Nitrogen distribution in diamonds from the kimberlite pipe No.50 at fuxian Eastern China: a CL and FTIR study

Abstract

CL (cathodoluminescence) and FTIR (Fourier Transform Infrared) techniques have been used to study internal structure, variations of N abundance and aggregation states of diamonds collected from kimberlite pipe No.50, Fuxian, Liaoning province, China. Three kinds of internal structures can be recognized: (1) single stage of growth structure, (2) multi-stage of complex growth structure, and (3) rare agate-like structure. Most of diamonds exhibit complex growth histories recording processes in the lithosphere over the residence time of diamond. Three diamonds were cut and polished parallel to (110). Diamonds with bright blue CL have higher N contents whereas dark green or green-blue CL duplicated oscillatory zone has moderate N content. The aggregation states and nitrogen data across the plate indicate that the diamond Lc35 (0.26 ct., 3.2×3.6 mm) experienced at least four growth stages. Two resorptions and one abrupt were occurred between four stages. N content varies from 679 to 244 atom ppm in this diamond. The variation of N contents is irregularly from core to rim, and it is individuals within single growth stage. N concentrations decrease form earlier to later growth in third and fourth stages respectively. The sharp increase of N contents have been found at the boundaries between second and third stages and decrease at third and forth stages indicating the different environments occurred during crystallized period of diamond. In the earlier period (may be before 1.3 to 1.4Ga) the environment was relative unstable with rapid growth rate of diamond, whereas in the later period (after 1.3 to 1.4Ga) the environment was stable with slower growth rate of diamonds and enhanced fluid activity. The results of calculated tMR supported the longevity of diamonds over than a thousand million years at mantle lithosphere.