Grain-size distribution of volcaniclastic rocks 1: A new technique based on functional stereology

The power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits. Grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits. However, the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record. Here we show that functional stereology can be used to obtain actual 3D volume fractions of clast populations from 2D cross-sectional images. Tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method. We show that the true grain size distribution is finer grained than its representation on a random 2D section. Our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes. We anticipate that the original grain-size distribution of welded ignimbrites can also be characterized by this method. Our method using functional stereology can be universally applied to any type of consolidated, weakly to non-deformed clastic material, regardless of grain size or age and therefore has a wide application in geology.

► The conventional sieving method does not allow calculation of grain size distribution for any clastic rocks. ► The grain size distribution of volcaniclastic rocks can be calculated by functional stereology. ► Very good fit was found between the grain size distributions calculated by functional stereology and sieving methods. ► Compaction of pumice clast into fiamme can be estimated in the calculation of the grain size distribution.