The origin of gigantic magma eruptions that led to global climatic crises and extinctions of species has remained controversial. Two competing paradigms explain these cataclysms, either by the splitting of tectonic plates at the Earth's surface or by the impacts of hot currents, called mantle plumes, from the planetary interior.
Picrite lava outcrops at the Luenha River, Central Mozambique [Credit: Arto Luttinen] |
"To our knowledge, the Luenha picrites are the first lava samples that could originate from the plume source that has been previously inferred from various geological and geophysical data on the Karoo province. Therefore they allow compositional analysis of this source," says Sanni Turunen, the leading author and a doctoral student at the Finnish Museum of Natural History, which is part of the University of Helsinki.
In the case of the Luenha picrites, named after the research area near the Luenha River, the geochemical compositions indicate a hot magma source that is in many respects different from previously reported magma sources in the Karoo province. They show compositional similarities to magmas formed in other deep mantle plume-related volcanic provinces worldwide.
Olivine crystal. Primitive lavas containing magnesium-rich olivine can record early events of the magmatic system [Credit: Sanni Turunen] |
"Previous studies of Karoo picrites in Africa and Antarctica by us and by other groups have suggested the generation of magmas in the upper mantle, but our new results indicate plume sources were also involved," adds Jussi Heinonen, an Academy of Finland fellow at the Department of Geosciences and Geography at the University of Helsinki.
Importantly, the Luenha picrites appear to represent the main source of the voluminous flood basalts of southern Africa. "We were fascinated to realise that the Luenha picrites revealed a type of magma source that was recently predicted using lava compositions, but which had not been confirmed by observational evidence," as characterised by Arto Luttinen, senior curator at the Finnish Museum of Natural History.
Professor Daud Jamal standing next to picrite lava outcrops on the Luenha River, Central Mozambique [Credit: Jussi Heinonen] |
Confirmation of the age and evolution of the primitive mantle-like source of the Luenha picrites requires further constraints from future isotopic studies.
"Whatever the exact nature of the Luenha source turns out to be, we feel confident that we have uncovered rocks that help to address the complex origin of large eruptions in new detail," Turunen concludes.
The research will be published in the journal Lithos.
Source: University of Helsinki [August 26, 2019]
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