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.Keywords:
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Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.
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Summary First results in the phlogopite + magnesite (KMASH-CO2) system demonstrate that a potassiumbearing fluid will be the metasomatic agent at sub-continental-lithospheric-mantle conditions with a continental geotherm of 40 mWm -2 . In this case, phlogopite can be stable to a depth of 200 km in the presence of carbonate, and will coexist with potassic fluids. Assuming a hotter geotherm of 44 mWm -2 , these fluids can be present to a depth of about 180 km. Beyond this depth, at the base of a thick sub-continental lithospheric mantle, a hydrous, potassium- and CO2-rich silicate melt would be the metasomatic agent. In this system, garnet is present above solidus as a residual phase, which implies that a K-CO2-H2O-enriched metasomatic fluid or melt could react with garnet peridotite to form phlogopite.
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23.0±1.2 Ma was obtained by Rb-Sr method for a phlogopite-bearing spinel lherzolite from the Horoman peridotite complex in the Hidaka metamorphic belt, Hokkaido, Japan. The age is essentially identical to the oldest ages of the country metamorphic rocks reported so far in the Hidaka metamorphic belt, and may indicate the time of a metasomatic event which occurred in the wedge mantle. This metasomatic event may have occurred during uplift of this mantle fragment under mantle conditions contemporaneously with elevation of the Hidaka metamorphic belt resulting from collision between the Eurasian and North American plates.
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