On the hydration of high-grade metamorphic terranes

The presence of water is one of the most important factors on our planet. Besides its necessity for any known form of life it also helps driving processes deep within our planet itself. So- called metamorphic rocks are `kneaded like dough deep underground, where they are exposed to enormous heat and pressure for millions of years, forcing the elements that make up the rocks to rearrange themselves into new minerals. Water plays a key role in this process because it allows the elements to move around efficiently, helping them to form new minerals. In our project, we aim to investigate this very process: how does the addition of water aid the rearrangement of elements into new minerals? How much water is required and where does it come from? Does the addition of water also release heat and does this heat accelerate the reshuffling of elements? To tackle these questions, we will sample rocks in Eastern Austria. Even though we find them on the surface today, have a long journey at their backs. They experienced intense `kneading, heating and compression, starting nearly 300 million years ago. Over their long lifetime, they had only limited access to water. But when they did, the water helped to nearly completely changing their face, or better the minerals within them. Through looking at these exciting samples we hope to better understand the role of water deep within the Earth and its contribution to the processes that have and are still shaping the planet as we see it today.

In this FWF individual project "On the Hydration of High-Grade Metamorphic Terranes," we explore the interaction between water and rock within the geological material cycle. The availability of water is crucial not only for the emergence of life but also as an essential component in geological processes deep within the Earth. Metamorphic rocks are deformed, heated, and compressed over millions of years by tectonic processes. Extreme heat and pressure cause the elements in the rocks to reorganize into new minerals based on the surrounding conditions. Water plays a key role here, as it greatly accelerates the mobility of elements, facilitating their redistribution into new minerals. This has significant impacts on the material properties of the Earth's crust, affecting the strength and density of rocks, thereby influencing the geological cycle where such density differences are critical. This project investigated these processes: How does water support element redistribution and new mineral formation? How much water is necessary for the process to be efficient? Where does the water deep in the Earth's crust come from? Is it possible that water input releases additional energy that further drives transformation? Studies of high-pressure rocks from the Kor- and Saualpe (Styria & Carinthia, Austria) and the Western Alps (Italy) have shown that even the smallest amounts of water can trigger the transformation into high-density rocks (so-called eclogites). The associated increase in density drives subduction in collision zones and is central to the global geological material cycle. This project demonstrated that, contrary to widespread belief, the required water doesn't always need to be externally supplied to drive the transformation and densification of the continental crust at depth. It is shown that the mineral transformations associated with density increases are driven solely by the breakdown of water-bearing minerals that are already present in the starting material, thus potentially occurring more widely than previously thought. Another aspect of the complex water-rock interaction is the release of heat when water is incorporated into the mineral structure. Similar to the release of hydration heat during cement curing, the incorporation of water into minerals heats the affected rock bodies. As a result, temperature-sensitive isotope systems used for age dating (e.g., argon-argon in micas) are disrupted, causing calculated ages to deviate from the norm. This project demonstrated that water-rock interaction can significantly distort calculated ages, making mineral ages appear up to 10% younger compared to the unaffected scenario. This has significant implications for interpreting geological processes, such as mountain uplift rates and especially surface processes like climate-linked erosion rates. Given that such data sets are often used to study past climate evolution and understand potential future developments, this finding is an important interdisciplinary contribution to understanding the Earth as integrated system.