The Rare Earth Content and Origin of Alkali-Rich Basalts

The rare earth content of alkali-rich, silica-undersaturated basalts erupted onto oceanic and continental plates is consistent with their derivation by slight amounts of partial melting of hydrous garnet peridotite at the top of the asthenosphere, within the low-velocity, low-Q zone of the mantle. The rare earths are fractionated during partial melting by retention of heavy rare earths relative to light rare earths in garnet and clinopyroxene in the mantle residue. We conclude from a melting model that the amount of melting of hydrous peridotite at the top of the asthenosphere and its clinopyroxene-garnet ratio increase with decreasing thickness of the overlying dry lithospheric plate. Thus, potassic basalts derived by 0.5% melting of peridotite with a clinopyroxene-garnet ratio of 0.3 come from deeper than nephelinites (1% melting, ratio of 1), and alkali basalts (2% melting, ratio of 5). Convection within the partly molten asthenosphere establishes an adiabatic geothermal gradient and localizes renewable magma sources into updwelling plumes, or hot spots, that remain relatively stationary with respect to the overlying lithospheric plates. Unlike the source regions of midoceanic ridge tholeiite basalts, the peridotite source regions of the alkali-rich basalts do not appear to have been previously depleted in large cations (Ba, Rb, light rare earths) and generally have more radiogenic Sr and Pb. If the peridotite source regions of the alkali-rich basalts have chondritic proportions of the rare earths, the partial melting model indicates two to five times chondritic abundance of the rare earths in the peridotite.