Finite Element Modelling of the Generation of Carbonatite Magmas:

Application to post-orogenic mantle processes

Moore, K.R., Ryan, P.D.R.

Department of Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland.

 

Key carbonatite localities that have been investigated as part of INTAS project 05-1000008-7938 Diamond and Graphite in Carbonate Magmas are: (1) Western Greenland, where the distribution of kimberlites, lamproites and carbonatites is correlated with depth to a fossil subducted slab (personal communication, Nielsen 2008); (2) the Chagatai Complex of diamond-bearing silicocarbonatite dykes and pipes at the western end of the Southern Nuratau Mountains (Tianshan Orogen), Uzbekistan; (3) Transbaikalia, Russia, where carbonatites are temporally correlated both with break-up of the Siberian craton by rifting (Windley et al., 2007) and also potentially with delamination of the lithosphere beneath Mongolia (Petit et al., 2002); and (4) The Kola Peninsula in Scandinavia, where the post-orogenic Baltic Shield is rifted and has been heated at the base of the lithosphere by mantle upwelling (Bulakh et al, 2004). These key localities can be tectonically summarised as suture zones that have later been subject to decompression or heating and produced carbonate magmas as a consequence.

 

Published (Lapin et al., 2005) and new (personal communication, Savatenkov and Konopelko 2008) isotopic data for sövite and silicocarbonatite dykes from Chagatai (Uzbekistan) are dominated by a mantle component, but have a significant crustal component regardless of the abundance of xenoliths. Some mechanism is required for contamination of the isotopic signature of carbonate magmas that, given the association of diamonds with silicocarbonatite (whether as xenocrysts or phenocrysts) must ultimately have a deep source (Djuraev and Divaev, 1999; Moore et al, 2009). Carbonatites emplaced into the Central Asian Orogenic Belt have both subducted oceanic lithosphere and subducted continental rocks (Windley et al., 2007) as potential source components. Release of CO2- or CO2-H2O-rich fluids of melts from a subducting slab has been linked to enhanced eclogitization, crustal isotopic components in a mantle wedge, and diamond formation in a subduction environment (Ducea et al., 2005; Timm et al., 2004). Furthermore, experimental petrology has shown that melts parental to carbonatite activity can be geochemically related to fusion of eclogitic mantle (Treiman & Essene, 1983; Dasgupta et al., 2003).

 

An alternative mechanism to place eclogite in the lithosphere beneath intra-continental carbonatite activity is the formation of deep crustal roots that are isostatically stable within the mantle if buoyancy is overcome by conversion to eclogite facies assemblages. Such assemblages may be preserved long after the end of the orogenic process (Ryan 2001) and numerical modelling shows that such roots are subject to pressures > 3GPa and temperatures in excess of 700oC (Ryan & Dewey 1997), which is considerably lower than the solidus of carbonated eclogites and potentially within the conditions of diamond stability. If a sufficiently dense eclogite layer forms, such that it becomes gravitationally unstable, detaches and sinks, it is replaced by upwelling mantle, providing a heat source at the base of the lithosphere.

 

Finite element modelling is used to investigate the production of carbonate magmas from carbonated eclogite in tectonic configurations anologuous to the post-orogenic scenario in Central Asia, and other INTAS localities, where:

1.        Subducted material remains at depth in the source region of carbonate magmas,

2.        Delamination has caused the emplacement of a high-temperature peridotite, heating the base of a thermally re-equilibrated lithosphere, and

3.        Crustal stretching and thinning decompress post-orogenic lithosphere.

Initial findings are that maximum preservation of carbonated eclogites occurs in cold thick continental lithosphere that has undergone minimal overstepping of the solidus, and that emplacement of hot mafic magmas at the base of the lithosphere causes local, rapid, short-lived melting of carbonated eclogites without necessarily producing associated silicate magmas.

 

This presentation was financially supported by INTAS project 05-1000008-7938.

 

References:

Bulakh, A.G., Ivanikov, V.V., Orlova, M.P., 2004. Overviewof carbonatite-[hoscorite complexes of the Kola Alkaline Province in the context of a Scandinavian North Atlantic Alkaline Province. Pp. 1-43 in: Phoscorites and Carbonatites from Mantle to Mine: the Key Example of the Kola Alkaline Province Wall, F., Zaitsev, A.N. (editors). Mineralogical Society Series 10.

Dasgupta, R., Withers, A.C., Hirschmann, M.M., 2003. Generation of carbonatitic melt through partial melting of carbonated eclogite under mantle conditions. Eurocarb Abstract Volume, 16-18

Djuraev A.D., Divaev F.K., 1999. Melanocratic carbonatites new type of diamond-bearing rocks, Uzbekistan. In: Stanley, C.J. et al. (Eds.), Mineral Deposits: Processes to Processing. Eds..Balkena, Rotterdam 1, 639-642.

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Moore, K.R., Wall, F., Divaev, F.K. and Savatenkov, V.M., 2009. Mingling of carbonate and silicate magmas under turbulent flow conditions: Evidence from rock textures and mineral chemistry in sub-volcanic carbonatite dykes, Chagatai, Uzbekistan. Lithos, in press, doi:10.1016/j.lithos.2008.11.013

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