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Geological observations and numerical models imply that Archean continents were mostly submarine. In contrast, approximately one third of modern earth's surface area consists of subaerial continental crust. To temporally constrain changes in the subaerial exposure of continents, we evaluate the eruptive environment (submarine vs subaerial) of 3.4–2.0 Ga continental large igneous provinces (LIPs). Our results indicate that up until 2.4 Ga LIPs predominantly erupted onto submerged continents. This period of low freeboard was punctuated by local subaerial eruptions at 2.8–2.7 Ga and 2.5 Ga. From 2.4 Ga–2.2 Ga, extensive subaerial continental volcanism is recorded in six different cratons, supporting widespread subaerial continents at this time. An increase in exposed continental crust significantly impacts atmospheric and oceanic geochemical cycles and the supply of nutrients for marine bioproductivity. Thus, the 2.4–2.2 Ga high-freeboard conditions may have triggered the earliest global glaciation event and the first significant rise of atmospheric oxygen.

The Archean-Proterozoic transition marks a time of fundamental geologic, biologic, and atmospheric changes to the Earth system, including oxygenation of the atmosphere (termed the Great Oxygenation Event; GOE), and the emergence of continents above sea level. The impacts of the GOE on Earth’s surface environment are imprinted on the geologic record, including the disappearance of mass-independent fractionation of sulfur isotopes (S-MIF). Temporally overlapping geologic and geochemical observations (e.g. a change in oxygen isotope ratio of sediments and an increase in subaerial volcanism) imply the widespread subaerial emergence of continents was coeval with atmospheric oxygenation. Here we present triple sulfur isotope ratios in pyrite and oxygen isotope ratios in garnet and zircon in a global suite of Archean and Proterozoic granitoids derived from the partial melting of sedimentary protoliths. These crustal melts record an increase in average garnet and zircon δ18O from 7.2‰ before 2.3 Ga to 10.0‰ post-2.3 Ga. Pre-2.3 Ga granitoids show small S-MIF signatures with Δ33S ranging from −0.29‰ to 0.13‰, whereas post-2.3 Ga granitoids record S-MDF (i.e. Δ33S = 0‰). The combination of sulfur and oxygen isotope signatures in the same sample with zircon U-Pb geochronology provides new insights on a potential causal link between the emergence of continents and Paleoproterozoic atmospheric oxygenation.

Subduction is a major process cycling material through Earth’s geochemical reservoirs. Although trends in chemical composition of arc magmas imply assimilation of metasediment, the degree of such assimilation and the loci of that metasediment contamination (whether via subducted sediment or country rock assimilation) are poorly understood. To address these issues, we explore compositional data of oceanic and continental arc systems from circum-Pacific subduction zones. We find that high-silica continental arc rocks of the circum-Pacific are associated with higher aluminium saturation indices interpreted to reflect higher degrees of metasediment assimilation, with Sr/Y suggestive of shallow emplacement levels within the crust. In contrast, high-silica oceanic rocks of the circum-Pacific display lower aluminosity and equilibrated at deeper levels within the crust. Continental arc basalts are often assumed to be the source of high-silica continental arc rocks. However, phase equilibrium modelling of partial melting and crystal fractionation of continental arc basalts yield results that question this assumption. Furthermore, continental arc rock compositions show that the assimilated metasediments have protoliths that are most probably felsic greywacke and pelite rather than mafic greywacke. These findings are consistent with the hypothesis that high-silica rocks in continental arcs are directly influenced by anatexis of metasediment at shallow crustal levels (

The Archean‐Proterozoic transition heralded a number of fundamental changes on Earth, including the oxygenation of the atmosphere, a marked emergence of continents above sea‐level, and an increase in δ¹⁸O of felsic magmas. The potential drivers for the latter are changes in the composition of supracrustal material or increased crustal reworking. Although the onset of subduction‐induced continental collision and associated enhanced crustal recycling could produce high‐δ¹⁸O felsic magmas, temporally constrained zircon δ¹⁸O reveals an increase in δ¹⁸O at ~2.35 Ga that predates the oldest widely recognized supercontinent. In this work, we use the O and Hf isotope ratios of magmatic zircon crystals in Archean and Proterozoic sediment‐derived granitoids of the North China Craton to track the incorporation of supracrustal material into magmas. The results are consistent with a Paleoproterozoic increase of continental freeboard producing sedimentary reservoirs with comparatively elevated δ¹⁸O that subsequently partially melted to generate the granitoids.

Numerous geologic proxies for Earth system processes track dramatic changes at the atmosphere-lithosphere or atmosphere-ocean interface during the early Paleoproterozoic Era. The presence of a geodynamic driver for these changes and how this might have affected the deeper lithosphere is more cryptic. Here we present temporally constrained δ18O and eHf in detrital zircon from Paleoproterozoic sedimentary successions in Western Australia and Canada that chart a rapid change in the oxygen isotopic composition from

The Archean-Proterozoic transition marks a time of fundamental geologic, biologic, and atmospheric changes to the Earth system, including oxygenation of the atmosphere (termed the Great Oxygenation Event; GOE), and the emergence of continents above sea-level. The impacts of the GOE on Earth’s surface environment are imprinted on the geologic record, including the attenuation of mass-independent fractionation of sulfur isotopes (S-MIF). Temporally overlapping geologic and geochemical observations (e.g. a change in oxygen isotope ratio of sediment melts) imply the widespread subaerial emergence of continents was coeval with atmospheric oxygenation. Here we present triple sulfur isotope ratios in pyrite and oxygen isotope ratios in garnet and zircon in a global suite of Archean and Proterozoic sediment-derived granitoids. These crustal melts record an increase in average 18O/16O isotope ratio and a disappearance of S-MIF in the Paleoproterozoic. The coupled behaviour of sulfur and oxygen isotope signatures imply a potential causal link between the emergence of continents and atmospheric oxygenation at ~2.3 Ga.

The detrital zircon oxygen isotope record has been used to describe 4.4 billion years of crustal evolution and maturation. Implicit in the application and use of large databases of oxygen isotope ratio ( δ 18 O) from detrital zircon is that such data are more representative of the history of the continental crust than are exposed rocks. Here we show that the δ 18 O of igneous zircon in preserved rocks varies greatly from the detrital zircon record over time, both in terms of the magnitude of δ 18 O and the secular variation. Starting from 4.0 Ga and lasting throughout the Archean, the detrital zircon record shows consistently higher δ 18 O values than the rock record. In post-Archean times, detrital zircon δ 18 O is conspicuously higher during periods associated with supercontinent amalgamation. Differences between detrital and igneous zircon δ 18 O records imply that the igneous source rocks from which high δ 18 O detrital zircon are derived have been entirely eroded and are not represented in the known geological record. We postulate that the ‘missing’ record of high δ 18 O magmatic rocks consists of upper crustal sedimentary-derived melts from collisional belts, such as that found in the Himalayan orogen. The observation of higher δ 18 O in Archean detrital zircon further requires a revision of the current paradigm for Archean magmatism. More broadly, recognition of a ‘missing’ component of the upper crustal rock record calls into question the degree to which the detrital zircon record accurately reflects the composition of the bulk continental crust.

ISSN:1525-2027

The mineral zircon is stable across a wide range of temperature and pressure conditions, and the most widely used geochronometer in felsic rocks. The oxygen isotope ratio recorded by zircon crystals is a sensitive tracer for crustal recycling in magmatic rocks and is a commonly used tool to address zircon petrogenetic questions (e.g. in studies on the evolution of geodynamics and continents). However, secondary processes (i.e. water uptake facilitated by radiation damage) can alter the oxygen isotopic composition of zircon. We present in-situ oxygen isotopic and 16O1H/16O data of natural zircon samples in combination with zircon trace element concentrations, and parameters that quantify crystal damage (i.e. Raman spectral parameters and alpha dose). In agreement with previous studies, our results demonstrate that the uptake of water into the zircon crystal structure strongly influences its recorded oxygen isotope ratio. Two distinct secondary processes are recorded which shift zircon δ18O values towards isotopically heavier and lighter values, respectively. Lower δ18O values associated with broadening of the zircon Raman bands and increasing 16O1H/16O are the result of a local charge-balance process during the interaction of radiation-damaged zircon with meteoric water. In contrast, an equilibrium fractionation process between zircon and percolating water leads to higher δ18O values with increasing 16O1H/16O. The disturbance of the oxygen isotopic system in zircon can be assessed through monitoring zircon 16O1H/16O; a parameter that is highly sensitive to the influx of water into radiation-damaged areas of the zircon grain, providing a powerful tool to access the validity of primary δ18O signatures.