Scientists at the University of the Witwatersrand’s School of Geosciences have proposed an explanation for a phenomenon that has long puzzled geologists. The researchers are Professor Rais Latypov and Dr Sofya Chistyakova and their research has been published in the journal Geology.

The puzzle has been the fact that some magmatic rocks have a mineral composition that is not normal. Usually, magmatic rocks are composed of fixed proportions of different minerals. Thus, one such rock would comprise 90% of one mineral and 10% of another. But some magmatic rocks were composed of minerals which were in totally random proportions. 

Such rocks were called non-cotectic. An example was chromite-containing anorthosite rocks found in South Africa’s famous Bushveld Complex. Normally, such rocks would contain a mere 1% of chromite. The Bushveld Complex anorthosites actually contain from 15% to 20% of chromite.

“Traditionally, these rocks with a ‘wrong’ composition were attributed to either mechanical sorting of minerals that crystallised from a single magma or mechanical mixing of minerals formed from two or more different magmas,” said Latypov. However, he and Chistyakova noted that both these explanations had significant problems.

Their explanation was both simple and totally different to previous suggestions. They proposed it happened when the molten rock (magma) moved upwards through feeder channels (or conduits) from deep chambers towards the surface. “While travelling up through the feeder channels, the magma gets into contact with cold sidewalls and starts crystallising, thereby producing more of the mineral(s) than what should be expected,” explained Chistyakova.

“It is possible that a clue to some other petrological problems of magmatic complexes should be searched for in the feeder conduits rather than in magma chambers themselves,” added Latypov. “This appealing approach holds great promise for igneous petrologists working with basaltic magma complexes.”