Petrological features of lithospheric mantle beneath Santo Antao (Cape Verde Archipelago)

Spinel peridotites in basanites from Sao Antao werestudied with the aim of characterising the lithospheric mantlebeneath this island, and working towards a betterunderstanding of the nature of the mantle under the CapeVerde archipelago. Most samples examined are four-phaselherzolites with modal clinopyroxenes up to 18% and noevidence of modal metasomatism. A few are characterized bythe presence of metasomatic textures such as i) glassy patcheswith tiny subidiomorphic crystals of secondary olivine andclinopyroxene, and ii) rare spongy pyroxenes. Texturally,clinopyroxene often occurs as clusters of small crystals (<200microns), whereas olivine and orthopyroxene are larger (up to1 mm), suggesting a secondary origin for this clinopyroxene.A few samples show clear evidence of host magmainfiltration, allowing us to discriminate the chemical effects ofthis process against processes related to mantle metasomatism,i.e. magma interaction at depth. Olivine, ortho- and clinopyroxenenear basalt or in contact with basaltic veins crossingthe samples systematically present lower mg# and Cr2O3, andhigher TiO2 and Al2O3 contents. Olivine ranges incomposition from mg# 90.1 to 92.0 and NiO from 0.31 to 0.44wt%, whereas the orthopyroxene has a more restricted mg#range (91.2- 92.2). Clinopyroxene has mg# varying from 91.7to 93.0, with Al2O3 from 0.98 to 4.43 wt%, without anycorrelation between the two parameters, which rules out theidea that it results from a simple depletion event. REEclinopyroxene patterns vary from ‘humped’to ‘spoon-shaped’at Yb at 3-6 x Ch, with strong LREE enrichment (La 7-117 xCh). Silicate glasses, unrelated to host basalt infiltrations arepresent just in one sample. They are rather homogeneous incomposition with silica contents between 59.6 wt% and 66.0wt%, and alkali contents up to 15 wt% with Na/K < 1. On thewhole they show fractionated trace element profiles with(La/Yb)N varying from 34 to 102. These preliminarypetrographic and geochemical data for Sao Antao spinellherzolites allow a comparison with the data and modelproposed for Sal, an island to the east where lherzolitescontain a sub-continental mantle component, and Santiago andFogo to the south where the lherzolite is interpreted as theresult of carbonatitic metasomatism and wehrlite is probablycumulate from a large magma chamber system, againinvolving carbonatitic activity.