by Evelien van de Ven Thursday, March 15, 2018
Massive volumes of rock called large igneous provinces (LIPs) have formed many times throughout Earth’s history, fed by some of the planet’s mightiest volcanic events. The volcanic eruptions, sometimes lasting millions of years and pouring hundreds of thousands of cubic kilometers of lava onto the surface, have influenced continental breakups, past climate change and mass extinction events. For everything that’s known about LIPs, however, many questions about them remain, including how far below the surface the erupted magma originate. In a recent study, researchers report that the origins of the Paraná-Etendeka LIP likely lay deep in Earth’s interior.
The Paraná-Etendeka LIP erupted roughly 130 million years ago during the breakup of the Gondwana supercontinent, which saw Africa, South America and other southern landmasses split apart. The source of the Paraná-Etendeka magmas is thought to be the Tristan volcanic plume, a long-lived plume that’s still active today under Tristan de Cunha Island in the South Atlantic Ocean. Because the LIP formed before Africa and South America had fully rifted, the rocks of the Paraná-Etendeka are, today, bisected by the Atlantic Ocean, with one portion located in Namibia (Etendeka section) and the other in South America (the Paraná section).
“The role that the Tristan plume played in the breakup of southern Gondwana has long been debated, with a key question being the depth of origin of the plume when it emplaced the Paraná-Etendeka LIP,” says Nicole Stroncik, a geochemist at the GFZ German Helmholtz Center for Geosciences and lead author of the recent study, published in Geology.
To understand the source of the plume, the team sampled the LIP’s most primitive exposed rocks: dikes in the Etendeka section. If the plume originated in the deep mantle, the chemical composition of these oldest rocks would reflect this, both in terms of their trace element contents and, more importantly, by a high ratio of helium-3 to helium-4. Alternatively, if the plume originated from a shallower source, the rock compositions would be different, with lower helium ratios.
Helium isotopic ratios can be used to assess the initiation depth of plumes because of the way the different isotopes are concentrated in different parts of Earth’s interior. The deep mantle’s high helium-3 to helium-4 ratio has been steady since the accretion of Earth, whereas ratios in upper mantle and lithospheric rocks vary substantially and are generally lower than in the deep mantle.
“If a plume is initiated in the deep mantle, it will start with a high helium-3 to helium-4 signature,” Stroncik says. “As it travels through the shallow mantle and crust, it can mix with these low helium-3 to helium-4 materials and acquire a lower helium isotope ratio itself,” she says. “So, samples with a low helium ratio can come from either the shallow mantle or from the deep mantle … But if a rock has a high helium ratio, it has to have come from the deep mantle.”
In its analyses of Etendeka rocks, the team found that while many samples had low helium ratios, a few had high ratios. The results show “that the Tristan Plume originated in the deep mantle and that the deep mantle was actively involved in the production of the magma erupted at the Paraná-Etendeka LIP 130 million years ago,” Stroncik says. “Though we can’t confirm the Tristan plume is currently sourced from processes in the deep mantle, it did originate there.”
The new research “is an important achievement, and a difficult one,” says Albrecht Hofmann, a geochemist at the Max Planck Institute for Chemistry in Germany, who wasn’t involved in the study. “The high helium-3 to helium-4 ratio indicative of a deep mantle source is not often preserved in old rocks, and thus, in the past, little effort was made to look for this signature in old rocks,” Hofmann says. “Since the team has now looked and found this signature, this may now encourage others to look for similar ratios in other old LIPs.”
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