EXPERIMENTAL STUDY ON MELTING PHASE RELATIONS AND DIAMOND FORMATION IN THE CARBONATE-RICH KIMBERLITE FROM MAJUAGAA,

SOUTHERN WEST GREENLAND

Butvina V.G.*, Nielsen T.F.D.**, Safonov O.G*., Litvin Yu.A.*

*Institute of Experimental Mineralogy, Chernogolovka, Russia; **Geological Survey of Denmark and Greenland, Copenhahen, Denmark

Carbonate-rich hypabyssal kimberlites of the Majuagaa dyke, Maniitsoq, southern West Greenland could be roughly characterized as a mixture of olivine and ilmenite (or Ti-magnetite) xenocrysts with the carbonate-rich (calcite and dolomite) matrix [1]. The kimberlites are characterized by high CO₂/H₂O ratio, while low silicate content in the groundmass is resulted from an active growth of melt-equilibrated olivine rims on olivine xenocrysts rather than primary serpentine [1]. Subtraction of olivine and ilmenite results in a composition, which is close to carbonatite rather than carbonate-silicate melt. Comparing this composition with the experimental data on melting of carbonated peridotite [e.g. 2], it is suggested that the Majuagaa kimberlitic melt, probably, has resulted from low-degree partial melting (about 0.6 %) of the pristine carbonated garnet lherzolite [1].

In order to further estimate a possible mantle source for the Majuagaa kimberlites, melting relations of the texturally homogeneous sample are experimentally studied at pressure 8.5 GPa and temperatures 1200 – 1750^oC using the anvil-with-hole HP apparatus and a thermogradient cell. The studied interval of temperatures includes conditions of subsolidus, subliquidus, and complete melting conditions for the kimberlite. Liquids forming both during partial and complete melting of the sample are homogeneous carbonate-silicate melts (L_CS). The quenched carbonate-silicate melt only (i.e. complete melting) is observed at temperatures above 1730^OC. Compositional characteristics of this melt and its MgO/CaO (~1.4) are very similar to the model Majuagaa kimberlite melt calculated from the rock bulk composition after subtraction of olivine composition [1]. It means that this melt would coexist with olivine. However, olivine Fo₉₀Fa₁₀ was firstly identified only at 1730^oC. Garnet and Mg-bearing ilmenite (?) appear at about 1710^oC forming olivine + garnet + ilmenite + L_CS cotectic assemblage. The melt at this temperature is more carbonatitic and shows lower MgO/CaO ratio (~0.8). Such evolution of the melt is consistent with that identified for kimberlite-related systems at high pressures [2-5]. Perovskite, apatite, and Ti-magnetite (?) subsequently join this assemblage at 1700-1680^oC. Appearance of carbonate phase, presumably, dolomite, at 1680^OC manifests the solidus of the system. At lowest temperature (down to 1200^OC), the subsolidus includes the assemblage olivine + garnet + ilmenite (Ti-magnetite) + Mg-aragonite(?) + Ca-Fe-magnesite + perovskite + apatite. No pyroxenes were observed in the solidus run products. Their absence is, probably, related to low activity of CO₂ in the experiments, stabilizing olivine with calcic carbonate (for example, 2olivine + 4CaCO₃ + 2CO₂ = 3dolomite + diopside). š

Thus, the present experiments indicate that the Majuagaa carbonate-rich kimberlite has originated from the carbonated garnet peridotite enriched in titanium. Experiments indicate very narrow temperature interval for formation of the kimberlite melt, i.e. 50-70^oC (~1760 - 1680^oC). It is consistent with experiments by Dalton & Presnall [2] in the model CMAS-CO₂ system, but strongly disagrees with the results of experiments on natural peridotite-carbonate systems [e.g. 3-5]. This narrow solidus-to-liquidus interval implies that the Majuagaa kimberlite, probably, is a near-eutectic (olivine + garnet + ilmenite + dolomite + L_CS) melt separated from the above carbonated garnet peridotite at very low degree of partial melting. Addition of carbonate to the peridotite is, possibly, related to metasomatic processes via CO₂-rich fluids or complex carbonatite melts. The presence of the second agents is supported by recent observations of chloride-bearing carbonatite inclusions in olivines from the Majuagaa kimberlites geochemically similar to the host kimberlites [6].

High efficiency of the Majuagaa carbonate-silicate melts for diamond spontaneous nucleation and growth in the case of oversaturation of this melt with dissolved carbon with respect to diamond is demonstrated with additional experiments at 6.5 – 8.5 GPa and 1650 – 1780^oC with the homogeneous mixture (3:2) of the kimberlite and spectral graphite. Formation of diamond crystals of octahedral habit were observed within the 5 – 12 min period.

ššššššššššššššš The study is supported by the INTAS project 05-1000008-793,8 RFBR grants 08-05-00110-a and 07-05-00499-a, the RF President grants MK-194.2008.5 and MD-130.2008.5.

References:

[1] Nielsen T.F.D., Jensen S.M. The Majuagaa calcite-kimberlite dyke, Maniitsoq, Southern West Greenland // Geological Survey of Denmark and Greenland, Report 2005/43. 2005.

[2] Dalton J.A., Presnall D.C. The continuum of primary carbonatitic-kimberlitic melt compositions in equilibrium with lherzolite: data from the system CaO – MgO – Al₂O₃ – SiO₂ – CO₂ at 6 GPa // Journal of Petrology. 1998. V. 39. P. 1953-1964.

[3] Ryabchikov I. D., Brey G. P., Bulatov V. K. Carbonate melts coexisting with mantle peridotites at 50 kbar // Petrology. 1993. V. 1. P. 189–194.

[4] Girnis A.V., Bulatov V.K., Brey, G.P. Transition from kimberlite to carbonatite melt under mantle parameters: an experimental study // Petrology. 2005. V. 13. P. 1-15.

[5] Brey G.P., Bulatov V.K., Girnis A.V., Lahaye Y. Experimental melting of carbonated peridotite at 6 - 10 GPa. Journal of Petrology. 2008. V. 49. P. 797-821.

[6] Kamenetsky V.S., Kamenetsky M.B., Weiss Y., Navon O., Nielsen T.F.D., Mernagh T.P. Alkali carbonates and chlorine in kimberlites from Canada and Greenland: evidence from melt inclusions and serpentine. 9^th International Kimberlite Conference. 2008. Extended Abstracts. A-00028.