International Geologiical Congress - Oslo 2008

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MPM-10 Fluids and melts in the Earth?s mantle: From natural observation to HT-HP experiment

 

The system garnet-clinopyroxene-H2O-CO2-KCl at 5 GPa and 1200-1300C

 

Valentina Butvina, IEM RAS (Russian Federation)
Oleg Safonov, IEM RAS (Russian Federation)
 

 

The present study at 5 GPa and 1200-13000Ñ with participation of the H2O-CO2 + KCl fluid is a first step to study melting of eclogites of diverse compositions. The following compositions of eclogites were chosen: (1) model eclogite (Prp80Grs20)50(Di60Jd40)50; (2) natural bimineral eclogite from the "Udachnaya" kimberlite pipe, Yakutia. An oxalic acid was taken as a source of the H2O-CO2 fluid.
The following regularities were found in the experiments with the model eclogite Prp80Grs20)50(Di60Jd40)50-(H2O)50(CO2)50. Firstly, quantity of the melt regularly increases with the increase of the fluid quantity in the system. At 6 % of the fluid, this melt coexists with pyroxene and garnet. At 8% of the fluid, the melt coexists with garnet only. At near complete melting, corundum and coesite occur. And at 18% of the fluid, no solid phases were observed. The same regularity, i.e. the increase of quantity of the melt with the increase of the H2O-CO2 fluid amount in the system, was established for the melting of the "Udachnaya" eclogite. At 8% of the fluid, the intergranular melt occurs, while at 15% of fluid, large pods of the quenched melt appear in addition to the melt films.
Following to the phase asemablges in the run products, the composition of the melt regularly changes. These regularities can be explained with the following scheme. The eclogite begins to melt with a formation of the melt, enriched in SiO2 and Na2O, reflecting the pyroxene-dominant composition of this melt. With increasing of the degree of melting and quantity of the "garnet component" in the melt, they accumulate Al2O3, CaO, MgO, but become less alkiline. Composition of the melts is shown on the ternary diagram (SiO2+Al2O3+TiO2) - (K2O+Na2O) - (MgO+FeO+CaO). They form regular trend. According to this trend, composition of the melts from products of eclogite melting (much SiO2-components) can form at much lower degree of melting, which correspond to low content of the fluid in the system. Nevertheless, the eclogite melting under influence of the H2O-CO2 fluid only can not explain all unique characteristics of melt composition, for example, their high alkali content with domination of K over Na.
The next stage of the experiments included the eclogite melting under influence of H2O-CO2 + KCl fluid. Even in the experiments at 1200 and 13000Ñ, the products of complete melting were observe: silicate melt, coexisting with immiscible chloride liquid. These melts are highly alkaline, K predominates over Na, while the Cl content in these melts reaches 3 mas.%. Predomination K over Na is explained by an intensive K-Na exchange between chloride and silicate liquids, the chloride liquid is being enriched in Na. This exchanges and high Cl concentration in the melts cause the abrupt increase of the melting temperature. These liquids correspond to some compositions of natural melts. Support RFBR: 07-05-0049 and the Program RAS 1P9.

 

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