EVALUATING THE SOURCE: USING DETRITAL ZIRCON TO INVESTIGATE PETROGENESIS IN SUBGLACIAL MAGMAS FROM VATNAJÖKULL, ICELAND

Iceland formed from the interaction of the Mid-Atlantic Ridge and the Iceland hotspot. Overlying these magma-producing features is Iceland’s largest glacier, Vatnajökull, which conceals ~5 central volcanoes and associated fissures. Additional, unconfirmed, volcanic systems may exist beneath the ice, but lack of access hinders investigation of their existence and evolution. Glacially sourced river sediments permit sampling of bedrock that is otherwise inaccessible. Detrital zircons are especially useful due to their physical and chemical robustness, ability to preserve age and evidence of source magma compositional, and their common presence in silicic (rarely basaltic) rocks. To investigate the presence and nature of previously unrecognized subglacial systems, we present preliminary detrital zircon U-Pb age and trace element SHRIMP-RG results from two sites along Jökulsá í Lóni, one of Vatnajökull’s large outflow rivers. U-Pb ages from the downstream sample range from ~1.7 to 6.2 Ma, while the upstream zircon ages are more restricted at ~4.8 to 5.4 Ma. Trace element analyses reveal a similar pattern, with more restricted compositions upstream (n = 30) than downstream (n = 84). Ti and Hf concentrations indicate upstream zircons generally crystallized from warmer, less-evolved magma(s) compared to downstream zircons: upstream Ti ~6-36 ppm vs. ~6-42 ppm downstream; upstream Hf 6500-9500 ppm vs. ~7000-12,500 ppm downstream). U/Yb in downstream zircon ranges from 0.06 to 0.5 and 0.03 to 0.21 in upstream sample grains, which extends the lower range of the previously recognized Iceland zircon array (Carley et al. 2014). Gd/Yb in upstream and downstream samples occupies the same range as the general Iceland zircon array (~0.08-0.2). It is unlikely that all grains originated from the same subglacial magmatic system based on the general spread in geochemical values and ages. It is likely that the more restricted compositions and ages of the upstream sample are most representative of any subglacial system(s). Future work will focus on sampling closer to the glacier, sampling catchment-bounding bedrock, and increasing the geochemical and chronological database to assess magmatic evolution trends in eastern Vatnajökull.