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Journal of Petrology Advance Access published online on August 10, 2006
Journal of Petrology, doi:10.1093/petrology/egl040
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received November 18, 2005
Accepted July 10, 2006

Article

Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P-T and Geochemical Studies on Xenoliths

BENJAMIN KAESER 1 *, ANGELIKA KALT 1, and THOMAS PETTKE 2

1 INSTITUT DE GÉOLOGIE ET D'HYDROGÉOLOGIE, UNIVERSITÉ DE NEUCHÂTEL, RUE EMILE-ARGAND 11, CASE POSTALE 158, CH-2009 NEUCHÂTEL, SWITZERLAND
2 ISOTOPE GEOCHEMISTRY AND MINERAL RESOURCES, DEPARTMENT OF EARTH SCIENCES, FEDERAL INSTITUTE OF TECHNOLOGY, ETH ZENTRUM NO, CH-8092 ZÜRICH, SWITZERLAND; Present address: Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland

* To whom correspondence should be addressed.
BENJAMIN KAESER, E-mail: benjamin.kaeser{at}unine.ch


  Abstract

Xenoliths hosted by Quaternary basanites and alkali basalts from Marsabit (northern Kenya) represent fragments of Proterozoic lithospheric mantle thinned and chemically modified during rifting in the Mesozoic (Anza Graben) and in the Tertiary-Quaternary (Kenya rift). Four types of peridotite xenoliths were investigated to constrain the thermal and chemical evolution of the lithospheric mantle. Group I, III and IV peridotites provide evidence of a cold, highly deformed and heterogeneous upper mantle. Textures, thermobarometry and trace element characteristics of minerals indicate that low temperatures in the spinel stability field (~750-800°C at <1·5 GPa) were attained by decompression and cooling from initially high pressures and temperatures in the garnet stability field (970-1080°C at 2·3-2·9 GPa). Cooling, decompression and penetrative deformation are consistent with lithospheric thinning, probably related to the development of the Mesozoic to Paleogene Anza Graben. Re-equilibrated and recrystallized peridotite xenoliths (Group II) record heating (from ~800°C to ~1100°C). Mineral trace element signatures indicate enrichment by mafic silicate melts, parental to the Quaternary host basanites and alkali basalts. Relationships between mineral textures, P-T conditions of equilibration, and geochemistry can be explained by metasomatism and heating of the lithosphere related to the formation of the Kenya rift, above a zone of hot upwelling mantle.

Keywords: East African Rift System; Anza Graben; in situ LA-ICPMS; peridotite xenoliths; thermobarometry.
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