In the last few years new data about the Icelandic mantleplume has emerged that forever has changed how we look at it, and how it affects Iceland.
In yon olden days, it was perceived through the eyes of the original plume model that was devised to explain the Emperor Seamount Chain and Hawaii. That means that it should have deep origin within the mantle, or even spring out from the boundary between the mantle and the outer core.
It was also believed that the Icelandic mantleplume would have emplaced a plume track in the same manner as the Emperor Seamount Chain. The idea was that it would be a remnant plume from the Alpha Ridge Large Igneous Province event that had meandered over to its current location via Greenland.
The only problem was that almost all evidence pointed away from this plume model as an explanation for Iceland. As science progressed it turned out that almost no mantleplume on earth behaves as the Emperor Plume does. It was the odd bird out and should never have been used as the defining plume for the mantleplume theory.
So, let us throw out the old and bring in the new and see where we stand.
Spinels and garnets on the plume
The first thing we need to do is to look at the general depth that the mantleplume burrows down to. The best way to do that is through petrochemical analysis of the lavas from Kistufell. The reason that it is best done from those lavas is that Kistufell is sitting on the centre point of the mantleplume upwelling and as such should contain the purest lavas if you wish for a high mantleplume derived magma origin of the lava.
Lavas from Kistufell shows that the plume derived magmas are like this: “The isotopic heterogeneity within the Iceland mantle plume may thus be viewed as a result of mixing between plume material rising from a layer of subducted slabs (which have partly maintained their geochemical integrity and heterogeneity) and lower-mantle material (FOZO) entrained in the initial stages of plume formation.” (Kresten Breddam, 2002, linked below)
The sentence above might be the most explosive sentence in contemporary volcanology. It is like someone had chucked the Tsar Bomba into the classical mantleplume model for Iceland. I will try to explain it by quoting myself… Massively.
“Kistufell is situated straight on top of the Icelandic mantleplume core. The petrochemical analysis gives at hand that a large part of the magma comes from the 670-kilometer discontinuity where it passes through subducted slab remnants and is consistent with a formative mantleplume in the lower mantle.
Now, what on earth is the 670-kilometer discontinuity? Well, material above that has the spinel crystal structure and below you have perovskite structure. In short, if your basic magma has spinels in it you have magma from above the discontinuity. If you have a marked lack of spinels the magma formed deeper than the discontinuity.
And the Kistufell magma is poor in chromium spinels, and the few that are seems to have come from xenoliths from the magma conduits rather than from the basic basalt (ol-tholeiite). Also, the high amount of Sr points towards a deep source.
Now over to garnets, they form at about 35 to 45 kilometers’ depth, and the Kistufell lava is very poor in garnets, so it is safe to assume that the magma has formed below that. This differentiate the Kistufell (and other mantleplume volcanoes) from other Icelandic volcanoes far away from the plume core.
There are also inclusions of material that points to the formative mantleplume punching through a tectonic slab graveyard situated above the 670km discontinuity.”
Is there any evidence that the mantleplume is indeed formative? Yes, there is. The amount of spinels increases with the age of the lavas tested in Iceland. Or, in other words, the older magmas came from an increasingly shallow depth as we progress backwards in time.
As such the mantleplume is not more than 14 million years old, at least in a way that we define as a mantleplume. That puts quite a spanner in the Alpha Ridge Theory, or any other theory stating that the Icelandic Plume has meandered over from somewhere else.
Locating the plumehead
Garnets fill yet another purpose, they are uniquely useful to decide where in Iceland the center of the plumehead lies. And it turns out that the further away you go from Kistufell, the more garnets you will find as you sample young lavas. The difference is not majestic, and there are anomalies.
There is a lower amount of Spinels in the southeastern quadrant of Iceland, and there is also a lower garnet count in a line that runs down via Eyjafjallajökull and Katla onwards down the Vestmannaeyjar Volcanic Line compared to volcanism west and northeast of Kistufell. There is also a low garnet line running to the North via the central volcanoes there.
This would indicate to any normal mind that there is a sub-crustal river of molten mantleplume derived magma flowing down to the south and to the north.
In Today’s Plume News
In a recently published paper by Schoonman et al titled: ‘Radial viscous fingering of hot asthenosphere within the Icelandic plume beneath the North Atlantic Ocean’, they presented a model discussing the formation of such sub-crustal rivers of magma.
The model is based on an old physics experiment where a less viscous fluid is injected into a more viscous fluid placed between two sheets of glass. In the experiment fingers of the less viscous fluid (more fluid) flows uniformly as tendrils (fingers) in all directions since the pressure is uniform due to the equidistance of the two sheets of glass.
In the real world, the pressure will be influenced by the thickness of the crust and crustal canyons giving differences in pressure.
In Iceland, the centre of the plumehead is at 35 kilometres’ depth below Kistufell. From there the naturally heat-buoyant plume derived magma will try to float towards shallower crustal parts following the path of least resistance.
The clearest visible finger follows the line via Eyjafjallajökull and Katla towards the WVL due to the crustal inverted canyons that have formed due to the spreading of Iceland. The bulk of this finger reconnects to the MAR SSW of Iceland, but a part of the finger splits off and meanders off towards Scotland.
This finger is interpreted in the paper as the reason that Northern Scotland is above the surface of the water since the buoyance of the hot magma would lift the crust upwards. The reasoning here is that the Scottish crust is not buoyant enough on its own, to be above ocean level.
There is also a large finger going northwards all the way to Jan Mayen where it forms a secondary plumehead. If there is further evidence of this being true, the Jan Mayen mantleplume will have been falsified. Time to test for spinel/garnet ratios on a line from Iceland to Jan Mayen methinks.
There is also a marked discontinuity towards Greenland negating any possibility that the plume has moved through Greenland to its current position since it would have left a remnant heat residual track. And there is just none to be had.
Now it is time to become critical of the paper. The authors indicate a very long finger running due east before it bends like a banana down the Norwegian coastline. They once again reason that what we see is distal plume derived magma lifting thin Norwegian crust upwards through heat buoyancy.
This seems to me to be a classic case of over-extending a valid model into a territory where it is no longer valid. Firstly, there is a marked discontinuity of the finger that is not explained. Secondly, they seem to forget that Norway is the leading edge of one of the world’s thickest crusts, the Baltic Shield. As such it is by far buoyant enough on its own to float above the oceanic surface.
So, what then would form the heat signatures we see? I will try to explain that. As the MAR split apart, it forms brand new crust that is pushing the Northern American continent apart from the Eurasian Continent.
This has two effects, one is that North America is moving to the south-southwest and that there is increased pressure on the western edges of the fairly immovable Eurasian continent. This has by now created what is believed to be proto-subduction faults. One of them is located near the Norwegian coastline.
It is as such far more likely that what we are seeing is that the subduction process has gone further than we previously believed and that the happy Norwegians will be the recipients of a few dozen explosive stratovolcanoes in a geological future.
Even though the paper in my view is over-extending the model by putting a finger in the nether regions of Norway it represents a major breakthrough in our understanding of the size and distribution of the Icelandic mantleplume.
It puts the last nail in the coffin of the wandering plume theory and it neatly seems to explain away the Jan Mayen mantleplume. In the other direction, it shows that the future of Icelandic volcanism will run through the Vestmannaeyjar and reconnect to the MAR south of Iceland.
It also explains why we do not have kilt-wearing fish-humans wielding claymore’s at passing ships whilst they emit burbling noises from sheep-bladders.
All in all, it has been a good week in volcanology.