ABSTRACT

The distribution of REE (La and Ce) in a hornblende cumulate was investigated with an electron probe microanalyser. The cumulate consists of cumulus kaersutite, augite, titanomagnetite and apatite and such intercumulus minerals as green hornblende, titanomagnetite, plagioclase, apatite and sphene. La and Ce are concentrated in grain boundaries between minerals as well as in apatite and sphene. Grains of cumulus apatite are homogeneous in REE contents, except for rims in contact with augite where REE contents conspicuously increase towards augite. Intercumulus apatite and sphene exhibit decreasing REE distributions from the core to the rim, which are followed by increases when the rims have contact with augite, kaersutite and green hornblende. The constant and the outward decreasing REE distributions may be best explained in terms of surface equilibrium crystallization from infinite main liquid and finite pore liquid, respectively. The formation of REE-rich rims of apatite and sphene and the grain boundary concentration of REE are hardly explained by the model, but may be attributed to a subsolidus diffusion process.