An experimental study of alkali metal distributions in feldspars and micas

Abstract

K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm 2 total pressure. Complete solid solution of RbMg 3AlSi 3O 10(OH) 2 in KMg 3AlSi 3O 10(OH) 2 exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated. Only limited solid solution of Rb AlSi 3O 8 in KAlSi 3O 8 has been found at all temperatures investigated. Distribution coefficients, expressed as K d = ( Rb/K) in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined K D values and their approximate Rb end-member mole fraction ( X RM ) ranges of constancy are summarized as follows: (°C) T K D Phlog/Vap. X RM K D Sandi/Vap. X rm (°C) T K D Phlog/Vap. X RM K D Sanid/Vap. X RM 500 0.64 ± 0.11 0–0.2 0.17 ± 0.04 0–0.07 700 1.11 ± 0.11 0–0.2 0.33 ± 0.04 0–0.1 800 1.28 ± 0.03 0–0.2 0.45 ± 0.06 0–0.1 Phlogopite-sanidine K D values calculated from this data increase from 2.9 at 800°C to 4.1 at 500°C, Analogous K D values calculated for biotite-alkali feldspar pairs from the literature are usually in the range 2.5–4.5 for igneous and metamorphic rocks with the latter having the higher values whilst volcanic rocks generally have the lowest values. Comparisons of the experimental data with available analytical data indicate that the distribution coefficient may be of value as a criterion for equilibrium and as a reference thermometer as well as for distinguishing metamorphic-metasomatie granitic rocks from magmatic rocks of similar composition. Preliminary results on K/Rb distributions between granitic melts and the chloride solutions at 800°C indicate a K D value close to unity and suggest that the crystal-vapour K D values may be used as a first approximation for crystal-melt distributions. Such tentative crystal—melt K D values emphasize the influence of mineralogy in controlling Rb/K trends during fractionation. Extrapolation of the tentative phlogopite-melt data to mantle temperatures indicate the possibility of deriving high K/Rb (1000) liquids from phlogopite bearing mantle material having an initial K/Rb of approximately 500. K/Rb values for successive liquid fractions derived by repeated fractional melting of such material should show a steady decrease and final residues should have low K/Rb (<200).