Sylvia is tracing the origins of the oldest crust

2016-03-02

Sylvia collects rock samples on Iceland. Photo: Steffi Burchardt

How did the first continental crust form? That is a question that has haunted geologists for decades. Sylvia Berg adressed this question in her dissertation that was successfully defended last week at the department of Earth Sciences.

The continental crust is mostly formed at subduction zones, but this model can not explain the ancient continental crust that formed in the hadean, prior to plate tectonics. To account for these very oldest crustal fragments, we need to explain how one gets large volumes of silicic magmas in an otherwise silica-poor environment. In her persuit of these questions, Sylvia travelled to Borgarfjörður Eystri on the North East coast of Iceland.

- Iceland is special as it in many ways resembles the geological conditions on the planet 4 billion years ago when the continental crust started forming, Sylvia says.

Iceland is made up of mainly basaltic - i.e. silica-poor – lava, which is the same rock type that dominated on the planet prior to the formation of the continental crust. The area around Borgarfjörður Eystri, in turn, is a bit different as it also hosts large volumes of silicic rocks in addition to the basalts.

- It is possible that the formation of these silicic rocks in Borgarfjörður Eystri is a modern analogue to the formation to the hadean continental crust more than 4 billion years ago, Sylvia explains. In my dissertation, we look at the oxygen isotope composition in the crystals hosted in the rhyolites. These geochemical signals tells us that the rhyolites were formed as a result of up to 30% partial melting of hydrothermally altered basalts.

The colourful rhyolites at Borgarfjörður Eystri. Photo: Sylvia Berg 

In her research, Sylvia has also dated the emplacement of the Borgarfjörður Eystri rhyolites by analysing the radioactive decay of uranium to lead inside individual crystals.

- Uranium can be found in the mineral zircon, Sylvia explains. Zircon acts like a stopwatch inside the rocks. This geological stopwatch lets us establish the window of time when the rhyolites erupted, and we find that this took place in a period of less than two million years! This might sound like a long time, but it is actually a blink of an eye in geological time. In other words, we have explained how it is possible to form large volumes of silicic rocks in a basaltic environment, in a short period of time, without the help of subduction processes! This is precisely what we need in order to explain the primordial crust from the hadean, Sylvia says. We actually also tested this further by analyzing the trace elements, such as titanium, inside the zircons. We find that, while the Borgarfjörður Eystri zircons differ from the icelandic norm, they are very similar to the hadean zircons, which lends further credence to this model.

In addition to her research on Iceland, Sylvia has also performed 3D tomographic reconstructions of xenoliths from the volcanic island El Hierro in the Canary islands archipelago. 

3D tomography of a sedimentary xenolith from El Hierro

- These volcanic fragments originate from an eruption in the ocean outside El Hierro in 2011, Sylvia explains. This was a rather special eruption, where volcanic bombs floated like rafts on the water surface, which was also bubbling like a jacuzzi!

These xenoliths originate from old sediments under volcanic island, and they were picked up by the magmas on their ascent to the surface. When the hot magma encounter the wet sediments, loads of gases are released, forming bubbles and a rapidly expanding melt. The sediments turn into inflated, foam-like fragments attached to the volcanic bombs. It is as if the volcanic bombs strap on life vests which allows them to float to the water surface!

- The 3D tomography of these xenoliths tells us that a considerable amount of the fluids associated to the sediments are released as gases when they are incorporated into the magmas, Sylvia explains. We already know that gas-rich magmas result in more explosive eruptions, and maybe these wet sediments provide a vital ”final push” that enables the magmas to reach the surface.  

Sylvias dissertation, that contain seven papers, is available at the Uppsala University library, and you can download the summary using this link.

Sylvias research was conducted at the Mineralogy, Petrology and Tectonics programme, and was funded by NordvulkVRKVA and MEMOVOLC.

/Börje Dahrén

News archive 2016