Carbon Isotopic Composition of Graphite Inclusion in Black Nephrite Jade from Xinjiang

Xinjiang is the most important metallogenic belt of nephrite jade in China, with a large number of high quality primary and secondary nephrite jade deposits. Two typical types of black nephrite jade from Xinjiang are classified. One is mainly composed of actinolite, and the other one is mainly composed of tremolite and graphite with its colour generally caused by graphite inclusions. It is well known that graphite plays an important role in the processes of organic and inorganic carbon cycle, which exhibits important scientific significance for the deep carbon cycle of the Earth, and the graphite inclusion is acted as a favorable object for the study of carbon accumulation, fixation and cycling under the complex background of Xinjiang. However, previous studies mainly focused on the genetic types and formation mechanisms of nephrite jade, and the carbon sources of graphite inclusions have not been determined, which is not conducive to the complete scientific understanding of the metallogenic regularity and tectonic evolution of nephrite jade. In this study, the mineral components and carbon isotope characteristics of graphite inclusions in black nephrites from Xinjiang were studied by means of microscopic petrography, electron probe microscopy, and carbon stable isotopes. The results show that the main mineral component of black nephrite jade is tremolite, and the accessory or secondary minerals are graphite, calcite, diopside, and apatite, etc. The carbon stable isotopes in graphite reveal that the δ¹³C_PDB values of graphite inclusions in black nephrite jade range from -21.3‰ to -8.5‰, with the average value -13.76‰. The δ¹³C_PDB of graphite inclusions exhibits relatively heavy carbon isotopic compositions, indicating that the carbon sources of graphite are both organic and inorganic. Combined with the regional geological environments, we infer that the relative heavy isotopic carbon derived from the mantle source was carried during the processes of intrusion of the Paleozoic intermediate-felsic magma into the Precambrian dolomite marble, leading to the crystallization or precipitation of graphite and tremolite. This study provides a new perspective for the ore-forming processes of the nephrite jade deposits and deep carbon cycle of the Earth.