Contrary to simple generalizations about their behavior, light rare-earth elements (LREE) do not act as incompatible elements in very felsic magmas. In fact, LREE concentrations typically decrease, often drastically, during differentiation of such magmas. The simplest explanation for this depletion involves the separation of minute, easily overlooked quantities of an extremely LREE-rich accessory mineral, either monazite or allanite. Our data indicate that felsic liquids with < 50 ppm LREE may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids. This accounts for incompatible behavior of LREE even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas. Partitioning of LREE into solid rather than liquid has important implications for trace-element and Nd-isotope modeling of crustal anatexis, as well as for magma differentiation.

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