DETRITAL ZIRCONS REVEAL THE GENESIS AND SURPRISING LONGEVITY OF ABUNDANT SILICIC MAGMATISM AT BREIDUVIK VOLCANO, EAST ICELAND

Breiduvik is an extinct volcanic complex in the Eastfjords of Iceland, containing the second largest abundance of silicic rock in the country (after Torfajokull). We use zircon to investigate the origins and longevity of Breiduvik’s silicic magmas. We minimized sampling bias and captured a representative view of the history of the volcanic center by using detrital samples. We performed the following in situ analyses on zircons from two major catchments: O isotopes (n = 133; Cameca-1270) to evaluate contributions of surface materials to magma generation; trace elements (n = 65; SHRIMP-RG) to elucidate magmatic conditions; U-Pb ages (n = 247; SHRIMP-RG) to constrain longevity of the magmatic system; and Hf isotopes (n = 73; LA-MC-ICPMS) to assess variability in mantle heritage.

Coherence of zircon elemental and isotopic compositions indicates a great volume of relatively uniform silicic material was produced in close spatial proximity between 11.2 ± 0.7 Ma and 15.0 ± 0.9 Ma, dominantly by partial melting of crust. Zircon populations from the two catchments are essentially identical in age (mean: 12.9 ± 0.5 Ma), Hf isotopes (mean: 14.7 ± 1.0 vs 14.9 ± 1.0 εHf), and O isotopes (mean: 3.1 ± 0.4 vs 3.3 ± 0.3 δ¹⁸O). Trace element (e.g. Ti, Hf, REE) compositions from the two catchments show some variability, but together they define a coherent population within a compilation of Icelandic zircon trace element data.

We applied Monte Carlo modeling to the Breiduvik detrital zircon results and found the age span to be statistically resolvable at 2.5 M.y. at the 2 sigma confidence level. This duration is the longest reported for an Icelandic volcano. A typical lifespan is considered to be ~0.5-1.5 M.y. (e.g. Jakobsson et al. 1987; Jakobsson 1979; Saemundsson 1978 and 1979; Thordarson & Larsen 2007). Our estimate of magmatic longevity is conservative because our ages only represent the zircon-saturated period of the volcanic center’s life. Magmas at Breiðuvík volcano may have been active for significantly longer.

This study demonstrates that detrital zircons are an effective, powerful, tool for assessing the history of zircon-saturated magmas at targeted volcanic complexes (in Iceland and globally). Our findings have important implications for understanding past, interpreting current, and predicting ongoing, magmatic activity.