Short- and Long-lived Silicic Volcanism at Árnes and Hafnarfjall-Skarðsheiði Central Volcanoes, Iceland

Abstract:

Relocation of Iceland’s subaerial rift zones over the last ~20 Myr has controlled the formation of voluminous silicic magmas in an intra-oceanic setting. While partial melting of hydrated basaltic crust plays an important role in generating silicic magmas at on-rift central volcanoes in Iceland, little is known about the timescales involved in generating silicic magmas or the petrogenesis of these evolved melts. In situ zircon analyses (U-Pb and trace elements by SHRIMP-RG, Stanford/USGS; δ¹⁸O by ims 1270, UCLA; εHf by Neptune MC-ICPMS, WSU) from silicic magmas from the Árnes and Hafnarfjall-Skarðsheiði (HS) central volcanoes in northwestern and western Iceland provide potential end members for investigating longevity and geochemical variation. Silicic magmatism at both large (>20 km diameter) centers began ~2 Myr after their parent rifts relocated and ≥20% of the erupted material is silicic in composition; however, longevity and petrogenetic processes vary between Árnes and HS. U-Pb ages reveal a short-lived (0.5 Myr) silicic lifespan for Árnes (12.8-13.3 Ma) vs. ~1.5 Myr life for HS (3.8-5.2 Ma). At HS, zircon δ¹⁸O is lowest in older units (averaging ~1.8‰ at 5.2 Ma), progressively increases to 3.3‰ in the middle units and decreases slightly in the youngest units (2.9‰; δ¹⁸O 2σ uncertainty for all samples is ~0.5–1‰), suggesting a history of silicic magma generation through partial melting of hydrothermally altered crust, transitioning toward either a greater role for fractional crystallization of basaltic parent magma or a less altered source over the volcano’s lifetime. Árnes units’ δ¹⁸O are homogeneous within error (3.2–3.6‰) and show no age correlation, consistent with a simpler petrogenetic history. Hf isotopic data reveal a more strongly depleted mantle signature for Árnes zircons (εHf₀ avg~12.3±1.7) than for HS magmas (εHf₀ avg~11.4±1.5).