Phlogopite- and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia
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Keywords:
Phlogopite
Fractional crystallization (geology)
Mineral redox buffer
Amphibole
Phlogopite
Amphibole
Metasomatism
Peridotite
Xenolith
Trace element
Incompatible element
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Experiments were performed from 1.0 to 8.5 GPa in a peralkaline system K2O–Na2O–CaO–MgO–Al2O3–SiO2–H2O (KNCMASH) to investigate the stability and composition of richteritic amphiboles in the MARID (mica–amphibole–rutile–ilmenite–diopside) assemblage amphibole + phlogopite + clinopyroxene. The results were compared with phase relations and the composition of natural MARIDs to assess possible mechanisms of formation for MARID-type rocks. K-richterite is stable in a wide range of bulk K/Na ratios in the MARID assemblage to 8.5 GPa and 1300°C. In this assemblage the amphibole can accommodate significant amounts of K on the M(4) site and shows a systematic increase in the K/Na ratio with increasing pressure. At P >7.0 GPa, K-richterite can coexist with garnet. Phase relations of K-richterite in a natural MARID composition are consistent with those in the simplified system and confirm the potential stability of K-richterite and K-richterite + garnet within the diamond stability field. The assemblage K-richterite + phlogopite + clinopyroxene is incompletely buffered in the KNCMASH system, resulting in a systematic relation between bulk- and mineral compositions observed in the experiments. Such a correlation, however, cannot be observed in natural MARIDs. Therefore, MARID-type rocks do not represent the bulk composition from which they formed and, hence, must be products of an open-system crystallization.
Amphibole
Phlogopite
Peralkaline rock
Ilmenite
Rutile
Diopside
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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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Amphibole
Metasomatism
Peridotite
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Phlogopite
Amphibole
Solidus
Peridotite
Metasomatism
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Amphibole
Mineral redox buffer
Fugacity
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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
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The stability of amphiboles on the join MgrSi.Orr(OH)r-FezSisorr(OH), has been hydrothermally investigated at 2 kbar fluid pressure as a function of oxygen fugacity and temperature. Atf;, defined by the MH buffer, the maximum extent of solution of Feend-member in amphibole is l4 and 22mole percent at725 and 630'C respectively; amphibole is unstable below 630'C, being replaced by the assemblage talc + quartz * magnetite + hematite. AtI), defined by the NNO buffer the extent of solid solution expands to 54,62, and 65 mole percent Fe end-member at 7250, 625', and 600C, respectively. Results obtained in this study have been combined with previously published data to produce a I-X section of the upper thermal stability of amphibole at2kbar andl,, defined by the FMQ buffer. Temperatures for the reaction: amphibole + pyroxene + quartz + vapor decrease from -765'C for the pure Mg end-member to -710'C for 62 mole percent Fe endmember. The breakdown reaction: amphibole - olivine + quartz + vapor, was observed for the more iron-rich amphiboles, and takes place at -675C for amphibole of 73 mole percent Fe end-member. Comparison of the experimental results to selected natural cases shows good agreement in maximum iron contents for the appropriate oxygen fugacity range. Estimates of temperatures of crystallization of metamorphic and igneous Fe-Mg amphiboles are also consistent with pnor reports.
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Mineral redox buffer
Pyroxene
Fayalite
Fugacity
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Amphibole
Mineral redox buffer
Phenocryst
Liquidus
Fugacity
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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.
Amphibole
Phlogopite
Peridotite
Ultramafic rock
Xenolith
Metasomatism
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