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Saline Fluids in the Mantle - Experimental Investigation of Their Role in Diamond Formation and Kimberlite Magmatism

Periodic Reporting for period 1 - SalFluMa (Saline Fluids in the Mantle - Experimental Investigation of Their Role in Diamond Formation and Kimberlite Magmatism)

Reporting period: 2018-02-01 to 2020-01-31

The majority of natural diamonds that have great economic importance for the gem industry are formed at great depth beneath old continents in the subcontinental lithospheric mantle (SCLM). The main carrier of these diamonds are kimberlites, volcanic rocks that transport fragments of the SCLM to the Earth’s surface. Both diamond formation and kimberlite genesis are still heavily debated processes in the Earth sciences. Growing evidence from natural samples, such as fluid inclusions in diamonds, suggest that Cl-bearing (i.e. saline) fluids may play an important role in diamond formation and possibly in kimberlite generation. These saline fluids are envisaged to percolate into the SCLM from slabs of oceanic crust subducted under continents. The main objectives of the SalFluMa project can be divided into the following research questions:
1. What is the nature of reactions between saline fluids and the SCLM? The project has successfully tested existing hypotheses for the formation of diamond-forming fluids by high-pressure, high-temperature experiments. Two distinct reactions between saline fluids and SCLM lithologies were investigated: a) saline fluids reacting with peridotite (i.e. lherzolite, harzburgite) to produce carbonatitic melts, and b) saline fluids reacting with eclogite to produce silicic melts. Our experiments have shown that these hypotheses are valid and could further explain compositional trends observed in other phases from kimberlites.
2. Can the introduction of saline fluids into the SCLM trigger kimberlite magmatism? Based on previous studies, a genetic link between the diamond-forming saline fluids and primary kimberlite melt seems plausible. This study has shed new light on this link by demonstrating a similarity in composition of melts resulting from the reaction of saline fluids with the SCLM to previous estimates of primary melts of kimberlites. This has underlined the possible involvement of saline fluids in the generation of kimberlites.
In summary, this project has significantly advanced our understanding of saline fluids in the mantle. Such fluids have recently gained more attention as a potent metasomatic agent in the SCLM (i.e. able to change the chemical characteristics of the ambient mantle). However, this notion was mostly based on observations on natural samples. The experiments performed here are thus not only completely novel, they are also indispensable in testing existing hypothetical models dealing with various mantle processes, including subduction zone processes, mantle metasomatism, diamond formation, and kimberlite magmatism. The Institute for Mineralogy at the University of Münster was an excellent match for this kind of research due to its outstanding research facilities and personnel. Realizing the project at the University of Münster should reinforce its reputation as a leading institute for mantle research in Europe.
High-pressure, high-temperature experiments were performed at the Institute for Mineralogy at the University of Münster to examine the reactions between saline fluids and different rock types that constitute the SCLM (i.e. lherzolite, harzburgite, eclogite). The reaction products (i.e. minerals and quenched melt) were evaluated by microanalytical methods, and their compositions compared to natural samples and to kimberlitic melts. These novel petrological experiments successfully tested existing hypotheses on diamond formation and kimberlite generation.
A Walker-type multi-anvil apparatus was used for the experiments. The synthetic starting powders were placed in Pd-Au capsules which in turn was placed within the multi-anvil assembly which has been calibrated for pressure. The temperature was held constant during the experiments using a Eurotherm controller. Samples were quenched by turning off the power supply, resulting in rapid cooling to below 500 °C in less than 1 s. Subsequently, the experimental samples were depressurized. Each run sample was removed from the assembly, mounted in epoxy and polished without the use of water in order to avoid dissolving chloride and alkali-carbonate phases. The samples were first examined using reflective light microscopy. Higher detail microscopic features and phase compositions were studied using the back-scattered electron mode (BSE) of a scanning electron microscope (SEM). In order to obtain quantitative analyses of phase compositions, a state-of-the-art electron probe microanalyser (EPMA) with a field emission gun was utilized. The Institute for Mineralogy at the University of Münster constitutes a unique location in Europe where all these analytical techniques are housed in the same building.
The results were presented at different stages of the research project at the following seminars or conferences: at a scientific seminar at Geoforschungszentrum (GFZ) Potsdam, Germany (June 2018), at the 16th International Symposium on Experimental Mineralogy, Petrology and Geochemistry (EMPG) in Clermont-Ferrand, France (June 2018); at the 3rd European Mantle Workshop in Pavia, Italy (June 2018); at the annual meeting of the Geological Association of Canada and Mineralogical Association of Canada (GAC-MAC) in Quebec City, Canada (May 2019); at the Goldschmidt Conference in Barcelona, Spain (August 2019). A manuscript for the journal Contributions to Mineralogy is currently under review.
While the SalFluMa research project constitutes basic research, understanding the processes of diamond and kimberlite formation is certainly relevant to the diamond industry which also constitutes an integral part of the European economy. For example, whereas much of the rough-diamond production is focused in countries like Russia, Canada, and South Africa, the diamond cutting and polishing business, as well as diamond jewelry retailing, is centered in European countries, such as Belgium. The results of the project could thus draw the attention of European companies related to the diamond industry and potentially of the public/government sector, as well.
An important pillar of the project was to reinforce the University of Münster as a center for mantle research in Germany, and Europe. Establishing and reinforcing the University of Münster as a center for kimberlite, diamond, and mantle research with strong ties to global players in the field, will contribute to the competitiveness of German and European research in general. This, in turn, will attract talented scientists and students, who will ensure that world-class research in the field of petrology will continue to be conducted in Germany.
SalFluMa cartoon