Why Anorthosite consists of only one mineral and how the white-colored layers in many intrusions worldwide were created

An example of a thick layer of stratiform Anorthosite (white) from the world-known Bushveld Complex in South Africa. Photo: Wits University

Scientists at Wits University in Johannesburg say it is the decompression that causes plagioclase-saturation of rising magma

Some Magmas crystallize into rocks that consist of only one mineral. A typical example is Anorthosite made up of more than 90% of plagioclase, a silicate mineral. Anorthosite in its many varieties is known for its iridescent crystals and is important for glass fiber manufacturing.

Anorthosites occur as very prominent, white-colored layers in many intrusions worldwide and, in particular, are common for the famous platinum-rich Bushveld Complex in South Africa in which these layers extend for hundreds of kilometers.

Since long, geologists have been puzzling about how these remarkable layers of pure anorthosites are produced.
„There were many attempts to solve this issue involving various processes that operate within the shallow magma chambers near the Earth’s surface, but they were not particularly successful,” says Professor Rais Latypov from the School of Geosciences at Wits University in Johannesburg, South Africa.

According to traditional teaching, in the magma chambers near the surface, the components of the liquid rock should separate from each other and the feldspar should form anorthosite. But how could such immense layers as in South Africa be created? Now, Latypov and his team have found an elegant solution to this long-standing petrological puzzle.

„We started searching for a mechanism to generate melts saturated in plagioclase alone outside of the shallow magma chambers,” says Rais Latypov.

„We realized that some melts rising up from deep-seated magma chambers may become saturated in plagioclase alone. This happens in response to decompression.” The detailed scientific explanation is published in Scientific Reports.
When these magmas arrive in a shallow magma chamber near the surface and cool there, they may crystallize stratiform layers of pure plagioclase composition like the ones we observe in the Bushveld Complex.

Latypov and his team believe that their work represents a significant advance in the understanding of the Earth’s magmatic systems.

„This study provides a long-missing bridge between volcanology – where we mostly deal with the generation of melts and their ascent – and igneous petrology that mainly focuses on crystallization of these melts within magma chambers,” says Latypov.

Source: Wits University

Scientific reports

(04.03.2020)