Experimentally determined partitioning of Rb between richterites and aqueous (Na, K)-chloride solutions

The distribution of Rb-Na and Rb-K between richterite and a 2-molal aqueous (Na, K, Rb)-chloride solution has been investigated with hydrothermal expe riments at 800 degrees C and 200 MPa. Experiments were performed as synthes es in which amphiboles grew in the presence of an excess fluid containing t he exchangeable cations Na+-Rb+ or Na+-K+-Rb+. The obtained amphiboles were large enough (up to 20 mu m in width) for reliable EMP analysis. They were chemically homogeneous and HRTEM investigations showed that they were stru cturally well ordered. The Rb, Na, K, Ca and Mg concentrations in coexistin g fluids were measured by ICP-AES. According to the possible incorporation of Na, K and Rb on the A-site, solid solutions in the ternary Na(NaCa) Mg-5 [Si8O22/(OH)(2)] (richterite)-K(NaCa)Mg-5[Si8O22/(OH)(2)] (K-richterite)-Rb (NaCa)Mg-5[Si8O22/(OH)(2)] (Rb-richterite) were expected. However, Rb-rich richterites always had significant amounts of A-site vacancy concentrations (X-square(amph) = square(A)/(Rb-A + K-A + Na-A + square(A)) of up to 0.42 in the K-free (Na,Rb)-richterites and of up to 0.67 in the (Na, K, Rb)-rich terites which corresponds to the same content of tremolite + cummingtonite- component. Amphiboles containing practically only Rb besides vacancies and no Na and/or K on the A-site were also synthesized, however. The Rb-Na and Rb-K exchange coefficients between fluid and richterites are similar. Rubid ium always fractionated strongly into the fluid phase. For low Rb-concentra tions in richterite (X-Rb(amph) < 0.1) a linear correlation between X-Rb(fl uid) and X-Rb(amph) exists. In this concentration range, the derived exchan ge coefficients KD(Rb-K)(amph-fluid) and KD(Rb-Na)(amph-fluid) were 0.08 +/ - 0.04 and 0.04 +/- 0.02, respectively. These low exchange coefficients sho w that significant amounts of Rb in amphiboles require a Rb-rich fluid phas e. The results indicate that K-Rb fractionation between alkali amphiboles a nd fluids is significantly different from K-Rb fractionation between alkali feldspar/phlogopite and fluid, with K(D)s of about 0.5 and 1.2, respective ly. Formation of richterites will drastically alter the K/Rb-ratios of flui ds or melts. These results may have important implications for the genetica l interpretation of various geological settings, e.g., MARID-type rocks.