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| Titel |
Eclogite-associated potassic silicate melts and chloride-rich fluids in the mantle: a possible connection |
| VerfasserIn |
O. Safonov, V. Butvina |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030672
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| Zusammenfassung |
| Relics of potassium-rich (4–14 wt. % of K2O and K2O/Na2O > 1.0) melts are a specific
features of some partially molten diamondiferous eclogite xenoliths in kimberlites worldwide
[1, 2]. In addition, potassic silicic melt inclusions with up to 16 wt. % of K2O are associated
with eclogite phases in kimberlitic diamonds (O. Navon, pers. comm.). According to
available experimental data, no such potassium contents can be reached by “dry” and hydrous
melting of eclogite. These data point to close connection between infiltration of essentially
potassic fluids, partial melting and diamond formation in mantle eclogites [2]. Among
specific components of these fluids, alkali chlorides, apparently, play an important role. This
conclusion follows from assemblages of the melt relics with chlorine-bearing phases
in eclogite xenoliths [1], findings of KCl-rich inclusions in diamonds from the
xenoliths [3], and concentration of Cl up to 0.5-1.5 wt. % in the melt inclusions in
diamonds. In this presentation, we review our experimental data on reactions of KCl
melts and KCl-bearing fluids with model and natural eclogite-related minerals and
assemblages.
Experiments in the model system jadeite(±diopside)-KCl(±H2O) at 4-7 GPa showed
that, being immiscible, chloride liquids provoke a strong K-Na exchange with silicates
(jadeite). As a result, low-temperature ultrapotassic chlorine-bearing (up to 3 wt. % of Cl)
aluminosilicate melts form. These melts is able to produce sanidine, which is characteristic
phase in some partially molten eclogites. In addition, in presence of water Si-rich Cl-bearing
mica (Al-celadonite-phlogopite) crystallizes in equilibrium with sanidine and/or potassic melt
and immiscible chloride liquid. This mica is similar to that observed in some eclogitic
diamonds bearing chloride-rich fluid inclusions [4], as well as in diamonds in partially molten
eclogites [2].
Interaction of KCl melt with pyrope garnet also produce potassic aluminosilicate
melt because of high affinity of Al and Si to potassium. Additional products of this
interaction are spinel and, possibly, olivine. These minerals are common products
of garnet breakdown within the zones of partial melting of eclogite xenoliths [1,
2].
It is evident that simultaneous action of fluid species (H2O, CO2) and chlorides would
produce much stronger effect. Following to this assumption, we further performed
experiments on melting of model and natural eclogites with participation of the
H2O-CO2-KCl fluids at 5 GPa. Comparison with the KCl-free melting (i.e. H2O-CO2
fluid only) shows that addition of KCl to the fluid intensifies melting. This effect is
related both to high Cl content (up to 3-5.5 wt. %) in the newly formed silicate
melt and its enrichment in K2O via K-Na exchange reactions with the immiscible
chloride melt. Owing to these reactions, the ratio K2O/Cl in the melts increases
with the increase of the KCl content in the system and reaches 2.5-3.5 in the melts
coexisting with immiscible chloride liquids. However, the KCl/(H2O+CO2) ratio in the
fluid does not influence on the K2O/Cl ratio in the melts suggesting that solubility
of KCl in the melts practically does not depends on a presence of the H2O-CO2
fluid.
Thus, the experiments imply that the KCl-bearing fluids or aqueous(±carbonic)
KCl liquids could serve as a possible factor assisting to formation of the K-rich
Cl-bearing aluminosilicate melts during the eclogite melting in the mantle. In turn, it
means that the KCl content in such rock-melt-fluid systems could exceed 5 wt.
%.
The study is supported by the RFBR (07-05-00499), the Leading Scientific Schools
Program (1949.2008.5), Russian President Grant MD-130.2008.5, and Russian Science
Support Foundation.
References: [1] Misra et al. (2004) Contrib. Mineral. Petrol. V. 146. P. 696-714; [2]
Shatsky et al. (2008) Lithos. 105. 289-300; [3] Zedgenizov et al. (2007) Doklady
Earth Sci. 415. 961-964; [4] Izraeli et al. (2001) Earth Planet. Sci. Lett. 5807. 1-10. |
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