Experimentally determined stability of alkali amphibole in metasomatised dunite at sub-arc pressures
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Phlogopite
Amphibole
Solidus
Peridotite
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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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Abstract The experimental study on the melting of potassic basalt and eclogite with about 2% water at 800—1300°C and 1.0—3.5 GPa shows that the solidi of both rocks are significantly lower than those obtained from the previous experiments of the same type of rocks under dry conditions, and the former which is enriched in potassium has a lower melting point than the latter. It is consistent with the previous study. The melting temperature of eclogite increases with pressure, whereas potassic basalt has similar properties only at 1.5—2.5 GPa and >3.0 GPa, and at 2.5— 3.0 GPa the melting temperature decreases with pressure. This can be explained as follows: (1) eclogite only has one hydrous mineral amphibole and the dehydous temperature is lower than the wet solidus of the rock. (2) Amphibole exists in potassic basalt at the pressures lower than 2.5 GPa and phlogopite exists at pressures higher than 2.5 GPa, and the special compositions of both minerals determine that amphibole has a dehydration temperature higher than or close to that of the wet solidus of the rocks, while phlogopite has a dehydration temperature lower than that of the wet solidus. On the other hand the features of the continuous solidus in the experiment of hydrous eclogite were produced by the fact that the dehydration temperature of its amphibole lower than or close to the melting temperature of the hydrous conditions. So the melting temperature lowers at higher pressures. Therefore, the composition of the rocks in the lithosphère and the types of hydrous minerals and their stable P‐T conditions are the important factors controlling the solidi of rocks. It can quite well explain the partial melting of rocks and the origin of the low velocity zone in the deep lithosphere.
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Phlogopite is widely accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P–T range. The paper reviews data on various phlogopite-forming reactions in upper-mantle peridotites. The review includes both descriptions of naturally occurring reactions and results of experiments that model some of these reactions. Relations of phlogopite with other potassic phases, such as K-richterite, sanidine and K-titanates, are discussed. These data are taken as a basis for thermodynamic modeling of the phlogopite-forming reactions for specific mantle rocks in terms of log(aH2O) − log(aK2O) diagrams (pseudosections) using the Gibbs free energy minimization. These diagrams allow estimation of potassium-water activity relations during metasomatic transformations of mantle rocks, prediction sequences of mineral assemblages with respect to these parameters and comparison of metasomatic processes in the rocks of different composition. This approach is illustrated by examples from peridotite xenoliths from kimberlites.
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Abstract This study reports halogen contents (F and Cl) of amphibole and phlogopite derived from mantle xenoliths and one peridotite massif, for amphibole and phlogopite megacrysts and ultramafic magmatic cumulates (hornblendites) found in alkaline volcanic rocks from 12 localities in Europe and Africa. Amphibole and phlogopite contain more F than Cl with F/Cl ratios reaching about 160 in phlogopites and 50 in amphiboles. Phlogopites are higher in F (median of 3400 μg/g) than amphibole (median of 1000 μg/g), while median Cl contents are higher in amphibole (290 μg/g) compared to phlogopite (180 μg/g). The Cl contents and the F/Cl ratios in amphibole and phlogopite from mantle xenoliths exhibit large differences between samples of the same region, recording very large variations of halogen contents in the continental lithosphere. We suggest that the halogen content in such samples largely depends on the initial composition of percolating melts and fluids in the continental lithosphere. During reaction of these agents with peridotitic wall-rocks, Cl is preferentially retained in the fluid as it is much more incompatible compared to water and F. This desiccation effect continuously increases salinity (Cl content) and decreases the F/Cl ratio in the agent with time, causing variable Cl contents and F/Cl ratios in amphibole and phlogopite at a specific locality. Subsequent partial melting processes may then sequester and re-distribute, especially Cl among amphibole, phlogopite and melts/fluids as a result of its strong incompatibility, whereas F is much less affected as it behaves slightly compatible. The impact of even small amounts of amphibole and mica on the total halogen budget in the continental lithosphere is significant and both minerals can effectively contribute to the high halogen contents typical of alkaline melts.
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