Impact of ephemeral cataclastic fabrics on laser diffraction particle size distribution analysis in loose carbonate fault breccia

Modern laser diffraction particle size analysers provide the possibility of fast particle size data acquisition over a wide size range by using a variety of analytical methods, named standard operating procedures. We performed specific tests on poorly coherent carbonate platform cataclastic rocks from a fault zone in the Central Apennines, Italy, by combining laser diffraction granulometry, thin section analysis, and optical morphometry. During laser diffraction granulometry tests, we used several wet and dry operating procedures that included different pump speeds, analyses with and without sample ultrasonication, and different dispersant liquids. The variability of particle size distributions from a given sample, as a function of the adopted operating procedure, has the same magnitude as that theoretically predicted in natural cataclastic rocks, from low- to high-deformation shear zones. Thin section image analysis and optical morphometry support mechanical disintegration of internally microfractured coarser particles in ephemeral cataclastic fabrics as the major cause of such a size variability.