Approaches to equilibrium in the distribution of trace elements among the principal minerals in a high‐grade metamorphic terrane

Abstract

X‐ray fluorescence, instrumental neutron activation, and particle‐induced X‐ray emission methods were used to determine the distribution of numerous trace elements among garnet (Grt), Ca‐pyroxene (Cpx), hornblende (Hbl), biotite (Bt), plagioclase (Pl) and K‐feldspar (Kf) in a high‐grade metamorphic terrane within the Grenville Province of the Canadian Shield. Results are presented as distribution formulae, e.g.Sr: Kf 1.1 Pl 16 Hbl 2.2 Cpx 1.0 Bt 1.2 GrtSc: Hbl 1.1 Cpx 1.0 Grt 7.8 Bt 22 Pl 2.6 KfV: Hbl 1.15 Bt 2.07 Cpx 6.0 Grt (1.4% CaO)>1 (Pl, Kf)Zn: Bt 1.6 Hbl 1.62 Cpx 2.9 Grt 10 PlGa: Bt 1.2 Hbl 1.2 Pl 2.5 Cpx 1.3 Grtwhere numbers are distribution ratios, e.g. ppm Sr in Hbl/ppm Sr in Cpx=2.2. Examples of inter‐element similarities and differences are (a) both Rb and Cs are concentrated in biotite relative to K‐feldspar, but for Rb the ratio is 2.3 and for Cs it is 16, (b) the distribution formulae for seven lanthanides are similar except for the position of garnet, e.g.Ce: Hbl 2.7 Cpx 2.8 Pl 1.1 Bt 11 Kf 16 GrtYb: Grt 2.8 Hbl 2.7 Cpx 9 Pl 1.0 Bt 7 Kfand (c) all of Sr, eight lanthanides, Zr, V and Cr are concentrated in hornblende relative to Ca‐pyroxene by a factor that lies in the narrow range of 2.2–3.1. There is a larger variation (departure from the mean) in some distribution ratios than in others. Thus the mean ratios (Hbl/Cpx) for each of six elements and in parentheses the percentage relative standard deviation are Zn 1.62 (8.6), V 2.38 (12), Cr 2.42 (18), Sr 2.7 (28), Ba 2.9 (36) and Ni 1.66 (38). We suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization (which facilitated the rearrangement of atoms), combined with rates of lattice and crystal‐boundary diffusion that are unique for the various elements, thus permitting some trace elements to approach equilibrium more closely than others.