Silicate, Hydrous and Carbonate Metasomatism at Lherz, France: Contemporaneous Derivatives of Silicate Melt–Harzburgite Reaction

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Complex multi-stage models involving silicate, hydrous and carbonate melts of distinct provenance have been invoked to explain the metasomatism observed in mantle rocks. In contrast, relatively simple models requiring polybaric crystallization of alkaline silicate melts have been proposed to explain the occurrence of veined mantle rocks. To address the spatial and temporal relationships between veins and wall-rocks, a sequence of drill cores was obtained from Lherz, France. In outcrop the vein (amphibole–garnet pyroxenite dyke) is spatially associated with hornblendite veinlets (lherzite), and proximal amphibole-bearing and distal apatite-bearing wall-rock peridotite. Considerable elemental and isotopic heterogeneity exists in these wall-rock peridotites, in many instances equivalent to, or greater than, that observed in mantle xenoliths from worldwide localities. A single stage of reactive porous flow best explains the elemental and isotopic heterogeneity in the wall-rock. In essence it is proposed that emplacement of the silicate melt (dyke) was inextricably linked to chromatographic fractionation/reaction of derivatives which led to the coexistence, in space and time, of silicate, hydrous and carbonate melts. This model elegantly and simply describes the formation of complex metasomatic aureoles around mantle veins and negates the need, in the case of basalt-hosted (and kimberlite-hosted) xenoliths, for complex multi-stage models involving several episodes of melt influx with each melt being of different provenance.

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