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Chemical effects of deuteric alteration in some Kenyan trachyte lavas

Published online by Cambridge University Press:  05 July 2018

W. B. Jones
W. B. Jones*
Affiliation:
Esso Exploration (Europe-Africa) Inc., St. Clements House, Church Street, Walton-on-Thames, Surrey, KT12 2QL
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Abstract

The Kenyan trachyte volcanoes Kilombe and Londiani show some unusual chemical features. Analyses of elements normally regarded as residual in salic peralkaline rocks do not show the linear covariance characteristic of these elements in other East African volcanoes. The scatter from expected trends is greatest for Y and the lanthanides, less for Nb and Rb, and not apparent for Zr. The lavas also show variable loss of Na, P, and Fe. It is suggested that this chemical deviation is the result of a deuteric event during the later stages of crystallization of the lavas. The flows showing the chemical deviations, in particular negative Ce anomalies, also tend to have their ferroaugite phenocrysts deuterically altered to a mineraloid closely resembling iddingsite.

During the crystallization of these trachyte lavas an H2O-, Cl-, and F-rich vapour separates from the liquid. This absorbs some elements, particularly Na, P, Fe, Y, and the lanthanides from the silicate liquid as water-soluble or volatile halides. Some of the absorbed material is redeposited in other parts of the flow and the rest is lost altogether as the vapour escapes from the surface.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 335 , September 1981 , pp. 279 - 285
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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