2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Geochemical heterogeneity of the lithosphere: oxygen fugacity on the basis of  REE variations in zircons from  the mantle rocks

Balashov Yu.A.*, Martynov E.V.*, Belousova E.A.**

* Geological Institute, KSC RAS, Apatity, Russia; balashov@geoksc.apatity.ru

^** Gemoc Arc National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia

The Ce⁺⁴/Ce⁺³ variations in zircons from kimberlite (from -0.76 to +16.4) and lamproite (from -0.32 to + 27.3) are nearly the same. In both cases the minimum ratios indicate the trifling amount of Ce^{+4 in zircons which suggests the reducing conditions of zircon generation ; on the contrary, the Ce+4}/Ce⁺³ maximum values indicate oxidizing conditions of their generation. This interpretation strictly agrees with the modern petrological diagrams of the lithosphere vertical zonality.

       A considerable variation range in oxygen fugacity (volatility) in the mantle and crust rocks effects the redistribution of elements with variable valence (Fe, Eu, V, Cr, Се…). The petrological buffer FMQ was proposed for mantle rocks from different depths and of various generation long ago ((Ballhaus, 1993; Galimov, 1998; Galimov, 2005; Kadik, 2006; Ryabchikov & Kogarko, 2009; Ryabchikov, 2009 et al). This buffer is basic for registration of the ƒО₂ variations,_, which corresponds to the decrease in oxygen volatility with the rise in the depth and temperature of mantle rocks; predominance of oxidized components (ОН^-, Н₂О) in the upper lithosphere and reducing components (Н, С,) in the mantle deep zones.

  We presents an attempt to use rare-earth elements for analyzing oxidation-reduction conditions on the basis of the Ce⁺⁴/ Ce⁺³ and Eu/Eu* variations in zircons from peridotite xenoliths, kimberlites, lamproites, basalts and alkaline complexes from different regions. The Ce⁺⁴/Ce⁺³ variations in zircons from kimberlite (from -0.76 to +16.4) and lamproite (from -0.32 to + 27.3) are nearly the same (Fig. 1). In both cases the minimum ratios indicate the trifling amount of Ce^{+4 in zircons which suggests the reducing conditions of zircon generation; on the contrary, the Ce+4}/Ce⁺³ maximum values indicate oxidizing conditions of their generation. This interpretation strictly agrees with the modern petrological diagrams of the lithosphere vertical zonality (Ashchepkov et al, 2004-2009; Kadik, 2006; Ryabchikov, 2009  et al;  ).

Fig.1. Contrast between the Ce⁺⁴/Ce⁺³ and Eu/Eu* variations in zircons from kimberlite (more than 150 analyses).

If we include the information on alkaline rocks and pegmatite (Fig. 2), the variation range for oxidized forms by cerium will reach gigantic values (intensity of the Ce⁺⁴/Ce⁺³ variation exceeds four orders): for the syenitic pegmatites from Norway, the Ce⁺⁴/Ce⁺³ range corresponds to the variation from 506 to 149, unlike the igneous carbonatite from Kovdor (from 1.36 to 0.14).

                  Fig. 2. Ce⁺⁴/Ce⁺³ variations in zircons from alkaline rocks and pegmatite.

The data on Eu⁺²/Eu⁺³ (or Eu/Eu*) in zircon are of some interest, since these ratios reflect the reduction degree of the natural mantle or crustal system, which in essence also correspond to the oxygen volatility and can bed used for petrological constructions. The Eu/Eu* minimum values should be observed in the uppermost lithosphere and maximum ones - at the bottom, where Eu⁺² should prevail. We have discovered this effect in a number of zircons from kimberlites  (Fig. 1). The distinct excess of europium agrees with the optimal deficit in cerium . Actually, it indicates reducing conditions of zircon generation in the lower lithosphere. It also agrees with petrological conclusions on expectations of such a regime of oxygen fugacity under conditions of H₂O great deficit and hydrogen excess (Galimov, 1998; Galimov, 2005; Kadik, 2006; Ryabchikov, 2009).

Thus, the information on the Ce⁺⁴/Ce⁺³ and Eu/Eu* variations can be considered as the independent geochemical ground for the existence of heterogeneity in the lithosphere.

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