Moving mountains – when rocks and fluids interact

Research notes

You cannot squeeze water from a rock, or sip it with a straw, but water is nevertheless a component of many rocks. Water in rocks drives geological processes both at and below Earth’s surface. Geologists call liquids and gas that occur naturally in geological material “fluids” or “volatiles”.

By: Ane K Engvik* // NGU Geological Survey of Norway. Synnøve Elvevold, Per Inge Myhre and Øyvind Sunde // Norwegian Polar Institute. Håkon Austrheim // Department of Geosciences, University of Oslo *Also affiliated with the Norwegian Polar Institute

In Dronning Maud Land in Antarctica, nuntaks – rocky outcrops that rise above the thick ice cap – provide excellent rock faces to study. Parallel to the edge of the East Antarctica ice sheet, an old mountain range extends for hundreds of kilometres. The bedrock of this region originally formed about 1100-1200 million years ago.

This old crust underwent an episode of mountain building, which modified the rock and caused intrusion of large masses of new melted rocks 500-600 million years ago. The rocks exposed at the surface today originally formed at about 25 km depth in the crust.

The high temperature and pressure deep within the mountains melted these rocks and transformed them into new varieties.

When rocks and fluid interact

The outcrops in the mountain chain through Dronning Maud Land provide unique insights into how and to what extent fluids infiltrate crystalline rocks in the deeper crust. Mühlig-Hofmannfjella and Filchnerfjella, two chains of mountains 50-250 km east of the Norwegian station Troll, have been visited by geologists during the Norwegian Antarctic Research Expeditions.

In this area the nunataks are predominantly of a dark brown colour due to a dry mineralogy stabilised by the high temperature of the rock during its formation. This anhydrous (water-free) crust is dominated by the minerals feldspar and pyroxene, and the dark colour of the rock is due to both the pristine quality of the minerals and the presence of orthopyroxene.

Conspicuous light-coloured zones frequently criss-cross the dark brown nunataks. A closer look at these paler stripes reveals “alteration halos” surrounding veins that cut through the rocks. These veins can be either fine- or coarse-grained magmatic dykes containing types of rocks called aplites and pegmatites – rocks that are typically formed during the final stage of the intrusion of magma (melted rock) into the crust. These “late” magmas have become enriched in volatiles. Alteration halos are centimetres-to-decimetres thick borders where the host rock has been bleached by exposure to hot volatiles emanating from the magma in the veins.

When samples from the alteration halos are studied under a microscope, we observe a high density of microveins running through cracks in the minerals.

The cracks have healed by recrystallizing and in this process the infiltrating volatiles were trapped as tiny inclusions. Such fluid-bearing inclusions can be studied using special microscopic techniques that reveal the composition of the fluids. We found that the inclusions were dominated by water and carbon dioxide, with some nitrogen. These volatiles penetrating from the veins into the host rock caused hydration and replacement of the originally dry mineral assemblage. The colour change occurred when orthopyroxene was replaced by the minerals biotite and amphibole, while feldspars were microcracked and filled with tiny particles of hydrous minerals and fluid inclusions.

Based on information from both field and laboratory investigations, we conclude that the area under study had been infiltrated by fluids which originated from volatile-enriched magma from underlying melt chambers.

Changing the mountains

The fluid–rock interaction in the nunataks of Dronning Maud Land occurred several hundred million years ago when the rocks were situated deep in the crust. Today these rocks are exposed at the surface, which allows us to combine field studies with detailed laboratory investigations of sampled rocks. Fluid–rock interaction has become a focus of international scientific study over the last decades, and the geology of the excellent outcrops in Dronning Maud Land allows us to increase our understanding of how fluids can interact with rocks even deep inside the crust.

The recent international focus on fluid–rock interactions has led to a general increase in knowledge on this scientific field.

We now know that fluid flow in rocks does not merely cause colour changes, but is also a significant factor in controlling mineral reactions, heat and mass transfer, and deformation within Earth’s crust. The insight that fluid–rock interactions influence and exert control over large-scale geological processes such as continent collisions and mountain building events makes these interactions an important field of study.

Our research in the Norwegian sector of Antarctica provides one more piece of the international puzzle and contributes new knowledge about fundamental questions of Earth’s evolution. The magnificent nunataks of Dronning Maud Land are impressive in their own right. It is only fitting that these rock exposures can also teach us so much about our planet’s history.