Iceland plume

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The Iceland Plume is an upcurrent of abnormally hot rock in the mantle below Iceland , the origin of which is probably at the border between the core and the mantle at a depth of about 2,880 km. According to the common doctrine, the plume theory of W. Jason Morgan , it is the cause of the formation of Iceland and of its volcanism , which has shaped the island to this day.

Geological development

Topography / bathymetry of the North Atlantic around Iceland

The plume that bears the name of Iceland and is now almost exactly below the center of the island is considerably older than Iceland. Volcanic rocks associated with it can be found on both sides of the south Greenland coast and, in the case of southwest Greenland, date back about 58–64 million years; they thus coincide with the opening of the North Atlantic in the late Paleocene and early Eocene . It is believed that volcanism was caused by the fact that hot material from the plume head flowed into areas below the lithosphere that had already been thinned out by previous rift formation and produced large amounts of melt there. The exact position of the plume during this period is disputed, but was probably under Central Greenland; It is also not yet clear whether the plume only rose from the deep mantle at this point in time or whether it is much older and also caused the ancient volcanism in northern Greenland, on Ellesmere Island and in the Arctic Ocean ( Alpha Ridge ).

As the North Atlantic opened up east of Greenland during the Eocene, North America and Eurasia began to drift apart; the mid-Atlantic ridge formed as an oceanic center of spreading and part of the submarine volcanic system of the mid- ocean ridge . With these plate movements, Greenland pushed itself over the Iceland plume; various studies locate the plume about 40 million years ago under or slightly east of the southeast Greenland coast (Scoresby Sound) and relate it to the North Atlantic flood basalt province. In the course of the further ocean opening and plate drift, the plume and the mid-Atlantic ridge approached each other, and finally part of the plume head got into the area of ​​the thin lithosphere on the ridge, where increased melt and crust formation occurred; this increased the more both moved towards each other. The Greenland-Icelandic and Faroe-Icelandic ridges, both areas of heavily thickened oceanic crust, are traces of this stage of convergence before Iceland was formed.

The oldest crust of Iceland itself is over 20 million years old and was formed at an old, now extinct, mid-ocean spreading center in the area of ​​the Westfjords (Vestfirðir). The westward migration of the plates and thus of the ridge across the plume and the strong thermal anomaly of the plume led to the fact that this old spreading center died off 15 million years ago and a new one further west, in the area of ​​today's Skagi and Snæfellsnes peninsulas formed; in the latter there is still some residual activity with the Snæfellsjökull volcano . The spreading center, and thus the main activity, shifted again to the east 7–9 million years ago and formed today's volcanic zones in the southwest (WVZ; Reykjanes - Hofsjökull - Vatnajökull ) and northeast (NVZ; Vatnajökull – Tjörnes). Currently, the activity in the VMS is slowly declining , while the volcanic zone in the southeast ( Katla –Vatnajökull), which was initiated 3 million years ago, is developing.

In addition to the formation of Iceland, the plume has also influenced the crust formation of the adjacent sections of the Mid-Atlantic Ridge, especially the Reykjanes Ridge southwest of the island. There one observes a clear thickening of the earth's crust and an abnormal rise in the sea floor, which is attributed to a hot mantle current emanating from the plume along the thin lithosphere below the ridge; the variations in crust thickness, which form a pattern of nested Vs pointing away from Iceland, show that this flow was not uniform. One of the different ways to explain this pattern relies on the interaction of the displacement of the center of spread with the plume head.

Geophysical and geochemical observations

Information about the structure of the deep interior of the earth can only be obtained indirectly using geophysical and geochemical methods. In addition to gravimetric and geoid investigations , especially seismological methods and geochemical analyzes of erupted lavas have proven useful for investigating the island plume as well as other plumes . Numerical models of the geodynamic processes attempt to integrate these observations into a coherent overall picture.

seismology

An important method for mapping large-scale structures in the earth's interior is seismic tomography , in which the area under investigation is "x-rayed" by registering earthquake waves from different earthquakes from as different directions as possible with a network of seismometers . The size of the network is decisive for the size of the area that can be reliably mapped. For the investigation of the Iceland Plume, both global tomography, in which the entire earth's mantle is mapped with the help of data from worldwide distributed stations with relatively low resolution, and regional tomography, in which a seismometer network limited to Iceland, covers the earth's mantle up to 400–450 km Higher resolution depth maps have been used.

Regional studies from the 1990s (ICEMELT, HOTSPOT) clearly show that under Iceland to a depth of at least 400 km there is a roughly cylindrical structure with a radius of 100–150 km, in which the speed of seismic waves by up to 3% (P -Waves) or more than 4% (S-waves) compared to the reference model; the latest analyzes of these data indicate an even greater reduction. If one converts this into a temperature anomaly with the help of rock-physical models, the result is that the earth's mantle there is 150–250 ° C hotter than normal. A certain uncertainty in the models results from the limited resolving power of seismic tomography: a hot, narrow plume is difficult to distinguish from a less hot, wider one.

Global tomography confirms that there is a strong anomaly with significantly reduced seismic velocities in the upper mantle below Iceland. For the lower mantle (below 660 km depth) the picture is more contradicting. In all investigations, the anomaly becomes significantly weaker there and has a more irregular shape; in some depth ranges it even seems to disappear completely, although this depth is not the same in all studies. Other seismological methods have also detected an abnormally hot area at the core-mantle boundary below Iceland, and the structure of the seismic discontinuities at 410 and 660 km below Iceland also indicate elevated temperatures. Therefore, the majority of scientists assume that the weaker signature of the plume in the lower mantle on the one hand with possible temporal variability of the plume and / or the change in the physical properties of the mantle with depth, and on the other hand with the limitations of the method and the available data can be explained, but the plume in any case extends through the entire depth interval of the earth's mantle.

geochemistry

Numerous studies have examined the geochemical signature of the lavas found in Iceland and the North Atlantic. They produce an extraordinarily complex, and in some cases still contradicting, picture, but they also agree on a number of important points. It is not disputed that the source of volcanism in the earth's mantle is chemically and petrologically heterogeneous: not only the normal peridotite of the upper mantle is involved in the formation of the melts , but apparently also eclogite , whose origin is assumed to be in the metamorphosed , very old oceanic crust which entered the Earth's mantle during the subduction of an ocean several hundred million years ago; one also finds z. For example, the isotope ratios of noble gases indicate that there is also a contribution from rock from the lower mantle.

The variations in the content of trace elements such as helium , lead , strontium , neodymium and others clearly show that Iceland is also geochemically an anomaly compared to the rest of the North Atlantic. The ratio of He-3 to He-4 has e.g. B. a pronounced maximum in Iceland, well correlated with geophysical anomalies, and the decay of these and other geochemical signatures with increasing distance from the plume makes it possible to estimate that the influence of the plume is about 1500 km along the Reykjanes Ridge and at least 300 km extends along the Kolbeinsey Ridge. Depending on which elements you are looking at and how large the area is from which the samples are taken, you can identify up to six different mantle sources, which, however, are never all to be found in one place.

Furthermore, some studies show that the content of water that is dissolved in the mantle minerals is two to six times higher in the area of ​​the island plume than in the mantle under undisturbed parts of the mid-ocean ridge, where it is estimated at around 150 ppm.

Gravimetry / geoid

The North Atlantic is characterized by strong, large-scale gravimetric and geoid anomalies , with Iceland at its center. The geoid rises up there in an approximately circular area of ​​hundreds of kilometers in diameter up to 70 m above the geodetic reference ellipsoid, or up to about 25 m above the hydrostatic reference figure of the earth. This anomalous 25 m or a part of it can be explained by the dynamic effect of the upflowing plume, which bulges the earth's surface outwards. The plume and the thickened crust also cause a positive gravity anomaly of approx. 60 mgal (= 0.0006 m / s²) (open air).

Open-air gravity anomalies in the North Atlantic around Iceland. For better representation, the color scale is limited to anomalies up to +80 mgal.

Geodynamics

Since the mid-1990s, several attempts have been made to explain the observations using numerical geodynamic models of mantle convection . The aim of these model calculations was, among other things, to resolve the contradiction that a broad plume with a relatively low temperature anomaly is more compatible with the observed crust thickness, topography and severity, while a narrow, hot plume is better suited to seismic and geochemical models. The latest models indicate that the plume is probably 180–200 ° C hotter than the surrounding mantle and that its trunk has a radius of about 100 km, ie the seismological models are confirmed; Crust thickness, topography and gravity can be explained with such a model, if one takes into account that the loss of water dissolved in the mantle rock during melting changes the flow behavior of the plume massively, so that the corresponding anomalies become wider and it generates less melt. However, previous models do not take into account petrological heterogeneity, or only do so in a very simplified manner.

The V-patterns of the crust of the Reykjanes Ridge mentioned in the section on geological development are explained by geodynamic models using pulsations of the plume, ie fluctuations in the mass flow through the plume trunk.

Alternative models

As mentioned at the beginning, the plume model is the prevailing doctrine to explain the formation of Iceland and its volcanism. In particular, the poor visibility of the plume in tomographic images of the lower mantle and the geochemical evidence of eclogite in the mantle source, however, have cast doubts on the validity of the plume model among some scientists such as Don L. Anderson . As an alternative, mechanisms are proposed that are limited to processes in the upper mantle.

According to one of these models, a large piece of the subducted plate of an earlier ocean has survived for several hundred million years in the uppermost mantle, and the oceanic crust that has become eclogite now causes excessive melt formation and the observed volcanism. However, this model is not based on dynamic modeling, is not enforced by the available data and also leaves questions such as the dynamic and chemical stability of such a body over such a period or the thermal effect of such massive melt formation unanswered.

Another model suggests that the upstream in the Iceland area is driven by lateral temperature gradients between the sub-oceanic mantle and the neighboring Greenland craton and is thus also limited to the upper 200–300 km of the mantle. However, under the conditions prevailing in the North Atlantic, this convection mechanism is presumably not large enough with regard to the spreading rate of the mid-Atlantic ridge and does not offer a simple explanation e.g. B. for the observed geoid anomaly.

literature

  • CJ Wolfe, I.Þ. Bjarnason, JC VanDecar, SC Solomon: Seismic structure of the Iceland mantle plume , Nature 385 (5727) (1997), pp. 245-247

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