Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thickness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consistent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation requires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar 'garnet signature' to all OIB melts (e.g. [Sm/Yb](N) > 1); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution. The dilution effect is also recorded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobarometry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such information. For plate ages up to similar to 80 Ma, we demonstrate that the geophysically defined base of the growing oceanic lithosphere corresponds to both an isotherm (similar to 1100 degrees C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H(2)O-CO(2)-bearing mantle peridotite is stable under conditions of T 1100 degrees C and P 3 GPa (similar to 90 km), this solidus is essentially isothermal (i.e. dT/dP similar to 0 in P-T space) with T similar to 1100 degrees C) at depths 90 km, but becomes isobaric (i.e. dP/dT similar to 0 in P-T space) at the similar to 90 km depth. The latter explains why older (> 70 Ma) oceanic lithosphere cannot be thicker than similar to 90 km without the need to invoke physically complex processes such as convective removal.