Thermobarometry of pyrope from the Eastern Priazovye kimberlites, Ukraine:

Lithosphere studies and diamond prospectivity

Zozulya D.*, Latsko V.**, Lehtonen M.***, Martynov E.*, Pakkanen L.***

*Geological Institute, Kola Science Centre, Apatity, Russia; **Priazovskaya GRP, Volnovakha, Ukraine; ***Geological Survey of Finland, Espoo, Finland. E-mail: zozulya@geoksc.apatity.ru

 

Based on the chemical composition of pyropes from the Nadiya, Novolospinskaya and Yuzhnaya kimberlitic pipes from the Eastern Priazovye block of the Ukrainian Shield, the P-T parameters of their crystallization are determined. Representing either mantle xenocrysts or constituents of mantle xenoliths, the pyropes contain valuable information on the composition of the lithospheric mantle, its thermal properties and diamond potential of kimberlites sampled.

Garnet major element compositions were determined by Cameca SX100 electron microprobe (EMP) at the Geological Survey of Finland (GTK), applying an acceleration voltage of 25 kV, probe current of 48 nA and beam diameter of 1 μm. Trace Ni data by EMP were obtained on pyrope grains employing 500 nA probe current, 600s counting times on peak plus background positions and were reduced by the CSIRO TRACE program for the SX50 (Robinson&Graham, 1992).

Based on commonly used classification systems (Sobolev, 1974; Gurney, 1984) it is established that the pyropes belong mostly to lherzolitic, and single grains - to harzburgitic parageneses. The single grain Ni-thermometry (Ryan et al., 1996) on pyropes gives a range of temperatures between 750-1200 °C (Fig. 1). The Mg# values of pyropes are highly variable in the Novolaspinskaya and Yuzhnaya pipes (0.83-0.89) assuming the different chemical content for peridotites at sampling interval. The Mg# values for pyropes from Nadiya pipe is relatively constant (0.84-0.85) assuming the homogeneous composition of peridotite at sampling interval. No correlation is observed for Mg# and temperature for all the pipes (Fig. 1), indicating on metasomatical processes in the lithosphere mantle. The mantle metasomatism is confirmed also by the overall elevated TiO₂ content (0.05-0.55 wt %, average 0.24 wt %) in pyropes (Fig. 1).

 

 

Fig. 1. Calculated temperature versus Mg# and TiO₂ for pyropes from the Eastern Piazovye kimberlites.

 

 

For pressure estimations the single grain geobarometer of Ashchepkov I.V. (2006) was used. It is established that the pyropes fall into the range of pressures between 20-60 kbar, corresponding to a sampling interval of ca. 75-190 km. Of those the predominant portion of pyropes has the calculated pressure values of 44-60 kbar, thereby falling into diamond stability field (Fig. 2). From this a stratified structure of the Eastern Priazovye lithospheric mantle is inferred: the deepest layer (130-190 km) is of lherzolitic compositions with insignificant impregnations of harzburgite; the shallower layer (75-130 km) is of presumably pyroxenitic composition with rare impregnations of lherzolite (as can be seen at ca. 100 km depth for Yuzhnaya pipe sampling interval).

The pyropes from the diamond stability field plot between 38 and 40 mWt/m² model geotherms of Pollack&Chapman (1977). The pyropes from the graphite stability field plot near 50 mWt/m² model geotherm, assuming the mantle heating during the kimberlite emplacement. The overall elevated heat flow calculated values are in accordance with measurements of Kutas et al. (1989) obtained for the Eastern Priazovye block and could be alternatively explained by the proximity of the region to the easternmost edge of the craton.

The data obtained show the relatively high diamond potential of the Eastern Priazovye kimberlites as they sampled the diamond “window” of 60 km thickness (130-190km depth), but the lithospheric mantle of the region was affected intensively by metasomatic processes which might caused the corrosion and destruction of the diamonds.

 

 

Fig. 2. Calculated P-T parameters for pyropes from the Eastern Priazovye kimberlites. Depth in km is converted from pressure according to Kukkonen et al. (2003).

 

References:

 

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