The Geochemistry and Cryptic Zonation of Pyrochlore from San Vicente, Cape Verde Islands

N. A. Hodgson* and M. J. Le Bas
Department of Geology, Leicester University, Leicester LE1 7RH
*Present address: British Gas Corporation, 59 Byranston Street, London W1A 2AZ.

Abstract: Zr-rich pyrochlore crystals in carbonatite from San Vicente (Cape Verdes Islands) show cryptic, concentric and rhythmic chemical zonation with Ca increasing and Ti, U and Zr decreasing towards the rims. In one carbonatite, taken from the Camile dyke, previously undocumented cryptic sector zonation of Ti, U and Zr is also observed in these crystals. The chemical variation is investigated by wavelength-dispersive electron microprobe, with both single spot and crystal map analyses.

The concentric and rhythmic zonation, marked by element substitution, was generated by magma heterogeneity and/or element diffusion kinetics, but it is suggested that the sector zonation, marked by differential site substitution, was governed by protosite variation between octahedral and cubic faces.

Keywords: pyrochlore • cryptic zonation • sector-zonation • carbonatite • Cape Verde Islands

Mineralogical Magazine; June 1992 v. 56; no. 383; p. 201-214; DOI: 10.1180/minmag.1992.056.383.06
© 1992, The Mineralogical Society
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