Alkalic pyroclast morphology informs on fragmentation mechanisms, Trindade Island, Brazil

Abstract

Quantitative morphological analysis of juvenile pyroclasts is commonly used to understand fragmentation mechanisms and eruptive styles. Pyroclast morphology has been chiefly studied on conventional tholeiitic and calc-alkaline suites, while the relationship is more poorly constrained for alkaline melt compositions. Here, we analyze the shape of high-Na alkalic juvenile pyroclasts from the Trindade Complex, Desejado, Morro Vermelho, and Paredão Formations on Trindade Island, Brazil, and classify particle populations using multivariate cluster analysis. We expand the field of particle morphology to low-viscosity alkaline suite and interpret fragmentation mechanisms and eruptive styles on products of dry magmatic and phreatomagmatic explosive eruptions. In contrast to tholeiitic and calc-alkaline series magmas, low-viscosity alkaline olivine nephelinitic compositions generate a wide variety of fragments. It includes achneliths, Pele's tears, spheres, sideromelane shards, and tachylyte, formed either by lava fountaining, Strombolian eruptions, or ash explosions, through ductile or ruptile magmatic and secondary fragmentation mechanisms. Phonolitic coarse ash and lapilli fall deposits are dominated by angular juvenile glassy pyroclasts with irregular contours and slightly rounded shapes resulting from dry magmatic fragmentation. The fine ash fall and surge phreatomagmatic deposits chiefly contain lithic clasts, crystal fragments, and blocky and angular juvenile glassy fragments.