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 (fO2) 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 °C, with a mean of 1182 °C. At these temperatures, the estimated fO2 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 °C) and fO2 (0.4–0.6 log units), whereas different kimberlite pipes present great changes of T and fO2 which can be up to 300 °C and 2 units respectively. The fO2 of kimberlite magma shows a strong negative correlation with the diamond grade of kimberlite, suggesting that the fO2 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 (fO2) can be a useful parameter in evaluating diamond survival in diamond exploration. [ABSTRACT FROM AUTHOR]