Metasomatism versus host magma infiltration: a case study of Sal mantle xenoliths, Cape Verde Archipelago

Based on phase geochemistry and Re-Os isotopic ratios, an exotic (in oceanic setting) K-rich silicate melt, named kimberlite-type, has been claimed as the metasomatising agent interacting with subcontinental lithospheric mantle (SCLM) fragments beneath the Cape Verde Archipelago. On the basis of textural features and major and trace element chemistry, key geochemical indicators able to discriminate percolation at depth of this exotic melt from infiltration of the host magma in Cape Verde mantle xenoliths have been constrained. Cape Verde Type A lherzolites and harzburgites show evidence of dissolution of the primary phases (mainly pyroxenes), the presence of large patches of secondary mineral (and glass) assemblages, and do not show textural evidence of host basalt infiltration. Cape Verde Type A mantle xenoliths frequently contain almost pure K-feldspar (An3.8–8.8, Ab6–24, Or72–89) in the secondary mineral assemblage. They have an anomalously high K content (up to 0.49 wt%), and a K/Na ratio generally >1, with respect to Cape Verde peridotites clearly affected by host basalt infiltration (Type B samples). The dichotomy between Na and K observed in the two textural types suggests that the Na- alkaline host basalt (K/Na<1), which infiltrated at low pressure, was able to modify the whole rock Na content of the xenoliths (Type B samples). In turn, a completely different K-rich alkaline melt, which interacted at depth with the peridotite, imposed its alkali ratio (K/Na>1) to the bulk composition and formed the Type A xenoliths. The kimberlite-type metasomatic agent, which reacted with the Cape Verde peridotite assemblage (manly orthopyroxenes) in those regions where the mantle xenoliths are entrapped in the host basalt (P=17Kb; T=1092°C), reasonably tends towards SiO2-saturated K-rich basic magmas (lamproite-type?) with K-feldspars as the “liquidus” phase. Isotopic data on separate clinopyroxenes, do not contribute to discriminate between metasomatism and infiltration processes, but certainly concur to reinforce the hypothesis that a fragment of SCLM domain has been preserved during the opening of the Atlantic Ocean, forming K-rich undersaturated silicate melts which percolate the peridotite matrix. Whole rock major and trace element and isotopic geochemistry alone would not contribute to the interpretation of the processes occurring in the mantle xenolith. The most reliable tool would be an in situ mineral (and glass) study, which would be valid for Cape Verde mantle xenoliths and others. Small melting degree undersaturated silicate melts percolating at depth are olivine-saturated and may form, by reaction and dissolution of pyroxene, Type A olivine without substantially modifying the original Fe/Mg peridotite ratio. By contrast, at low pressure (< 1.5 GPa)/high temperature regimes, olivine silicate melts infiltrating the mantle xenoliths form Type B olivine, whose Fe/Mg ratios will be controlled by fractionation. Mantle diopsides interact (at depth) with undersaturated silicate melts, re-arranging the most fusible elements in a new diospide composition: Type A cpx. By contrast, diopsides which interact with melts at progressively lower pressure, react and are locally rearranged in a new chemical structure, able to accommodate the high diffusive elements (i.e. Fe and Ti): Type B aegirine-augites. Fe3+ in spinel is a key element to investigate the processes acting on Cape Verde mantle xenoliths. As a metasomatic product, secondary chromian spinel tends towards a Fe3+ buffered compositon, mainly depending on pressure and chemistry of the magma. A decompression system is able to change the percolation regime from porous flow to open conduct. At this stage, the chromian spinel would be the low pressure phase able to accommodate larger amounts of Fe3+. Type A glasses have exceptionally high K2O content and when associated with K-feldspars, they are buffered at ~ 9 K2O wt%, in a silica range of 55.7-66.8 wt%. By contrast, Type B glasses follow a hypothetical major element trend towards the host basanites. In conclusion, the compositional features (in particular major elements) of minerals and glasses in relation to their chemical behaviour in mantle systems, is the most efficient tool to distinguish metasomatism-related (Type A) from infiltration-related (Type B) samples and consequently to place the mantle xenoliths in a correct genetic framework.