Faculté des sciences

Mantle xenoliths from the Marsabit volcanic field : a case study on the evolution of the lithospheric mantle in a continental rift environment

Kaeser, Benjamin ; Kalt, Angelika (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2006 ; 1917.

Mantle xenoliths, rock fragments sampled by magmas during their ascent from depth to the surface, provide direct information on the nature and composition of the Earth’s mantle. This thesis is the result of a petrographic, geochemical and petrological case study on mantle xenoliths hosted by Quaternary basanitic and alkali basaltic scoriae of the Marsabit volcanic field (northern Kenya).... More

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    Summary
    Mantle xenoliths, rock fragments sampled by magmas during their ascent from depth to the surface, provide direct information on the nature and composition of the Earth’s mantle. This thesis is the result of a petrographic, geochemical and petrological case study on mantle xenoliths hosted by Quaternary basanitic and alkali basaltic scoriae of the Marsabit volcanic field (northern Kenya). Magmatic activity is related to the development of the East African rift system. Results from previous seismic, geological and petrological studies show that continental rifting in East Africa is strongly controlled by pre-existing structures in the lithosphere. Further, the nature of the lithosphere has been shown to play a crucial role for the locus and composition of volcanic rocks, as magmas partly derive from, or at least interacted with the lithospheric mantle. The xenoliths from Marsabit provide a direct window in the mantle and allow constraining the nature of the East African Rift lithosphere. The xenoliths comprise several groups of ultramafic (peridotite) and mafic (pyroxenite and gabbro) rocks. Peridotite includes porphyroclastic or statically recrystallised, formerly garnet-bearing lherzolite (Group I and II, respectively), porphyroclastic spinel harzburgite and dunite (Group III) and mylonitic spl harzburgite and lherzolite (Group IV). Mafic rocks comprise garnet-bearing and garnet-free pyroxenite (Group V and VI, respectively) and gabbro (Group VII). The integration of textural and compositional data, together with results from thermobarometry and evaluation of mineral zoning indicate a complex evolution of the lithospheric mantle. The possibly oldest features are preserved in the formerly garnet-bearing lherzolites (Group I and II) and in the garnet pyroxenites. These rocks provide evidence of an earlier high-pressure / high temperature stage (~970-1100°C at depths around 60-90 km), similar to non-rifted sub-continental lithospheric mantle such as actually present underneath southern Kenya. This stage most likely corresponds to the lithospheric conditions prior to continental rifting which started during Mesozoic times with the formation of the Anza Graben (an older rift perpendicular to the present-day East African rift). The garnet pyroxenites formed prior to rifting as well. It is suggested that the garnet pyroxenites represent the products of high-pressure crystallisation of opx-saturated melts, possibly formed during Pan-African (Neoproterozoic-Paleozoic) orogenesis. Crustal rocks issued from this time period make up most of the present-day crystalline basement of the Marsabit area. All peridotite types, as well as the garnet pyroxenites were subjected to later cooling, decompression and pervasive deformation (to very low mantle P-T conditions of ~700-800°C at depths ~30-40 km). These features are best explained by continental rifting during Mesozoic-Paleogene times that led to the formation of the Anza Graben Subsequently magmatism and metasomatism related to the development of the Tertiary-Quaternary East African rift obliterated features related to Mesozoic-Paleogene rifting. Evidence for this comes, for example, from the statically recrystallised lherzolites, where textural annealing is associated with a young heating event (up to 1100°C). Heating was accompanied by cryptic metasomatism (i.e., enrichment of clinopyroxene in Fe-Ti and incompatible trace elements). The metasomatising melts were compositionally similar (and possibly genetically related) to the Quaternary basanites erupted at the surface of Marsabit. Probably in the same period, garnet-free pyroxenites (Group VI xenoliths) crystallised from alkaline melts, presumably in dykes within the shallow mantle or at the mantle-crust boundary (between ~30-60 km depths). Also these alkaline melts were most likely related to the lavas erupted at the surface. Further evidence for Tertiary-Quaternary metasomatism can be found in the porphyroclastic Group III peridotite xenoliths (Group III), which show a textural transition from virtually non-metasomatised spl harzburgite to modally metasomatised amphibole dunite. The latter contain rather unusual mantle minerals such as apatite, graphite, Na-rich phlogopite and katophorite (amphibole). The phase assemblage, as well as major and trace element characteristics indicate that this type of metasomatism resulted from the infiltration of volatile (H2O, CO2)-rich silicic melt and/or fluid in a pre-existing heterogeneous and probably reduced mantle. Such melts may have evolved from previous melt-rock reaction processes. In a very late stage (i.e., shortly before the xenoliths were transported to the surface in their host magma), the metasomatic minerals partially melted. This led to the formation of patches consisting of newly formed minerals (microlites) and glass (quenched melt). The metasomatised harzburgites and dunites are further strongly enriched in the low-atomic mass elements (light elements) Lithium, Beryllium and Boron. Therefore, these elements were investigated with special emphasis. The light elements are now widely used to trace recycled components in mantle and volcanic rocks in subduction zone settings. In the case of Marsabit, the light element systematics could potentially be interpreted as reflecting such components in the mantle, added by ancient, pre-rift subduction events. The detailed investigation of Li, Be and B systematics in minerals from the Marsabit xenoliths, however, clearly points to young disequilibrium features and to modification of light element budgets during very late-stage melting events (i.e. the formation of melt pockets). These results highlight that the application of light element systematics to trace subduction-related components is not un-problematic. This applies in particular to xenoliths where a careful quantification of late-stage metasomatic events with respect to the light elements is necessary.