Microdiamonds in Alkalic Dolerites from the North China Craton
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Abstract:
Diamonds on the Earth mainly occur in volcanic rocks such as kimberlites and lamproites[1-8], but can also be found in ultrahigh-pressure metamorphic rocks[9-10], meteorites[11] and alluvial deposits[12]. In recent years, diamonds have been recovered from ophiolites[13-16] and alkalic dolerites[17-18]. The discovery of ophiolitic diamonds and alkalic dolerites diamonds has drawn significant research interests to explore the origin of this new class of diamond source and to infer the evolution of their hosting rocks[19-21]. This new type of diamond had been initially considered as a result of contamination. However, more and more evidences either directly or indirectly demonstrate that these diamonds are of natural origin[16, 18, 19, 22-24].
During a geological survey from 2012 to 2015, the geologists from Nanjing Centre of China Geological Survey discovered a large number of yellow microdiamonds in the Langan area in northern Anhui Province[18, 25-29]. The diamond-bearing rocks of these microdiamonds mainly include dolerite and olivine basalt. From 2016 to 2018, four microdiamonds in basic rocks were recovered again in the prospecting work for primary diamond deposits in the Tashan and Zhangji areas in Xuzhou, which is geographically close to Langan[30]. All these microdiamonds are similar in colour and shape to ophiolite type diamonds[31], and show different characteristics of kimberlite and lamproite type diamonds.
Cai, et al. (2019) reported the petrological characteristics of the diamondiferous rocks[17, 21, 30]. In this paper, the morphology, infrared spectrum, and carbon isotope compositions of microdiamonds were analysed and discussed by Fourier infrared spectroscopy and carbon isotope test. The types of microdiamonds found in the North China Craton, the age of mantle occurrence, and the source of carbon isotopes were revealed.
In the past, many deposits of macro-diamonds, mostly of TypeⅠ a or Ⅱ a, were found in the North China Craton, and they have been extensively studied. Microdiamonds recovered from the alkalic dolerites of the North China Craton were studied by FTIR and carbon isotopic.
These diamonds are usually light yellow to yellow, with a few colourless, and cubic, octahedral or rhomboidal dodecahedron, and octahedron in shape. The surface characteristics of diamonds, such as dissolution, can be observed. The overall N concentration is not high, with an average of 173×10-6. The infrared spectra show that most of these diamonds are Type Ⅰ b, and C centres are found at 1 344 cm-1. Three diamond samples are classified as Type Ⅰ a/Ⅰ b, because of A centres and C centres in these diamonds. Two diamonds are classified as type Ⅰ aAB because B, B′ and A centres are found co-existing. FTIR microscopic measurements from the core to the edge of the Type Ⅰ aAB diamond suggest a mantle residence time of approximately 550 Ma. The C isotopic analysis reveals that these diamonds are strongly depleted in 13C. These low δ13C values of dolerites-hosted diamonds overlap with the lower ends of peridotitic diamonds and metamorphic diamonds, and the upper end of the ophiolitic diamonds. Additionally, the reason for the strong deficit δ13C shown by the carbon isotope should be studied in the future.
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Keywords:
- microdiamond /
- FTIR /
- carbon isotopic /
- alkalic dolerite /
- North China Craton
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Table 1 Types of tested microdiamonds and the data of the IR spectrua
Sample No. Major Peak/cm-1 Type of C-N Type N Concentration /10-6 Ntotal/10-6 AD1 1 280, 1 344, 1 976, 2 852, 2 925 A, C ⅠaA/Ⅰb 143.0(C centre) 143.0 AD2 1 172, 1 288, 1 361, 1 650, 1 976, 1 851, 2 919 A, B', B ⅠaAB 60.9(A centre), 134.2(B centre) 195.1 AD3 1 650, 1 976, 2 850, 2 919 A Ⅰa 4.5(A centre) 4.5 AD4 1 130, 1 344, 1 976 C Ⅰb 10.7(C centre) 10.7 AD5 1 128, 1 344, 1 650, 1 976, 2 850, 2 919, 3 290 C Ⅰb 76.9(C centre) 76.9 AD5 1 130, 1 284, 1 345, 1 645, 1 977, 2 848, 2 920, 3 201, 3 394 A, C ⅠaA/Ⅰb 23.6(C centre) 23.6 AD7 1 128, 1 272, 1 344, 1 976, 2 848, 2 921 C Ⅰb 393.5(C centre) 393.5 AD8 1 128, 1 282, 1 344, 1 596, 1 976, 2 854, 2 919 A, C ⅠaA/Ⅰb 240.9(C centre) 240.9 AD9 1 344, 1 645, 1 976, 2 848, 2 919, 3 191, 3 394 C Ⅰb 137.0(C centre) 137.0 AD10 1 128, 1 272, 1 344, 1 600, 1 976, 2 848, 2 917 C Ⅰb 503.5(C centre) 503.5 
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