Water transport to the Earth’s interior: Clues from high-pressure Alpine rocks (Zermatt, Switzerland)

6.6.2018

Water in the Earth’s interior influences many geological processes. The evolution of our planet and the development of life is tightly linked to the deep water cycle. Rocks from Zermatt (Switzerland) document formation at the ocean floor, followed by subduction to 80 km depth prior their incorporation into the Alpine belt. The detailed investigation of these rocks by researchers from the University of Bern provides evidence how and how much water can be incorporated in minerals at these subduction zone conditions and how water might be transported to even greater depth. The results are published in the journal “Geology”.

The shallow water cycle that links atmosphere and hydrosphere is crucial for life on Earth. However, there exists also a deep water cycle in which water is transported over millions of years through the Earth’s interior. Hydrothermal alteration of oceanic crust results in the formation of minerals that incorporate water into the crystal structure. Through plate tectonics and subduction of such oceanic crust, the hydrous minerals are transported to increasingly greater depths. As the rocks are heated up during burial, the hydrous minerals break down at 50-100 km depth and are replaced by anhydrous minerals resulting in the liberation of the stored water. Would it be possible that traces of water are still incorporated into these newly formed minerals? Would this provide a mechanism to transport water to even greater depth and how would this influence the very long term, hundred million years water cycle?

To answer these questions, PhD student Elias Kempf and Prof. Jörg Hermann from the Institute of Geological Sciences at the University Bern investigated rocks from a former subduction zone in the Zermatt area with sophisticated analytical techniques.

Traces of water in metamorphic olivine

The rocks from Zermatt document the metamorphic transformation of the hydrous minerals brucite and serpentin to „anhydrous“ olivine, which took place at temperatures of 550°C and 80 km depth. The detailed investigation of infrared light absorption at microscopic scale showed that 100-140 ppm (0.01- 0.014 weight percent) water is incorporated in the newly formed olivine. Water incorporation into olivine has mainly been studied in laboratory experiments conducted at much higher temperatures. Olivine from the Earth’s mantle, brought to the surface by volcanic eruptions, contain also traces of water. However, the process of water incorporation cannot be observed. The amount of water found in the olivine from the Zermatt serpentinites are among the highest measured so far in natural olivine. The current study is also the first one to demonstrate that metamorphic olivine in subduction zone rocks can host significant amounts of water.

Important for the deep water cycle

The new data provides evidence for a transfer of water from serpentine to olvine in a subduction zone environment. While hydrous phases such as serpentine are only stable to about 100 km depth, olivine is the main constituent of the upper mantle. Therefore, water incorporated into olivine can be subducted to even greater depth and replenish the mantle with water. A mass equivalent to 1/3 of the oceans could be stored in the Earth’s interior if the entire mantle would contain 140 ppm water. At this stage it is still unclear, weather the deep water cycle is in a dynamic equilibrium in which water input through subduction zones is balanced by water loss through volcanism.

Publication:

Elias Kempf & Jörg Hermann: «Hydrogen incorporation and retention in metamorphic olivine during subduction: Implications for the deep water cycle», Geology v.46, no 6, p571-574 Publication on 1.6.2018, doi.org/10.1130/G40120.1

Further information:

Prof. Dr. Jörg Hermann
Institut für Geologie
Universität Bern
Baltzerstrasse 1+3
3012 Bern
Tel.: +41 031 631 8493 / joerg.hermann@geo.unibe.ch

Elias Kempf
Institut für Geologie
Universität Bern
Baltzerstrasse 1+3
3012 Bern
Tel.: +41 031 631 4738 / elias.kempf@geo.unibe.ch