Using a compilation of mantle peridotites (n = 2153), we statistically evaluate the frequency of Si enrichment in cratonic and other types of mantle lithosphere. We define an empirical parameter (ΔMg/Si) describing the degree of Si enrichment or depletion in mantle lithosphere relative to residue trends defined by high-pressure, high-temperature melting experiments. Silica enrichment (a strong skew to negative ΔMg/Si) is absent in the majority of cratonic xenoliths, and mostly occurs only in those from southern Africa, and in abyssal peridotites. The Si-rich composition of cratonic mantle, where it occurs, can be linked to the hydration of its protoliths on the Archean ocean floor before being subducted or imbricated to form a craton root. Oxygen isotopic shifts that correlate with bulk Mg/Si in mid-Atlantic ridge seafloor rocks parallel those seen in the few such data for cratonic peridotite xenoliths, in support of our hypothesis. Chemical variability in the mantle is canonically viewed to have originated from the bottom up by percolating melts. We turn this idea on its head, and explain how Si enrichment in the cratonic lithosphere could have originated by a top-down chemical exchange during weathering or hydrothermal activity when such peridotites resided on an Archean ocean floor.