A huge outpouring of the earth's interior that occurred over much of present-day India 65 million years ago came from the boundary between the earth's lower mantle and its molten iron core some 1,800 miles beneath the surface, scientists have determined. The work is published in today's issue of Science.
By measuring the oldest samples of helium ever obtained -- helium trapped in rocks 68.5 million years old -- scientists at the University of Rochester and the Institute of Human Origins in Berkeley, Ca. show that volcanic activity incubated for 3.5 million years before erupting, rapidly blanketing the Indian subcontinent with more than 1 million cubic kilometers of lava. The stack of lava is now known as the Deccan Traps.
Scientists have long debated the source of such huge outpourings of lava, known as flood basalt volcanism. Some scientists believe that such volcanoes begin within the earth's upper mantle or lithosphere just beneath the crust, while others believe they begin 1,800 miles beneath the surface at the base of the lower mantle. The Science paper gives some of the most compelling evidence yet that such volcanism begins as a plume, a long column of gas-bearing rock, within the lower mantle near the earth's core. Asish R. Basu and Robert J. Poreda of the University of Rochester and Paul R. Renne of the Institute of Human Origins in Berkeley, Ca., believe that other flood basalts, such as ones that covered Siberia, parts of Brazil, the Karoo Province in southern Africa, and the northwestern United States, also have a plume origin.
Such outpourings of lava often coincide with the biggest extinctions in earth history. This paper gives the most precise date yet for the end of the main pulse of Deccan volcanism -- 64.9 million years ago, give or take 100,000 years -- and shows that it occurred precisely at the time between two geologic time periods, known as the Cretaceous and Tertiary periods (the K/T boundary), when dinosaurs and many other animals became extinct. Two years ago in another Science paper, Renne and Basu showed that the extensive volcanism in Siberia 248 million years ago occurred at precisely the time of the largest extinction in earth's history.
"This paper's primary importance is on how flood basalts form and how they evolve," says Renne, who notes nevertheless that "the results will be interpreted in different ways by different people" seeking to explain the demise of the dinosaurs. "No definite link between the Deccan volcanic activity and the disappearance of the dinosaurs has been proven," he notes.
Isotopic measurements show that the lava originated at the boundary between the earth's lower mantle and the molten outer core. Super-hot, buoyant rock pushed upwards and rose in a narrow column like a plume hundreds of miles to the earth's crust.
"As the plume rises in the mantle as a narrow column, it becomes less compressed, begins to melt and gets bigger," says Basu. "When it approaches the earth's crust it continues to build, like a giant mushroom with a long tail. Finally it breaks through the crust, and vast floods of lava occur." Such lava flows, known as flood basalts, are the biggest outpourings of the earth's interior the world has ever seen. The Deccan eruption alone would have covered the entire surface of the Earth with a lava layer 10 feet thick if it had spread evenly over the continents and oceans.
To determine the origin of the Deccan Traps, Basu and colleagues analyzed the ratio of certain isotopes of helium and strontium in rocks brought up during volcanic activity. Several rocks dating from 68.5 million years ago, 3.5 million years before the main eruption, have the "isotopic signature" characteristic of the lower mantle. "This shows that the mantle beneath India was melting millions of years before the bulk of the eruption occurred," says Basu.
The same lower mantle signature also typifies several "hot spots," volcanic areas beneath earth's plates responsible for forming chains of volcanoes as the plate gradually slides above. Reunion Island in the Indian Ocean is thought to be the hot spot that formed the Deccan Traps. Other hot spots with a lower mantle source are believed to have formed Hawaii and Iceland.
Basu carefully combed India for just the right rocks, looking for samples that had held their helium since the time of the dinosaurs. He found intact helium samples inside bubbles of carbon dioxide encapsulated within one-millimeter grains of the mineral pyroxene. Basu chose samples from the "early froth" of the plume, which were carried upward virtually intact, were not contaminated by melting, and which never made it to the surface to release their helium. Helium isotopic composition was measured by Poreda, associate professor of geology at Rochester.
"The high helium-3 to helium-4 ratio points to a lower mantle source for the Deccan flood basalts," says Poreda. "The helium-3 is a remnant from the formation of the earth 4.5 billion years ago; such high ratios are found only in plumes from the lower mantle."
Renne dated the rocks that contained the helium using the argon-argon laser heating technique applied to individual grains of the minerals biotite and hornblende. With this technique, one of the most precise available, researchers determine how old a rock is by determining how much argon has been produced by the radioactive decay of potassium in that rock.
Working on the project besides Basu, Renne and Poreda were graduate student Friedrich Teichmann at Rochester and Deb DasGupta of Presidency College in Calcutta, India. The work was supported by the National Science Foundation and the Institute of Human Origins.