Mineralogical studies of pyropes in kimberlites from China |
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| Authors: | Dong Zhenxin |
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| Institution: | (1) Geological Museum of China, 100034 Beijing |
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| Abstract: | Based on the measurements of refractive index, specific gravity, unit cell parameter, and mineral chemistry and infrared absorption
spectrum analyses of pyropes in kimberlites from China, systematic studies of the physical properties and compositional variations
of pyropes of different colors and diverse paragenetic types, within and between kimberlite provinces have been undertaken.
The origin of pyropes in the kimberlites and the depth of their formation have been discussed.
Pyropes of the purple series are different from those of the orange series in physical and chemical properties, for example,
pyropes of the purple series are higher in a0, RI, SG, Cr2O3, MgO, Cr/(Cr+ Al), Mg/(Mg+ Fe), and Mg/(Mg+ Ca), and lower in A12O3, Fe2O3 + FeO than those of the orange series. The classification of garnets in kimberlites from China by the Dawson and Stephens’
method (1975) has been undertaken and clearly demonstrates that pyropes of diamond-rich kimberlites contain much more groups
than those of diamond-poor, especially diamond-free kimberlites. The higher in a0, RI, SG, Cr2C3, Cr/(Cr + Al), knorringite and Cr-component the pyropes are, the richer in diamond the kimberlites will be. The infrared
absorption spectrum patterns of pyropes change with their chemical composition regularly, as reflected in the shape and position
of infrared absorption peaks. Two absorption bands at 862–901 cm-1 will grade into degeneration from splitting and the absorption band positions of pyropes shift toward lower frequency with
increasing Cr2O3 content and Cr/ (Cr+ Al) ratio of pyropes. LREE contents of orange pyrope megacrysts are similar to those of purple pyrope
macrocrysts, but the former is higher in HREE than the latter, showing their different chondrite-normalized patterns. The
formation pressures of pyropes calculated by Crcomponent, Ca-component, knorringite molecules of pyropes show that some pyropes
of the purple series in diamondiferous kimberlites fall into the diamond stability field, but all pyropes of diamond-free
kimberlites lie outside the diamond stability field. The megacrysts were formed through early crystallization of kimberlites
magma at high pressure condition, the majority of the purple pyrope macrocrysts have been derived from disaggregated xenoliths,
but the minority of them appear to be fragments of the discrete megacryst pyropes, or phenocrysts.
This research project was supported by the Geological Museum of China, the Chinese Academy of Geological Sciences, and the
National Natural Science Foundation of China (No. 49472094). |
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| Keywords: | pyrope kimberlite diamond China |
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