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 delta O-18 (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 (epsilon(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 delta O-18 values (2.5%-4.8%, n = 9 of 15) across a range of ages (3536-2598 Ma; epsilon(Hf) = -18.2 to + 0.4). The low delta O-18 grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low delta O-18 magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low delta O-18 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 delta O-18 zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low delta O-18 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 delta O-18 magma from which zircon crystallized. Ultimately, in either scenario, the low delta O-18 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.