DIAMOND GROWTH IN ALKALI BRINES

The composition of the fluid or melt that facilitates diamond growth in the mantle remains poorly constrained. High density fluid inclusions (HDFs) found within diamonds may be trapped samples of the diamond growth medium and therefore could help narrow down the composition of the diamond growing fluid. HDFs have three end member compositions: saline, carbonatitic, and silicic. A recent study (e.g., Weiss et al. 2015 Nature 524:339) suggested that the saline end member is parental to the carbonatitic and silicic end members, which are a result of fluid-rock interaction between the saline fluid and eclogitic and peridotitic mantle respectively. Therefore it is of interest to study the growth of diamond in the proposed parental saline fluid.

Our study aims to determine the effect of salinity on diamond growth by conducting experiments using mixed KCl-NaCl brine system within the diamond stability field (1450°C, 7 GPa). Three series of experiments were run using NaCl, NaCl-KCl (1:1), and KCl hydrous brines at salinities of 23, 38, and 52 wt% with graphite added as a carbon source. Seed diamonds were imaged on the SEM and weighed before and after each experiment.

In the mixed brine system, maximum diamond growth occurred at 38 wt% salinity; increasing salinity further led to a decrease in diamond growth. The NaCl brine promoted more diamond growth than the mixed system with 32% more diamond growth. In contrast, dissolution dominated in the KCl brine experiments. It is possible that the smaller ionic radius of Na relative to K allows the NaCl system to better facilitate C dissolution into the brine, explaining the large difference in diamond behaviour between the two systems. These experiments show that brines that have an intermediate salinity and are more Na-rich as opposed to K-rich best facilitate diamond growth.