Abstract
Oxygen isotopic ratios are largely homogenous in the bulk of Earth's mantle but are strongly fractionated near the Earth's surface, thus these are robust indicators of recycling of surface materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ(18)O in the shallowest continental lithospheric mantle since the Archean, no change in Δ'(17)O is observed. Younger samples document a decrease and greater heterogeneity of δ(18)O due to the development and progression of plate tectonics and subduction. We posit that δ(18)O in the oldest Archean samples provides the best δ(18)O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ(18)O may explain the increasing δ(18)O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ(18)O sediment accretion and low-δ(18)O mantle in subduction zones.