Evolution of low-18O Icelandic crust

The Krafla central volcano in the neovolcanic zone of Iceland hosts a chemically diverse suite of magmas characterized by anomalously low delta O-18 values. A rhyolite magma intercepted by the Iceland Deep Drilling Project (IDDP) exploratory well at 2.1 km depth provided a unique opportunity to investigate the origins of an unerupted rhyolite melt in the primarily basaltic central volcano at Krafla. Here we compare whole rock hydrogen and oxygen isotopes of this melt to those of lavas within and near the caldera of the Krafla central volcano ranging from recent fissure eruptions to Plio-Pleistocene age (including analyses of 18 new samples, plus previously published values) in order to evaluate the petrogenesis of low-O-18 magmas within the neovolcanic zone of Iceland. Oxygen isotope values of the IDDP-1 melt (delta O-18 = 3.2 +/- 0.2 parts per thousand) are within the range of Krafla eruptives that have a bimodal composition of olivine-tholeiite and rhyolite (delta O-18 = +1.6 parts per thousand to +4.5 parts per thousand). Lavas show significantly more variability in hydrogen isotope values (delta D = -161 parts per thousand to -92 parts per thousand) than the IDDP-1 melt (-121 +/- 2 parts per thousand), whose 613 is comparable to local hydrothermal epidote (-127 to -108 parts per thousand), and show significantly lower water contents than IDDP-1 (0.1-1.1 wt%, in contrast to similar to 1.8 wt%). Basaltic to dacitic lavas from the proximal Heidarspordur ridge volcanic zone have 6180 between +3.4 parts per thousand and +4.2 parts per thousand and delta D between -105 parts per thousand and -99 parts per thousand. Uniformity of oxygen isotopes in the Heidarspordur ridge lavas suggests that their magmatic compositional variations are a consequence of fractional crystallization. The delta D of the glass sampled by IDDP-1 unequivocally identifies the source of silicic low-O-18 melts like those erupted from within the caldera of the Krafla volcano as anatexis of meteoric-hydrothermally altered basalts resulting from the intrusion of mantle-derived basaltic magma. Finally, mantle-derived basalts in both the Krafla central volcano and Heidarspordur ridge (MgO > 5 wt%) have delta O-18 values lower than the typical MORB. There is convincing evidence favoring low-O-18 compositions in primitive Icelandic basalts as also being the result of assimilating meteoric hydrothermally altered Icelandic crust, particularly in the roots of central volcanic complexes, where high-level intrusions develop. Here hydrothermally altered basalts have a high probability of undergoing partial melting. If this is the case, we hypothesize that there should be a notable secular variation in the delta O-18 values of upper crustal igneous products of the Iceland mantle plume over the sub-aerial extrusive geologic history of Iceland, in which progressively younger lavas exhibit decreasing delta O-18 values. Synthesis of published oxygen isotope analyses of unaltered whole rock and mineral separates of lavas from Tertiary age to present lends support to this hypothesis. (C) 2013 Elsevier B.V. All rights reserved.