GENERATING THE WORLD’S LOWEST MAGMATIC ZIRCON δ18O: MELTING OF INTENSELY HYDROTHERMALLY ALTERED SOURCES AT AUSTURHORN INTRUSIVE COMPLEX, SE ICELAND

The 6.4 Ma Austurhorn intrusion, a shallowly emplaced (~2 km) complex exposed over ~15 km² in SE Iceland, comprises large bodies of granophyre, gabbro, and a mafic-felsic composite zone (MFCZ) that exemplifies magma interactions common in Icelandic silicic systems.

Oxygen isotopic analyses of zircon and primary quartz, feldspar, and pyroxene from samples representing the range of compositions in the MFCZ, from gabbros to high-silica granophyres (HSG), reveal extreme oxygen variability but small mineral-mineral fractionation, consistent with a hydrothermal-magmatic transition (>600°C). In most samples, individual zircon δ¹⁸O falls between +2.5 and +4.5 ‰. These values suggest major contributions from hydrothermally (meteoric)-altered crust in generating parental silicic magmas. A subset (~15%) of in situ zircon analyses, all from HSG, preserve lower values (to -14.6 ‰, the lowest δ¹⁸O yet measured in magmatic zircon). Major minerals and bulk HSG also display low and variable δ¹⁸O (e.g. +6 to -3 ‰ in quartz), supporting magmatic origin for the lower δ¹⁸O zircons. We interpret zircon δ¹⁸O diversity and greater homogeneity of major minerals and whole-rocks to indicate construction of the intrusion from multiple, isotopically diverse magma increments.

The HSG with the lowest magmatic δ¹⁸O zircon values comprise nearly pure granophyric quartz and feldspar (+ zircon). They are exposed near the highest structural level of the MFCZ as thin felsic dikes and pods that magmatically interacted and mingled with less silicic granophyres. We interpret these to represent batches of small volume melts generated by pure melting of intensely altered crust that underwent nearly complete (large water/rock ratio), high-T exchange with ~-13‰ hydrothermal meteoric water.

Heterogeneity in δ¹⁸O in zircon and major minerals is consistent with field relationships and U-Pb geochronology at Austurhorn, suggesting that sustained melting activity, as a consequence of continuous mafic recharge, was the primary mechanism in the generation and accumulation of silicic magmas throughout the lifetime of the Austurhorn system. Diverse δ¹⁸O in zircons and other minerals provide important insights into magmatic-hydrothermal transition zones as well as physical mechanisms of shallow magma petrogenesis by amalgamation of diverse melts.