First cosmic-ray muography of a crust-mantle transition zone

Abstract Studying ophiolitic crust–mantle boundaries provides key insights into oceanic lithosphere formation, yet their internal density structure remains poorly constrained due to a scale gap: laboratory measurements capture centimeter-scale densities, whereas geophysical techniques average properties over kilometers and lack intermediate-scale continuity. Here, we use cosmic-ray muography to provide spatially continuous density constraints over tens to hundreds of meters, bridging this gap. We present the first muographic imaging of the crust–mantle transition zone (Moho Transition Zone, MTZ) at Wadi Fizh in the northern Samail Ophiolite. A multi-wire-proportional-chamber-based muography system was operated for 171 days at a distance of 400 m, producing a density image with ~ 3.5 m resolution. The mean density of layered gabbros is 3.03 g cm -3 , while lower densities (2.72 g cm -3 ) are observed within the MTZ, consistent with enhanced alteration, and higher densities (3.38 g cm -3 ) in the lower section indicate peridotitic rocks beneath a thin gabbroic cover. These results indicate that the crust–mantle transition at Wadi Fizh is spatially heterogeneous rather than a simple gradual boundary. Thus, muography enables continuous, in situ mapping of density variations at intermediate scales and provides a valuable complementary approach for studying the structure and evolution of the oceanic lithosphere.