The lanthanide tetrad effect in apatite from the Altay No. 3 pegmatite, Xingjiang, China: an intrinsic feature of the pegmatite magma

Apatite and associated minerals, such as spessartite, tourmaline, beryl and alkali feldspar, from the Altay No. 3 granite pegmatite in Xinjiang, China, have highly variable concentrations of rare earth elements (REEs) and possess chondrite-normalized REE patterns that show a clear convex tetrad effect. The Y/Ho and Sr/Eu ratios of the apatite samples are extremely fractionated relative to the corresponding chondritic values. Based on the La/Lu, Sr/Eu and Y/Ho ratios of these minerals, the crystallization zones of the pegmatite can be divided into two groups that are interpreted to reflect two main stages of magma evolution, namely, a magmatic and a magmatic hydrothermal transition stage. gree of the lanthanide tetrad effect in REE patterns of the apatite, expressed by the quantification factor TE1,3, ranges from 1.3 to 1.9. The size of the tetrads correlates with Y/Ho, Eu/Eu* and Sr/Eu ratios. In addition to the mineral samples from the pegmatite, the REE patterns of the altered wall rocks also show a convex tetrad effect. A mass balance calculation was carried out to constrain the REE partitioning between the aqueous fluid, crystalline phases and the residual melt. It was found that the REE patterns of these three phases are dominated by the concentration of the REEs in the initial pegmatite-forming melt. Accordingly, it is proposed that the REE tetrad effect and the fractionation of the chemically coherent isovalent element pairs in the studied mineral separates represent intrinsic features of the granitic pegmatite-forming melt. It appears unlikely that these geochemical characteristics developed as a result of melt/mineral/fluid interactions taking place during magma crystallization after its intrusion. Interaction of an external fluid with pegmatite magma before emplacement or with parent materials of the highly evolved granitic rocks before melting would be favorable for producing the REE tetrad effect in highly evolved granitic rocks.