Hidden currents: For a long time it was a mystery why there are hydrothermal springs and outgassing in Panama – because there are no volcanoes there. Now researchers have solved the riddle: There is a 1,500-kilometer underground connection between Panama and the volcanic hotspot of the Galapagos Islands. This long-distance transport is made possible by a special feature in the actually separating subduction zone, as the scientists discovered.
Usually volcanoes are fed by magma reservoirs that lie directly below them in the earth’s crust or in the upper mantle. Magma and volcanic gases then rise from these reservoirs, and the heat in the subsurface often also creates hydrothermal springs. The isotope composition of the volcanic gases and the ejected lava can be used to determine which volcanic area or magma reservoir they originate from – the isotope signature is like a fingerprint.
Hot springs puzzles without volcanoes
But it was precisely this “fingerprint” that brought surprising things to light in Panama and Costa Rica. There are numerous hydrothermal springs and outgassing, although there are no active volcanoes there. That’s why researchers around David Bekaert from the Woods Hole Oceanographic Institution (WHOI) wanted to find out where the heat and volcanic gas come from. To do this, they took samples from 65 hot springs in Panama and Costa Rica and subjected them to isotope analyzes.
The result: “We measured an unexpected composition of the volcanic gas,” reports Bekaert. The volcanic gases dissolved in the liquids had an unusually high proportion of the light helium isotope helium-3. However, this indicates that the source of this material is not in the upper mantle, as is typical for many volcanoes, but far deeper, as the team explains.
Where does the volcanic material come from?
The hot springs in Panama are therefore twice as unusual: They are not linked to nearby volcanoes and their material comes from the area of the lower mantle. However, roots that extend this deep usually only have mantle plumes – volcanic hotspots, where particularly hot currents transport magma upwards from the deep areas of the lower mantle. Such mantle plumes are responsible, among other things, for the volcanoes on Hawaii, Iceland and the Galapagos Islands.
However, there is no such hotspot under Panama. So how do the hot springs there get helium isotopes from the lower mantle? Looking for an explanation, Bekaert and his team next examined lava samples from the Galapagos Islands for their isotope signatures. Although these are around 1,500 kilometers away from Panama, they come from the closest hotspot. In fact, the analyzes showed striking similarities.
Remote connection in the earth’s mantle
According to scientists, this suggests that there is a hidden connection between the Galapagos hotspots and Panama’s hot springs. Volcanic material has to be transported from the mantle plume to Central America through this 1,500-kilometer “pipe” in the upper mantle. Such lateral transport of plume material has been discussed for a long time, but has so far hardly been investigated, as Bekaert and his colleagues explain.
In the case of Panama and the Galapagos plume, the geochemical signatures suggest such a “long-distance transport”: “Our data match a deep eastward expansion of material from the Galapagos plume,” the researchers write. This explains why material from the lower mantle appears in central Panama, even though there are no active volcanoes there.
“Window” in the barrier
However, there is a problem with this: Between the Galapagos and Panama there is a barrier that extends deep into the earth’s mantle – the subduction zone. At this plate boundary, the oceanic crust pushes itself under the continental plate of South America and the Caribbean and is pushed into the depths. The tectonic plates, which descend diagonally downwards, stand like a wall in the upper mantle of the earth.
“From Mexico to Patagonia, subducted ocean plates and thick continental roots form an almost continuous barrier that prevents the flow in the upper mantle,” explain Bekaert and his colleagues. In order for volcanic material to get to Panama from the Galapagos Plume, it must have somehow overcome this barrier.
In fact, in the region under Panama, there is apparently a kind of “window” in the subducted plate, as the research team found out. This gap in the submerged piece of crust allows material to pass through from the side. “Our data indicate that small pieces of material from the lower mantle were carried through this window by a kind of ‘jacket wind’,” says Bekaert.
Also relevant for other coat plumes
According to geologists, their results demonstrate that volcanic material can be carried laterally by plumes than long believed. “Such lateral transport can transport geochemical mantle heterogeneities from volcanic hotspots over thousands of kilometers,” the researchers write. This also has potential significance for other volcanic regions and mantle plumes. (Proceedings of the National Academy of Sciences, 2021; doi: 10.1073/pnas.2110997118)
Quelle: Woods Hole Oceanographic Institution, PNAS