Experimentally determined partitioning of Rb between richterites and aqueous (Na, K)-chloride solutions |
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Authors: | Stefan Melzer Matthias Gottschalk Wilhelm Heinrich |
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Institution: | (1) GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany; Phone: 0331-288-1483; Fax: 0331-288-1402; E-mail: smelzer@gfz-potsdam.de, DE |
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Abstract: | 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 experiments at 800∘C and 200 MPa. Experiments were performed as syntheses in which amphiboles grew in the presence of an excess fluid containing
the exchangeable cations Na+-Rb+ or Na+-K+-Rb+. The obtained amphiboles were large enough (up to 20 m in width) for reliable EMP analysis. They were chemically homogeneous
and HRTEM investigations showed that they were structurally well ordered. The Rb, Na, K, Ca and Mg concentrations in coexisting
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) Mg5Si8O22/(OH)2] (richterite)-K(NaCa)Mg5Si8O22/(OH)2] (K-richterite)-Rb(NaCa)Mg5Si8O22/(OH)2] (Rb-richterite) were expected. However, Rb-rich richterites always had significant amounts of A-site vacancy concentrations
(X
□
amph=□
A
/(RbA+KA +NaA+□A) of up to 0.42 in the K-free (Na,Rb)-richterites and of up to 0.67 in the (Na, K, Rb)-richterites 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. Rubidium always fractionated strongly into the fluid phase. For low Rb-concentrations in richterite (X
Rb
amph<0.1) a linear correlation between X
Rb
fluid and X
Rb
amph exists. In this concentration range, the derived exchange coefficients K
D(Rb−K)
amph−fluid and K
D(Rb−Na)
amph−fluid were 0.08 ± 0.04 and 0.04 ± 0.02, respectively. These low exchange coefficients show that significant amounts of Rb in amphiboles
require a Rb-rich fluid phase. The results indicate that K-Rb fractionation between alkali amphiboles and fluids is significantly
different from K-Rb fractionation between alkali feldspar/ phlogopite and fluid, with KDs of about 0.5 and 1.2, respectively. Formation of richterites will drastically alter the K/Rb-ratios of fluids or melts.
These results may have important implications for the genetical interpretation of various geological settings, e.g., MARID-type
rocks.
Received: 6 October 1997 / Accepted: 7 July 1998 |
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Keywords: | |
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