image: Flood basalts in Dronning Maud Land, Antarctica, come from an exceptionally deep mantle source. see more
Credit: Arto Luttinen
Large magma eruptions have produced large floods of basaltic lava on continents throughout Earth’s history. Conventionally, the largest flood basaltic eruptions are thought to be possible only in regions where the continental tectonic plates are unusually thin, so that material from the deep mantle is able to rise near the surface of the land In these 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 erupting magmas,” explains Dr. Jussi Heinonen of the University of ‘Helsinki, lead author of the recent article in the Journal of Petrology. describing this study.
It further specifies that the relative amounts of rare earth elements in many flood basalts point to magma formation in the presence of low-pressure mantle minerals.
Support from computer simulation
The new study was carried out as part of a research project focusing on the origin of the flood basalts that erupted in southern Africa and Antarctica when these continents joined together as parts of Pangea about 180 million years ago.
“We were intrigued by the occurrence of most flood basalts in regions where the African and Antarctic tectonic plates are thick rather than thin,” explains Dr Arto Luttinen, team leader from the University of helsinki “Furthermore, we found that many flood basalts that have rare-earth element compositions, suggesting high-pressure formation conditions, are actually found in thin regions of plate tectonics.”
The idea for an alternative hypothesis began to form after the team’s discovery of a type of flood basalt in Mozambique that shows compositional evidence of exceptionally high eruption temperatures.
“These flood basalts led us to consider the possibility that melting of an exceptionally hot mantle could lead to the formation of high-pressure magmas with similar trace element characteristics to those of low-pressure magmas,” adds l PhD student Sanni Turunen from the University of Helsinki.
The researchers decided to test their hypothesis using the geochemical modeling tool REEBOX PRO, which allows a realistic simulation of the behavior of minerals, melts and their trace element contents during mantle melting.
“We were excited to find that the simulations supported our hypothesis by predicting the total consumption of garnet, a mineral diagnostic of high-pressure conditions, when mantle melting occurred at the high temperatures indicated by the basalts of the flood,” says Dr. Eric Brown of Aarhus. University, co-author of the article and one of the developers of the REEBOX PRO tool.
Thus, magmas formed at high pressure can chemically resemble low-pressure magmas when the mantle source is very hot. Furthermore, the results indicated the survival of garnet at relatively low pressures when a different type of mantle source was selected for modeling.
“Our results help us understand the apparent controversy between the occurrence of southern African and Antarctic flood basalts and their trace element characteristics. Most importantly, we show that voluminous basalts can form of flooding in regions of thick tectonic plates and that the trace element compositions of Flood basalts are unreliable messengers of magma generation depths unless the influences of temperature and composition of the mantle,” the authors conclude.
magazine
Journal of Petrology
Research method
Computational simulation/modelling
Research topic
Not applicable
Title of the article
Heavy rare earth elements and the sources of continental flood basalts
Publication date of the article
27-Sep-2022
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