Rare-earth Partition Between Allanite and Glass in the Obsidian of Sandy Braes, Northern Ireland

C. K. Brooks, P. Henderson and J. G. Rønsbo
Institut for Petrologi, Øster Voldgade 10, DK-1350 København K, Denmark
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, UK
Institut for Mineralogi, Øster Voldgade 5–7, DK-1350 København K, Denmark

Abstract: Allanite phenocrysts and co-existing glass from the perlitic obsidian of Sandy Braes have been analysed for nine rare earths (RE), uranium, and thorium by instrumental neutron activation analysis and for the major elements by electron microprobe. The chondritenormalized RE plot for the allanite shows a steep slope with a negative Eu anomaly. Allanite/glass partition coefficients show a smooth variation with ionic radius (except for Eu), the variation spanning two orders of magnitude. The partitioning behaviour, which is distinct from that shown by the RE in sphene, apatite, and zircon, can be explained by the allanite structure. The pronounced affinity of the light RE for allanite makes this an important mineral in considerations of RE concentrations during the evolution of granitic liquids.

Mineralogical Magazine; June 1981 v. 44; no. 334; p. 157-160; DOI: 10.1180/minmag.1981.044.334.07
© 1981, The Mineralogical Society
Mineralogical Society (www.minersoc.org)