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.

Eleven synthetic silicate and phosphate glasses were prepared to serve as reference materials for in situ microanalysis of clinopyroxenes, apatite and titanite, and other phosphate and titanite phases. Analytical results using different micro‐analytical techniques showed that the glass fragments were homogeneous in major and trace elements down to the micrometre scale. Trace element determinations using inductively coupled plasma‐mass spectrometry (ICP‐MS), multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS), laser‐ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ionisation mass spectrometry (SIMS) showed good agreement for most elements (Li, Be, B, Cs, Rb, Ba, Sr, Ga, Pb, U, Th, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Er, Tm, Yb, Lu, Zr, Hf, Ta, Nb) studied and provide provisional recommended values.

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Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of the lithospheric mantle in this region. Garnet-bearing rocks are amphibolebearing websterite with *5-10 vol% orthopyroxene. Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (*970-1,100°C at *2.3-2.6 GPa to *700-800°C at *0.5-1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P-T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling. In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya rift.

[1] The light elements Li, Be, and B have been analyzed in situ in minerals from three groups of peridotite xenoliths hosted in Quaternary basanites from the Marsabit volcanic field (northern Kenya). Group I and II are fertile lherzolites that experienced deformation, decompression, and cooling in the context of Mesozoic rifting (Group I), followed by heating, static recrystallization, and associated cryptic metasomatism (Group II) as a result of Tertiary-Quaternary rifting and magmatism. Group III xenoliths are spinel harzburgites and dunites that experienced strong cryptic and modal metasomatism. The Li-Be-B systematics in minerals of Group I and II are similar to unmetasomatized subcontinental lithospheric mantle. In contrast, Group III samples are characterized by significant enrichment in all light elements and disequilibrium partitioning between different phases. Light element concentrations levels are similar to that expected for mantle rocks metasomatized by melts and fluids released from subducting slabs, while light element/rare earth element ratios (especially Li/Yb) approach those of typical Island Arc basalts. However, detailed investigation of textures and chemical zoning shows that at least Li concentrations in primary minerals were modified (i.e., decoupled from Yb) during late-stage melting and/or fluid percolation related to Tertiary-Quaternary alkaline magmatism in Marsabit (formation of melt pockets consisting of silicate glass, clinopyroxene, olivine, and chromite), ultimately followed by xenolith entrapment and transport to the surface. Mass balance calculations show that the melt pockets formed at the expense of earlier metasomatic phases. During this process the melt pockets mostly preserved the B, Be, and rare earth element budget of the precursor phase assemblage, whereas Li was added. Elevated B/Be and low Ce/B of metasomatic phases prior to late melting could result from metasomatism by a slab fluid. However, similar characteristics are expected for evolved Si-and CO 2 -rich fluids derived from basanite melt-peridotite interaction, not related to any subduction zone process. The results of this study imply that the inference of a ''slab signature'' exclusively based on trace element data of metasomatized peridotite is ambiguous.