Experimental determination of radium partitioning between leucite and phonolite melt and 226Ra-disequilibrium crystallization ages of leucite

Radium and other trace element (i.e. Cs, Rb, Ba, Sr, Cd, Cu, Pb, Zn, La, Lu, Nd, Sm, Y, Sn) partition coefficients have been experimentally determined at atmospheric pressure for leucite in equilibrium With a phonolitic melt. Ra bulk concentrations of similar to 0.25 ppm are sufficient for precise LA-ICP-MS measurements of Ra in leucite and melt. Partition coefficients increase for the alkalis from Rb to Cs, and for the alkaline earths from Sr to Ba to Ra, with the maximum of both lattice strain parabolas at a radius of 1.89 +/- 0.01 angstrom, i.e. near Cs and Ra. At 1190 degrees C, D-Ra(leucite/melt) was determined to 2.3 +/- 0.7 and 1.8 +/- 0.5 for two different experiments, the average D-Ra/D-Ba is 4.2 +/- 0.5. Hitherto, crystallization ages calculated from the isotopic Ra-226 disequilibrium assumed that D-Ra equals D-Ba. The experimentally determined partition coefficients lead to an order of magnitude younger Ra-226 crystallization ages for leucite. Employing our partitioning data in combination with the isotopic results from Black, S., Macdonald, R., DeVivo, B., Kilburn, RJ., Rolandi, G. U-series disequilibria in young (A.D. 1944) Vesuvius rocks: Preliminary implications for magma residence times and volatile addition. J. Volcanol. Geotherm. Res. 82, 97-111, leucite crystal ages from several phonothephrite lavas of the 1944 eruption of Vesuvius become undefined, testifying for a more complicated process than equilibrium leucite crystallization and subsequent undisturbed evolution of these lavas. (C) 2008 Elsevier B.V. All rights reserved.