Temperature and oxygen state of kimberlite magma from the North China Craton and their implication for diamond survival

The grade and morphological character of kimberlite-hosted diamonds were compared to crystallization temperature (T) and oxygen fugacity (fO(2)) estimated from groundmass spinels in six kimberlite pipes in the North China Craton (NCC). Crystallization temperatures calculated at an assumed pressure of 1 GPa are in the range of 1037-1395 degrees C, with a mean of 1182 degrees C. At these temperatures, the estimated fO(2) varies from 1.2 to 3.1 log units below the nickel-nickel oxide (NNO) buffer. Generally, individual kimberlite pipe shows a small variation of the T (50-100 degrees C) and fO(2) (0.4-0.6 log units), whereas different kimberlite pipes present great changes of T and fO(2) which can be up to 300 degrees C and 2 units respectively. The fO(2) of kimberlite magma shows a strong negative correlation with the diamond grade of kimberlite, suggesting that the fO(2) plays an important role in diamond resorption, whereas the T shows no relationship with the diamond grade, indicating the T plays no role in diamond resorption. The conditions of kimberlite crystallization (fO(2)) can be a useful parameter in evaluating diamond survival in diamond exploration.