Chronometry in plutonic rocks: cooling rates of ancient oceanic crust

• Applicants: Dr. Kathrin Faak; Dr. Maria Kirchenbaur
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405665352

Since the onset of plate tectonics the formation of new oceanic crust at mid-ocean ridges is a principal mechanism of cooling of the Earth's interior. However, one of the central unsolved problems is how heat is efficiently transferred during the formation of the plutonic section of the oceanic crust. Several models for the generation of the plutonic section of oceanic ridges have been put forward, which mainly revolve around cooling of the oceanic crust by conduction vs. advection of fluids through the crust. Prominent end-member models include the gabbro-glacier model and the sheeted sill model. Attempts to test the models have led to inconclusive results so far. For example, while evidence of deep hydrothermal flow is found in the rocks, cooling of the overall crust appears to be more compatible with conductive cooling. Therefore, it is necessary to map out the distribution of cooling rates as a function of distance from documented fluid pathways, while simultaneously quantifying the fluid flux. We propose to address this problem with a comprehensive, multiscale approach, combining diffusion chronometry and isotope geochemistry on samples from focused fluid flow zones (FFFZ) within the layered gabbros of the Wadi Gideah reference profile in the Oman ophiolite. If such zones have the potential for an effective cooling of the deep crust, we expect strong cooling of layered gabbros at the contact of the FFFZs. Moreover, if such fluid flow is capable of enhancing the cooling rate of the surrounding crust as a whole, such high cooling rates would be recorded even in the far field region with respect to the FFFZ. Alternatively, if high cooling rates are obtained only in the near-field region, it may explain why signatures of conductive cooling for the crust as a whole are obtained even when evidence of fluid flow is present.Therefore, we aim to quantify cooling rates in multiple profiles beginning at the contact to a FFFZ and progressing away into the massive layered gabbro. Cooling rates will be determined on olivine, plagioclase and pyroxenes in various gabbro samples using multiple diffusion chronometers. These include, for example, the well-established Ca-in-olivine geospeedometer together with the recently developed geospeedometers based on trace element diffusion in plagioclase (Mg, Ba, Sr, La, Ce) as well as any possible chronometers using pyroxenes (e.g. Fe-Mg, Sr or Li diffusion in clinopyroxene). Additionally, we propose to apply the fast-diffusing stable Li isotope system, which is a highly suitable tool for the determination of hydrothermally promoted fast cooling rates in the system olivine-pyroxene-plagioclase and can as such be used to simultaneously determine cooling rates as well as fluid fluxes. In addition to providing insights into the cooling mechanism of the oceanic crust, the study would enable us to check the consistency and robustness of different diffusion chronometers applied on the same samples.

• DFG Programme: Research Units
• Subproject of: FOR 2881: Diffusion Chronometry of magmatic systems
• Co-Investigator: Professor Dr. Jürgen Koepke