Experimental Study of Eclogite Melting with Participation of the H₂O-CO₂-KCl fluid.

Butvina V.G.*, Safonov O.G.*, Litvin Yu.A*.

*Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow district,Russia.

Recent studies prove that the partial melting in some eclogite xenoliths in kimberlites is closely related to formation of diamonds in these rocks at 4-6 GPa and 1150-12500C (Misra et al., 2004; Shatsky et al., 2008). Along with specific mineral assemblages, the products of the eclogite partial melting commonly include relics of potassium-rich silicic melts (45-65 wt. % of SiO2, 4–14 wt. % of K2O and K2O/Na2O > 1.0) (Misra et al., 2004; Shatsky et al., 2008).

Available experimental data, however, demonstrate that such melts can not be produced by “dry” or hydrous melting of a common eclogite. It implies that partial melting and conjugate diamond formation in mantle eclogites was triggered by infiltration of potassic fluids/melts. Assemblages of Cl-bearing phases and carbonates in eclogite xenoliths (Misra et al., 2004), and eclogitic diamonds (Izraeli et al., 2001; Zedgenizov et al., 2007; Tomlinson et al., 2006; Weiss et al., 2009) suggest that these agents were chloride-carbonate-H2O melts or/and chloride-H2O-CO2 fluids. In order to characterize interaction of both types of liquids with eclogites and their minerals, experiments in the eclogite-related systems with participation of CaCO3-Na2CO3-KCl-H2O or H2O-CO2-KCl are reviewed. Melting relations in the system eclogite-CaCO3-Na2CO3-KCl-H2O follow the general scheme proposed earlier for chloride-carbonate-silicate systems (Safonov et al., 2009). Below 12000C, Grt, Cpx and phlogopite (Phl) coexist with LCC only. Formation of Phl and Ca-rich Grt after Cpx indicate active reactions of Cpx with LCC accompanied by CO2 degassing and depletion of the clinopyroxene in jadeite. Subsequent dissolution of silicates in LCC at >1200OC results in formation of potassic silica-undersaturated carbonate and Cl-bearing melt (LCS) (37-40 wt. % of SiO2, 10–12 wt. % of K2O, 3.5 wt. % of Cl) immiscible with the LCC. Compositional feature of this melt is very comparable to those of low-Mg carbonate-silicate melt inclusions in diamonds (Weiss et al., 2009). However, it is not relevant to the melt relics preserved in the partially molten eclogite xenoliths.

Melting of eclogites with participation of the H2O-CO2-KCl fluid at 5 GPa at 1200-13000C (Butvina et al., 2009) produces CO2-depleted aluminosilicate melts with up to 46 wt. % of SiO2, 9–10 wt. % of K2O, 2-5 wt. % of Cl, whose SiO2 and K2O contents resemble the silica-poor varieties of melt relics in the eclogite xenoliths (Misra et al., 2004; Shatsky et al., 2008). Presence of KCl in the fluid intensifies melting, that is related both to high Cl content in the melt and its enrichment in K2O via K-Na exchange reactions with the immiscible chloride melt. 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. No additional crystalline phases, except Grt, Cpx, and Phl, were observed in the above experiments. However, experiments in the model system jadeite-diopside-KCl-(H2O) at 4-5 GPa shows, that KCl liquids provoke formation of ultrapotassic Cl-bearing silica-rich (i.e. 63-65 wt. % of SiO2) melt, which is able to produce sanidine and Al-celadonite-phlogopite mica, which are observed in partially molten eclogites (Shatsky et al., 2008). Dissolution of pyrope in KCl-rich liquids results in formation of spinel and olivine, which are also common products of garnet breakdown within the zones of partial melting in eclogite xenoliths (Misra et al., 2004; Shatsky et al., 2008).

Thus, the reviewed experiments imply that the KCl-bearing liquids could serve as triggers for formation of the wide varieties of K-rich aluminosilicate and carbonate-silicate melts during the eclogite melting in the mantle. Nevertheless, compositional variability of the produced melts, as well as formation of some crystalline phases (sanidine, mica, spinel, olivine) during this process could be a result of highly localized action of these liquids.

Support RFBR: grants 10-05-00040, 08-05-00110, grant to the leading scientific school 3634.2010.5 (A.A. Marakushev).

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