Paleoproterozoic increase in zircon δ18O driven by rapid emergence of continental crust

Coupling of the atmosphere and sediment melts across the Archean-Proterozoic transition

“…Given the small equilibrium fractionation (smaller than the analytical uncertainty) of oxygen isotopes between almandine-rich garnet and zircon at temperatures typical for granitoid melts 27 (Figure 1) δ 18 O values recorded by garnet and zircon, respectively, in the sediment-derived granitoids of this study are directly comparable. The range and average δ 18 O values of sediment-melts increases post-2.3 Ga (Figure 3), in accord with previous studies that interpret the rise of average δ 18 O to be the result of continental emergence and a concomitant change in sediment composition 24,25 . Low δ 18 O values in sediment melts (equal or lower than the mantle value of 5.3 ± 0.6‰; 22 , like in 19GH9 and 19GH11B, are interpreted to re ect hydrothermal alteration of the protolith 37 .…”

“…An increase in average δ 18 O of zircon in the Paleoproterozoic has been recognized for over a decade 20 . Contrasting models have been suggested to account for this increase in δ 18 O; 1) the formation of high δ 18 O sediments (and sediment-derived melts) potentially linked to enhanced subaerial weathering and erosion related to the emergence of continents above sea-level 24,25 , and 2) enhanced crustal recycling associated with the onset of collisional tectonics 26 . However, high-δ 18 O sediment melts do not show a concomitant depletion in radiogenic 176 Hf, as would be expected if the increase in δ 18 O was related to enhanced supracrustal recycling 25 .…”

Emergence of continents above sea‐level influences sediment melt composition

“…Archean zircon has uniform δ 18 O values averaging 5.8 ± 1.0‰ (1σ, n = 2,644), whereas post-Archean zircon δ 18 O is more variable and on average higher (6.8 ± 1.8‰; 1σ, n = 13,314) (Spencer et al, 2014;Valley et al, 2005). Statistical change-point analysis of 2.8-1.9 Ga detrital zircon data (n = 452) suggest that the initial rise in average zircon δ 18 O occurred at ~2.35 Ga (Spencer et al, 2019). One way to achieve higher δ 18 O felsic magmas is through enhanced assimilation of supracrustal material (Kirkland et al, 2010;Spencer et al, 2014) driven by a change in the geodynamic regime (i.e.…”

“…Sediment-derived granitoids with crystallization ages <2.0 Ga have a wider range of zircon δ 18 O from 4.7 ± 0.9‰ to 11.4 ± 1.0‰, but on average have higher δ 18 O than older samples (Figure 4). (Spencer et al, 2019). This is not surprising given detrital zircon are sourced from diverse magmatic systems, with both mantle and supracrustal components.…”

“…A model involving crustal reworking has been invoked to explain the rise of average zircon δ 18 O at ~2.35 Ga (Spencer et al, 2019).…”