K/Na Variation in Phlogopite and Amphibole of Upper Mantle Peridotites Due to Fractionation of the Metasomatizing Fluids

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Abstract
K/Na ratios of metasomatic phlogopite and amphibole of the upper mantle origin are high in garnet peridotites and low in spinel or plagioclase peridotites. Combined with the observation that the phlogopite/ amphibole volume ratio is high in garnet peridotites and low in spinel or plagioclase peridotites, it is concluded that the bulk K/Na ratio of the metasomatized part decreases upwards in the upper mantle. This vertical variation of the K/Na ratio may be due to the fractional crystallization of phlogopite and amphibole from the metasomatizing fluids. The upward decrease of the K/Na ratio of the bulk metasomatic minerals is, at least partly, responsible for the layered structure of the upper mantle in terms of the K/Na ratio.
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