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SIR-So-called hotspot magmas, including ocean island basalts and continental flood basalts, differ isotopically from midocean-ridge basalts (MORB) and require a reservoir, or a component, that has been isolated from the MORB source for more than 2 billion years. It has been widely assumed that a suitable isolated reservoir might be the continent lithosphere. Most authors refer to'lithosphere'as the strong outer shell of the _Earth. Geophysics1 and rock physics2 show that the silicates of the mantle have little strength at temperatures greater than 650±100 C or about half the absolute melting temperature. Some authors, however, refer to the thermal boundary layer as' lithosphere'even though it extends to about 1,300 C, near or above the silicate solidus3. The thermal boundary layer has a close to critical Rayleigh number and cannot remain attached to the plate or accumulate long-term isotope anomalies3 unless it …