Zircon Evidence Reveals that Iceland is Not a Modern Analogue for Earliest Crustal Construction (Hadean, >4 Ga)

Tamara Carley, Lafayette College, Easton, PA, United States, Calvin F Miller, Vanderbilt Univ, Earth and Environmental Sciences, Nashville, United States, Joseph Wooden, Stanford University, Los Altos Hills, CA, United States, Abraham J Padilla, Vanderbilt University, Nashville, TN, United States, Axel K Schmitt, University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, Rita C Economos, University of California Los Angeles, Earth and Space Sciences, Los Angeles, United States, Ilya Bindeman, University of Oregon, Department of Earth Sciences, Eugene, OR, United States and Brennan Jordan, South Dakota School of Mines and Technology, Black Hills Natural Sciences Field Station, Rapid City, SD, United States

Contact First Author: Tamara Carley; carleyt@lafayette.edu

Previously Published Material: This conference submission is based on work published earlier this year: Carley, T.L., Miller, C.F., Wooden, J.L., Padilla, A.J., Schmitt, A.K., Economos, R.C., Bindeman, I.N., and Jordan, B.T. (2014) Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record. Earth and Planetary Science Letters Vol. 405, pp. 85–97.

Abstract ID#: 34306

 

English Abstract:
Tangible evidence of Earth’s earliest (Hadean; >4.0 Ga) crust, and the processes and materials that led to its formation, exists almost entirely in detrital zircon. Iceland’s crust (hot, thick, juvenile, dominantly mafic, substantial silicic component) resembles what is often intuitively visualized for Hadean crust. Because zircon crystals provide a unique view of the >4.0 Ga world, a zircon-based comparison of Icelandic and Hadean zircon data is a first-order test of this model. We present the first extensive dataset for Icelandic zircon, with O isotopes and trace elements for samples spanning the island’s history and tectonic settings. Comparison of isotopic and trace elemental data suggest fundamental differences in magmatic environments. Deviations from juvenile δ18O reveal that surface waters interacted with magmatic source materials for many Hadean and most Icelandic zircons, but under very different circumstances. Preserved Hadean zircons have dominantly normal to high δ18O (median value 6‰; 85% have δ18O ≥ 5.3‰). Protolith material for host magmas may have been altered at low T by subaerial meteoric water or submarine seawater with δ18O ≥ 0‰. Abundant low- δ18O Icelandic zircons (98% ≤ 5.3‰; median 3‰) imply high T alteration of protolith by meteoric water. Titanium abundances in zircon are also distinct (Iceland median 12 ppm, Hadean 5 ppm), suggesting that Hadean zircons crystallized in cooler magmas (median difference ≥75°C). Other trace elements, e.g. MREE and HREE (Iceland >> Hadean zircons), are also consistent with distinct origins of zircon-bearing magmas. Perhaps Iceland-like settings and magmas existed in the early Earth, but evidence for this is very rare in >4 Ga zircons. Iceland is thus not representative of Hadean crust. Based on zircon, mid-ocean ridges, evolving continental rifts and continental hotspots are equally poor matches. Arc zircons provide a closer fit, although >4 Ga zircons appear to be from even cooler, wetter, magmas.
Back to: Planetary Thermal History and the Transition of Stagnant Lid to Plate Tectonics I