Continental igneous rock composition: A major control of past global chemical weathering.

The composition of igneous rocks in the continental crust has changed throughout Earth's history. However, the impact of these compositional variations on chemical weathering, and by extension on seawater and atmosphere evolution, is largely unknown. We use the strontium isotope ratio in seawater [(87 Sr/86 Sr)seawater ] as a proxy for chemical weathering, and we test the sensitivity of (87 Sr/86 Sr)seawater variations to the strontium isotopic composition (87 Sr/86 Sr) in igneous rocks generated through time. We demonstrate that the 87 Sr/86 Sr ratio in igneous rocks is correlated to the epsilon hafnium (εHf) of their hosted zircon grains, and we use the detrital zircon record to reconstruct the evolution of the 87 Sr/86 Sr ratio in zircon-bearing igneous rocks. The reconstructed 87 Sr/86 Sr variations in igneous rocks are strongly correlated with the (87 Sr/86 Sr)seawater variations over the last 1000 million years, suggesting a direct control of the isotopic composition of silicic magmatism on (87 Sr/86 Sr)seawater variations. The correlation decreases during several time periods, likely reflecting changes in the chemical weathering rate associated with paleogeographic, climatic, or tectonic events. We argue that for most of the last 1000 million years, the (87 Sr/86 Sr)seawater variations are responding to changes in the isotopic composition of silicic magmatism rather than to changes in the global chemical weathering rate. We conclude that the (87 Sr/86 Sr)seawater variations are of limited utility to reconstruct changes in the global chemical weathering rate in deep times.