Novel Isotopic Insights Into Continent Evolution: tracing juvenile and reworked crust with titanium stable isotopes.

The Earth is the only planet in the solar system having continents roaming at its surface. The presence of continental landmasses in contact with the atmosphere has played a pivotal role in the evolution of the Earth surface over the entire Earth history, and ultimately its habitability. Yet, it is currently unknown what sequence of events led to the development of these landmasses. This research aims to answer a key yet unresolved question: How did continents form? We will do so by using a novel isotopic tracer that is sensitive to specific chemical reactions occurring during the formation and evolution of continental crust.
Continent ages tend to cluster around specific dates in Earth history, but the reasons for this remain unclear. Is it because continents formed episodically in environments akin to present day oceanic plateaux? Or is it because crustal rocks were preferentially preserved during episodes of continental collision? To date, attempts to solve these questions have mainly focused on explaining patterns in the ages of continental rocks or by estimating the composition of the continental crust through time. These studies hint that the mechanism of continent formation may have changed through time with the onset of plate tectonics 2.5 to 3 billion years ago, yet direct evidence for this is lacking.

Here, we propose to use a novel geochemical tool, purposely designed by the PI, to directly identify the processes involved in the generation of juvenile crust and its reworking.
First, we will identify the tectonic setting in which the magmas forming juvenile crust were formed through time. Existing models each imply a very different role for rutile, a Titanium-rich mineral for which involvement in magma generation can be traced through its impact on the titanium stable isotope composition of magmas. By measuring magmatic rocks forming juvenile crust throughout Earth's entire history, we will determine the role played by rutile in their generation and link it with a specific tectonic setting.
Second, we will directly determine when reworking of continental crust started. Reworking of pre-existing continental crust occurs during continental collision. It affects the composition of the upper level of the continental crust, which can be accessed by measuring terrigenous sediments through time. The titanium stable isotope composition of these sediments will reflect the relative amount of juvenile and reworked crust and indicate the onset of continent collisions. Therefore by combining these two objectives we will learn about where crust is formed and survives.

Overall, answering these questions will provide new evidence for the mechanisms by which continental crust formation occurs, and will greatly enhance our understanding of how processes occurring in Earth's interior have influenced Earth's surface.

The proposed research, including the methods involved, is likely to impact positively on the following groups outside of academia:

- Private sector:
This research may provide a positive stimulus to the mining industry by providing new tools to investigate the formation of economically important natural resources and help direct future exploration strategies.
The titanium stable isotope technique used in this project can potentially be used to investigate the source of diamond-bearing magmas (i.e., Kimberlites and Lamproites), which to this day remains debated. Much of the debate hinges on the role of Ti-bearing oxides (e.g., rutile, ilmenite amongst others) in the genesis and evolution of these magmas. The unique sensitivity of titanium stable isotopes to oxide-melt equilibrium could therefore be used to unravel this question. Moreover, Ti-bearing oxides are the main ore for titanium, an element of extreme industrial and commercial importance. Ti is a key component of alloys needed for a wide range of industries, pigments and photocatalysts. Further development of the Ti stable isotope system during this project will make possible future projects focusing on petrogenesis of Ti-ore bodies which will have the potential to guide the exploration industry.

- Policy makers:
There is currently a growing concern over the availability of natural resources to mankind. Areas where old continental crust can be found, which are the focus of our study, are the most fertile localities for mineral resources. Understanding the processes that formed our landmasses and how they played a role in creating an environment able to host life is a way to increase public awareness about the conditions necessary for life to be sustainable and how the current modification of our environment could affect us. The outcomes of this research may therefore complement other efforts by policy makers concerned with these issues.

- Media, general public and Science teachers:
In recent years, the interest of the general public in the origin of life has been brought into focus by a number of space missions, such as New Horizons (which operated the first-ever fly-by of Pluto), Rosetta (recently completed its successful mission around comet Tchouri) or the Mars rover missions (Curiosity, Opportunity, Spirit). This is clearly demonstrated by the large number of television programmes and popular publications that expound these scientific achievements to the general public. The research proposed here will directly contribute to these endeavours and will also be presented at events such as the British Science Week and the Earth Science week, which are annual events that aim to stimulate interest and encourage wider participation in science where researchers present and discuss their research with the public.
In addition, key results will also be explained in teaching material and illustrations. The aim will be to capitalise on the general public's current enthusiasm about blue-skies research by generating further interest in secondary school pupils about STEM university and career pathways. This teaching material will be designed for and made available to pupils and school teachers through the Cardiff School Curriculum Support.