2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Diopside melting in sodium vapor: an experimental study

Borisov А.А.

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM),

Russian Academy of Sciences

aborisov@igem.ru.

In present paper we present first experimental results on diopside melting in sodium vapor, conducted with crucible supported loop technique. The results may be interesting for understanding of possible changes in mantle rock xenoliths in alkali basalts with assuming of perfectly mobile behavior of sodium.

The crucible supported loop technique suggested by Borisov et al. (2006) allows working with alkali-containing systems at 1 atm total pressure even at very reducing conditions, where the “classical” loop technique results in complete loss of alkalis. The method happened to be useful in many experimental investigations:  the effects of sodium on metal solubilities (Borisov et al., 2006), sodium partitioning between olivine and melt (Borisov et al., 2008), Na, K and Rb partitioning between miscible melts (Borisov, 2008, 2009).

In brief, glass of diopside composition on platinum loop (about 3 mm in diameter) was suspended in quartz crucible above the melt of Na₂Si₂O₅ under a platinum lid. The crucible was lowered into the tube furnace and held under desired temperature (1200-1315ºС) and oxygen fugacity (about IW buffer). During experiment (1-2 h) diopside glass was exchanged with small diopside crystals (5-20 µm) which then reacted with sodium vapor arising during sodium evaporation from Na₂Si₂O₅ melt.

New phases were reveled in experimental products: wollastonite (pseudowollastonite) and glass. The wollastonite contains about 3 wt.% MgO, which is equivalent to Wo₉Di₁ (mol.). Essential isomorphic Ca exchange with Mg is known in CaSiO₃ structure (e.g., Shinno, 1974).

The glass (up to5 µm) marks the boundaries of diopside crystals. It is heterogeneous in composition (maximum SiO₂ content is 70 wt.% and Na₂O content is 8 wt.%).

The mantle glass-containing xenoliths are well known with silica content in glass reaching 72 wt.% (e.g., Ishimara and Arai, 2009). The origin of such glasses is under discussion. Our experiments demonstrate the possibility of silicic melts to be obtained by incongruent melting of xenoliths crystal during its reaction with alkalis-containing fluids.

References:

Borisov A.A. Experimental Investigation of K and Na Partitioning between Miscible Liquids // Petrology. 2008. V. 16. P. 552-564.

Borisov A.A. Influence of SiO₂ and Al₂O₃ on the Activity Coefficients of Alkalis in Melts: An Experimental Study // Petrology. 2009. V.17. P. 579-590.

Borisov A., Lahaye Y. and Palme H. The effect of sodium on the solubilities of metals in silicate melts // American Mineralogist. 2006. V. 91. P. 762-771.

Borisov A., Pack A., Kropf A. and Palme H. Partitioning of Na between olivine and melt: An experimental study with application to the formation of meteoritic Na₂O-rich chondrule glass and refractory forsterite grains // Geochimica et Cosmochimica Acta. 2008. V. 72. P. 5558-5573.

Ishimaru S. and Arai S. Highly silicic glasses in peridotite xenoliths from Avacha volcano, Kamchatka arc; implications for melting and metasomatism within the sub-arc mantle // Lithos. 2009. V. 107. P. 93-106.

Shinno I. Unit cell dimensions and infra-red absorption spectra of Mg-wollastonite in the system CaSiO₃-CaMgSi₂O₆ // Mineralogical Journal. 1974. V. 7. P. 456-471.