PAIRING AGES AND ISOTOPES IN THE ICELANDIC ZIRCON RECORD: ADDING CRITICAL INSIGHT TO FELSIC PETROGENESIS WITH U-PB, U-TH, O AND HF

Iceland’s abundant (~10%) felsic rocks have been the focus of extensive study but until recently (e.g. Carley et al. 2011, Bindeman et al. 2012) there was a paucity of zircon-based investigations. Zircon is a robust geochronometer and recorder of magmatic evolution; its chemical durability is especially valuable in Iceland where high heat flux and hydrothermal alteration are major considerations.

We use high spatial resolution measurements of U-Pb and U-Th ages, trace elements, O isotopes (SIMS), and Hf isotopes (LA-MC-ICPMS) in zircon to elucidate: the timing and longevity of magmatism; melt evolution; contributions from surface (O) and mantle (Hf) sources; and open and closed system processes. Multiple analyses are performed on individual growth zones, resulting in a high-resolution view of magmatism through time.

We have analyzed >1000 zircons from 9 volcanic, 6 intrusive, and 10 detrital samples. To assess spatial and temporal trends, samples span Iceland’s history (15.5 Ma to present) and tectonomagmatic settings (e.g. on vs. off rift). Volcanic samples are valuable for focused study of environment (e.g. rift-effect); detrital samples define an “Iceland-type” zircon population, globally-distinct in composition (e.g. Y, Yb, Nb, Hf; cf. Grimes et al. 2007).

Icelandic zircons are strikingly low in δ¹⁸O, a feature commonly attributed to hydrothermal alteration of source material by meteoric water (less commonly, as evidence of a unique mantle, e.g. Thirwall et al. 2006). Of 710 oxygen analyses (average 3.0 ‰), 695 fall below the mantle range (5.3 +/- 0.3 ‰). Despite pronounced cooling through Iceland's climate history (warm-temperate to full glaciation), we see no evidence in the zircon record for changing δ¹⁸O with time.

Most (367 of 376) Hf analyses fall within a continuous range of 10-17 ε_Hf (average 13.4; low outliers). We see a correlation between tectonic setting and ε_Hf in active systems (13-16 on-rift; 10-13 off-rift), but this relationship is obscured in extinct systems. Unlike O, Hf isotopes decrease with time; we hypothesize that coupled U-Pb ages and ε_Hf will help better-constrain tectonomagmatic settings for Iceland’s older magmatic centers. Sparse yet tantalizing zircons (anomalously old, high δ¹⁸O, low ε_Hf) prompt future investigation into non-juvenile explanations of Iceland’s unique magmas.