Temporal variation in oxygen isotopes of peraluminous granites derived from sedimentary sources

Abstract

The oxygen isotopic composition of magmatic rocks provides critical constraints on the contributions of crust versus mantle in their genesis. Peraluminous granites derived from partial melting of sedimentary rocks offer a unique archive to examine how the oxygen isotope composition of sedimentary rocks recycled into magmas has changed throughout Earth history. In this study, we explore this record through a two-pronged approach. First, we compile a comprehensive dataset of oxygen isotope bulk-rock and mineral analyses from globally distributed peraluminous granites and volcanic rocks ranging in age from 3.2 billion years to 6 million years. Second, we model the isotopic fractionation between sedimentary source rocks and derivative melts, as well as minerals in equilibrium with the melt during anatexis. Our compilation demonstrates a progressive increase in δ¹⁸O values of zircon and garnet from the Archean to Paleoproterozoic and again in the latest Neoproterozoic to Phanerozoic. Specifically, the average δ¹⁸O of zircon from peraluminous granites increases from 6.7 ± 1.0 ‰ to 9.5 ± 0.8 ‰ at ∼ 2.35 Ga, followed by another increase to 10.4 ± 0.7 ‰ after 600 Ma (errors are ± 1 s.d.). These observations align with the sedimentary rock record which suggests enhanced weathering and clay deposition resulting from tectonic and biological forcings. Furthermore, the increase in δ¹⁸O of peraluminous granites broadly mirrors similar secular increases in the δ¹⁸O values of siliciclastic sedimentary rocks. However, using our modeled fractionations between melt and source rock, the reconstructed source-rock δ¹⁸O values capture only the lower end of the siliciclastic sedimentary rock range, suggesting that sedimentary rocks recycled into magmas have, on average, lower δ¹⁸O values than coeval clay-rich shales. We propose that any global analyses of oxygen isotopes in magmatic rocks or zircon should use the average values of calculated source rocks for peraluminous granites in defining crustal contributions. Furthermore, our study highlights the broader implications of the observed trends, including the progressive contamination of the mantle and elevation of its δ¹⁸O values.