PAIRING AGES AND ISOTOPES IN THE ICELANDIC ZIRCON RECORD: ADDING CRITICAL INSIGHT TO FELSIC PETROGENESIS WITH U-PB, U-TH, O AND HF
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 εHf (average 13.4; low outliers). We see a correlation between tectonic setting and εHf 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 εHf will help better-constrain tectonomagmatic settings for Iceland’s older magmatic centers. Sparse yet tantalizing zircons (anomalously old, high δ18O, low εHf) prompt future investigation into non-juvenile explanations of Iceland’s unique magmas.