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Tangible evidence of Earth's earliest (Hadean; >4.0 Ga) crust, and the processes and materials that contributed to its formation, exists almost entirely in a record of detrital zircon from Jack Hills, Western Australia, and a few other locations. Iceland, with its thick, juvenile, basaltic crust and relatively abundant silicic rocks, is considered a potential modern analog for the Hadean magmatic environment where >4 Ga zircon formed. We present the first extensive dataset for Icelandic zircon, with trace element and oxygen isotope compositions from samples that span the island's history and full range of tectonic settings. This statistically robust zircon-based comparison between Iceland and the early Earth reveals distinctions in chemistry that suggest fundamental differences in magmatic environments. Whereas the δ18O signature of Hadean zircons generally exceed that of zircons equilibrated with mantle-derived magma ( 85 % ≥ 5.3 ‰ ; median 6‰), almost all Icelandic zircons are characterized by a “light” oxygen signature ( 98 % ≤ 5.3 ‰ ; median 3‰). Deviations from “juvenile” oxygen values indicate that many Hadean zircons and almost all Icelandic zircons grew from magmas with substantial contributions from materials that had interacted with surface waters. In the Hadean case, the interaction occurred at low temperatures, while in Iceland, it was a high-temperature interaction. Icelandic and Hadean zircons are also distinct in their Ti concentrations (Icelandic median concentration 12 ppm, Hadean median 5 ppm). Titanium in zircon correlates positively with temperature of crystallization, and this difference in median Ti concentration suggests a temperature difference of at least 50 °C. Other differences in trace elements compositions are consistent with the interpretation that Icelandic and Hadean zircons grew in magmas with very different origins and histories (e.g., the heavy rare earth element Yb is almost an order of magnitude higher in Icelandic zircon). A comparison with elemental data for Phanerozoic zircon from different environments demonstrates that the Hadean population is unusually depleted in Ti, but otherwise similar to zircons from continental arc settings. Zircons from Iceland, and from modern evolving rift environments where oceanic lithosphere and upwelling asthenosphere are replacing continental lithosphere, are compositionally intermediate between mid-ocean ridge and continental arc zircon populations. The elemental distinctions are consistent with fractionation of zircon-bearing magmas under hotter and drier conditions in Icelandic, mid-ocean ridge, and evolving rift environments and cooler and wetter conditions in arc and, especially, Hadean environments. |
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