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Journal of Petrology Advance Access originally published online on July 25, 2007
Journal of Petrology 2007 48(9):1725-1760; doi:10.1093/petrology/egm036
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)

Benjamin Kaeser1,*, Angelika Kalt1 and Thomas Pettke2

1Institut De Géologie Et D’Hydrogéologie, Université De Neuchâtel, Rue Emile-Argand 11, CH-2009 Neuchâtel, Switzerland
2Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012, Bern, Switzerland

RECEIVED AUGUST 4, 2006; ACCEPTED JUNE 15, 2007


  Abstract

Mantle-derived xenoliths from the Marsabit shield volcano (eastern flank of the Kenya rift) include porphyroclastic spinel peridotites characterized by variable styles of metasomatism. The petrography of the xenoliths indicates a transition from primary clinopyroxene-bearing cryptically metasomatized harzburgite (light rare earth element, U, and Th enrichment in clinopyroxene) to modally metasomatized clinopyroxene-free harzburgite and dunite. The metasomatic phases include amphibole (low-Ti Mg-katophorite), Na-rich phlogopite, apatite, graphite and metasomatic low-Al orthopyroxene. Transitional samples show that metasomatism led to replacement of clinopyroxene by amphibole. In all modally metasomatized xenoliths melt pockets (silicate glass containing silicate and oxide micro-phenocrysts, carbonates and empty vugs) occur in close textural relationship with the earlier metasomatic phases. The petrography, major and trace element data, together with constraints from thermobarometry and fO2 calculations, indicate that the cryptic and modal metasomatism are the result of a single event of interaction between peridotite and an orthopyroxene-saturated volatile-rich silicate melt. The unusual style of metasomatism (composition of amphibole, presence of graphite, formation of orthopyroxene) reflects low P –T conditions (~850–1000°C at < 1·5 GPa) in the wall-rocks during impregnation and locally low oxygen fugacities. The latter allowed the precipitation of graphite from CO2. The inferred melt was possibly derived from alkaline basic melts by melt–rock reaction during the development of the Tertiary–Quaternary Kenya rift. Glass-bearing melt pockets formed at the expense of the early phases, mainly through incongruent melting of amphibole and orthopyroxene, triggered by infiltration of a CO2-rich fluid and heating related to the magmatic activity that ultimately sampled and transported the xenoliths to the surface.

KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass

*Corresponding author. Present address: School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK. Telephone: 0044 (0) 131 650 7339. Fax: 0044 (0) 131 668 3184. E-mail: v1bkaser{at}staffmail.ed.ac.uk


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[Abstract] [Full Text] [PDF]


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