HR: 0800h
AN: U11A-0017    [Abstracts]
TI: Early Formation of Terrestrial Crust
AU: * Harrison, T M
EM: tmh@oro.ess.ucla.edu
AF: University of California at Los Angeles, UCLA-Department of Earth and Space Sciences Box 951567, Los Angeles, CA 90095-1567, United States
AU: Schmitt, A K
EM: axel@ess.ucla.edu
AF: University of California at Los Angeles, UCLA-Department of Earth and Space Sciences Box 951567, Los Angeles, CA 90095-1567, United States
AU: McCulloch, M T
EM: Malcolm.McCulloch@anu.edu.au
AF: Australia National University, Australian National University, Research School of Earth Sciences, Canberra, A.C.T. 260, Australia
AU: Lovera, O M
EM: lovera@ess.ucla.edu
AF: University of California at Los Angeles, UCLA-Department of Earth and Space Sciences Box 951567, Los Angeles, CA 90095-1567, United States
AB: Early (≥4.5 Ga) Formation of Terrestrial Crust T.M. Harrison¹, A.K. Schmitt¹, M.T. McCulloch², and O.M. Lovera¹ 1Department of Earth and Space Sciences and IGPP, UCLA, Los Angeles, CA 90095, USA; 2Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2601 AUSTRALIA Large deviations in \varepsilon_Hf(T) from bulk silicate Earth seen in >4 Ga detrital zircons from Jack Hills, Western Australia, have been interpreted as reflecting a major differentiation of the silicate Earth at ca. 4.4 to 4.5 Ga. We have expanded the characterization of ¹⁷⁶Hf/¹⁷⁷Hf (Hf) in Hadean zircons by acquiring a further 116 laser ablation Lu-Hf measurements on 87 grains with ion microprobe ²⁰⁷Pb/²⁰⁶Pb ages up to 4.36 Ga. Most measurements employed concurrent Lu-Hf and ²⁰⁷Pb/²⁰⁶Pb analyses, permitting assessment of the use of ion microprobe data to characterize the age of the volumetrically larger domain sampled by laser drilling. Our new results confirm and extend the earlier observation of significant negative deviations in \varepsilon_Hf(T) throughout the Hadean, although no positive \varepsilon_Hf(T) values were documented in this study. These data yields an essentially uniform spectrum of single-stage model ages between 4.54 and 4.20 Ga for extraction of the zircons' protoliths from a chondritic reservoir. We derived the full error propagation expression for a parameter, \varepsilon_o, which measures the difference of a sample from solar system initial (Hf) (Hfo), and from this conclude that data plotting close to (Hfo), are statistically meaningful and consistent with silicate differentiation at 4.540±0.006 Ga. δ¹⁸O and Ti thermometry for these Hadean zircons show little obvious correlation with initial (Hf), consistent with their derivation through fusion of a broad suite of crustal rock types under near water-saturated conditions. Together with the inclusion assemblage and other isotopic and trace element data obtained from these ancient zircons, our results indicate essentially continuous derivation of crust from the mantle from 4.5 to 4.2 Ga, concurrent with recycling into the mantle and internal crustal re-working. These results represent further evidence that by 4.35 Ga, portions of the crust had taken on continental characteristics.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1194 Instruments and techniques
SC: Union [U]
MN: 2007 Fall Meeting