On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth's earliest magmatic differentiation

Whitehouse, M.J. and Kamber, B.S. (2002) On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth's earliest magmatic differentiation. Earth And Planetary Science Letters, 204 3-4: 333-346. doi:10.1016/S0012-821X(02)01000-2

Author Whitehouse, M.J.
Kamber, B.S.
Title On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth's earliest magmatic differentiation
Journal name Earth And Planetary Science Letters   Check publisher's open access policy
ISSN 0012-821X
Publication date 2002-12-15
Sub-type Article (original research)
DOI 10.1016/S0012-821X(02)01000-2
Volume 204
Issue 3-4
Start page 333
End page 346
Total pages 14
Editor E. Bard
E. Boyle
Place of publication The Netherlands
Publisher Elsevier Science BV
Collection year 2002
Language eng
Subject C1
260301 Geochronology and Isotope Geochemistry
780104 Earth sciences
Abstract We present whole-rock and zircon rare earth element (REE) data from two early Archaean gneisses (3.81 Ga and 3.64 Ga) from the Itsaq gneiss complex, south-west Greenland. Both gneisses represent extremely rare examples of unaltered, fresh and relatively undeformed igneous rocks of such antiquity. Cathodoluminescence imaging of their zircons indicates a single crystallisation episode with no evidence for either later metamorphic and/or anatectic reworking or inheritance of earlier grains. Uniform, single-population U/Pb age data confirm the structural simplicity of these zircons. One sample, a 3.64 Ga granodioritic gneiss from the Gothabsfjord, yields a chondrite-normalised REE pattern with a positive slope from La to Lu as well as substantial positive Ce and slight negative Eu anomalies, features generally considered to be typical of igneous zircon. In contrast, the second sample, a 3.81 Ga tonalite from south of the Isua Greenstone Belt, has variable but generally much higher light REE abundances, with similar middle to heavy REE. Calculation of zircon/melt distribution coefficients (D-REE(zircon/melt)) from each sample yields markedly different values for the trivalent REE (i.e. Ce and Eu omitted) and simple application of one set of D-REE(zircon/melt) to model the melt composition for the other sample yields concentrations that are in error by up to two orders of magnitude for the light REE (La-Nd). The observed light REE overabundance in the 3.81 Ga tonalite is a commonly observed feature in terrestrial zircons for which a number of explanations ranging from lattice strain to disequilibrium crystallisation have been proposed and are further investigated herein. Regardless of the cause of light REE overabundance, our study shows that simple application of zircon/melt distribution coefficients is not an unambiguous method for ascertaining original melt composition. In this context, recent studies that use REE data to claim that > 4.3 Ga Hadean detrital zircons originally crystallised from an evolved magma, in turn suggesting the operation of geological processes in the early Earth analogous to those of the present day (e.g. subduction and melting of hydrated oceanic crust), must be regarded with caution. Indeed, comparison of terrestrial Hadean and > 3.9 Ga lunar highland zircons shows remarkable similarities in the light REE, even though subduction processes that have been used to explain the terrestrial zircons have never operated on the Moon. (C) 2002 Elsevier Science B.V. All rights reserved.
Keyword Ianthanides
Distribution Coefficients
Geochemistry & Geophysics
Southern West Greenland
Crystal X-ray
Synthetic Zircon
Detrital Zircons
Amitsoq Gneiss
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Earth Sciences Publications
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Created: Tue, 14 Aug 2007, 18:11:05 EST