Retrieving past geodynamic events by unlocking rock archives with μ-XRF and μ-spectroscopy

Abstract

Rocks are commonly polycrystalline systems presenting multi-scale chemical and structural heterogeneities inherited from crystallization processes or successive metamorphic events. This work illustrates how spatially resolved analytical techniques coupled with thermodynamic approaches allow rock compositional variations to be related to large-scale geodynamic processes. Emphasis is placed on the contribution of quantitative chemical imaging to the study of 2.2-2.0 Gy old metamorphic rocks from the West African Craton. A thorough analysis of elemental chemical maps acquired on rock thin sections enabled high pressure relic minerals to be located and re-analyzed later with precise point analyses. The pressure-temperature conditions of crystallization calculated from these analyses are typical of modern subduction zones. These results push back the onset of modern-style plate tectonics to 2.15 Gy, i.e. more than one billion years earlier than was consensually accepted. The second part of the paper describes the imaging capabilities offered by the new ultra-bright diffraction limited synchrotron sources. Experimental data acquired with the Maia detector at beamline P06 at Petra III as well as simulations of μ-XRF spectra that will be generated at the SRX beamline at NSLS-II are presented. These results demonstrate that cm2 large chemical maps can be acquired with submicron spatial resolution and a precision suitable for thermobarometric estimates, with dwell time smaller than 1 millisecond. The last part of the paper discusses the relevance of utilizing recent X-ray fluorescence nanoprobes for diagenetic to low grade metamorphism applications.