International Geologiical Congress - Oslo 2008

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MPM-06 Melts and glasses in mineralogy and petrology

 

Fluid-bearing evolved silicate melt inclusions in peridotite xenoliths from the Carpathian-Pannonian region

 

Karoly Hidas, Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eotvos University (Hungary)
Csaba Szabo, Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eotvos University (Hungary)
Eniko Bali, Bayerisches Geoinstitut (Germany)
Tibor Guzmics, Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eotvos University (Hungary)
Zoltan Zajacz, University of Maryland (United States)
Kyoung-hee Yang, Division of Earth Environmental System, College of Natural Sciences, Pusan National University (Republic of Korea)
Istvan Kovacs, Research School of Earth Sciences, The Australian National University (Australia)
 

 

A large number of silicate melt inclusions (SMI) were found in the rims of clinopyroxene (cpx) and along healed fractures of orthopyroxenes (opx) from two equigranular textured, amphibole-bearing spinel lherzolite xenoliths representing the subcontinental lithospheric mantle (Szigliget, Pannonian Basin, Hungary). The SMI, regardless of host minerals or host xenoliths consist of glass, hydrous mineral phase (mica?), clinopyroxene and individual fluid phase. Similarly to the petrography, the major element composition of the glass phase, as well as the bulk trace element composition of the SMI shows no notable differences either in the host minerals, or in the host xenoliths. The major element composition of glass in SMI, regardless of xenoliths and host minerals, cover a wide range, mostly with trachyandesitic composition and they are extremely enriched in incompatible elements (particularly in U, Th, La, Zr) with a slight negative Hf anomaly. As a conclusion, the SMI of cpx and opx differ only in their petrographic appearance and the presence of cpx wall-crystals in the SMI of cpx (as a result of post-entrapment crystallization), which results in the ratty shape of these inclusions.
Together with the SMI, coeval and cogenetic fluid-rich inclusions were also trapped both in cpx and opx. Microthermometry and Raman analysis of the individual fluid inclusions and the fluid phases of SMI indicate that the main components are CO2, H2O (± H2S).
Electron microprobe analysis reveals in both xenoliths that cpx and opx are zoned, especially in terms of basaltic major elements. Cores of both cpxs show trace element distribution close to the primitive mantle. Rims display an overall enrichment with high positive anomalies in Th, U and moderate LREE content, as an indication for metasomatism. The trace element composition of the SMI indicates an originally mafic character. The pargasitic amphiboles, formed after rims of the cpx, exhibit elevated Rb, Ba, Nb, Ta and moderate LREE content.
The development of zoned pyroxenes and the trapping of SMI are the result of multiple mixing of different melts in the mantle. We suggest that the initial melt would have a basaltic character and during its migration toward the surface, turned into an evolved, C-O-H-S fluid-bearing andesitic-trachyandesitic melt, which triggered partial melting in the studied clinopyroxenes and filled the fractures of orthopyroxenes. This process leaded to the formation and entrapment of SMI, as well as the chemical zonation of clinopyroxenes. As its trace element composition indicates, the formation of amphibole was a later, independent event.
Our model indicates migration and entrapment of C-O-H-S bearing fluids and melts at upper mantle levels beneath the central Pannonian Basin.

 

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