UNDERSTANDING MAGMA FORMATION AT ÞINGMÚLI (THINGMULI) VOLCANO, ICELAND BASED ON ZIRCON ANALYSES

Iceland is one of the few places on Earth where a spreading center and mantle hot spot coincide, leading to abundant magmatism and production of silicic rocks. There are two processes thought to be dominantly responsible for generating silicic magmas in Iceland: fractional crystallization and partial melting of pre-existing crust. Þingmúli (Thingmuli) volcano, an extinct volcano in eastern Iceland, is viewed as an archetype for rhyolite generation via fractional crystalization based on whole rock analyses (Carmichael, 1964, 1967; Charreteur et al., 2013). We present the first zircon-based oxygen (O) isotope compositions and U-Pb ages for Þingmúli. Whole rock samples (n=8) were collected from mapped Þingmúli silicic units and zircon was separated via standard crushing, sieving, and conventional magnetic and density separation methods, followed by hand-picking under a microscope. Oxygen isotope compositions in zircon grains were then analyzed at UCLA and U-Pb geochronology and trace element compositions were measured using the Stanford–USGS SHRIMP-RG. Zircon U-Pb ratios indicate ages ~9–10 Ma. Zircon δ¹⁸O in seven samples is restricted to ~3.1–3.9‰, suggesting rhyolite petrogenesis at Þingmúli can be dominantly attributed to fractional crystallization of mantle-derived basaltic magmas, which is in line with previous whole-rock-based findings. The sample with the oldest U-Pb age has a substantially lower δ¹⁸O weighted mean (~2.1‰), with grains (n=10) having δ¹⁸O ranging from ~1.8–2.3‰. Titanium—which is a proxy for crystallization temperature in zircon—ranges from ~8–17 ppm and Hf ranges from ~9,000–16,000 ppm across all samples, both of which are typical for Icelandic zircon. However, the oldest, low- δ¹⁸O unit has an average Ti concentration of ~14 ppm, which is 2–4 ppm higher than the other sample averages. While overall these zircon data provide support prior interpretations for rhyolite petrogenesis at Þingmúli (a history dominated by fractional crystallization), our data also hint at a warmer, early phase of magmatism in which incorporation of low- δ¹⁸O material (perhaps by partial melting) was a significant contributor to these oldest zircons’ parent melt.