ALBERT

Description: The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated 18 O (6.0–7.5 relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf ( Hf = –6.6 to –4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low 18 O values (2.5–4.8, n = 9 of 15) across a range of ages (3536–2598 Ma; Hf = –18.2 to +0.4). The low 18 O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low 18 O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low 18 O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low 18 O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low 18 O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low 18 O magma from which zircon crystallized. Ultimately, in either scenario, the low 18 O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time.