2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Djerfisherite as an indicator of enrichment of kimberlite melts in chlorine

 Sharygin I.S., Golovin A.V., Pokhilenko N.P.

V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

igor.sharigin@gmail.com

The identification of the initial kimberlite mineralogy and primary compositions of kimberlitic melts are the main problems in kimberlite petrogenesis. Solving of these problems is hampered mainly due to strong postmagmatic alteration of kimberlite worldwide. The results of the study of altered rocks reported in numerous publications do not reflect the primary chemical and mineral compositions of kimberlites. Unique unaltered kimberlites characterized by low water contents (<0.5 wt %) and unusually high concentrations of alkalis (up to 8 wt % Na₂O + K₂O) and chlorine (up to 6 wt %) were discovered in the Udachnaya-East pipe. These rocks do not contain the minerals typical for the secondary processes (e.g., serpentine). The study of radiogenic isotopes in kimberlites (Kamenetsky et al., 2009) provides evidence for the fact that the system was closed from the moment of pipe formation (~370 Ma). The above mentioned peculiarities allow us to attribute these kimberlites to a rocks unaltered by postmagmatic processes. Thus, the chemical composition of the groundmass of unaltered kimberlite from the Udachnaya-East pipe may be considered as the closest to the primary composition of kimberlitic melt at the moment of rock crystallization. A number of petrological, geochemical, mineralogical and melt inclusion studies (Golovin et al., 2007; Kamenetsky et al., 2007, 2008, 2009) demonstrated that kimberlitic melts of the Udachnaya-East pipe were initially enriched in alkalis and chlorine of mantle origin. This result is hardly acceptable by the geoscience community due to following major reasons. First, Udachnaya-East pipe is an only example of such unusual kimberlite rocks. Second, there is a possibility that Udachnaya-East kimberlites are enriched in alkalis and chlorine due to either contamination by the saline deposits of the sedimentary cover of the Siberian Platform or brines, which were documented in sedimentary rocks enclosing the pipe.

An enrichment of Udachnaya-East kimberlites in alkalis and chlorine is caused by the presence of groundmass halite, sylvite, sodalite Na₈Al₆Si₆O₂₄Cl, djerfisherite K₆Na₈(Fe,Cu,Ni)₂₄S₂₆Cl, shortite Na₂Ca₂(CO₃)₃, zemkorite (Na,K)₂Ca(CO₃)₂, aphthitalite NaK₃(SO₄)₂, rasvumite KFe₂S₃, and phlogopite (Golovin et al., 2007; Kamenetsky et al., 2007, 2008, 2009; Sharygin et al., 2007). In addition, chlorine-bearing carbonate, northupite Na₃Mg(CO₃)₂Cl, was recognized in olivine-hosted melt inclusions from this kimberlite (Golovin et al., 2007). Among the main chlorine concentrators revealed in unaltered kimberlites from the Udachnaya-East pipe, chlorides and chlorine-bearing alkaline carbonates are water-soluble minerals, whereas sodalite is usually decomposed under near-surface conditions. Evidently the preservation of these minerals in kimberlite is unlikely to occur under the near-surface conditions. At the same time chlorine-bearing sulfide, djerfisherite K₆Na₈(Fe,Cu,Ni)₂₄S₂₆Cl, was registered in groundmass of unaltered kimberlite and xenoliths as well from pipes of the Siberian (Fig.1), Slave (Canada), Karelian (Finland) and Kaapvaal (South Africa) cratons (see references in Sharygin et al., 2011; Sharygin et al., in press), i.e., this mineral may be preserved for a long time in rocks modified to various degrees. The appearance of djerfisherite in magmatic rocks reflects the high chlorine contents in the melt at the moment of crystallization of this sulfide (see references and review in Sharygin et al., 2007). Thus, the presence of djerfisherite in kimberlites altered to various degrees is an indicator of the high chlorine concentrations in kimberlitic melts, even if the rocks are depleted in chlorine now as a result of postmagmatic alterations and do not contain other chlorine-bearing minerals. Recent studies of unaltered mantle xenoliths from kimberlite of the Udachnaya-East pipe (Sharygin et al., 2007; Sharygin et al., in press) demonstrated that the origin of djerfisherite in xenoliths resulted from the influence of the kimberlitic melt. Consequently, the presence of djerfisherite in mantle xenoliths is also an indicator of high chlorine concentrations in kimberlitic melts.

By analogy with unaltered Udachnaya-East kimberlites, minerals of the groundmass from altered kimberlites worldwide could originally contain other chlorine-bearing minerals in addition to djerfisherite, such as chlorides and alkaline carbonates, which were removed by postmagmatic alterations later. It is demonstrated for the active sodium-carbonatite Oldonyo Lengai volcano (Tanzania) that under the near-surface conditions alkaline carbonates and chlorides crystallized from the melt disappear over several years, months, and even days; as a whole the chemical composition of rocks varies significantly (Zaitsev et al., 2008).

Presence of djerfisherite in xenoliths and groundmass of kimberlites from pipes of Kuoika field (western surrounding of the Olenekskoe uplift) (Fig.1), for which assimilation of saline deposits or interaction with brines are excluded, can be considered as an unambiguous argument supporting the mantle nature of chlorine in kimberlite melts (Sharygin et al, 2011). Pipes of the Kuoika field at the modern erosion level are localized in Precambrian and Lower Cambrian rocks do not contain saline deposits (Fig. 1). Middle Cambrian, Permian, Triassic, and Jurassic rocks were there at the moment of kimberlite eruption. This is evident from datings of xenogenic sedimentary material, as well as reconstructed epochs of denudation and sedimentation at the Siberian Platform. Proterozoic and Cambrian saline deposits are absent in the area of the Olenekskoe uplift and at the neighboring territories, whereas saline deposits were not registered in Permian and Mesozoic series on the whole territory of the Siberian Platform. In addition, these pipes are located outside the area of abundance of brines (Fig. 1).

Thus, the presence of djerfisherite in multi-age kimberlite pipes worldwide supports the idea about significant chlorine concentrations in kimberlites, and this element would be an important component in kimberlite worldwide rather than at the Udachnaya-East pipe only. Finds of djerfisherite in pipes of Kuoika field, for which assimilation of saline deposits or interaction with brines are excluded, can be considered as an unambiguous argument supporting the mantle nature of chlorine in kimberlitic melts.

This study was financially supported by RFBR (grant № 10-05-00575a) and IGM SB RAS (grant No. VMTK-13).

References:

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