Experimental melting of carbonated K-rich garnet harzburgite and origin of kimberlite melts
Bulatov V.K.a, Girnis A.V.b, Brey G.P.c
Institute of Geochemistry and Analytical Chemistry, Russian
b Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny 35, Moscow, 119017 Russia
cInstutut für Geowissenschaften, J.-W. Goethe Universität, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany
Experimental melting of carbonated K-rich garnet harzburgite was carried out at 6 and 10 GPa, 1100–1600oC in a Walker-type multianvil apparatus. The main goal of the experiments was to determine the composition of melt near the solidus of harzburgite with addition of potassium and magnesium carbonates as a proxy for metasomatized mantle peridotite. The starting material was a mixture of natural olivine, orthopyroxene, and garnet with 5% MgCO3 + K2CO3 (~1 wt % K2O). It was found that the near-solidus mineral assemblages of this material could not be reliably established because of the small grains size and low amounts of K-bearing phases. Therefore, in order to clarify possible phase relations near the solidus of K-bearing carbonated peridotite, an experimental series was performed with the SC1 lherzolite (Brey et al., 2008) blended with 10% MgCO3 and 10% K2CO3.
In the both systems, the beginning of melting was detected at ~1100œC
at 6 GPa and ~1200œC at 10 GPa. Only olivine, orthopyroxene,
garnet and magnesite were found in the products of
harzburgite-MgCO3-K2CO3 experiments. The
amount of K2O in the starting mixture (1.4 wt %) was too high to be
accommodated in the silicates and carbonates. Therefore, a K-bearing phase (or
phases) must be present near the solidus of this mixture. Experiments with the
K richer starting material based on the SC1 peridotite showed that two
K-bearing phases are stable near the solidus of the lherzolite
(SC1)–MgCO3–K2CO3 system. Phase X was found in
the subsolidus experiments at 8 and 10 GPa, and K-Mg carbonate, K2Mg(CO3)2,
crystallized at 6–10 GPa. In the 10 GPa experiments it coexisted with magnesite over a wide temperature range. The stability
field of magnesite expands considerably with
increasing pressure in the SC1–MgCO3–K2CO3
system. In the harzburgite system, magnesite is stable up to about
A comparison of the experimental carbonated silicate melt with the supposed primary kimberlite magmas (Becker, Le Roex, 2006) has led us to the conclusion that the latter cannot be produced by single-stage melting of the asthenospheric mantle. In order to explain the origin of “protokimberlite” carbonated silicate melts, we proposed a two-stage scenario, including the formation of a liquid enriched in volatile and incompatible components in the asthenospheric (lherzolitic) mantle and its interaction with depleted harzburgites (possibly, preliminarily metasomatized) in the lower part of the continental lithosphere.
Becker, M., Le Roex, A.P. (2006). Geochemistry of South African on- and off-craton, group I and group II kimberlites: Petrogenesis and source region evolution. Journal of Petrology 47, 673–703.
Brey, G.P., Bulatov, V.K., Girnis, A.V., Lahaye, Y. (2008). Experimental melting of carbonated peridotite at 6–10 GPa. Journal of Petrology 49, 797–821.