Fig. 1: Geological map, field images and microstructures of the studied omphacitite.

a Geological map of the Lanzo Massif, Italian Western Alps. The star highlights the location of the outcrop (GPS coordinates: 45°17'44“N 7°30'36“E), the red square the approximate geological map location (modified from31). Figure created with Adobe Illustrator CS6 (https://www.adobe.com/products/illustrator.html). b Outcrop of omphacitite layers, a few meters thick and tens of meters long, bounded by serpentinites and discontinuous talcschist-rich layers. c Polished slab of omphacitite displaying a fracture network (light green) and graphite-garnet-bearing jadeitite veins (silvery gray). d Enlargement of the contact between omphacitite and talcschist-rich layer. e Thick section scan highlighting the brecciated structure, with omphacitite clasts surrounded by a matrix of jadeite, omphacite, grossular, titanite, and graphite. The yellow arrow shows a graphite-enriched stylolitic structure between two interpenetrated clasts. The location of fluid inclusion shown in Fig. 6a.