Ion Microprobe Determination of Rare Earth Elements in Perovskite from Kimberlites and Alnöites

Roger H. Mitchell and Stephen J. B. Reed
Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada
Bullard Laboratories, Department of Earth Sciences, Madingley Road, Cambridge CB2 0EZ, England

Abstract: Ion microprobe analysis ofperovskite from kimberlites and alnöites permits the accurate determination of La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb at the part per million level. Other rare earth elements (REE) are subject to interferences in the mass spectrum caused by matrix-derived molecular ions and cannot easily be determined with comparable precision. Chondrite-normalized plots of the ion probe REE data are smooth curves, confirming the superiority of this technique over electron microprobe methods at the levels of REE abundance found in these perovskites. The perovskites analysed contain between 2.8 and 7.1 wt. % REE oxides and are highly enriched in the light REE, having La/Yb ratios of 577–3229. These La/Yb ratios are not representative of the parental magmas but result from REE fractionation during crystallization of the perovskite. Parental magma La/Yb ratios are estimated to be of the order of 120–650. The lower La/Yb ratios (80–200) found for whole-rock kimberlites are considered to result from contamination by relatively heavy REE-enriched crustal material.

Keywords: ion microprobe • rare earth elements • perovskite • kimberlite • alnöite

Mineralogical Magazine; June 1988 v. 52; no. 366; p. 331-339; DOI: 10.1180/minmag.1988.052.366.04
© 1988, The Mineralogical Society
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