Constraining P-T conditions using a SEM Automated Mineralogy based workflow – an example from Cap de Creus, NE Spain

The spatial distribution of mineral phases in a thin section provides information about the mineral reactions and deformation history of the sample. This information is often difficult to obtain using classical optical microscopy or SEM analyses, as the spatial resolution is too small to provide the necessary overview. SEM Automated Mineralogy (AM) delivers false colour mineral phase maps at the full thin section scale. Combined with full-sized PPL and XPL thin section scans, this provides an exceptional high-resolution overview of the mineral content and microstructures. Moreover, SEM-AM provides quantitative information about the mineral and bulk rock compositions, which can subsequently be used in thermodynamic modelling to establish P-T conditions for the entire, or a subset of, the rock sample.

The structural geology group at Utrecht University recently acquired a SEM-EDS system with Automated Mineralogy capabilities. The accuracy of the EDS system was compared against WDS microprobe measurements, while the SEM-AM based bulk rock composition of the thin section was compared against XRF data from the corresponding sample dummy. Subsequently, the SEM-AM bulk rock composition was used as input for thermodynamic modelling using Perple_X. Independent temperature estimates were established using; i) SEM-EBSD based CPO results on quartz, in conjunction with the quartz recrystallization mechanisms and recrystallized grain size; and ii) titanium-in-quartz using nano-SIMS analyses. Further constraints on fluid-rock-melt interactions were obtained by using LA-ICP-MS.

This workflow is applied to samples from the Cap de Creus region in northeast Spain. Located in the axial zone of the Pyrenees, the pre-Cambrian metasediments underwent HT-LP greenschist- to amphibolite-facies metamorphism, are intruded by pegmatite bodies, and overprinted by greenschist-facies shear zones. The SEM-AM workflow allowed to further constrain the prograde and retrograde P-T conditions in the different metamorphic zones. In addition, at the thin section scale, the results show temporal and spatial variations in the mineral reactions that occurred.  

In the near future, this workflow will be refined and included in the broader correlative microscopy workflow that will be applied in the H2020-funded EXCITE project (https://excite-network.eu/), a European collaboration of electron and x-ray microscopy facilities and researchers aimed at structural and chemical imaging of earth materials. The data will be made available in a FAIR manner through the EPOS (European Plate Observing System) data publication chain (https://epos-msl.uu.nl/).