| 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 [e.g. 1,
2]. 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) [1, 2]. 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 [1], and
eclogitic diamonds [3-6] 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 [7]. 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 [6].
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 [8] 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 [1, 2]. 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 [2]. 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 [1,
2].
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.
The study is supported by the RFBR (10-05-00040), 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] Izraeli et al. (2001) Earth Planet. Sci. Lett.,
5807, 1-10; [3] Zedgenizov et al. (2007) Doklady Earth Sci., 415, 961-964; [5] Tomlinson et
al. (2006), Earth Planet. Sci. Lett., 250, 581-585; [6] Weiss et al. (2009), Lithos, 112S,
660-674; [7] Safonov et al. (2009), Lithos, 112S, 260-273; [8] Butvina et al. (2009), Doklady
Earth Sci., 427A, 956-960. |