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.
