Low δ18O zircon grains in the Neoarchean Rum Jungle Complex, northern Australia: An indicator of emergent continental crust

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 δ 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 (ε 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 δ 18 O values (2.5‰-4.8‰,n = 9 of 15) across a range of ages (3536-2598 Ma; ε Hf = -18.2 to +0.4).The low δ 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 δ 18 O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low δ 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 δ 18 O zircon.In the fi rst scenario, assimilation of these altered rocks during Neoarchean magmatism generated low δ 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 δ 18 O magma from which zircon crystallized.Ultimately, in either scenario, the low δ 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.