Karoo flood basalts have deep roots

A mantle plume fed the volcanic mega-eruption 180 million years ago

The Karoo eruptions 180 million years ago left a thick layer of basalt in southern Africa and the Antarctic, here in the Drakensberg in South Africa. © JMK / CC-by-sa 3.0
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But a hotspot: The Karoo flood basalts in South Africa and the Antarctic are one of the largest volcanic areas on earth - and they have deeper roots than previously thought. Because much of the lava that spewed out of this volcanic province 180 million years ago, comes from the lower mantle, as now reveal analyzes. The Karoo volcanism is thus the result of a Mantelplumes and was caused not only by the breaking of the Urkontinents Pangäa.

Tidal basalt are witnesses of the most powerful volcanic catastrophes of the prehistoric times. Several times in the history of the earth it came to the eruption of entire volcanic provinces. Lava, ash and volcanic gases penetrated from hundreds of chimneys, leaving hundreds of meters thick lava strata. Some of these mega-outbreaks, including the eruption of Dekkan-Trapp in India and the Siberian Trapp, may have contributed to mass extinctions.

At the Luenha River in Mozambique, the researchers found evidence for a mantle plume as magical supplier of the Karoo province. © University of Helsinki

Eruption as Pang a broke

One of these huge primeval volcanic provinces is today in the south of Africa. The lava strata of the Karoo flood basalts come from an eruption that occurred on the supercontinent Pang a 180 million years ago. According to estimates, the vents spewed five to ten million cubic kilometers of lava to this day the remains of this lava can be found in the south of Africa and the Antarctic.

But what triggered this primeval volcanic disaster? For the Dekkan Trapp is known that his magma was fed by two hotspots. These mantle plumes transported the glowing rock from the lower mantle to the surface. And also the Columbia flood basalts in North America got their magma supply from such a mantle plume.

R tsel around the third source

In the Karoo flood basalts, however, the cause was unclear. In spite of the enormous volume of the lava flows, geochemical analyzes seemed to indicate an origin of the material partly in the crust and partly in the upper mantle. "But these two sources alone can not explain the various types of magmatic Karoo province, " explain Sanni Turunen of the University of Helsinki and his colleagues. "More than half the volume of this flood basalt must come from a third source." Display

But from which? To answer this question, Turunen and his colleagues studied lava from the vicinity of the Luenha River in Mozambique, which belongs to the karoo basalts. "These flood basalts are extensive, but so far little studied, " say the researchers. For their study, they took samples from different parts of the area and analyzed them for their elemental composition and mineral structure.

Geochemical signature of a mantle plume

The result: The lava rock from this northern part of the Karoo flood basalts has a composition similar to that of other, from the deep mantle originating volcanic rock. It contains relatively little titanium dioxide, much magnesium, as the researchers report. The structure of these rocks also indicates that this magma once melted at temperatures of around 1, 530 degrees.

All this together, according to the researchers, suggests that this magma has been transported to the surface by a mantle plume from the deep mantle. "The Luenha Pikrite thus provide us with the first direct geochemical test of a significant mantle plume source for the volcanic Karoo province, " Turunen and his colleagues note.

This deep mantle reservoir is likely to be the long-sought third source of karoo flood basalts. Scientists estimate that more than half of the lava in this volcanic province could come from this hotspot. (Lithos, 2019; doi: 10.1016 / j.lithos.2019.105152)

Source: University of Helsinki

- Nadja Podbregar