Crustal thickening and clay: Controls on O isotope variation in global magmatism and siliciclastic sedimentary rocks

- Whole rock δ18O data suggest increased clay production since the Archean.
- Stochastic models are used to assess variation of δ18O composition of zircon.
- Models of δ18O data indicate no secular change in mantle input into global magmatism.
- Changing δ18O of magmas and sediments is consistent with increased crust thickening.

New compilations of global O isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in δ18O values in both systems since the late Archean. This is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks. Each of these factors can arguably be achieved by increased crustal thickening in the late Archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks. Despite the suggested change in tectonic regimes in the late Archean, stochastic modelling in this study demonstrates that δ18O data do not provide evidence for a secular decrease in the proportion of mantle-derived magmas in granitoid rocks. Instead, best-fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term (100-200 Myr) tectonic cycles preserved in the continental crust. Hence, major step changes in global tectonic regimes in the post-Hadean, such as the initiation of subduction in the mid- to late Archean, are not supported by global zircon O isotope datasets and instead minor, progressive changes are indicated for Earth's tectonic regimes.