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The geochemistry and cryptic zonation of pyrochlore from San Vicente, Cape Verde Islands

Published online by Cambridge University Press:  05 July 2018

N. A. Hodgson  and
M. J. Le Bas
N. A. Hodgson
Affiliation:
Department of Geology, Leicester University, Leicester LE1 7RH
M. J. Le Bas
Affiliation:
Department of Geology, Leicester University, Leicester LE1 7RH
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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

pyrochlorecryptic zonationsector-zonationcarbonatiteCape Verde Islands
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 383 , June 1992 , pp. 201 - 214
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

*

Present address: British Gas Corporation, 59 Byranston Street, London W1A 2AZ.

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