KRAFLA ZIRCON: INSIGHTS INTO GENERATION OF SILICIC MAGMA AT AN ICELANDIC CENTRAL VOLCANO
We sampled Gaesafjallarani, one of three subglacial rhyolite ridges formed outside the Krafla caldera rim and interpreted to postdate caldera formation (Jonasson, 1994). This lava is compositionally typical of the main post-caldera phase of rhyolitic volcanism (74 wt% SiO2, 2.6% K2O, 550 ppm Zr), with a phenocryst assemblage dominated by plag + fayalite + cpx.
After CL imaging, we analyzed zircon from Gaesafjallarani lava for trace elements and U-Th ages (by SHRIMP-RG), and O and Hf isotopic compositions (SIMS, LA-MC-ICPMS)(all analyses in situ). U-Th model ages indicate that Gaesafjallarani lava is ~100 ka, consistent with post-caldera age, but ages range upward to at least 200 ka, possibly equivalent to the age of the caldera-forming event. The elemental compositions of Gaesafjallarani zircon fall within a distinctive compositional field inhabited by other Icelandic zircon, low in both Yb/Nb and U/Yb, that is readily distinguished from those of MORB, arc, and intracontinental settings (Grimes et al, 2007, 2011). Measured zircon eHf (14-17) matches whole rock values for Krafla basalt (14-15; Koornneef et al 2012), but δ18O (1-2‰) is considerably lower than basalt counterparts (~4-5‰; e.g. Nicholson et al 1991). This supports the proposal that silicic magmas at Icelandic rifts are generated by partial melting of highly altered crust (Martin & Sigmarsson, 2007).