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Alkaline carbonate-silicic fluid as a crystallization medium of diamond coats from Sytykanskaya kimberlite pipe

Skuzovatov S.Yu., Zedgenizov D.A.

Institute of geology and mineralogy SB RAS

Novosibirsk, Russia

 justsquall@gmail.com

 

Due to its chemical and physical resistance a diamond is an ideal container for the transporting of mineral-forming medium and mantle rock minerals. Study of high-density fluids (HDF), preserved as microinclusions  in diamonds with cubic habit and coated diamonds, gives a unique opportunity  for composition and evolution reconstruction of deep-seated liquids, responsible for mantle metasomatism, diamond formation and upper mantle rock partial melting.  Such microinclusions are found in cubic habit diamonds, central parts of octahedral crystals, showing a morphology change from cubic to octahedra, and coated diamonds (IV mineralogical variety according to Yury Orlov classification) [Boyd et. al., 1989; Boyd et. al., 1991]. Coated diamonds from some Yakutian kimberlite pipes are of a high proportion relative to the whole selection, but no dedicated studies of such crystals were made earlier. Here we report the results of study of coated diamonds from Sytykanskaya kimberlite pipe.

Studied crystals have a complex internal structure: there are clearly distinguishing octahedral core with a linear zoning and outer fibrous coat. It is obvious, that a growth mechanism changed from tangential (layered) to normal as a carbon oversaturation value and a growth rate increased in fluid/melt at crystallization process

Major defects distribution in diamond crystals is not uniform. The total nitrogen content varies both between different crystals and within an each single crystal (between a core and a coat), and has a value of 150-200 ppm. Within a single crystal, a nitrogen concentration is either higher in a rim relative to a core (a coat-core difference is up to 750 ppm), or prominently higher in a core, or core and a coat values are comparable. This variety in distribution heterogeneity trends may be due to a changing nitrogen concentration in a growth medium.  At the same time the nitrogen aggregation state (into a IaB type) values show quite a definite regularity: highly-aggregated central parts of crystals (up to 80-85% IaB), in almost all cases containing B2-defects, are contrast to low-aggregated coats (5-20% IaB). The band, corresponding to C-H vibrations (3107 cm-1), is observed in all spectra. Its intensity is also not uniform within crystals: hydrogen defect content always has a higher value in a crystal core and sharply decreases in a coat. It is notable, that different crystal cores vary significantly in their impurity composition characteristics, whereas fibrous coats characteristics are similar.

In addition to major defects absorption bands, those corresponding to water, carbonates and silicates are detected in IR-spectra. Nevertheless, absorption intensity of these phases is much lower as compared to cubic habit diamonds. In coat spectra the presence of water is detected both in the form of stretching vibrations of OH-groups in microinclusions minerals and in the form of deformation vibrations HOH, corresponding to molecular water. A water/carbonate ratio in microinclusions is 0,14-0,30. In spectra of diamonds, enriched by water and silicates (absorption bands in 1000-1200 cm-1 range), SiO2 presence is clearly distinguished (810 and 783 -1 for quartz at 1,7 GPa).

High SiO2 concentration from these crystals is confirmed by EDS analysis of major elements content. Microinclusions in studied diamonds are enriched in Ca and Fe as compared to Mg. A positive correlation of alkalis with chlorine content lets us suggest K and Na to enter partially a composition of chlorides No dependence between bivalent cations content and Si+Al and between alkaline cations and Si+Al was observed. Based on (K+Na)/Cl ratio (3-9), quite a high proportion of these cations enter a composition of phases, which are stoichiometrically differ from alkalis chlorides, e.g. silicates or more compositionally complex minerals. It has been distinguished earlier, that high potassium content can also depend on the presence of glass, enriched by K, and fluid bubble, partly or mainly composed by KOH among microinclusion phases [Logvinova et.al. 2008].

At present it is noted, that the bulk microinclusion composition ranges continuously between hydrous-saline and carbonate end members, and also between carbonate and hydrous-silicic ones [Navon et al., 1999]. Microinclusion composition in studied diamonds corresponds to a transition between carbonate and hydrous-silicic end members. (Fig. 1, a dashed-line field marks growth medium composition data on diamonds from various mines worldwide). It is notable, that for three diamonds average compositions of fluid/melt show a relative enrichment by alkaline component. Data received are close to compositions, detected for cubic diamond population from Jwaneng (Botswana) [Schrauder, Navon, 1994], Internationalnaya (Yakutia) [Zedgenizov et. al., 2009] pipes and Brazil placers [Shiryaev et. al, 2005]. This fact is consistent with considerations about growth process of cuboids, showing in most cases a fibrous structure, close in time to the kimberlite eruption. It was already reported earlier, that a growth of fibrous coats, containing numerous microinclusions of mineral-forming medium, takes place directly before the kimberlite eruption episode [Boyd et. al., 1989; Boyd et. al., 1991]. It is also confirmed by the nitrogen aggregation data, received for studied diamond coats. At the same time diamond cores are characterized by much longer times/higher temperatures of a residence in mantle.

 

Fig. 1. Growth medium composition of studied diamond coats in comparison with data on diamonds from various pipes worldwide.

 

It is suggested, that the interaction between deep-seated alkaline fluids/melts and upper mantle rocks determines the formation of kimberlite and lamproites melts. The generation of carbonate-silicic fluids/melts, which are the growth medium for Sytykanskya pipe diamond coats, may be due to metasomatic processes in the upper mantle, and also due to a partial melting of carbonated peridotites and eclogites. Compositional variations, having quite a wide range, may be caused also by a fluid fractionation and a mixing of fluids/melts with different compositions

 

This study was financially supported by RFBR grant 09-05-00985 and SB RAS integration project 51).

 

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