öræfajökull suite is in strong contrast to the low δ¹⁸O of evolved basalts and rhyolites affected by contamination by anatectic melts in the subsiding hydrated basement in the volcanic rift zone systems (19, 65).