Magma floods erupt from deeper sources than previously thought

An international team of geologists has demonstrated with computer simulations that massive magma eruptions can be initiated deeper below the Earth’s surface than previously thought. Such flood basalt eruptions have caused many global climate changes and major mass extinction events in the past.

Large magma eruptions have caused large floods of basalt lava on the continents throughout Earth’s history. Conventionally, the largest flood basalt eruptions are thought to be possible only in areas where the continental tectonic plates are unusually thin, so that deep mantle material can rise close to the Earth’s surface. In such low-pressure environments, melting of the hot mantle can generate very large amounts of magma.

A new study by researchers from the University of Helsinki and Aarhus University challenges this widely held view.

“The idea that flood basalt eruptions generally require mantle melting under low-pressure conditions is largely based on the trace element compositions of the eruptive magmas,” explains Dr. Jussi Heinonen, University of Helsinki, the lead author of the recent Journal of Petrology article describing this study.

It further clarifies that the relative amounts of rare earth elements in many flood basalts indicate magma formation in the presence of low-pressure mantle minerals.

Support from computer simulation

The new study was conducted as part of a research program focusing on the origin of flood basalts that erupted in southern Africa and Antarctica when those continents were connected together as parts of Pangea about 180 million years ago.

“We became curious about the occurrence of most flood basalts in areas where the African and Antarctic tectonic plates are thick rather than thin,” says Dr. Arto Luttinen, leader of the University of Helsinki team. “In addition, we discovered that many flood basalts that have rare-earth compositions, indicating high-pressure formation conditions, are actually located in thin regions of the tectonic plates.”

The idea of ​​an alternative hypothesis began to take shape after the team’s discovery of a type of flood basalt in Mozambique that shows compositional evidence for extremely high eruption temperatures.

“These flood basalts led us to consider the possibility that extremely hot mantle melting could lead to the formation of high-pressure magmas with trace element characteristics similar to those of low-pressure magmas,” adds Ph.D. student Sanni Turunen from the University of Helsinki.

The researchers decided to test their hypothesis using the geochemical modeling tool REEBOX PRO, which allows realistic simulation of the behavior of minerals, melts and their trace element contents during mantle melting.

“We are excited to find that the simulations supported our hypothesis by predicting the total consumption of garnet, a diagnostic mineral in high-pressure conditions, when mantle melting occurred at the high temperatures indicated by the flood basalts,” says Dr. Eric Brown. Aarhus University, co-author of the article and one of the developers of the REEBOX PRO tool.

High-pressure magmas can thus chemically resemble low-pressure magmas when the mantle source is very hot. In addition, the results showed survival of garnet at relatively low pressures when a different type of mantle source was chosen for modeling.

“Our results help us understand the apparent controversy between the occurrences of South African and Antarctic flood basalts and their characteristic trace elements. More importantly, we show that massive flood basalts can form in areas of thick tectonic plates and that the trace element compositions of flood basalts are unreliable proxies of magma generation depths unless the effects of temperature and mantle composition are taken into account,” the authors conclude.