Collaborative Research: Tracing ca. 4 billion years of volatile cycling in magmas and fluids: insights from halogens in synthetic and natural zircons

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Halogens (e.g., chlorine and fluorine) are key ingredients encountered in everyday life. They are found in consumer-based polymers (plastics), drinking water, toothpaste, swimming pools, cleaning supplies, and table salt. In the geosciences they represent one of the key components in ocean water; deeper into our planet, their concentration can affect whether a rock is a molten magma or crystalline at a given temperature. Halogens also affect the mobility and transport of critical elements in the crust like rare earth elements, cobalt, chromium, and copper etc. While researchers have some basic understanding of halogen interactions between the solid earth and the ocean/atmosphere, there is scant information about distribution and behavior of halogens during the very earliest times of our planet. These early times are important to understand because it is this information that serves as inputs into models that seek to understand the present and future state of Earth. This research will help to understand the complex connections between: (1) halogens inside of Earth’s rocks and (2) the ocean/atmosphere, and the movement between (1) and (2) through time. Since there is precious little information from the earliest Earth, the researchers will rely upon a robust geochemical repository (the mineral zircon which resists physical and chemical weathering with ages that extend back to 4.4 billion years), carefully designed laboratory experiments, and mass spectrometry. This work will educate undergraduate/graduate students in mineralogy, state-of-the art experimental geochemistry/materials science, and mass spectrometry. These training opportunities have applications that extend beyond the geosciences. Halogen measurements of these Archean and Hadean zircons will be combined with high-temperature and high-pressure laboratory partitioning experiments to provide context for halogen-in-zircon geochemistry. Together, this information will be used to explore different halogen cycling models that may have prevailed on the early Earth, the outcome of which would have direct consequences for the composition of the early crust and possibly early Earth surface water chemistry. This research will yield the first ever detailed F and Cl partition coefficients between zircon-melt and zircon-fluid. Results will also be used to constrain the F and Cl contents of early Earth igneous systems (zircon-melt). In addition, zircon-fluid partition coefficients will be applied to a previously identified suite of ~3.9 billion- year-old zircons, which crystallized below the wet granite solidus and may record significant early Archean metamorphic or fluid-flow events. The partitioning studies will enable a broad characterization of halogen exchange between the crust-hydrosphere system and the mantle of the early Earth. Diffusion studies for Cl and F in zircon will be carried out to better establish the conditions under which zircon will retain primary Cl and F. Finally, new Cl- and F-in-zircon analyses will be undertaken, to better characterize terrestrial halogen cycling. The researchers will train a PhD student in high P and T experimentation, electron microscopy, and mass spectrometry. Second, the course “Principles of Experimental Geochemistry”, will be offered as an upper division hybrid lecture- laboratory-based course designed as an active learning ‘studio’ format. Third, an undergraduate student will conduct laboratory research directly related to the objectives of this proposal, leading to a senior research thesis. And finally, the PI will design and build a simple petrographic microscope to enable mineral viewing in plane polarized and cross polarized light to develop a simple educational lesson, using the features of this microscope, for 8-12th grade students. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.