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Partial melting of fluid-bearing peridotite at high pressures: textural and mineralogical particularity at under- and over-critical T and P.

Gorbachev N.S., Kostyuk A.V., Nekrasov A.N.

Institute of Experimental Mineralogy, Chernogolovka, Russia

gor@iem.ac.ru;

 

             Experimental studying of partial melting of peridotite at high Р and Т has the important value for understanding of phase parities(ratio) of the upper mantle and processes of mantle magma formed. Fluids render effective influence on phase relatiohship and melting of a mantle. There are two types of fluid-bearing silicate systems, normal, at under-critical Р and Т conditions and anomal, over-critical Р and Т conditions.

With the purpose of modelling melting of fluid-bearing mantle the system peridotite-basalt-volatile (H2O, H2O+CO2) was carried out in temperature range T=1250-1450°C, and pressure 1.5-4 GPa.

Experiments were carried out using of apparatus high pressure (piston cilinder and anvil with hole) by a quenching technique. It was used two ampoules (Pt+peridoite) method. Depending on Р-Т-Х conditions quench  samples have prominent features of a texture and composition.

             Normal, under-critical Р and Т conditions of fluid-bearing magmatic systems.

In dry conditions and at the presence of H2O or H2O+CO2 fluid at unde-rcritical Р and Т (up to 1450°C, ≥ 4 GPa at H2O+CO2 fluid , and up to 1450°C, and ≤ 2.5 GPa at H2O fluid) not observe dectruction of peridotite ampoules. The quench samples are characterized by the massive texture caused by that intergrain silicate glass, cements silicate and oxide  minerals of liquidus (fig 1).

 

  

 

             Anomal, over-critical Р and Т conditions of fluid-bearing magmatic systems.

At over-critical Р-Т (≥ 1400°C, ≥ 3.8-4 GPa at H2O or hydrous alkalic-carbonaceous fluids) observe dectruction of peridotite ampoules without intergrain silicate glass. Quench samples consist of not connected quenching mix microlites of silicate minerals and their aggregate needle, dendrite forms, with globules a silicate glass and sulfides (fig. 2).

 

  

 

The over-critical fluid-melts interaction with liquidus minerals of  peridotite (Ol, Opx) with form reaction zonality Ol-Opx-CaCpx-KCpx (fig.3).

 

 

  

 

The reaction zonality indicate high reactionary ability critical fluid-melt phases.

This study was financially supported by RFBR grant  09-05-01131, grant ОSE РАН т.2