2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Interaction of the model CMAS pyrolite with the H₂O-KCl fluid at  2.5 GPa and 900-1200^ОС

 Safonov О.G.

 Institute of Experimental Mineralogy RAS, Chernogolovka, Russia; oleg@iem.ac.ru

Experimental study of phase relations in the model pyrolite with participation of the H₂O-KCl fluid at 2.5 GPa and 900-1200^ОС showed that presence of KCl led to disappearance of garnet, amphibole, and orthopyroxene, which were stable in the hydrous (4.4 wt. % of H₂O) pyrolite without KCl. The characteristic assemblage in presence of the H₂O-KCl fluid is phlogopite+clinopyroxene+olivine. Addition of KCl results in a decrease of the melting temperature of the hydrous pyrolite by more than 100^ОС.

In the upper mantle, KCl and NaCl are constituents of concentrated aqueous solutions (brines), as well as chloride-carbonate and carbonatite melts. Their influence is recorded in mineral assemblages coming from diverse depth levels. These assemblages show that mobile (K, Na)Cl-bearing fluids are able to provoke intensive modifications in the peridotitic mentle accompanied by melting. Unfortunately, experimental studies on influence of brine fluids on mineral equilibria in the peridotitic mantle are rather scarce (Stalder et al., 2008; Bernini et al., 2009; Chu, 2010). These studies indicated that influence of chlorides on water activity in a fluid equilibrated with forsterite и enstatite at pressures above 2 GPa is very similar to the effect of these components at lower pressures (Aranovich, Newton, 1997): decrease of the H₂O activity with an increase of the salt content results in an increase of the melting temperature of silicates. Nevertheless, the above experiments were mostly performed using Al-free silicates. Presence of Al would provoke an active interaction of alkali chlorides with silicates with formation of new K-Al-bearing phases. The most significant phase would be phlogopite. Evidently, appearance of this phase would influence on the melting of complex assemblages.

In order to investigate an influence of KCl on modifications of the Al₂O₃, CaO, Na₂O-rich hydrous peridotite and on stability of garnet, pyroxenes, and amphiboles, in particular, experiments on interaction of the model CMAS pyrolite Fo₅₇En₁₇Prp₁₄Di₁₂ (+0.3 wt. % of Na₂O) with the H₂O-KCl fluid were performed at 2.5 GPa in the temperature interval 900-1200^ОС. Mixtures of synthetic forsterite (Mg₂SiO₄) and diopside (CaMgSi₂O₆) and gels of enstatite (MgSiO₃) and pyrope (Mg₃Al₂Si₃O₁₂) compositions in the above weight ratio were used in the experiments. 14 wt. % of Mg(OH)₂ corresponding to 4.4 wt. % of H₂O in the system and KCl (2.4, 3.7 and 5.0 wt. %) were added to silicate mixture. Experiments were conducted using a piston-cylinder apparatus with ½-inch talk high-pressure cells equipped with graphite heaters and soft ceramic sleeves as pressure medium. Pressure in the cells at high temperatures was calibrated via brucite = pericalse + H₂O and albite = jadeite + quartz equilibria curves. Temperature was controlled with accuracy ±1^ОС with the W₉₅Re₅/W₈₀Re₂₀ thermocouple. Spherical and tube Pt capsules with 0.2 mm-thick walls were used in the experiments. Run products were analyzed using CamScan MV2300 (VEGA TS 5130MM) electron microscope equipped with EDS INCA-Energy-250.

The subsolidus assemblage of the model pyrolite (< 1025^OC) containing 4.4 wt. % of H₂O at 2.5 GPa includes forsterite (Fo), clinopyroxene (Cpx), orthopyroxene (Opx), pargasite-tschermackite amphibole (Amp), and pyrope-grossular garnet (Grt). It is consistent with the results of experiments with amphibole-bearing lherzolite at 2.5 GPa (Niida, Green, 1999). Reaction relations 3/2Opx + 1/2Fo + 1/2Amp = Grt + Cpx + 1/2H₂O are observed in the run samples. Melting apparently begins at temperatures 1025-1050^ОС and results in gradual disappearance of amphibole (Niida, Green, 1999) in this temperature interval. In general, these relations are available in presence of 2.4 wt. % of KCl. However, reaction 6Opx + Fo + Amp + KCl = [Cl-Phl + Phl] + Grt + 2Cpx is accompanied by formation of Cl-bearing phlogopite, Phl (up to 1 wt. % of Cl). Garnet, orthopyroxene, and amphibole are totally absent in the run samples at the bulk KCl content 3.7 wt. %, while the assemblage of Cl-bearing phlogopite with clinopyroxene and olivine is stable. The forming phlogopite is found to be stable at higher temperatures (apparently, >1200^ОС) with respect to amphibole, consistently with the experimental data on melting of phlogopite and amphibole-bearing peridotites at pressures >1.5 GPa (Modreski, Boettcher, 1973; Mysen, Boettcher, 1975; Mengel, Green, 1989). Nevertheless, the solidus temperature of the H₂O-bearing pyrolite with addition of KCl is below 900^ОС, that is more than by 100^ОС lower of the melting temperature of the H₂O-bearing pyrolite without KCl. Apparently, decrease of the temperature is related both to the formation of phlogopite, which forms low-temperature eutectics with other silicates, and to solubility of Cl in the melts. Unfortunately, it is difficult to analyze reliably composition of the quenched melt. However, quench products show that the “phlogopite” component is predominant in these melts.

Thus, the preliminary experimental data show that KCl decreases the solidus temperature of the hydrous peridotite. This result contradicts with the experiments on melting of the Fo+Opx assemblage in presence of H₂O+KCl at 5 GPa (Chu, 2010), which showed that the temperature of melting increases with an increase of KCl content in the system. The present experimental results indicate an important role of alumina as a component forming phlogopite, which, in turn, seems to be responsible for the decrease of the melting temperature.

The study is supported by Russian Foundation for Basic Research (project #10-05-00040) and RF President Grant for young scientists (project #MD-380.2010.5).

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