On the behavior of rare-earth elements during partial melting of granitic rock

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Rare-earth element (REE) analyses are reported for a suite of granitic partial melts and their residual parent gneisses. Before melting, the gneiss possessed a light-REE-enriched pattern, with a negative Eu anomaly. With increasing melting, light REE were partitioned into the melt ( D > 1) while Eu and heavy REE were retained in the solid ( D > 1). The crystallised melt is not, however, significantly enriched in light REE or depleted in heavy REE relative to the residue. This is probably a consequence of variations in the partition coefficients concerned during fractional melting, in which the partition coefficients of light REE decreased from greater than to less than unity, while those for heavy REE rose from less than to greater than unity. The partial melt underwent fractional crystallization, during which Eu and light REE were partitioned into the solid ( D > 1), while heavy REE concentrated in the melt ( D > 1), leading to concave-up REE patterns in the more fractionated solids. These variations are not due to major minerals as the proportion of these are virtually constant. The data suggest that the behaviour of the REE in the suite studied (Eu excepted) is almost entirely controlled by minor minerals (other than refractory zircon and apatite) which have very large crystal/melt partition coefficients for the REE. Light REE are probably largely located in early melting minor mineral(s). This inference is supported by good coherence between La and Th. Heavy REE may be influenced by this early melting mineral(s) as well as by refractory minor minerals. The influence of such minerals severely restricts the use of REE as a petrogenetic tool in felsic rocks.