An international group of researchers, led by geologists from the University of the Witwatersrand (Wits), have found that the Bushveld Complex in South Africa functioned as a “big magma tank” in the Earth’s crust in ancient years.

In a paper published in Scientific Reports, the scientists confirm that large, long-lived and entirely molten magma chambers did exist in the crust of the Earth.

Wits School of Geosciences Professor Rais Latypov says the group set out to answer a vital question and, through this process, deduced that the Bushveld Complex started as a 400-km-diameter magma body, a conclusion which is at odds with the current school of thought that large, long-lived and largely molten lava magma chambers are non-existent in Earth’s history.

“While re-examining thin sections of Bushveld chromitites, we noticed a very puzzling observation: chromite often occurs as individual grains that are seemingly ‘suspended’ within matrix minerals. This observation leads us to a critical question: why have the chromite grains failed to sink towards the chamber floor despite being much denser than the host melt?”

To answer this question, the researchers studied chromitite in three-dimension (3D) using high-resolution X-ray computed tomography, which revealed that nearly all chromite grains are closely interconnected to form a single continuous 3D framework.

“This gave us an answer to the above question: chromite grains are not able to settle freely towards the chamber floor simply because they are all bound together in self-supporting 3D frameworks attached to the chamber floor,” notes Wits School of Geosciences Dr Sofya Chistyakova.

There is only one process that may result in the formation of such 3D frameworks of chromite crystals. This is an in situ self-nucleation and growth of chromite grains, for example, when all new chromite grains nucleate and grow on pre-existing chromite grains directly at the chamber floor.

This happens from the parental melt that is saturated in chromite as the only crystallising phase.

“This logically brought us to a long-known chromium mass balance issue – normal basaltic melts contain only a very small amount of chromium so that the formation of thick chromitite layer requires extraction of chromium from a very large volume of liquid that must be present as a thick melt layer in the chamber.

“Simple mass balance calculations indicate that a 1-m-thick layer of chromitite will require a magma column of 2 km to 4 km thick,” says Latypov.

Latypov and his co-authors believe the enormous lateral extent of chromitite layers indicate that during the formation of massive chromitites the Bushveld chamber was operating as a giant magma body of more than 400 km in diameter, with a column of the resident melt likely attaining a few kilometres in thickness.

BACKGROUND TO THE THEORY

For more than a century, the classical paradigm of magma chambers has underpinned all models of the Earth’s magnetism. This paradigm envisages a magma chamber as a large body of the molten, long-lived and slowly fractionating magma enclosed in crustal rocks.

In recent years, this classic view of a magma chamber has been challenged by a new view that largely molten big tank magma chambers were either short-lived or never existed in Earth’s history.

Most volcanologists have abandoned the classic paradigm because geophysical surveys have failed to detect any present-day eruptible magma bodies in the Earth’s crust.

Enter the Wits-led study, which presents field and microtextural data on chromitites from the Bushveld Complex whose formation requires many times their own volume of magma to supply the key component – chromium.

The Bushveld Complex is the largest mafic-ultramafic layered intrusion in the Earth’s crust, totalling up to 600 000 km³ of igneous rocks.

It contains about 80% of the world’s known chromium resources.

The researchers stress that such intrusions are quite rare through the whole of geological time, so it is not surprising that there are no known examples of equivalent magma chambers that are active and detectable in the present-day Earth’s crust.

“We conclude that it is too early to discard the classical paradigm of a magma chamber developed by several generations of petrological luminaries.

“Rather, we suggest re-directing our efforts to find out how new geophysical, geochronological and thermal/diffusion modelling can be logically reconciled with the classical paradigm.”