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| Highlights |
Brittany
Meagher
Research Training Program, 2003
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Looking South of Iceland through Volcanic Glasses Geologic hot spots are places where a column of upwelling hot rock, known as a plume, rises from deep in the mantle toward the Earth's surface. When a hot spot occurs in the ocean, it builds shield volcanoes like the Hawaiian Islands. These islands are constructed on the deep ocean floor and build up until plate movement has shifted the volcano off of the hot spot. A new volcano starts forming only a little distance from the old now dying volcano. Although most Hot Spots form in the center of a tectonic plate, they can also occur along mid-ocean ridges. Iceland is an example of such a hot spot. At a mid-ocean ridge the oceanic crust is spreading apart allowing magma to come to the surface. This upwelling of magma leads to new crust being formed. Iceland sits astride the Mid-Atlantic ridge, making a place where large amounts of magma has erupted to build the volcanic structure above sea level.
Iceland has been hypothesized to not only be a hot spot but also a wet spot. This would mean that water contents of Mid Ocean Ridge Basalts (MORBs) would increase as Iceland is approached along the Mid-Atlantic Ridge from the south. To test this hypothesis twenty-eight samples were obtained from the Smithsonian Sea-Floor Glass Collection. The majority of these samples run along the Reykjanes Ridge just south of Iceland. These samples were analyzed for H2O using Fourier Transform Infrared Spectroscopy. Upon analyzing the results of this method, it was found that H2O increases as Iceland is approached from the South along the Reykjanes Ridge. Similar increases can also be seen in the abundances of K2O and P2O5. This is what was expected to be found since K (potassium), H (hydrogen), and P (phosphorus) are all incompatible elements in basaltic systems. One way to explain
this increase in incompatible elements is to argue that there is a lower
percent of melting happening beneath Iceland compared to the southern
Reykjanes Ridge. However, since Iceland is known to be a very active
hot spot, where anomalously large volumes of magma have erupted, the
idea of lowering the percent melting to provide an explanation for the
abundance of incompatible elements does not make sense. The logical
alternative explanation is that the Icelandic hot spot must come from
an enriched mantle source, in turn showing that Iceland is indeed not
only a hot spot but also a wet spot.
The Mid-Atlantic Ridge runs southward through Iceland down to Antarctica, splitting the Atlantic Ocean in two. This ridge is one of the best-known examples of a divergent-plate boundary. In the North Atlantic it separates the North American Plate from the Eurasian Plate, and in the South Atlantic it separates the South American Plate from the African Plate. In the 1950s, the ridge was discovered, leading to the idea of sea-floor spreading, and to the acceptance of Wegener’s theory continental drift. The Reykjanes Ridge
is located southwest of Iceland along the Mid-Atlantic Ridge. It is
approximately 1,100 km long, extending between Iceland and the Bight
fracture zone. In a paper published in Earth and Planetary Science Letters,
Nichols stated “the water content increases northwards along the
Reykjanes Ridge towards Iceland”(Nichols et al. 2002). This led
him to believe that Iceland “does indeed appear to be a wet spot
compared to the southern end of the Reykajanes Ridge”(Nichols et
al. 2002). We seek to confirm this recently proposed hypothesis that
the Iceland hot spot is also a wet spot.
Incompatible elements
are those elements that are not significantly incorporated in the minerals
present in a magmatic system. Instead these elements are concentrated
in the melt (glass). For basaltic systems K (potassium), P (phosphorus),
and H (hydrogen) are all strongly incompatible. This common incompatibility
is the reason that K2O, P2O5, and H2O are expected to behave similarly.
It is interesting
to note that on the plots of K2O and P2O5 in figures 2a and 3a. There
is a definite drop off in the contents of K2O and P2O5 north of Iceland.
However, proceeding farther north along the Mid-Atlantic Ridge the contents
tend to increase. This trend peaks sharply around the Jan Mayen hot
spot (71o N latitude), which also appears to have an enriched source.
Total water contents were analyzed by Fourier Transform Infrared Spectroscopy in twenty-eight samples of quenched pillow basalts from the Mid- Atlantic Ridge, obtained from the Smithsonian Sea-Floor Glass Collection. The samples show an increase in the amount of H2O as Iceland was approached from the south. This increase in H2O parallels previously known trends for K2O and P2O5. H, K, and P are all considered to be incompatible elements in basaltic systems and all of them were found to have the same increasing trend approaching Iceland. Since Iceland is
known to be a very active hot spot, the idea of lowering the percent
melting to provide an explanation for the great abundance of incompatible
elements does not make sense. This leaves the idea that the melt comes
from an enriched mantle source as the only other explanation for this
high level of incompatible elements coupled with high magma-production
rate.
I would like to thank Jim Luhr, Tim O’Hearn, Bill Melson, and the whole Mineral Sciences Department at Smithsonian National Museum of Natural History. I would also like to thank the National Science Foundation for their generous donation to the Research Training Program.
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