Enrichment of incompatible elements at grain-boundaries of olivine in an olivine-nephelinite

Abstract

Variations in the concentrations of trace elements at and near the subgrain-boundaries within olivine phenocrysts in an olivine-nephelinite were inspected with an electron microprobe. The olivine phenocrysts show homogeneous core (Fo₈₅) surrounded by the Fe-richer rim. The core contains 160ppm Ti, 210ppm Al, 170ppm Cr and 140ppm Na, whereas the subgrain-boundary in the core contains 350ppm Ti, 310ppm Al, 490ppm Cr and 590ppm Na. The trace elements with their olivine/bulk-rock concentration ratios smaller than unity (i.e., Ti, Al, Cr, La, Ce, Zn, Ca, Na and K) are enriched at grain-boundaries and cleavages, whereas the concentrations of elements with the ratios larger than unity (i.e., Ni, Mn and Co) are approximately constant. The grain-boundary enrichment of the incompatible elements can be explained in terms of their kinetic segregation on cooling. The present study suggests that the partition coefficients obtained from element concentrations in the phenocrystic minerals and matrix of volcanic rocks do not essentially represent those in magmatic processes in nature. This is important when partitions of trace elements are used in modeling magmatic processes.