X-ray microdiffraction mapping on experimentally synthesized clinopyroxenes

Alexandre Cugerone , Luca Caricchi , Michael Stueckelberger, Evan Musterman, Andrew Kiss, Laure Guénée, Martin Miranda Muruzabal, Clothilde Leeloo Jost, Peter Ulmer

Crystallographic parameters serve as fundamental archives of crystallization environments, with pressure generally acting as a key variable governing crystal formation. In magmatic systems, tracking crystallographic and chemical parameters in clinopyroxene is critical for robust magmatic thermobarometry and reconstructing crystal life histories. While chemical thermobarometry is limited by melt dependencies, crystallographic alternatives with the measurement of lattice parameters have historically faced significant analytical barriers too. Specifically, traditional single-crystal X-ray diffraction (SC-XRD) relies on averaging bulk crystal fragments (>40–50 µm) and cannot resolve the intra-crystalline crystallographic zoning necessary to decouple pressure-induced contraction from localized chemical substitutions. Here, we present synchrotron-based X-ray microdiffraction (s-µXRD) coupled with a suite of micro-analytical tools (µ-XRF, EPMA, SC-XRD) and applied to clinopyroxene experimentally crystallized at 0.001 to 35 kbar and 1000 to 1200 °C. During s-µXRD analyses, a fixed-stage energy-scanning setup (13.2–22.0 keV) captured micrometer-resolution, 3D reciprocal-space diffraction data for clinopyroxene within multiphase charges.
The obtained datasets is complex and heavy (multi-gigabyte) and we developed a well-structured automated computational workflow integrating: (1) localized diffraction features and vectorization across energy steps; (2) phase segmentation using µXRF elemental masks (Ca, Cr, and Ca/Fe ratios); (3) density-based spatial clustering (DBSCAN) in reciprocal space; and (4) pixel-by-pixel non-linear least-squares refinement of monoclinic (C2/c) lattice constants constrained by at least 30 robust anchor reflections.
Our results demonstrate that while strongly zoned samples yield crystallographic questionable bulk values via SC-XRD, s-µXRD explicitly resolves distinct core and rim unit-cell volumes and β angles. In 15 kbar crystals, s-µXRD decouples a constrained core (cell volume between 433.45-435.32 Å3) from a more variable rim (432.88-436.78 Å3), correlating with localized variations in jadeite and wollastonite.
Unlike bulk-averaging SC-XRD, s-µXRD isolates distinct crystallographic domains by quantifying unit-cell volumes at the micron scale. This in-situ approach preserves spatial context, allowing estimation of lattice parameters from discrete cores and rims. This technique provides a transformative pathway to refine thermobarometric models in dynamic volcanic systems, offering a robust analytical framework for deciphering complex open-system magma plumbing architecture and the localized physicochemical triggers governing orthomagmatic ore deposit formation.