Composition and paragenetic types of pyropes from kimberlites of East Peri-Azovian
Tsymbal S.N.*, Tsymbal Y.S.*, Bondarenko V.A.**, Latsko V.G.**
Institute of geochemistry, mineralogy and ore formation of
N.P.Semenenko, NAS of Ukraine, Kiev, Ukraine;
Pyrope is the most characteristic mineral of kimberlites and xenoliths of deep peridotites. It is considered as one of the major associated mineral of diamond and effectively used for searches of kimberlites, evaluation of their diamond content and reconstruction of mineralization conditions at the upper mantle.
During many years searches for kimberlites are based on dispersion halo of this mineral and are carried out worldwide and in Ukraine as well. Here these searches resulted in discovery of Petrovsk, Nadiya, Southern and Novolaspinsk kimberlite pipes within the eastern part of Peri Azovian megablock of the Ukrainian Shield. They comprized by kimberlite breccias and massive porphyritic kimberlites of Devonian age are also found here.
Pyrope was found in all kimberlite bodies, but in different amount and different proportion with other deep minerals. This mineral is rarely found in Nadiya pipe and most frequently found in Petrovsk and Southern pipes.
Pyrope is represented by oval fractured grains with magmatic (fritted) surface, their clasts and fragments of irregular shape. Some of them show the presence of preserved kelyphitic rim or its relics. Kelyphitic rims are changed in different degree by superimposed processes. Surface of pyropes is smooth or thinly matted under kelyphitic rim and is well sculptured out of rim due to metasomatic and hypergene dissolution processes. Pyramidally-tiled, hackly and blotched sculptures are most widely developed. The size of pyrope grains range from 0,1-0,2 to 5-7 mm and with average values of 0,5-2,0 mm. Most grains have violet-lilac, lilac and lilac-pink colouring. Pyropes of pink, red and reddish-orange colour as well as pyrope-almandine of orange and orange-yellow colours are widely abundant.
Optical-spectroscopic and colorimetric investigations of pyropes sampled from kimberlites of Petrovsk (Matsyk et.al., 1983) and Southern (Tsymbal et.al., 1986) pipes as well as from Novolaspinsk pipe and associated dike (our personal unpublished data) have shown, that predominant amount of pyrope grains should be related to paragenesis of pyrope and chromeshpinel-pyrope lherzolites. There are also varieties of pyropes typical of harzburgites, vehrlites, websterites, magnesian and magnesian - ferrous eclogites.
Chemical composition of pyropes from all known kimberlitic bodies is studied comprehensively enough but the data available are published insufficiently (Tsymbal et.el., 1996; Panov et.al., 2000; Panov, 2001). The results of more than 2000 microprobe analyses of pyropes testify for their composition to be various over a wide range. On CaO-Cr2O3 diagramme offered by N.V.Sobolev they occupy fields of pyropes of lherzolite, vehrlite and a dunite-harzburgite types.
Pyropes of lherzolitic type make 95 % of all the investigated grains. Contents of Cr2O3 in them ranges from 0,3-0,5 to 10-12 %, and CaO does from 4-5 to 7-8 %. Based on the content of Cr2O3 in these pyropes they can be subdivided into low-, middle- and high-chromium varieties.
Low-chromium varieties are rarely developed. Contents of Cr2O3 in them as a rule, is less than 2 % and CaO is about 4,0-5,5 %. They have a little raised contents of alumina (Al2O3 ‑ 20-23 %) and titanium (TiO2 ‑ 0,4-0,8 %) and low content of magnesium (MgO ‑ 18-22 %). Content of FeO varies from 5 to 10 %, and Fe2O3 does from 0,8 to 3,6 %. Knorringite component is absent. On composition they are similar to pyropes found in xenoliths of granular lherzolites with high contents of clinopyroxene and websterites from kimberlites found in some other regions.
Middle-chromium pyropes are considerably prevail over other varieties. The contents of the major oxides in them are following (%): Cr2O3 ‑ 2-8; CaO ‑ 4,5-6,5; MgO ‑ 19-23; Al2O3 ‑ 18-23; FeO+Fe2O3 ‑ 5,5-7,5; TiO2 ‑ 0,10-0,65. There is distinct positive correlation established between CaO and Cr2O3, as well as negative one established between Al2O3 and Cr2O3, MgO and CaO. At the same time MgO and FeO as wel as TiO2 and FeO do not show any correlation between them.
The amount of knorringite component in pyropes does commonly not exceed 10 mole %, that allows us to assume their formation at pressures of 25-35 kBаr. Pyropes that are characterized by 5-7 % of Cr2O3 and 5,5-6,5 % of CaO sometimes form intergrowths with high -magnesian chromite (Cr2O3 ‑ 52-58 %). Pyrope and chromeshpinel-pyrope lherzolites with medium and lowered contents of clinopyroxene were interpreted as possible host rocks for middle-chromium pyrope variety.
High-chromium pyropes of lherzolitic type are rarely found. High contents of Cr2O3 (8-12 %) and CaO (6,5-8,0 %) and low contents of MgO (17-20 %), Al2O3 (14,0-17,5 %) and TiO2 (<0,2 %) are typical of them. The amount of knorringite ranges from 10 to 20 mole %. Inclusions of high-magnesian high-chromium chromites (Cr2O3 ‑ 54-61 %, MgO ‑ 11-13 %) are established in pyropes of this variety. Granular lherzolites with low contents of clinopyroxene that are formed at pressures of 35-45 kBar are supposed to be possible host rocks for them.
Pyropes of vehrlitic type make about 2,5 % from all the grains studied. There are three varieties established among them, that form separate fields by plotting on diagramme CaO-Cr2O3. Among common feature established for each variety is much higher contents of CaO in comparison to pyropes of lherzolite type with the same content of chromium.
Pyropes of the first variety is characterized by low content of Cr2O3 (3,2-3,6 %), raised CaO (5,8-6,2 %), FeO (8,0-8,4 %) and TiO2 (0,32-0,37 %), slightly lowered contents of MgO (19,6-19,9 %) and Al2O3 (21,0-21,3 %). The content of Na2O admixture sometimes reaches 0,11 %. Whole absence of Fe2O3 and knorringite component is characteristic for them.
Pyropes of the second variety are characterized by 4,4-5,2 % of Cr2O3 and 6,7-7,1 % of CaO. In comparison to the first variety they show more high contents of iron (FeO+Fe2O3 ‑ 9,7-10,4 %) and accordingly more less contents of magnesian (MgO ‑ 16,8-17,3 %) and alumina (Al2O3 ‑ 20-21 %). The contents of TiO2 admixture does not exceed 0,1 % with Na2O reaching 0,14 %.
Pyropes of the third variety is different from pyropes of the first and second ones by much high contents of CaO (6,5-8,8 %) and Cr2O3 (6,7-10,3 %) and low contents of Al2O3 (14-19 %) and FeO+Fe2O3 (5,4-7,5 %). The content of Fe2O3 is usually less than 2 % with MgO reaching 18-20 %. They are characterized by raised contents of Na2O (to 0,12 %) and TiO2 (sometimes to 0,7-0,8 %).
Pyropes vehrlitic type are characterized by their enrichement in uvarovite component, with the most chromium bearing varieties showing their enrichement in knorringite component (up to 10-15 %). Their compositional features indicate for their crystallisation at high pressures conditions and low fugacity of oxygen.
Pyropes dunite-harzburgitic type make about 2 % of all studied grains of minerals and are represented by moderate chromium varieties (Cr2O3 ‑ 2,8-7,0 %). The contents of CaO in them varies within the range from 1,6 to 4,5 %. They differ from pyropes of other types by much higher contents of magnesium (MgO ‑ 22,5-24,3 %) and low contents of iron (FeO ‑ 4,6-6,3 %). Content of Al2O3 is 17,0-22,0 %. And there is some distinct inverse relation established between Al2O3 and Cr2O3. Content of TiO2 admixture does not exceed 0,1 % with Na2O reaching 0,12 %. All pyropes of this type show enrichment in knorringite component (to 15 mole %). On CaO - Cr2O3 diagramme some analysis of pyropes of this type are localized within the field of pyropes typical of diamond association.
LA-ICP MS investigations of pyropes of lherzolitic type sample from Petrovsk, Nadiya, Southern and Novolaspinsk pipes have shown, that they are very similar between themselves on "set" and concentration values of admixture elements. The contents of these element are following (ppm): V ‑ 164-318; Sc ‑ 110-212; Ni ‑ 25-70, sometimes 110-132; Co ‑ 28-44; Ga ‑ 2-10; Y ‑ 2-33; Zr ‑ 6-100, rarely 100-156; Hf ‑ 0,3-3,9; Nb ‑ 0,1-1,3; Sr ‑ 0,2-7,9 (Panov et. al., 2000; Panov, 2001). These data testify for the fact that among pyropes of lherzolitic type there are varieties of pyropes depleted and enriched in incompatible elements. Based on Ni contents, it is possible to suppose that pyropes were formed at various temperature. For the majority of them the temperature values were about 1000-1150о C. But there are also more low-temperature (800-900о) and more high-temperature (1200-1400о) varieties. The presence of pyropes containing Y <10 ppm and supposition of their crystallization at Т=1000-1150о C, allows us to assume, that possible thickness of lithosphere where known kimberlite pipes and dikes might be formed did not exceed 140 km in Devonian time.
The typical feature of Peri-Azovian kimberlites is rare occurrence of magnesian-ferrous garnets that are related in composition to pyrope-almandine minerals from xenoliths of magnesian and magnesian-ferrous eclogines, developed in kimberlites of Yakutia and other provinces. By microprobe analysis following elements are defined in them (%): SiO2 ‑ 39,9-42,5; TiO2 ‑ 0,0-0,4; Al2O3 ‑ 22,0-23,8; Cr2O3 ‑ 0,06-0,15, rarely to 0,3; FeO+Fe2O3 ‑ 10,7-18,3; MnO ‑ 0,3-0,5; MgO ‑ 12,0-19,3; CaO ‑ 3,7-8,0. Their relation to eclogitic type is confirmed by data of optical spectroscopy and colorimetry (Tsymbal et.al., 1996).
On the basis of available data some general conclusions should be.
1. All studied kimberlite pipes and dikes of East Peri-Azovian host compositionally similar macrocrysts of pyropes. Among them varieties of lherzolitic type are predominant. Uvarovite-pyropes of vehrlite type and knorringite-pyropes of dunite-harzburgitic type are developed in small amount. Pyrope-almandines of eclogitic type are very rarely found.
2. On typochemical features pyropes might subdivided into two groups, depleted and enriched by incompatible admixture elements. Presence of such groups of pyropes testifies for the fact that upper mantle rocks localized under eastern part of the Peri-Azovian megablock of the Ukrainian Shield have experienced high-temperature metasomatism.
3. Pyropes are crystallised at pressure range from 25 to 45 kBar and temperature range from 800-900 to 1150-1200 °C. At kimberlite formation during the Devonian time thermal flow values was about 40-45 mW/m2 (Panov et. al, 2000).
4. Kimberlite pipes and dikes of East Peri-Azovian have common source of kimberlitic melt which focus were localized at the depth of more than 140 km, near to the boundary of graphite-diamond transition. Presence of knorringite-pyrope diamond association allows us to consider these kimberlites as potentially diamond containing formations and area of their occurrence might be considered as perspective for continuation of prospecting works.
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Panov B.S., Griffin V. L, Panov Yu.B. Р-Т conditions of formation of Cr-pyropes from kimberlites of the Ukrainian Shield // Reports of NAS of Ukraine. 2000. № 3. P. 137-143. (in Russian)
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Tsymbal S.N., Tatarintsev V.I., Kniazkov A.P. Minerals of deep paragenesises from Southern kimberlite pipe (East Peri-Azovian) // Mineral. journ. (Ukraine) 1996. 18, № 5. P. 18-45. (in Russian)