Variations in carbon isotopic composition in the subcontinental lithospheric mantle beneath the Yangtze and North China Cratons: Evidence from in-situ analysis of diamonds using SIMS

Abstract

The components and evolution of subcontinental lithospheric mantle beneath the North China Craton and the Yangtze Craton is a current topic in the geological study of China and the carbon isotopic composition of diamond is one of the most direct probes into cratonic lithospheric mantle processes. In this paper, in-situ SIMS (Secondary Ion Mass Spectrometry) techniques were used to analyze the carbon isotope compositions at different internal growth zones of diamonds from Shandong and Liaoning in the North China Craton and Hunan in the Yangtze Craton. It was found that the carbon isotopic range of diamonds from the North China Craton are rather distinct from those of the Yangtze Craton; the former has a range of 6.0‰ to 2.0‰ (relative to VPDB) with an average value of 3.0‰ in their core areas, which is consistent with global peridotitic diamonds; the diamonds from the Yangtze Craton, however, have a carbon isotopic range from 8.6‰ to 3.0‰ with an average value of 7.4‰ in their core areas, being more consistent with global eclogitic diamonds. The variations of carbon isotope ratios between different internal growth zones in individual diamonds were different in the three diamond localities studied. There was a clear correlation between changes in carbon isotopic composition and phases of diamond dissolution and new growth, while no correlation was observed between   13 C and internal inclusions. The variations suggest that the carbon isotopic compositions of mantle fluids were changing during the process of diamond crystallization, and that the heterogeneity of the carbon isotopic composition in mantle carbon reservoirs was a more important factor than carbon isotope fractionation in controlling the carbon isotopic compositions and their variation in diamonds. In addition, the preliminary results of in-situ nitrogen analyses demonstrated that the variation of carbon isotopic compositions between the core and outer growth zones does not correlate with nitrogen abundances, implying either that diamonds crystallized in an open environment or that the carbon isotopic composition and nitrogen contents in mantle fluids were controlled by other, not yet understood factors. The experimental results provide hints that the isotopic composition of carbon and its original sources were different in metasomatic fluids controlling diamond formation in the mantle beneath the North China Craton and the Yangtze Craton.

Thermobarometry of Diamond Inclusions: Evidence Mantle Evolution beneath Siberian Craton and Archean Cratons Worldwide.

“…In China, the diamonds and hosting kimberlites were discovered mainly within the North China (Sino-Korean) craton. The main deposits are located in the Mengyin kimberlite field (Shandong province) which contain also diamonds with inclusions [155].…”

Microdiamonds in Alkalic Dolerites from the North China Craton: FTIR and C Isotopic Characteristics

“…The North China Craton (NCC) is one of the oldest cratons on Earth [32][33][34]. It was amalgamated after the collision of Eastern Block and Western Block at ca.…”

“…Several diamond deposits have been reported in the North China Craton [33,35]. The two most significant deposits are in Wafangdian of Liaoning province and Mengyin of Shandong Province [1].…”

“…The requisite nitrogen concentrations and aggregation states were determined via FTIR analyses (see above). The mantle residence temperature was assumed to be in the range of 1200-1225 • C [33].…”