Magmatic δ18O in 4400-3900 Ma detrital zircons:: A record of the alteration and recycling of crust in the Early Archean

Ion microprobe analyses of delta O-18 in 4400-3900 Ma igneous zircons from the Jack Hills, Western Australia, provide a record of the oxygen isotope composition of magmas in the earliest Archean. We have employed a detailed analysis protocol aimed at correlating spatially related micro-volumes of zircon concordant in U/Pb age with delta O-18 and internal zoning. Simultaneous analysis of 180 and 160 with dual Faraday cup detectors, combined with frequent standardization, has yielded data with improved accuracy and precision over prior studies, and resulted in a narrower range of what is interpreted as magmatic delta O-18 in > 3900 Ma zircons. Preserved magmatic delta O-18 values from individual zircons (Zrc) range from 5.3 parts per thousand to 7.3 parts per thousand (VSMOW), and increasingly deviate from the mantle range of 5.3 +/- 0.3 parts per thousand as zircons decrease in age from 4400 to 4200 Ma. Elevated delta O-18 (Zrc) values up to 6.5 parts per thousand occur as early as 4325 Ma, which suggests that evolved rocks were incorporated into magmas within similar to 230 Ma of Earth's accretion. Values of magmatic delta O-18 (Zrc) as high as 7.3 parts per thousand are recorded in zircons by 4200 Ma, and are common thereafter. The protoliths of the magmas these zircons crystallized in were altered by low temperature interaction with liquid water near Earth's surface. These results provide the strongest evidence yet for the existence of liquid water oceans and supracrustal rocks by approximately 4200 Ma, and possibly as early as 4325 Ma. The range of magmatic delta O-18 values in the 4400-3900 Ma zircons is indistinguishable from Archean igneous zircons, suggesting similar magmatic processes occurred over the first two billion years of recorded Earth history. Zircons with sub-solidus alteration histories, identified by the presence of disturbed internal zoning patterns, record 6180 values both below (4.6 parts per thousand) and above (10.3 parts per thousand) the observed range for primary magmatic zircon, and are unreliable indicators of Early Archean magma chemistry. (c) 2005 Elsevier B.V. All rights reserved.