Possible effects of grain-boundary REE on the REE distribution in felsic melts derived by partial melting.

Abstract

Abundances of rare earth elements (REEs) in the whole rock and coexisting accessory minerals from a sample of the Cretaceous Naegi granite from central Japan, were determined with isotope dilution and electron microprobe methods. The granite contains the following amounts of accessory minerals: 853.8 mg fluorite, 215.2 mg apatite, 166.5 mg ilmenite, 145.5 mg zircon, 78.1 mg monazite, 10.5 mg fergusonite, 9.5 mg allanite, 3.6 mg rutile, 1.7 mg columbite and 1.4 mg thorite per 1 kg. REEs in the granite reside mainly in the accessory minerals, but the contribution of the accessories to the whole-rock REE abundance becomes less important toward the light REE end; the sum of REEs in the accessories accounts for only 53±6% of the whole-rock La, 59±6% Ce, 62±8% Nd, 66±10% Sm and 74±11% Gd-Yb. The unaccountable REEs in the rock are stored at the grain-boundaries of constituent minerals. The chondrite-normalized REE pattern for the grain-boundary (whole-rock minus sum of accessories) of the granite is light REE-enriched and consists of two linear segments with turning point at Gd. Since grain-boundaries are considered to melt first at partial melting, the early-formed melt will retain the REE characteristics of grain-boundaries. Some granitoids, which possess REE patterns consisting of two linear segments with turning point at around Gd and are considered to be formed by partial melting of crustal rocks, may have originated from source rocks with a grain-boundary REE distribution like that of the Naegi granite.