JUVENILE SILICIC MAGMATISM AND CRUST CONSTRUCTION: ICELAND AS A MODEL FOR EARLY-EARTH OR ICELAND AS A UNIQUE PETROGENETIC ENVIRONMENT?

Iceland, as an oceanic environment with abundant silicic magmatism, is unique on modern Earth. It provides an opportunity to evaluate the genesis of silicic magma in the absence of ancient crust or appreciable sedimentary or silicic igneous rocks; its heritage is juvenile and mafic. Icelandic petrogenesis may thus illuminate the formation of the earliest felsic crust in the Hadean-early Archean. Zircon, which has elsewhere underpinned our understanding of crustal evolution, is largely absent from Icelandic studies. We have conducted in situ trace element (SHRIMP) and O isotope (SIMS) analyses of zircon from 15 active and ancient volcanoes, intrusions, sandstones, and modern river sands, together spanning Iceland’s range of ages and tectonic settings.

Elemental and isotopic compositions display sample-to-sample variability, but the population is coherent enough to suggest a signature of the Icelandic environment. In U/Yb vs Y space (Grimes et al. 2007, 2011), Icelandic zircon straddles the boundary between MORB and continental fields and overlaps the low-U/Yb portion of the Hadean-early Archean field. In Yb/Nb vs U/Yb, with rare exceptions, they are restricted to the ocean island field and distinct from MORB, arc, and continental fields. Icelandic zircons are typically ~3x richer (~15 vs 5 ppm) and more variable in Ti than Hadean-early Archean zircons, suggesting higher crystallization T. Their range of δ¹⁸O (~0 to +5‰ magmatic, to -6‰ hydrothermal) is very low by global standards, and notably lower than Hadean-early Archean values (~4.5-7.5‰, >3.8 Ga, Valley et al. 2002, Trail et al. 2007). There is a correlation between volcanic setting and zircon δ¹⁸O: lowest on-rift (typically <3‰) and highest off-rift (>3‰). This may support the hypothesis of Martin & Sigmarsson (2007): silicic magma is produced by fractional crystallization off-axis and by partial melting on-rift. Low δ¹⁸O is attributed to source or contaminant material altered by meteoric water in this cool climate. No correlation between O and time is evident, despite the climate-related decline in δ¹⁸O of Icelandic water through the ~12 m.y. covered by our data.

Our data set, while not precluding petrogenetic similarities between Icelandic and early-Earth silicic magmas, demonstrates differences in zircon that call into question very close parallels.