Chlorine isotope fractionation in Earth's mantle and crust - Experimental investigation on their role in magmatic ore formation processes and deep volatile cycling

Project Information

ExCliso

Grant agreement ID: 101017762

DOI

10.3030/101017762

Project terminated on 30 September 2024

EC signature date 14 April 2021

Start date 1 April 2022

End date 31 March 2025

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 270 876,48

EU contribution

€ 270 876,48

Coordinated by

JOHANN WOLFGANG GOETHE-UNIVERSITAET FRANKFURT AM MAIN
Germany

Periodic Reporting for period 2 - ExCliso (Chlorine isotope fractionation in Earth's mantle and crust - Experimental investigation on their role in magmatic ore formation processes and deep volatile cycling)

Reporting period: 2024-04-01 to 2025-03-31

Summary of the context and overall objectives of the project

ExCliso is a three-year MSCA Postdoctoral Global Fellowship that experimentally studies chlorine (Cl) isotopes in magmatic systems. So far, the Earth’s deep Cl cycle remains poorly understood. A major amount of Cl is brought into the mantle by subduction, but a significant fraction is returned to the surface as Cl-rich hydrothermal fluids e.g. in volcanic systems. These Cl-rich fluids likely play central roles in transporting certain metals and other elements to form magmatic ore deposits. Cl isotope signatures are generally understood as a signature for the origin of these fluids. Understanding the behavior of Cl isotopes in magmatic systems can help to better understand magmatic ore formation. The project investigates the deep Cl cycle from an experimental perspective, focusing Cl stable isotope fractionation between fluids, melts, and relevant mantle minerals.

Each of the three research objective addressed a scientific question that is faced with a specific type of high-pressure-high-temperature experiments that mimic Earth’s mantle conditions:
Q1: How do Cl isotopes fractionate between saline fluids and hydrous mantle phases?
Q2: How do Cl isotopes fractionate between silicate melts and hydrous mantle phases?
Q3: Which impact do Cl-rich phases have on ore deposit isotope signatures?

The high pressure-temperature experiments are conducted at the Australian National University, Australia and the University of Frankfurt, Germany, with Cl analyses and isotope fractionation enabled by state-of the-art instruments (SHRIMP in Canberra and SIMS in Heidelberg, Germany). These innovative experiments will help quantify the Earth’s deep halogen cycle, while also providing constrains to test hypotheses on magmatic ore deposit formation from a perspective that is significantly different from those currently available.

Figure 1 shows the backscattered electron image of an experimental sample: Synthetic Chloro-apatite reacted with a hydrothermal fluid of different Cl-isotope composition. The fluid is not visible. While the core of the apatite mineral remained in its original composition (bright, smooth center of the mineral), the rim reacted with the fluid and altered its composition and Cl isotope signature (darker, patchy area). A systematic set of about 30 experiments will give insight into the Cl isotope signature change (fractionation) with respect to pressure, temperature, and chemical composition.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Experiments on Q1 were planned for the second phase of the project. Due to delays in other parts of the project no experiments have been conducted with the early termination of the project.

Q2 reached the testing phase. Crystals (or other phases) grown from a basaltic melt remain too small for further analyses, but new results on similar work from a researcher group in Sydney look promising. Together with the researchers from Sydney a publication of the findings is likely in the future - after the end of the poject.

All planned experiments have been successfully conducted to study chlorine isotope fractionation during the reaction between apatite and saline fluids. The samples have been analysed with SHRIMP in Canberra, Australia and data are currently processed. Publication of the results in a scientific paper are postponed to a date after the end of the project.
This experimental study will give a good insight into the behavior of chlorine isotopes in ore-forming related systems, as well as their analytical and experimental challenges.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

So far, three articles on related datasets have been published in peer-reviewed journals. One more is currently under review.

The conducted and planned experiments will significantly advance our understanding of the behaviour of chlorine in the mantle and of the formation of related ore deposits. As our knowledge about chlorine isotopes is mainly based on observations of natural samples, appropriate petrological experiments are urgently needed to investigate the role of Cl-rich fluids as metasomatic agents. So far, no other experimental data have been published on this research topic.
The experimental results are supposed to lead to an improved understanding of the role of high-temperature magmatic fluids in the development of economic relevant ore deposits as chlorine is the main carrier for these metals. This brings together different research fields in the Earth sciences, including stable isotope geochemistry, experimental petrology, and economic geology. The results can thus be expected to raise widespread interest within the scientific community and the mining industry alike.

Figure 1