The world's largest volcanic eruptions appear to be derived from a reservoir of magma at least 4.5 billion years old. The magma represents a remnant of the composition of the early Earth. The findings suggest that the Earth's overall mineral composition is different than previously thought.
Researchers believe that several of the largest volcanic eruptions in Earth's history were fueled by magma from a primitive reservoir in the mantle that has remained unchanged for 4.5 billion years, a remnant of the material that formed the early Earth.
Punctuating the last 250 million years, these staggering eruptions sometimes lasted several million years and were so large that a single event could double the total amount of magma on the Earth's surface.
The most recent of the resulting formations (called flood basalts) is found on Baffin Island in Canada and West Greenland, created by eruptions 62 million years ago. Samples from here gave researchers the first indication that the ancient mantle reservoir existed.
In a report published last year, researchers found unexpected element concentrations and isotope ratios that matched what is predicted for the primitive mantle -- the homogeneous matter that surrounded the Earth's newly-formed metal core before the mantle cooled enough to separate the crust from the modern mantle. In the new report, published Wednesday in Nature, the researchers find that world's other largest eruptions tapped this same reservoir.
"We decided to look at other flood basalts." said Matthew Jackson of Boston University who authored the new study with Richard Carlson of the Carnegie Institution of Washington.
They started with the biggest one, the Ontong Java plateau, a Pacific Ocean, undersea plateau the size of Alaska, 20 miles thick and as big as the other flood basalts combined. They studied its isotope ratios and element concentrations, as with the previous case.
"It had a lot of the same characteristics as the Baffin Island ones that we looked at last year" he said. They looked at four others around the world, with similar results.
Taken together, "The world's largest volcanic events in the last 250 million years all seem to have compositions that are consistent with sampling an early Earth reservoir," Jackson said.
The findings make sense, he notes, because the undisturbed, never-melted primitive mantle material would have retained high concentrations of radioactive elements to provide a lot of heat, as well as relatively high levels of easily melted materials, compared to already-depleted mantle reservoirs.
"Together these are the perfect recipe for generating enormous quantities of melt," Jackson said.
Recent work by others indicates that the largest volcanic events all erupted over one of two "superplumes" in the mantle, one under Africa and one in the South Pacific. It may be that these represent the locations of primitive mantle reservoirs, Jackson noted.
The findings also raise questions about what the Earth is made of, Jackson said. "We've always thought that Earth's primitive mantle would be the same as chondrites," carbon-containing meteorites thought to contain material formed in the early solar system. "People always thought the earth was just an agglomeration of chondrites."
But the new findings suggest Earth's early mantle does not have a chondritic origin. "If the Earth is not chondritic, we developed a very specific hypothesis for what its composition would be. Isotope compositions in Baffin Island lavas perfectly match that composition."
"If it is true, it is going to be a big paradigm shift," said Mukul Sharma of Dartmouth College in Hanover, NH. "All textbooks in geochemistry will have to be revised," Still, he added, other types of evidence need to be found before this can be confirmed.
by "environment clean generations"
Posted in: 
0 comentarii:
Post a Comment