2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Interphase partitioning of rare elements in diamondiferous eclogite-carbonatite and peridotite-carbonatite mantle system: experiment at 7.0 – 8.5 GPa

Kuzyura A.V..*, Okoemova V.Yu.**, Vasiliev P.G.**, Litvin Yu.A.*, Jeffries  T.***, Wall F.****

* Institution of The Russian Academy of Sciences Institute of Experimental Mineralogy RAS, Chernogolovka, Russia; shushkanova@iem.ac.ru

** Geological department of Moscow State University, Moscow, Russia

*** Natural History Museum, London, Great Britain

**** Cornwall Campus of University of Exeter, Cornwall, Great Britain

Using methods of scanning electron microscopy and electron microprobe (IEM RAS) as well as LAICPMS (London) peculiarities of partitioning of rare elements in pairs mineral/melt and mineral/mineral in eclogite-carbonatite and peridotite-carbonatite systems that were modeled experimentally were determined. A comparison of obtained experimental results with literature ones was done.

Data about trace elements partitioning numbers between mineral and melt phases of diamond forming systems of the Earth mantle are extremely low

Researches of partitioning of rare elements in diamondiferous mantle systems were relates to model eclogite-carbonatite and peridotite-carbonatite systems. In this connection series of experiments on experimental research of equilibrium participation of rare elements “mineral-melt” and “mineral-mineral” in eclogite-carbonatite and peridotite-carbonatite systems were carried out with using of apparatus “anvil-with-hole”-like (IEM RAS) at 7-8.5 GPa.  Prepared in special way mixture of rare elements, mainly in oxides:  Li, Rb, Cs, Ba, Th, U, Ta, Nb, La, Ce, Pb, Pr, Sr, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu, Sc, и Zn was added to the systems. Starting proportions of components were [[CPx_40-64Grt_16-40(SiO₂)₂₀]_59.3Carb_39.3]_98.6RE_1.4 for eclogite-carbonatite system and [[Ol_36-60OPx₁₆CPx_12-24Grt_12-24]₃₀Carb₇₀]₉₉RE₁ for peridotite-carbonatite system. Electron microprobe and SEM researches were carried out on the polished surfaces with carbon covering in IEM RAS. Contents of trace elements in grains on the same samples (carbon covering has been removed from) were determined using of method of LА-ICP-MS in mineralogical department of the London Natural History Museum. We analyzed homogeneous melt areas and isometric grains of minerals.

Quite large (more than 100 µm) garnet, clinopyroxene, and olivine crystals crystallized form melt in experiments at  7.0 – 8.5 GPa (fig. 1). Mostly the crystals appear surrounded by melt in quenched samples. Melt of model eclogite-carbonatite and peridotite-carbonatite systems quenches as cryptocrystalline aggregate. Most cryptocrystalline areas were chosen for analyses, analyses was done on squares.

Based on results of analyses of content of rare elements in experimental phases by LA-ICP-MS-method, coefficients of interphase participation (Kd) garnet-melt, clinopyroxene-melt, and garnet-clinopyroxene were calculated. There is a diagram demonstrating participation of rare elements between garnet and carbonate-silicate melt, clinopyroxene and carbonate-silicate melt, and garnet and clinopyroxene on Fig. 2.

The main feature of the obtained picture of interphase partitioning of trace elements is quite different behavior of light (La, Ce, Pr) in relation to medium and heavy (Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) rare-earth elements. While light elements are mainly distributed in melt phase, the heavy ones go to garnet. A number of elements including LILE (Rb, Sr, Ba), Sc, as well as Zn, Ta, Pb, Th, and U Sc, V, Y, Zr, and Hf have a clearly expressed affinity to carbonate-silicate melt. A curve format on the diagram of participation garnet-melt is practically confirms a curve format for garnet-clinopyroxene participation, this testifies to that main participants of rare elements distribution are garnet and carbonate-silicate melt, clinopyroxene plays a part of Zn-concentrator in the process.

By preliminary data, garnet and clinopyroxene are main collectors of rare elements in peridotite-carbonatite system.

After comparison of obtained partitioning pictures with data of different researchers (Sweeney et al., 1992, 1995; van Westrenen et al, 1999; Walter et al, 2008) there were find out a similarity in contrast behavior of light and heavy REE and other RE at comparison of distribution of rare elements between garnet and carbonate-silicate melt of eclogite-carbonatite system, from one side, and garnet and model silicate melt, from other side (van Westernen et al, 1999).

 Data about trace elements partitioning numbers between mineral and melt phases of diamond forming systems of the Earth mantle were unknown until the present work. These investigations have also more general geochemical value as formation of growth media for a great bulk of natural diamonds and syngenetical minerals included in them are a part of general process of magmatic evolution of mantle substances and is integrated physico-chemically and spatially to it.

Financial support: RFBR 10-05-00654, 11-05-0040, grant of The President RF № МК- 913.2011.5

References:

Sweeney R.J., D.H. Green,  S.H. Sie (1992), Trace and minor element partitioning between garnet and amphibole and carbonatitic melt,  Earth and Planetary Science Letters, V. 114, № 1-2, pp.1-14

Sweeney R.J., V. Prozesky, W. Przybylowich (1995) Selected trace and minor element partitioning between peridotite minerals and carbonatite melts at 18-46 kb pressure, Geochimica et Cosmochimica Acta, V. 59, pp. 3671-3683

Van Westrenen W., J. Blundy, B. Wood (1999) Crystal-chemical controls of trace element partitioning between garnet and anhydrous silicate melt, American Mineralogist, V. 84, pp. 838-847

Walter M.J., G.P. Bulanova, L.S. Armstrong, et al (2008), Primary carbonatite melt from deeply subducted oceanic crust, Nature, V. 454, pp. 622-626.