Early Earth Mirrored in Zircon

Abstract

GEOLOGY The oldest minerals on Earth—a few dating as much as 4.4 billion years ago—are igneous zircons that have been eroded and incorporated into comparatively younger 3.8-billion-year-old sedimentary rocks. Several studies have examined the geochemistry of these zircons and inferred conditions on early Earth and the composition of its early crust. Initial results based on the zircon oxygen isotopic compositions and trace element chemistry have implied that liquid water was abundant and that some magmas were cool and water-rich. Two related studies provide expanded data and additional constraints. Harrison et al. examined the titanium content of zircons, which can be related to the crystallization temperature and thus the water and silica content of a magma. Through a comparative study of zircons in younger granites, they argue that the overall distribution of data in the very old zircons is most consistent with derivation from magmas that formed by remelting of water-rich crust. Trail et al. examined the oxygen isotope compositions and confirmed that several grains have high 18O/16O ratios, implying derivation of the host magma from water-altered crust or sediments. Together the data imply that early Earth had a vigorous rock cycle involving water, erosion, and burial and heating of sediments, like that operating today. — BH Geology 35 , 635 (2007); Geochem. Geophys. Geosyst. 8 , 10.1029/2006GC001449 (2007).