The researchers made an interesting discovery when re-examining thin sections of Bushveld chromitites: chromite is often found as individual grains that seem to be suspended inside matrix minerals, which they thought was strange. Because of this result, the researchers are forced to ask a difficult question: why were the chromite grains unable to sink to the chamber floor regardless of the fact that they are far denser than the host melt?
Utilizing high-resolution X-ray computed tomography, the researchers investigated chromitite in three dimensions (3D), and they discovered that virtually all of the chromite grains are tightly linked, forming a single continuous 3D structure. This provided them with an explanation to the aforementioned issue: chromite grains are unable to drop easily towards the chamber bottom merely as they’re all bonded collectively in self-supporting 3D structures that are connected to the chamber floor.
In the case of chromite crystals, there is only one procedure that may result in the production of such three-dimensional frameworks. For instance, once all fresh chromite grains nucleate and develop on pre-existing chromite grains right on the chamber floor, this is referred to as in situ self-nucleation & development of chromite grains. This occurs as a result of the parental melt being saturated with chromite and becoming the sole crystallizing phase.
“This logically brought us to a long-known Cr mass balance issue — normal basaltic melts contain only a very small amount of Cr so that the formation of thick chromitite layer requires extraction of Cr from a very large volume of liquid that must be present as a thick melt layer in the chamber,” explained Professor Rais Latypov from the School of Geosciences at Wits University.
A column of the resident melt could have reached several kilometers in thickness, according to the researchers, and this would explain the immense lateral extent of the massive chromitite layers. The Bushveld chamber was believed to have operated like giant magmatism with a diameter of more than 400 kilometers, and the chromitite layers had a lateral extent of several kilometers, according to the scientists.
