SILICIC PETROGENESIS AT SNÆFELL VOLCANO, ICELAND, REVEALED BY ZIRCON ANALYSES

Silicic magma petrogenesis in Iceland is enigmatic and the subject of ongoing study. The vast majority of magmatism—especially silicic magmatism—occurs along Iceland’s active rift zones. Snæfell is the northernmost volcano in the Öræfi Volcanic Belt (ÖVB), an off-rift area of Quaternary magmatic activity located ~70 km east of the Eastern Rift Zone. While the ÖVB is hypothesized to represent an incipient rift, evidence is lacking for a close relationship between volcanism and rifting at Snæfell. Zircons obtained from several silicic units exposed at Snæfell provide insight into rhyolite residence times, pre-eruption growth histories, and potential eruption triggers that may illuminate petrogenesis in off-rift systems. Snæfell zircon crystals are invariably euhedral, and internal morphology is broadly consistent across samples. Roughly 10% of grains display penetration twinning and CL-bright, c-axis-parallel features commonly occur in all samples. Grains commonly exhibit faint oscillatory and sector zoning, and many zircons display clear internal boundaries that truncate existing zoning patterns. U-Pb ages indistinguishable from zircon interiors and unpolished rims and have weighted mean sample ages from ~271 to ~364 ka. Snæfell zircon Hf concentrations are generally low and range from ~7,000–12,000 ppm. Ti concentrations in zircon rims of several samples average ~5 ppm higher than in grain interiors. Low overall Ti (avg. ~5 ppm) suggests crystallization in the Snæfell system occurs at lower temperatures than is typical for Icelandic zircon, but that rims crystallize at slightly higher temperatures. Oxygen isotope ratios for Snæfell zircon (n=31) range from ~3.08‰ to ~4.36‰, which corresponds to whole rock values from those outcrops. These values suggest that a dominant proportion of relatively high-δ¹⁸O (i.e. close to mantle value) melts with minimal contribution from low-δ¹⁸O recycled crust is required to produce Snæfell silicic magmas. Textural and compositional data hint at protracted cooling histories prior to a final rapid and hot crystallization event. Two mechanisms of formation, (1) fractional crystallization from a mantle-derived, little-altered melt and (2) partial melting of an older crust, will be discussed to provide a more nuanced view into silicic magma genesis during incipient rifting.