Zoning in columbite-tantalite crystals from the granitic pegmatites of the Eräjärvi area, southern Finland

Abstract

Electron microprobe analyses of zoned columbite-tantalite crystals from the granitic pegmatites of the Eräjärvi area in Orivesi, southern Finland indicate wide compositional variation within the series FeNb 2O 6-FeTa 2O 6-MnNb 2O 6-MnTa 2O 6, especially in specimens from thin pegmatite dikes. Most crystals show gentle progressive zoning characterized by small-scale variations in the major elements. Where the compositional variation is large, backscattered electron images indicate oscillatory or patchy zoning or various replacement textures. The zones in oscillatory zoned crystals are usually 1–50 μm in width, exceptions reaching 50–120 μm. The wider zones often consist of a group of very narrow subzones of only slightly different composition. Zoning is due mainly to the compositional variation in Ta and Nb. In two of these crystals, the oscillations in Mn follow the strongest oscillations in Nb content. Patchy zoned crystals exhibit corroded cores of early columbite-tantalite, surrounded by later zones enriched in Ta. The mottled appearance of such crystals results from two or more successive replacements. Replacement tongues or network-like replacement textures are typical in the rims of some crystals. The zoning of the columbite-tantalite is related to the complex crystallization history of the pegmatite dikes. The main factors controlling oscillatory zoning are considered to be the growth dynamics of the crystals, the concentration and diffusion of the main elements, and the successive flows of the magma in an intrusion channel. The generation of a corrosive supercritical vapor phase at the end of magmatic crystallization caused resorption, patchy zoning and the replacement of the columbite-tantalite.