2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Mg-ilmenites: evidence from kimberlite pipes with different diamond grade (Daldyno-Alakit area, Yakutia)

Kargin A.V. * Golubeva, Yu. Yu.**

* The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Moscow, Russia; kargin@igem.ru

** Central Institute of Geological Exploration for Base and Precious Metals (TsNIGRI), Moscow, Russia jugolubeva@gmail.com

Mg-ilmenites from high diamond grade kimberlite are different from very low grade by higher Cr and Fe3+ contents and moderate MgO contents. This suggests with different sequence of crystallization of Cr- and Mg-rich phases of megacryst association. This sequence of crystallization depends on presence of carbonate-rich melts in megacryst-forming zone of high diamond grade kimberlite.

Ilmenite is a typical mineral megacryst association of kimberlites. Its origin and characteristics of the composition have a close origin relationship with host kimberlite. Kostrovitsky S.I. (Kostrovitsky et al., 2006 and references therein) and our previous studies (Kargin et al., 2009) shown that the variation of composition of ilmenite correlate with geochemical characteristic of host kimberlite in a field, group (cluster), pipe kimberlites and reflect evolution of kimberlite melt.

The object of this research is the study of composition of ilmenites from kimberlite pipes with different diamond grade for study of changing the composition of ilmenites within the transition from high diamond grade kimberlite to very low grade kimberlite in the Daldyno-Alakyt region.

In this work have been studied a suite of ilmenites from four kimberlite pipes of Daldyno-Alakyt region with different diamond grade (in brackets the number of ilmenites study): high diamond grade Komsomol'skaya kimberlite pipe (97); moderate diamond grade Ukrainskaya (81) and Dal'nyaya (450) kimberlite pipes; very low grade Ugadayka kimberlite pipe (99). The determination of the composition of ilmenites was performed on a JEOL JXA-8200 (Japan) in IGEM RAS. We used the data of composition of ilmenites high diamond grade Yubileynaya and Zarnica pipes and very low grade Ozernaya kimberlite pipe from (Ashchepkov et al., 2010) for comparing with the results obtained in this study. All presented in a suite of minerals are Mg-ilmenite with high MgO content (more 7 wt. %) and the Cr₂O₃ content from 0.01 to 5 wt. %. Cr-poor (generally < 0.2-0.3 wt. % Cr₂O₃) ilmenite occur in eclogite xenoliths (Tollo, 1982). Cr-rich (generally > 0.2-0.3 wt. % Cr₂O₃) ilmenites comparable to ilmenites from megacryst association which close origin relationship with host kimberlite melts (Moore, Belousova, 2005; Kostrovitsky et al., 2006; Ashchepkov et al., 2010).

Ilmenites from high diamond grade kimberlite are characterized by relatively high Cr₂O₃ content at rather low MgO content. They tend to be left branch of the parabola on the fig (a). Ilmenites from very low diamond grade kimberlite are characterized by relatively moderate Cr₂O₃ content at rather high MgO content. They tend to be right branch of the parabola on the fig (b). In addition ilmenites from high diamond grade kimberlite are characterized high Fe³⁺ content rather than ilmenites from very low diamond grade kimberlite. This is reflected in lower values ​​of the ratio of Fe²⁺/(Fe²⁺+Fe³⁺) (fig. c, d).

The presence of two trends on MgO vs. Cr₂O₃ diagram for mg-ilmenites of high diamond grade kimberlite and very low diamond grade kimberlites may be explained by the different sequence of crystallization of ilmenite and silicate phases rich in Cr and Mg (garnet, clinopyroxene, olivine, orthopyroxene) in megacryst association. Ilmenites from very low diamond grade kimberlites are characterized by high MgO content and moderate Cr₂O₃ content. These ilmenites may be crystallized after the crystallization of Cr-rich phases (garnet, clinopyroxene) and before the crystallization to the main Mg-rich silicate phase (Moore, Belousova, 2005). Ilmenites from high diamond grade kimberlite are characterized by relatively high Cr₂O₃ content at rather low MgO content. These ilmenites are crystallized simultaneously with Mg-rich silicate phase (olivine, orthopyroxene) and before crystallization of Cr-rich phase (garnet, clinopyroxene). This is possible explained by the presence of fluid or melt enriched CO₂ in the area of occurrence of megacryst association. In this case the mineral-melt Mg-Fe distribution coefficient (K_d) is higher for carbonate-bearing liquids than silicate magmas. This presence promotes to early crystallization Mg-rich phases. The increased activity of Fe³⁺ is the result of increasing oxygen fugacity during the interaction of CO₂-rich fluid or melt (accompanying kimberlite melt) and host of mantle peridotites (Moore, Belousova, 2005).

Thus, Mg-ilmenites from high diamond grade kimberlite are different from very low grade by higher Cr and Fe3+ contents and moderate MgO contents. This suggests with different sequence of crystallization of Cr- and Mg-rich phases of megacryst association. This sequence of crystallization depends on presence of carbonate-rich melts in megacrysts-forming zone of high diamond grade kimberlite. The obtained data are consistent with the study of the geochemical characteristics of kimberlites of Daldyn-Alakit region (Kargin et al., 2011). In this study have established that the at occurrence high diamond grade kimberlite there is an intensive interaction kimberlite melt with fluid/melt enriched CO₂ and the lithospheric mantle.

Fig. MgO vs. Cr₂O₃  (a, b) and atomic Fe2⁺/(Fe²⁺+Fe³⁺) vs. MgO (c, d) in ilmenites from high diamond grade kimberlite (a, с) and very low grade kimberlite (b, d) from Daldyno-Alakyt region.

1-7 ilmenites from kimberlite pipes: 1 – Komsomol'skaya, 2 – Yubileynaya, 3 – Zarnica, 4 – Ukrainskaya, 5 – Dal'nyaya, 6 – Ugadayka, 7 – Ozernaya. The date for Yubileynaya, Zarnica and Ozernaya pipes from (Ashchepkov et al., 2010). Dotted gray line in fig. (a, b) from shows the parabola from (Haggerty, Tomkins, 1983). Gray field on the fig. (d) show the area of high diamond grade kimberlite in fig. (c).

This study was financially supported by grant of the President of the Russian Federation the state support of young Russian scientists, MK-310.2010.5.

References

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